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What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?
7

What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

(OP)
I'm a designer - there is a recurring theme with some parts we've seen recently, where there is a sharp step right where we'd least want one. We battle with other departments - they say it meets the print, we say it doesn't.

For example, we might design a bracket with a fillet/blend specifically to avoid a sharp internal corner for stress reasons, but we control the size of the radius or the adjacent flat surfaces with a profile tolerance - say .015 (we use inches). I realize this allows for considerable surface variation (like they draw in the GD&T books!), but we also have a block tolerance surface finish of 63 or 125. Here's an image of it (blue is as-built, grey is as designed):


You can see there is a step near the root of the fillet. Likely, this was machined with two different tools or from two directions, so I understand HOW it happens. At prior jobs, quality would have rejected or reworked the part. At the current job, Quality says "because the step is within the surface profile tolerance, it meets print."

I originally thought block tolerance surface finish of 125 Ra should cover it. I then thought maybe we should say this is a controlled radius, but we see steps at the tangency point. The more I consider it, our surface finish should call out an Rmax that prevents this. I'm particularly concerned if this happens on aluminum parts prone to fatigue.

How do other people control for this type of manufacturing artifact with the print?

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Fire the company that makes parts like that. That works as it is at least a workmanship problem, but I see where QA/QC would weasel on the profile tolerance.

I guess you could try "NO STEP CUTS" allowed, but it is certainly outside the surface roughness requirement.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

1. Try to specify "non-uniform" profile zone (see picture).

2. If your shop still try to justify abrupt changes within the tolerance zone, the "per unit" control is not only for straightness / flatness:


"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

SBlackBeard,

I would argue that your step is a feature you did not specify, therefore you reject the part.

In a court of law, your vendor may be able to prove his part meets your specifications. Don't deal with that vendor ever again.

--
JHG

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Out side diameters and adjacent shoulders it is very simple to turn the radius, and adjacent shoulder. Where it get difficult is inside diameters and adjacent shoulders.
Where on a single tool can be used leaving no step.
In an id if there is no clearance it may require two or more tools and a step may be unavoidable. I suggest in such cases a .005 max step on all surfaces. Excuse the typing ol cell phone.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

When milling steps are unavoidable and have to be smoothed out by hand. Requiring hand blend. And is time consuming and expensive.
So only add no steps allowed when absolutely necessary.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

SBlackBeard,

I just thought of something else.

Does that notch conform to your surface finish specification?

--
JHG

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Only areas with steps would be transition from a surface that transitions to an other surface like a radius.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

To make it clear to inspect and not be interpreted wrong make it clear no steps allowed or .005 max step permitted.
So this is clear and concise.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Would specifying a controlled radius work here?

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

(OP)
Thanks for all the replies.

@3DDave and others - I have little/no control over firing a vendor. I could make a stink on some parts and ask for "return-to-vendor" that would maybe start to hurt a vendor's score card, but it's hard when Quality is saying "what's the violation?"

@ many - I could put "no steps" - but I'd be adding this note to every print with a fillet. Then the next scratch on the print would make me put "no scratches" in the notes. Then the swirl, then the ding, then the yadada... It seems like big OEMs should have it figured out how to prevent a stress riser in radiused brackets and shafts. But maybe this is still the way to go.

@drawoh - yes, I agree, surface finish was my go-to, but I think the standard block tolerance uses Ra as 125 - letter 'a' below. My thought was that maybe I could put an Rmax of .0002 on the print? That would be where letter 'b' is below, so I'd have to change block tolerances or add a new note.

@SDETERS - I'd thought that too, but this step is really at the tangency point. How do you specify the quality of an implied tangency?

I feel like I just want to write a nasty-gram to Quality and the Vendor, but I'd rather make a change to all my prints in the future that is defensible and parts don't get here that I need to argue about. So far, Rmax or "No steps" are the leading candidates. Appreciate the opinions, keep them coming!

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Talk to your legal department about what happens when a fatigue crack starts at the step. Let them talk to QA.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

If you have to tell a vendor things you shouldn't have to, it's time to part ways.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

If we fire every supplier that made a part that complies with the drawing yet doesn't meet your expectation, we would have no suppliers left.

Quote (SBlackBeard)

Quality says "because the step is within the surface profile tolerance, it meets print."
This is 100% correct.
The question of "What's the violation?" is 100% valid. This is a drawing issue, not a supplier issue. You are correct to strive to fix the drawing rather than strive to fix the supplier. Everybody wants craftsmanship but nobody wants to pay for it, so being specific about the part requirement on the drawing is crucial. I've seen some good suggestions with other responses. CR is one possibility. Controlling the radius with profile on a per unit basis is another possibility. Something as simple as the note that was suggested is a good one too.

Quote (SBlackBeard)

We battle with other departments - they say it meets the print, we say it doesn't.
Why do you say it doesn't? What part of the drawing is being violated?

Quote (SBlackBeard)

...but I'd rather make a change to all my prints in the future that is defensible and parts don't get here that I need to argue about.
THIS RIGHT HERE!!!

Keep your supplier and fix the print. If they can't meet your newly stated requirement then a supplier change is in order.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

powerhound - I see the value in your specific suggestion on dealing with this. I'll put that note on drawings from now on. That fix you suggest is brilliant and should be a chapter all its own in the next revision.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

(OP)
3DDave, TheTick, powerhound - thanks for the replies.

I looked at this case in more detail, because it did seem like surface finish wasn't met... even though block tolerances call out Ra and not Rmax.

Working in theory and general terms is nice, but this specific one had a .003" step.
I drew it with the fillet, but it likely works about the same assuming it's flat.

In a spreadsheet I wrote out the deviation as .003 for 8 lines (arbitrary) and .000 for 12 lines. The average value is .0012. Ra is the average of the absolute value of the difference between the average and the actual for each line. So a step of .003 minus average of .0012 = .0018 for the first 8 lines, then for the next 12 lines, the absolute value of .000-.0012 = .0012. The average then equals (8*.0018+12*.0012)/20 = .001226 or an Ra of 1226 microinches. I did this again assuming it was a ramp from 0-.003 across the first 8 lines (.0009/.0012/.00015/.../.003) and then .000 for the next 12... that's an Ra of 806 microinches. I then dialed down the step height until the Ra calc equaled 125 microinches - you'd have to have a ramp of 0-.0004" or a step function of .00026 to meet an Ra of ~125. Even if you play with the number of lines that contribute, the answer stays in that ball park. If 19/20 lines are .00026 and one is 0, Ra is 153 microinches. If 1/20 lines is .00026 and the rest are .000, the Ra is 107. Anyway, not sure everyone will follow that but the summary is that it looks like if you were to swipe a profilometer across any step that is a few tenths high, you'd fail surface profile requirements, even when they are expressed as an Ra and not Rmax. I'll see if I can validate this with an actual profilometer and a .0005 feeler gauge.

I don't have time right now, but the next step will be to dig into what the stress analysis answer would be.

BTW, My intent is not to come up with a mountain of data and storm in waving it in someone's face - my intent is to figure out "what's right" - if stress folks don't care on certain parts, then let the vendor do what they need to. If a step like this is a big deal for stress on other parts, I'd prefer to find a clean and defensible way to state it on the print.

Thanks for the continued input.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Dave,
I'm not at all surprised by your response. I knew it was coming but didn't really care. The reason is because most of your responses over the years have been hard nosed and angry. I don't know why you have that attitude but your "specific suggestion" of firing the supplier lines up with it perfectly.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Your response is expected. No value added to the original problem, just attacking those who point it out. Pandering to the incompetent suppliers that make their problems the problems of engineers to guess just how they will do the damage and require thousand page documents to cover every possible way they can screw up.

Had one like that - engineering in my company decided the supplier had a flow chart to follow of excuses. The supplier decided not to passivate stainless parts (per the drawing,) which showed up in the fog-chamber test as rust. Their response: Fog chamber must be contaminated. Spent many hours on outside testing to prove that wasn't the case. Then it went to salt-fog. Rusted up like a furry caterpillar. Their excuse: they used a special anti-corrosion treatment, but just missed this one part. That treatment? Overnight oil for use in machining parts. It went on and on. We specified sealed bearings - they "sealed" them with grease. The best one was a gear driven synchro. The assembler took the item from their packaging and the gear fell off. They said it was mishandled. We checked the rest and the supposedly press fit gear (designed to spec not to dimensional drawings, so their design responsibility) was loose on all of them. Because by that time we demanded that the housings were passivated. So they had pried the gears off, and put them back on. Loose. So their excuse: we didn't specify an axial retention force. Sure - we specified that they survive a 30 g impact load, but just 1G was too much.

I'm tired of watching people do crap work and make crap excuses. If you prefer to cover for the incompetence of others that's certainly yours to do, but it does erode the industry - it's an acid that cuts and destroys and putting up with poor performance and excuses hurts everyone.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Here's how a reasonable company deals with a defect like that:
Company: Is that step a problem?
Engineer: Yes, it is.
Company: We will re-work the parts to remove that step.
Engineer: Great. What can I put on the drawing to tell you not to allow a step?
Company: We use a general note "NO STEPS LARGER THAN THE SURFACE FINISH ALLOWED."
Engineer: OK - then we'll do that.

(Actual outcome)

New design goes to another company and they ask why that note is on the drawing when surface roughness covers it. The engineer shrugs and the other company guy says "You were buying from XYZ, weren't you?" The engineer says they cannot comment. Both laugh.

When the response from a company is - We. Don't. Care. About. You. that's a reason to fire them. When the part they made is one they would reject if they were buying parts for their own use but want you to pay. That's a reason to fire them. When they refuse to offer an answer about how to avoid this and the procurer has to wander into the wastelands looking for a solution to this one supplier problem. That's a reason to fire them. When that part, in fatigue sensitive aluminum, has a stress riser that might crack and ultimately kill someone and they act like this. That's a reason to fire them. Did the company call and inform that their process would create a step in that location and ask if that was suitable before accepting the contract? If not, that's a reason to fire them.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

To your first response: If passivation was on the drawing and they ignored that requirement, that's indefensible. They didn't do what the drawing said to do. I'm not arguing about that. Bad on them.

To your second response to me which deals with the OP: that's a drawing change that was actually one of your suggestions that I mentioned was a good one in my first post on this thread (albeit not to the value you recommend). As far as surface roughness being sufficient to prevent that step goes, according to ASME B46.1-2009 [1-1.2], that step is only considered a flaw if it has been agreed upon in advance by the the supplier and customer. In the absence of that agreement, it's not a flaw so rejection of the part shouldn't be based that as long as everything else is within the specified tolerance zone. It's likely that B46 is not actually invoked on the drawing though so nothing actually locks in, by default, the step being controlled by the surface roughness. The other thing to consider is that since Ra is an average, the step can actually be larger than 125 and by how much depends on the condition of the rest of the surface.

Regarding the last paragraph of your second post to me: I agree with everything in it. Is that actually the case here though? Did any of that actually happen? Is the severity of this flaw such that a failure will cost lives? Maybe this is just a cosmetic issue.

And finally, I wasn't attacking you. I just think it's a bad idea to fire suppliers who make parts that comply with the requirements they are given. If you participate in a forum like this, you can't be so deeply offended when someone doesn't agree with you. In my opinion, adding the note you suggested is sufficient but not to 125 microinches. Now inspection will reject any part that has a step larger than .000125". That doesn't seem to make sense to me if this is just a cosmetic feature.

John Acosta, GDTP Senior Level

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Look at the title of the thread: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

I don't care about -this- part. For all I know it's a paperweight. And cosmetic features aren't a worry for stress concentrations or crack initiation, the subject of the thread.

I get that you tolerate suppliers who have a bucket of excuses for making parts that do not conform to the topology as shown on the drawing. What I do know is the Y14.5 committee has zero answers for this problem that aren't going to end up as a thousand page specification to cover every possible imaginable way for a supplier to screw the customer.

Example: I would love to specify cylindricity on every hole with a tolerance of 0.0001 so that they don't use a single tooth nibbler that leaves sharp crack initiation and corrosion sites. Or maybe that's how they profile a part with each facet having surface roughness of say 63 microinch but overall rough enough to use as a wood rasp.

On flat surfaces one can use flatness per area, but one needs to use several of those at different scales to keep the more clever suppliers from screwing that up. On non-flat surfaces I don't know of any similar control, so one is back to a supplier who takes the parts back and either reworks them or replaces them for not being what they would personally accept for their own use.

Oh - we didn't require them to passivate. We required them to pass salt fog and they chose not to passivate the stainless steel parts after assuring us they would when they chose not to paint with the paint schedule we provided.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

3DDave & PowerHound
apples and oranges.

The drawing has to be clear and concise yet there are a lot of suppliers will not spend extra time and money just to make a part look pretty,
they will make it to the drawing requirements. my opinion making it no steps permitted is clear. they have no idea what the part does or if stress risers are critical.
it's up to the engineer to supply that on the drawing.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

the example given by the OP is a way to simplified drawing, there are very complicated parts which are very difficult to make.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Has been indicated that profile of the surface could work.
Per another discussion pmarc recommended to use composite profile with dynamic profile modifier on the lower segment.

Re:"About usage of composite profile, I think that even more appropriate control could in many cases be the dynamic profile (ASME) or the Offset Zone, OZ profile (ISO) as an additional
requirement to a more generous regular profile callout. Quite often, the design intent is to not allow the fillet curvature to go below the minimum radius (max radius limit is of less importance) but still have its shape/form tightly controlled to avoid abrupt flats or reversals.


Lower segment of the composite profile will not allow the "absolute size" of the fillet to fully vary from the min to max established by the upper segment."


If only regular profile is used then we can get this issue - per Mark Foster- and as the OP said the surface pretty much can be any shape and form within the specified tolerance zone, hence pmarc's idea to used dynamic profile

“I'm working with a client right now in an attempt to document this little-known revelation. If you don't understand what I'm trying to state with words or if you simply don't believe me, lay it out in a CAD system. Draw two boundaries with radius 99.95 and 100.05 in a 90-degree arc. Then draw the largest radius surface that you can fit into that 90-degree arc and the smallest radius surface that you can fit within those boundaries. Now measure those "surfaces" that you just drew. You will find that the radius of the actual surface can be substantially smaller than 99.95 and larger than 100.05 with all the points on the surface remaining within that 0.1-wide tolerance zone.“

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

An actual part that affected me - I bought a gas can with the EPA mandated sealed, spring loaded, interlocked spout. To operate there is a latch lever that needs to be overcome before the plunger can be moved and of course they are integrated.

Really, a very nice design. So I go to use this, pressing the lever with the palm of my hand and pushing directly on the face of the plunger - which didn't move. At all. Since this was the first time using it and being the middle of the night putting gas into a car I ended up using a channel lock plier to dismember the thing to get it to dispense the gasoline.

Later, in examining the broken fragments to see why such a simple task was such a problem I find that the designer of same had gone to great lengths to ensure that this problem could not happen, only to be undone by the designer/maker of the molding tooling.

The core has a plastic guide rod that fit into a hole in the plunger to keep the plunger from going crooked. That hole was made with a nice lead-in chamfer to avoid digging into the guide rod.

But that's where the mold tooling guys stepped in. They made the chamfer part of the mold separate from the remainder of the hole pin with a larger contact face than the hole pin, leaving a razor sharp edge that, because the release tab is off center, meant that the slight off-center load forced that razor sharp edge into the guide rod. Push harder and it gouges in more. The plunger is a rigid plastic and becomes self-locking. They did put a fillet between the face of the part and the chamfer plug; too bad this doesn't touch anything.

To make it worse, there are finger grips to brace the load being applied to the plunger against the force from the EPA hard seal spring - using that obvious feature ensures the load is far off center; about a 30 degree angle to drive the gouging edge in. It really only works easily if, as I did, the spout is disassembled (it is designed to not be disassembled) and the sharp edge gets carved away or if the plunger is pushed on the side that doesn't have the release lever, which means a second hand to hold the locking lever to start it.

Had the hole and chamfer plug been made as a single piece, there would be no razor sharp edge. Or they could have made the hole plug larger than the mating truncation on the chamfer plug so the step would be on the back side of the chamfer so it could not dig in.

Now I'm sure they had mold mismatch requirements, but I bet they didn't foresee the maker adding an additional step feature at the one place that would ruin the part function. They made the chamfer feature in the mold with a face a tiny bit larger than the matching rectangular plug that formed the rest of the hole, probably to eliminate flash - which would have had no effect on the function of the part.

The maker did send a replacement but about 2-3 years later, the ones on store shelves still have the problem despite my telling them what the symptom was and where to look to fix it. I guess they decided to just keep making them.

I'll not be buying that brand again if I have any other alternative, though they seem to have a lock on the market since Blitz went broke.

---

Also - funny story. I had a Blitz can leak in my car. Filled it up set it where I could ensure it would not tip and I could see it and the side of the can was wet. It took a bit to figure out why. Found a slice right along the edge of the label, so perfectly as if it was planned. So the next trip to buy a replacement I looked at the shipping/display. They used a full cardboard box with a "cut here" line marked at just about exactly the same height my can was slashed.

I dropped Blitz a note that stores have people with box cutters and maybe lining up the "cut here" with the side of the blow-molded cans was a bad idea. No response, but, a couple of months later I was in the store for something else and noticed that they were now in a half-box with a clear shrink wrap so that employees could see where they were cutting. Coincidence? The first one I'd had for about 5 years or more and they had kept that shipping design at least that long.

Though seemingly very responsive Blitz got sued out of existence because they didn't have an anti-flashback feature and some users poured gasoline on open flames, so it is likely their legal team took product liability issues very seriously. I also understand the lack of a response - no need to generate a paper trail. I think they went under the next year after I bought a Blitz replacement (carefully checked for knife wounds before purchase.) Now I have this complicated monstrosity. Can't get sued if the fuel won't come out. There are places that sell cheater spouts that must love the EPA and this complicated self-jamming design. Which doesn't have a flame arrester.

https://www.nytimes.com/2012/10/05/business/in-a-s...

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Quote" An actual part that affected me - I bought a gas can with the EPA mandated sealed, spring loaded, interlocked spout. To operate there is a latch lever that needs to be overcome before the plunger can be moved and of course they are integrated."

man I really dislike the new pour spouts that come with the plastic gas cans. they are really bad

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Quote (CheckerHater)

2. If your shop still try to justify abrupt changes within the tolerance zone, the "per unit" control is not only for straightness / flatness:

I had to look up this figure so that I could include it for anyone interested (attached).

Question as to the application: does 'length' become an absolute measure or a linear measure--and how does that translate to a profile on a radius? (arc-length?)

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

What is the difference between "absolute" and "linear"?

As far as I can tell there is no similar control for arcs/arbitrary profile shapes. Just flat and straight.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Well for a theoretically straight part the absolute length deviates from a linear length as it becomes less straight. ( vs. |

That said, that's just a point of curiosity--the applicability of a profile as it pertains in a similar situation (tolerance per length) that is not nominally straight is what is of interest in how it would be applied and interpreted. The bit CheckerHater included alludes to there being a similar control for profile for curved parts (or more specifically, to changes in profile tolerances--but the context is for curved parts).

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

The "Note" I quoted is part of Para. 8.3.2.2 which refers to Fig.8-11(b)

So I guess control can be extended to curved parts (At least creators of Y14.5 think so)

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

On unrelated matter - mfgenggear, you need a third hand to operate those spouts.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

8.3.2.2 refers to a tolerance zone-to-tolerance zone transition, not a transition within a zone. While that might handle the OPs exact image it does not help with the thread titled problem.

The Note about tolerance per unit length is alluding to a dynamic profile zone without requiring it to by dynamic. Since dynamic profile was not part of the 2009 version it is ambiguous of how it would be specified and inspected. It would also exclude, for the '2009 notion, a smooth but sloped change in profile.

For 2018 the only 'per unit length' mention is straightness.

The standard is missing surface curvature continuity controls that would essentially represent the nose radius of the tools used to cut/machine the part and, separately, a way to exclude the curvature in scratches or other microscopic features where local curvature is very high but the transverse excursion is very small.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Quote (3DDave)

For 2018 the only 'per unit length' mention is straightness.

...and I just realizing I was looking at the 2009 version and not even considering the 2018. Since this is not an issue I've run into before it wouldn't have dawned on me to compare these particular differences in the standard. So thanks for that, I guess. 2thumbsup

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

My hot take:
It sounds like the vendor is producing the part this way because the less expensive machining option creates a form that meets the tolerance zone but can leave a step. I agree, I can't imagine many design reasons to leave an internal radius in a bracket that would be compatible with the step feature. But the vendor cannot be expected to know that.

I think the solution lies with the drawing. A variety of solutions are here, but I like the understandable one, which is to add a note "internal radius features must blend with no step greater than the surface finish". Do your surface finish specs allow them to grind or polish the step using a belt sander? If so, that could be very helpful.

It sure is nice to work with vendors who don't "cut corners", who in this case, did it literally. But at the end of the day, we want vendors who consistently produce the cheapest, most consistent, fastest parts that meet the definition on the print. And we designers are responsible to make the best designs and communicate our needs via the drawing itself. Unless you're in a cost-insensitive market, you can't afford to have vendors who spend your money making everything nicer than specified, because for every one instance where it helps, there are likely nine others where it's waste. There will of course, be disconnects. We must be humble and listen and our drawings must evolve as these little wrinkles present themselves.

As for crappy gas cans, try No-Spill brand. They're a very secure design. My only critique is they drain slowly, which is annoying when I'm filling the entire 5 gallons into a large mower tank and the gas slowly gurgles down the spout because air is forced to inrush back through it. For filling small tanks the slow draining is beneficial. I installed an aftermarket air vent valve in each tank and now they are perfect. I had a couple of Blitz cans and they were junk. None of them worked as advertised for more than a month, and I'm sure the result of the regulations & actual products being sold, we lost more fuel and fuel vapors than before.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Tmoose, That can, but only for this particular case; it's usually called a relief and is frequently used. If the machinist decided to form the step anywhere else along the straight section then that would not work. For the general case Y14.5 doesn't have solution to forcing continuity on a nominally continuous surface.

For example on a simple cylinder with size limits the machinist apparently is OK to form a series of ridges that conform to MMC and are divided by LMC segments that may or may not be coaxial; or a sawtooth pattern with sharp edges like a screw thread. The only defense is as previously mentioned is to specify a ridicioulsly small cylindricity, but that would eliminate the ability for the potentially acceptable case of a smooth shaft with a slight bend as is typically depicted for illustrating the envelope principle.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

You could add a general profile of surface tolerance in the notes, without any datum references, which is just as tight as the maximum step you would accept.
If you already have a single segment general profile tolerance with datum references, you could change it to composite or two segments (in the case of composite, simultaneous requirement doesn't apply anyway to the lower segments, so an unintentional tight pattern isn't an issue).

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Burunduk: that could put a very tight constraint on the angular relationship between the vertical and horizontal faces if it was not previously critical.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

BiPolarMoment, why would it?
Without any pattern-creating notation such as the number of places (nX) or datum references that could invoke a simultaneous requirement, the tolerance would apply to each feature separately. It would only impose a standalone form control for each feature.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Perhaps I'm having a brain-fart in visualizing this. Does not the continuous feature (vert-curve-horizontal) have to fall within your suggested profile of a surface? If the tangency points of the surface are not included in the profile then I don't see how a discontinuity (i.e. step) would now be impermissible by adding the profile.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

BiPolarMoment,
Imagine that each of the three features (the vertical surface, the radius, and the horizontal surface) were associated with a separate feature control frame of surface profile without datum references. Would you then say that the angular relationship between the vertical and horizontal surfaces is controlled by those profile tolerances? A general profile note without datum references would act the same. 

I sort of get why you are thinking that the tangency points (which are actually lines in 3D) could fall through the cracks, but I don't really consider it as a real issue, let me tell you why: The tangency line is only there in theory. On a real part, if it's made to print, the best you could have is some narrow border zone, not precisely defined in any way, where one surface blends into another. This zone can be considered as part of both surfaces, and points on this zone could be inspected as part of the control of one surface, or the other, or both. So if there is a step right between the radius and one of the planar surfaces on the part, it could be detected and treated as a violation of either the profile tolerance of the radius or the profile tolerance of the planar surface. 

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Okay, this is a little bit of mixed message... the profiles are not connected, but the area in question has to be in one (or both) of them... sounds like they are defacto connected at that (each) junction? In any case the fact that it can be inspected as "one [...] the other, or both" still makes it ambiguous, no?

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

The main problem is that this would limit the overall form of the part to be within some tiny amount of the true shape. Which solves one problem by spending a large amount of money machining it to be nearly perfect and a larger amount in inspecting it and fitting all the data to see that it conforms. The more ideal goal is to allow the form to vary a lot (while remaining functional) but not to allow irregularities, like steps, which are features that don't appear on the drawing.

I might not care, for example, if a hand rail is 2.00 inches or 2.25 inches in diameter, but I do care if there is a sharp step that gouges my hand if I slide my hand along the hand rail. Sure, making the tolerance to be 0.001 inch will make the rail smooth, but at considerable expense for the overall dimensional control when all I really want is smooth transitions if the size/form varies.

Y14.5 doesn't deal with that requirement. I think it is because they don't see that as acceptable workmanship. The closest they come is profile tolerance zone transitions that say, effectively, a part with a smooth transition is acceptable, but it does not force the surface to have a smooth transition.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

BipolarMoment,
This is a really minor issue. Yes, I could imagine a case where it could be ambiguous which feature is non-conforming if the violation is right on the border.
But there would be no way for the separate profile controls to affect the angular relationship allowance between the two planar features.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Quote (3DDave)

The main problem is that this would limit the overall form of the part to be within some tiny amount of the true shape.

If you mean that the datumless general profile note would act similarly to profile all-over, then no, it would apply to features individually. The note would effectively control form, UOS. Cylindrical features can be excluded. If this is too costly or troublesome, the control could be applied directly in the drawing views only to some features that are at risk of stress concentration or where irregularities need to be tightly controlled from any other reason.

The handrail example could be solved by a cylindricity requirement. Plus, any sharp defects that may cut hands can be eliminated by an appropriate surface treatment.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Perhaps you've never used a hand rail, but they don't need to be perfectly straight and can be somewhat oval, but sure, using hydraulic shaft tolerances to steady me on the stairs would be fine and very expensive. I don't believe a 0.25 inch sharp shoulder bump can be taken up with a surface treatment.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

3DDave,
You didn't comment on the first part, but I hope you now agree that a datumless general profile note would not "limit the overall form of the part to be within some tiny amount of the true shape".

As to the handrail example and similar cases - straightness, cylindricity, total runout, or profile applied on a cylindrical part would control bumps/steps, and other form errors. If this is not desired, and you are looking to limit only bumps/steps without limiting other form error types such as bend or ovality, there is no other choice but to have some document or notation defining what a bump/step is and specify a custom control to limit it.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

The profile tolerance is the deviation from the nominal surface. If that number is small then the overall form must be within some small deviation from the true shape. Since it applies to the entire surface it is a simultaneous requirement for the entire surface.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

3DDave,
Looks like you are using the term "surface" in the general sense, like the outside layer of an object.
In the tolerancing language terms (ASME Y14.5), the typical part has more than one surface. Each part feature is a surface or a collection of surfaces.
"A simultaneous requirement for the entire surface" - if that's one at a time - then yes. Also, not for all surfaces of the part, but for each surface to which the tolerance applies - those not otherwise specified, if the general tolerance note includes the UOS wording as it does typically.

There would be no simultaneous requirement or any sort of pattern that connects the tolerance zones, since the profile tolerance in the note would not include datum references to impose a simultaneous requirement (per section 4 of Y14.5-2009) and so none of the following would apply: "nX, n COAXIAL HOLES, ALL OVER, A ↔ B, n SURFACES, simultaneous requirements, or INDICATED" to create a pattern per the section 1 definition (note the plural in "SURFACES").

But if that's not clear enough from some reason, "INDIVIDUALLY" could be added for clarification, and the problem is solved.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

I think you two are now finally arguing about where I was tripped up... my interpretation was that those three segments constitute a singular surface due to the illustrated tangency unless you intentionally segment it (e.g. A↔B, B↔C).

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

BiPolarMoment,
It is the opposite.
By default, a profile tolerance applies to a single surface/feature, unless you group several of them by A↔B, all-around, all-over, the number of places where the tolerance applies being specified in one way or another, indication of features as belonging to a group by a labeling letter, or if a simultaneous requirement is in force.
The argument with 3DDave is actually empty, as the standard is quite clear about it.
Hope this clarifies the matter, and you aren't tripped up anymore.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Can someone explain to me why adding a note of "Max Rz 125 microinches" doesn't cover this requirement? (In practice - if you want 125 microinches Ra then you should allow a bit more; 200 microinches(?) Rz.)

I get why Ra and RMS won't cover this issue. Those are varying methods of calculating an average. Rz is the maximum difference between a peak and trough. This specific issue is why Rz can be called out?

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

I think a Max Rz is perfectly correct solution. However, it will work best if the areas of concern can be directly measured with a profilometer. I think our profilometer probably can do it, which makes it appealing to me.

I think that's the element of many of these ideas that will separate really good solutions from the technically correct solutions. 1) will it be readily understood by a machinist with common working knowledge of GD&T (and not an advanced degree) and 2) can success be measured directly using common tools? Short of situations involving a Tier-1 supplier producing thousands of parts, I find these questions to drive most of how I tolerance and control a part design.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Burunduk - you would be less confused if you pointed to a use of profile without a datum feature reference. In any case in every other use of datumless references all the surfaces/features involved have a simultaneous requirement.

Perhaps you mean the standard is clear because it has an example of how you believe it is to be interpreted? The null feature reference is still a reference, just like is used elsewhere in the standard.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

What's also bad about Burunduk parroting of some verbiage is that that interpretation (an incorrect one) just comes back to the original problem - allowing a discontinuity between adjacent portions of the part. Either argument produces a failure of the Burunduk suggestion - either very expensive or failure to fix the problem. Good job.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Quote (3DDave)

you would be less confused if you pointed to a use of profile without a datum feature reference.

Had you read what I explained earlier, you would learn that profile without datum references controls only the form of the feature. See fig. 11-19 in ASME Y4.5-2018 to get yourself familiar with this tolerancing practice.

Quote (3DDave)

In any case in every other use of datumless references all the surfaces/features involved have a simultaneous requirement.

If that was true, any time anyone would specify a datumless profile tolerance on basically dimensioned rounds or fillets connecting planar features at different locations on a part (which is a scheme chosen to avoid the more ambiguous directly toleranced radius dimensions), it would force the creation of a pattern of these rounds/fillets.

Quote (3DDave)

The null feature reference is still a reference, just like is used elsewhere in the standard.

You would be more convincing if you pointed to that "elsewhere", specifically where the lack of datum references or, as you call it, the "null feature reference" is the sole grouping method to specify a pattern.

As for "allowing a discontinuity between adjacent portions of the part", this suggestion of yours is based on the poorly thought-through assumption that where there is a step produced between two tangent-designed features, it is legitimate and feasible to "chop" each of the two tolerance zones exactly where the step begins from each direction, to prevent each of the two profile controls from detecting that step. This is a faulty premise that was already covered in this discussion.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Quote (B)

If that was true, any time anyone would specify a datumless profile tolerance on basically dimensioned rounds or fillets connecting planar features at different locations on a part (which is a scheme chosen to avoid the more ambiguous directly toleranced radius dimensions), it would force the creation of a pattern of these rounds/fillets.

Yes, it would. That's what the symbol/phrase "SEP REQT" is for if that's not what you want. OTOH, if you have used datum features and profile tolerances on the surfaces then any radii not sharing that datum reference frame don't need to even be on the part - quite the problem that you have created.

Quote (B)

This is a faulty premise that was already covered in this discussion.

No. It wasn't. If there are two profile controls, the step is allowable as conformance to the zones is independent. There is no requirement to meet both zones at the same time nor to be continuous in curvature.

You know exactly what figures there is a tolerance controlling multiple features with no datum feature references. How can that be if datum features (plural) are required? That means at least two datum features are required to establish a Simultaneous Requirement. Perhaps the committee got that wrong and you alone have it right?

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

3DDave,
"SEPT REQT" is inappropriate with profile or position tolerances that don't include any datum references. Based on the definition of a simultaneous requirement, at least one datum reference has to be included in all tolerance specifications that invoke that requirement.

Quote (3DDave)

You know exactly what figures there is a tolerance controlling multiple features with no datum feature references

No I don't.
I suggested you to provide evidence to support your "null feature reference is still a reference" theory, which you haven't done. I still encourage you to point to any text wording or even a single example figure in the standard where a pattern is created exclusively based on common lack of datum references, without any official (text supported) or even unofficial (such as the single FCF with multi-leader, no longer appearing in '18) pattern grouping techniques.

Quote (3DDave)

There is no requirement to meet both zones at the same time nor to be continuous in curvature
And there need not be. As I explained, either of the tolerance zones for the two features nominally meeting at the common tangency can drive the rejection of an irregularity located wherever someone may assume that the tangency is supposed to be on a physical part.

Quote (3DDave)

OTOH, if you have used datum features and profile tolerances on the surfaces then any radii not sharing that datum reference frame don't need to even be on the part - quite the problem that you have created.

Curious idea - nonexistent radii are conforming, and I'm the one who created that problem. Is that unique to radii, or can other types of features be missing from the part too? According to your logic, if one of the part features is out of tolerance, and it's not in a simultaneous requirement with anything else, I could rework the part and plainly cut that feature off solving the problem. Per the same logic, in some cases I may even be able to cut a part in half and hand over a half part to inspection, and it would be found conforming.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Sure you do. You scan every word and diagram looking for flaws and I know you have done so for this. You won't admit to this as it undercuts your argument. If I point it out you will claim it doesn't apply to this case, but if it supported your argument it would be there in your reply front and center.

Per the definition of simultaneous requirements in your understanding it requires at least two datum feature references. One is insufficient. You read that paragraph. It's plural. That's what you wrote was the case.

How are the radii non-existent? It's your example which you are now denying. Good job.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Quote (3DDave)

Per the definition of simultaneous requirements in your understanding it requires at least two datum feature references.

This is not per my understanding, rather it is your misunderstanding.
If the same primary datum feature is referenced in several location-controlling tolerances, it is still a simultaneous requirement, regardless of the lack of any lower precedence datum references. It doesn't help you to be pedantic about the plural verbiage in the simultaneous requirement definition because your "null feature reference" is also singular. And it is definitely not an excuse to deny that explicit datum references are needed.
If the plural verbiage is a flaw, you are the one who made the effort to find it in order to base an argument on it, not me. I don't need to scan every word and figure in the Y14.5 standard to know what's in it, because I use it. Yes, I can bring a reference to the document to address a specific point if needed, and I appreciate when others do the same. Why would anyone choose to criticize this, is beyond me.

I suggested you get familiar with profile tolerances without a datum reference, have looked into it? Your learning process of this concept will not be complete until you realize that they only control the form of the feature, and not the mutual location and orientation between similarly toleranced features, unless the features are grouped to a pattern by one of the acceptable methods.

If you want to promote any other interpretation, it is up to you to present the back-up for it, rather than hiding it based on your predictions of my reactions.

Quote (3DDave)

How are the radii non-existent? It's your example which you are now denying. Good job.

Well thank you, but it was your assertion that "any radii not sharing that datum reference frame don't need to even be on the part", not mine, I don't deserve the credit

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Your understanding is a literal reading of the exact wording in the standard - is that correct or do you think it doesn't mean exactly what it says?

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Isn't an "all over" profile simultaneous despite possibly (probably?) having no explicit datum references?

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

that's the problem with GD&T , people try to make it perfect and it is not. the more complex and the more it become interpretation.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?


Quote (3DDave)

Your understanding is a literal reading of the exact wording in the standard - is that correct or do you think it doesn't mean exactly what it says?

That's not my approach, but it may be yours if you suggest that the plural in the wording of the simultaneous requirement definition may invalidate a single datum reference.

When reading the standard, just like with everything else, you have to get the gist of what's written, and understand the words in their context. That's why the plural in the simultaneous requirement definition doesn't mean that two or more datum references are needed. The context of the plural wording is that the datum precedence order has to be the same, and it's difficult to talk or write about any precedence order without plural wording. But it is elementary that where only a primary datum reference is used, and it is the same one for several controls, it is still primary for all of them, in the sense that the part is mated to the simulated datum the same way it would mate to a primary simulated datum when lower precedence datums are referenced, so it is within the definition. The goal is that all specifications taking part in a simultaneous requirement are to be controlled in the same datum reference frame, and in one inspection set-up, in which datum feature simulators are involved. If that datum reference frame is established by a single datum reference, it is not less valid.

Quote (BiPolarMoment)

Isn't an "all over" profile simultaneous despite possibly (probably?) having no explicit datum references?
BiPolarMoment,
"All over" is a different type of pattern than a simultaneous requirement. All over doesn't require a datum reference and in its common datumless application which is supported by the standard all degrees of freedom are available to make a best fit evaluation. In a simultaneous requirement, all considered features are controlled in one datum reference frame with identical constraints of degrees of freedom.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Quote (Burunduk)

When reading the standard, just like with everything else, you have to get the gist of what's written, and understand the words in their context. That's why the plural in the simultaneous requirement definition doesn't mean that two or more datum references are needed.

There is an example of using only a single datum in this application in Fig. 7-47 (Y14.5-2018), although that is a datum axis so two degrees of freedom are controlled with a single reference.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

BiPolarMoment, that's true, your example supports the point that it is useless to nitpick on the plural verbiage of the definition.

The correction to what you said is that the number of degrees of freedom constrained by the primary datum axis is 4. Only the axial translation and the rotation about the axis are unconstrained.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Yes well...I was thinking in 2d. tongue

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

B, you've argued as a nitpicker before. Just when it suits your purpose. But it's clearly plural. File a change with the committee to fix it and support, as you have previously demanded be done, the exact wording until it is changed.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

3DDave,
The closest I've done to nitpicking is insisting that tolerancing terms should be used according to their defined meaning.
You on the other hand don't hesitate before resorting to the most blatant type of nitpicking wherever you think it may benefit your position, even if the nitpicking ends up completely nonsensical, like dissecting the use of plural vs. singular words in a paragraph of the standard while ignoring the context, and being persistent with that even after the context was clearly explained to you. And for your information, one (1) is also a number that can describe the quantity of "common datum features referenced in the same order of precedence at the same boundary conditions". If you think there is anything wrong with the wording, you file the change. You probably already filed changes in the past with no good reason, go on and do it again.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

I have filed changes they were accepted. That's why there is an example of using three levels in a composite tolerance after nearly 40 years of showing only two.

I haven't felt the need to deal with this issue because I thought most users would be intelligent enough to know that zero is a number and that zero references means that all callouts with that same reference for position and profile of surface would be simultaneously applied. I overestimate users as you have clearly shown. So thanks for pointing out where users are unable to understand for themselves a simple concept.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Expanding the composite tolerance to include more than two segments was widely discussed prior to taking place in the 2009 revision, so don't take all the credit to yourself and your change file.

Most users are intelligent enough to know that if a geometrical control specified with zero datum references, it means that it has no datum references, and any concepts involving datum references are irrelevant to it. This is exactly why a simultaneous requirement as defined is not the only recognized grouping method to invoke a pattern, there is a list of them.
But I guess this doesn't help those users who are too lazy to learn the terms and understand the differentiation between a simultaneous requirement and the other types of patterns.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

I made a contribution that was accepted and used. What have you contributed? I don't know if it was discussed - you weren't at the meetings so you could not know. What I do know is I corresponded with Alex Krulikowski before it was in the standard - he did not indicate that it was being considered, though he was on the committee, but he did indicate that it should be acceptable.

You haven't gotten to the case where no datum references would be used and there is no pattern. Since you are good at parroting then I expect you won't figure that case out. Until you do that's it. Please don't attack my opinion in the future when you offer nothing of value in return. Thanks.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

It was widely discussed in the industry and therefore brought to the attention of the committee at multiple occasions.

A simultaneous requirement based on no datum references - there is no such case. If you say there is, it is still up to you to provide the evidence, not up to me to find one that isn't there.

How many degrees of freedom are constrained by the "null reference"?

You were the one who attacked my opinion and the profile control suggestion I made to help the case of the OP. You couldn't explain why it would not work, so you went on to some useless nitpicking of the standard and personal accusations. Thank you for your valuable input.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

I don't see the example you should have been able to show, so you didn't write anything worth reading. If you don't understand the subject and don't offer workable solutions then go back to being a reciter of only what is written and leave the problem solving to engineers.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

At least I offered more than writing an ambiguous note, or suggesting that any step on the surface is a violation of surface roughness requirements.

But your main engineering problem-solving suggestion was firing the vendor. Yeah, keep telling us about your workable solutions.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

SBlackBeard,
As I recently found out thanks to a comment by chez311 on another thread, if a general datumless profile is specified per my suggestion from 29 Apr 22 08:28, and the drawing is based on the 2018 version of the ASME Y14.5 standard, the notation "INDIVIDUALLY" has to be placed following the profile requirement. Otherwise, it can be interpreted as profile All Over.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Regarding whether or not position and profile without datum references should be considered a pattern per simultaneous requirements, that was argued heavily. I was at the meeting where it reared it's ugly head. I discussed this with a brilliant mind in this community and he gave me this analogy: He asked me who my favorite squash player was. I said I didn't know since I don't follow squash. So he asked me "So you don't have a favorite squash player?" I said "No". He said "Neither do I. Does that mean we have the same favorite squash player?"

John Acosta, GDTP Senior Level

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

In the lowest level of a composite frame when there is no datum feature reference does the same thing apply? I know they say it's a refinement or some such - however -

Suppose there's an extruded bar with two patterns of holes that will be used as the datum features to finish the outside. They have no datum feature references as the entire outer surface will be machined relative to them. Are the two patterns simultaneous?

I read feature control frames left to right - the tolerance zone shape - the tolerance zone size - establishes all the related tolerance zones which are located/oriented per the basic dimensions between them, then aligned/oriented to the first reference using any relevant basic dimensions , further aligned/oriented to the second one until they are as desired.

They are not dandelion seeds blowing randomly in the wind suddenly organized by noting the Earth, but a diagram on a page being moved and located in unison according the the frame of reference the group belongs to. Unless they are related to different datum feature references they should all be on the same sheet of paper (or the CAD model of same.)

If they are loose like dandelions then I wonder where they are blown to when they are UOS.

Was this a guy named W.(B) T.? It's an interesting analogy, but being a favorite or having no selection is not the case in a drawing of a part as those are subjective opinions. There should be chains of basic dimensions and/or implied angle controls between them.

It still doesn't solve the continuity problem unless they are simultaneous and then it does so by making the part much more expensive than it might have to be. I can level a table with a match book or a gauge block; there's no need for parallelism in the millionths of an inch. I don't see an answer within Y14.5.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Quote (powerhound)

He asked me who my favorite squash player was. I said I didn't know since I don't follow squash. So he asked me "So you don't have a favorite squash player?" I said "No". He said "Neither do I. Does that mean we have the same favorite squash player?"

That's a spot-on analogy.
I am surprised it was argued, let alone heavily.
Any beginner starting to understand what a simultaneous requirement on a practical level and not just per the dry definition and how to specify it on a drawing semantically, realizes it has to do with common constraints of degrees of freedom for all the features in a simultaneous requirement.
This is why datum references are part of it. Of course it differs from a pattern established without datum references and implies an unconstrained best-fit for the grouped features.
Furthermore, even if a specific degree of freedom remains unconstrained such as the rotation in the example of the shaft with the two keyslots aligned by the simultaneous requirement, the degrees of freedom that ARE constrained allow the alignment anyway by making the tolerance zones related to each other in a common measurement even when both are not clocked around the datum axis.

I believe that the confusion that evolved into the "null datum reference" is rooted in the tendency of people to interpret concepts literally, i.e "what it sounds like". And since all patterns can be associated with simultaneity, they think: "The green cylinders are in a simultaneous requirement, but the standard says there need to be common datum references, then let's be sophisticated and say that a datum reference is there in a way even though it is not there, and call it the null datum reference". What they don't realize is that if there was no "3X" and the position tolerance would be applied separately to each feature it would become meaningless. But it would not be meaningless if it was profile, it would then become a pure form control, separate for every feature.

3D,
The low levels of different composite tolerances are not in a simultaneous requirement, by definition.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Quote (B)

The low levels of different composite tolerances are not in a simultaneous requirement, by definition.

Really? None of the features within that lower level are in a simultaneous requirement? You certainly mean that they are isolated from other simultaneous requirements, by definition and by default, not that within the pattern they aren't simultaneous.

As usual you changed to an argument I did not make.

When you said "none" is not a number, but argue that "one" is plural I think you need more time learning to design things and less time spent on this.

I'll look for the exception to the rule that position and profile are subject to simultaneous requirements, but only where it makes sense and not otherwise. Perhaps that can be found somewhere.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

3D,
Simultaneity between the features inside a specific pattern considered by itself is not a simultaneous requirement. Simultaneous requirement is about simultaneity and basic relationship between the tolerance zones of different geometric tolerances applied to different features or patterns, where the unifying factor between them is the common datum references in the same order and same boundary conditions. Simultaneous requirement is not a synonym to a pattern, although it is one type of pattern. Why do you fail to differentiate between simultaneity the word and simultaneous requirement as a defined concept?

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

I recall a President of the US arguing about what "is" meant. You could have backed his argument.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Quote (3D)

It still doesn't solve the continuity problem unless they are simultaneous and then it does so by making the part much more expensive than it might have to be.

This was addressed, but I'll try one more time.
If two features such as a radius and a planar feature are designed tangent, and are controlled by what even you would consider as separate requirements, such as profile tolerances referencing different datum references. On an actual part produced according to the design, can you tell where one tolerance zone starts and another one ends?
There is no way to know for sure where the limit is, and the controls can slightly overlap. A measured surface ends as far as the last sampled point on it is located.
Now if there is a step between the radius and the surface, is it reasonable to say, "hey that step is exactly the limit between the radius and the planar feature that I was looking for, therefore it is not part of any of them! Let it be, and both features comply". That's not reasonable. The step is not even supposed to be there, why let it govern where the inspection of the radius ends

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Section 4.9 of 2009 says the lower segments of a composite FCF are not a simultaneous requirement. Is that what we’re talking about here? That shouldn’t even be a question. I think I missed part of this.

John Acosta, GDTP Senior Level

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

powerhound,
I mentioned this, but the counterargument that I get here is that there is still "simultaneity" within the pattern for the lower segment when considered by itself.
But that is not a "simultaneous requirement" per the definition of the concept. It may look like a fussy differentiation, but the lack of it is exactly what leads to the "null datum feature reference" misunderstanding and interpreting separate profile tolerances without datum references as imposing mutual relationship requirements.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

That's great and it greatly restricts the remainder of the surface in ways that are often not required. As I pointed out - you can use that very expensive approach, but it is still very expensive because it controls not just transitions, but everywhere a larger tolerance was allowable, even if you don't feel that applying to all surfaces means at the same time.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

3D, it may not be more expensive than what they are willing to spend to prevent the problem.
It depends on the maximum size of the step they would allow and whether the entire part must be inspected for it or if it applies locally where the risk is.
If applied as a general tolerance on an individual basis for UOS surfaces it can be used to limit relatively large steps.
If a tight control is required they can apply it locally, as I mentioned.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Quote (B)

It was widely discussed in the industry and therefore brought to the attention of the committee at multiple occasions.

Do you attend the committee meetings and have first hand knowledge? The attendees aren't supposed to discuss the meeting contents.

I was looking at where you suggested limited areas. You tend to make a wall of text and bury whatever is important in a bunch of copy-paste.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

"If this is too costly or troublesome, the control could be applied directly in the drawing views only to some features that are at risk of stress concentration or where irregularities need to be tightly controlled from any other reason."

This was part of short post, 29 Apr 22 19:56.

Do you think the committee is isolated from the rest of the industry or introduces changes based on a single change file?

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Quote (3DDave)

I recall a President of the US arguing about what "is" meant. You could have backed his argument.

Sarcasm aside, but it is you who were preaching in a recent thread1103-494217: GD&T Training Recommendations? that dimensioning and tolerancing is like a programming language. What you wrote there could lead someone to believe that you are aware of the importance of knowing how to encode something precisely to get the expected outcome. It is not consistent with your tendency to mix between spoken language associations and defined terms. Simultaneous requirement has a specific defined meaning, you aren't supposed to relate everything you associate with the word simultaneity to it, interpreting unintended patterns from a drawing or thinking that you specified one when you didn't do so. I bet you are one of those who used to specify symmetry where position is appropriate because "the feature has to be symmetric to the datum".

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Burunduk,

I glanced through the responses between you and Dave and didn’t see what I’m about to say. Simultaneous requirements is a way of invoking a pattern. Another way of doing that is by stating the number of instances (i.e., 3X, 5X, etc.). That number stated with the composite FCF makes the holes a pattern. That pattern has to comply with all segments. The 5X—or whatever it is—keeps them a pattern, nothing about the DRF in the lower segments do. I know you know this but maybe the words “pattern” and “simultaneity” are more different than we think.

John Acosta, GDTP Senior Level

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

powerhound,
You are absolutely correct.
This is why I have been saying repeatedly that controlling surfaces separately by profile tolerances without datum references or the number of places indicated does not impose a simultaneous requirement between them or any other type of pattern. It is essentially a form control that applies separately to each surface, to limit among other things the step the OP is concerned about.

Quote (myself, 30 Apr 22 06:23 )

There would be no simultaneous requirement or any sort of pattern that connects the tolerance zones, since the profile tolerance in the note would not include datum references to impose a simultaneous requirement (per section 4 of Y14.5-2009) and so none of the following would apply: "nX, n COAXIAL HOLES, ALL OVER, A ↔ B, n SURFACES, simultaneous requirements, or INDICATED" to create a pattern per the section 1 definition (note the plural in "SURFACES").

I also argued that if there is a pattern invoked by nX or the other methods and no datum references are specified for the features in the pattern, it doesn't mean that there is a simultaneous requirement invoked by the "null datum reference". A pattern can be a pattern without being a simultaneous requirement (the low segment is an example of a pattern not in a simultaneous requirement, even when datum references are used). I asked a question - how many degrees of freedom does the "null datum reference" constrain? Still didn't get an answer to that.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Burunduk,

Could you point me to the place in any Y14 standard (or ISO - honestly curious if they have handled it rigorously) where it is clearly defined what happens where two adjacent/tangent tolerance zones meet?

I ask because I've found its something the standard lacks in clarity, however you seem to be very sure about it being obvious what happens at these transition points, especially points of tangency, or at the very least a non-issue.

I'm not sure I follow your logic. Hoping you can expand on it a bit.

Quote (Burunduk, 29 Apr 22 15:32)

The tangency line is only there in theory. On a real part, if it's made to print, the best you could have is some narrow border zone, not precisely defined in any way, where one surface blends into another. This zone can be considered as part of both surfaces, and points on this zone could be inspected as part of the control of one surface, or the other, or both. So if there is a step right between the radius and one of the planar surfaces on the part, it could be detected and treated as a violation of either the profile tolerance of the radius or the profile tolerance of the planar surface.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

chez311, it's a good question.
The strongest support for this would be common sense and some experience.
If you ever measured parts, even on a basic optical comparator, you could have seen that for tangent surfaces under great magnification it is very difficult to tell one feature ends the other one starts. You don't have those nice looking lines that are there on the model. Does it mean that violations can fall between the cracks? In my opinion - it's the contrary. 

Here's why,
The Fundamental Rules paragraph section in Y14.5-'18 states "UOS, all tolerances and datum features apply for full depth, length, and width of the feature" (and a similar rule was in the previous versions). It is mandatory that the tolerance zone covers the entire as produced feature. So when you can't tell exactly where that depth/length/width ends, and you detect an irregularity of the surface approximately where the limit might be, you have no reason to assume you are no longer in the tolerance zone of the feature you are inspecting, or not yet at the tolerance zone of the next feature - one of the zones should surely reject a violation larger than both tolerance zones. 

More support for this can be found in the recently published standard, ASME Y14.45. The draft version from July 2020 defines what a measured surface is:  "measured surface: a collection of all measured points of a feature that separates the feature from another feature or from free space." 
In other words, what separates an inspected feature from another feature is the last point you choose to measure on it. If the violation happens to be on the border, why let it escape the measurement, especially since you can't tell for sure where the border lies.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Quote (Burunduk, 16 May 22 21:06)

you detect an irregularity of the surface approximately where the limit might be, you have no reason to assume you are no longer in the tolerance zone of the feature you are inspecting

Quote (Burunduk, 16 May 22 21:06)

what separates an inspected feature from another feature is the last point you choose to measure on it. If the violation happens to be on the border, why let it escape the measurement

These are qualitative evaluations, not quantitative ones. It only confirms what my experience has been, that measurements in these border regions - especially points of tangency - are lacking a rigorous definition without supplementary documentation, and typically are unfortunately left up to the inspector to muddle through.

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

chez311,
It may be so, and possibly if nothing is unusual, with adequate transition between the features, this area might not get any special attention.
But if an irregularity is clearly detected, (otherwise there would be no issue and this thread wouldn't exist) can't the specified tolerances on both surfaces justify a rejection? Practically speaking could the vendor claim the step is not covered by any of the tolerance zones and that the part is compliant to the drawing? Or would it force him to adjust his process to avoid the step?

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

Hi All,

Sorry I'm late joining this thread - you guys have gone very deeply into patterns and simultaneous requirements. I've been even deeper into this rabbit hole myself for several years. I remember the Dallas meetings in 2016 that powerhound mentioned - there was definitely a lot of discussion of the "more than one datumless FCF" case.

I'll start by acknowledging that Y14.5 doesn't explicitly address the issue of whether or not two (or more) datumless Position or Profile FCF's qualify as a simultaneous requirement. There are two camps - one saying that there would be a simultaneous requirement and the other saying that there would be separate requirements. However, after examining the evidence and following the consequences of each interpretation, I concluded that the hypothesis that datumless FCF's are separate requirements is far more defensible and consistent with the content of the Y14.5 standard. I basically agree with what powerhound and Burunduk said in their latest posts.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: What Prevents Inadvertent Stress Concentrations or Crack Initiation on most prints?

If the topic regarding the numbers 0 and 1 for datum feature reference count was discussed and left unresolved then a pox on both their houses.

If the datum reference frame is that important in order of evaluation then it should be the left hand portion of the feature control frame to indicate that the features cannot be evaluated for any characteristic without datum references.

I think the term "pattern" needs to go. They are related features and may be randomly located during the design process. Per the convention use it can only apply to identical elements in some uniform arrangement - therefore excluding a profile and position tolerance as "pattern".

What I really want to know is why '2018 was so rushed. On the typical schedule it should not have been until 2022 or later; if on the official schedule it was about a year early after previous allowance for procrastination. 2016 must have been the last meaningful input before closing all discussion other than - "This is what we are doing; vote for it" in 2017.

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