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Dimensioning for Interchangeability

Dimensioning for Interchangeability

Dimensioning for Interchangeability

(OP)
Would like to start a long overdue discussion of the significance of the ASME Y14.5, paragraph 2.7.1(a) statement: “The surface or surfaces of a feature shall not extend beyond a boundary (envelope) of perfect form at MMC. This boundary is the true geometric form represented by the drawing” (para. 2.7.1.(a)). From my perspective, based upon this statement, designing and dimensioning at MMC should be a design goal. See attachment.

RE: Dimensioning for Interchangeability

No attachment. You have to copy/paste the attachment link into the message.

RE: Dimensioning for Interchangeability

I would rather see designers and engineers exercise awareness of the state of their designs. But the pro-competence faction doesn't attract many rabid or even casual followers.

RE: Dimensioning for Interchangeability

Paul,

Of course adding the missing attachment would possibly help determine what you mean, but I'm not clear on what about the bolded statement you take issue with or want to discuss. Could you clarify a bit?

It also always helps to state the version in question, in your case Y14.5-2009.

RE: Dimensioning for Interchangeability

Oop. The attachment function doesn't like files with the ampersand or I think other symbols (ie: GD&T is a common error around here), I tried to guess at the file name to try and pull up the link but no luck, I would try to reattach without any symbols.

Also, can you expand on more specifically what you want to discuss about Rule#1, "true geometric form" and/or drawing practices? Your OP is pretty general.

RE: Dimensioning for Interchangeability

Replace the offending "&" with "%26" (not the quotes, just the contents) in the download url that shows up when clicking on the link.

RE: Dimensioning for Interchangeability

Paul Finch,

Interchangeability is the whole point behind GD&T! All this stuff about drafting standards kicked in after World War II. To give you an excellent example, Rolls Royce made their Merlin engines in factories in Derby (32377), Crewe (26065) and Glasgow (23647). Ford of England made 30428 in Manchester, and Packard in the USA made another 55523. Forget the American fasteners versus British Standard Whitworth. How many of the components were interchangeable?

The ASME Y14.5 standard accomplishes something actually more important than interchangeability. It creates a precise interpretation of all the stuff we apply to drawings. The text you have quoted is Rule #1. A rectangular feature with plus/minus dimensions is surprisingly ambiguous, until Rule #1 is applied.

--
JHG

RE: Dimensioning for Interchangeability

Paul Finch,

I am reading your attachment. I hope it is not a slide show. You are almost certainly violating most of the rules for a good slide presentation.

In your section Throwing Away Good Parts — Accepting Bad Parts, you discuss throwing away good parts and accepting bad parts. This has nothing to do with MMC tolerances, and everything to do with bad drafting. As drafters, we need to do our tolerance stacks, and understand how our parts must work. Too many companies leave drafting to CAD monkeys. MMC tolerances will not fix that problem. Your notes mention "at least 50% more functionally good parts". This problem shows a lack of comprehension of geometry by your drafters. All of the tolerancing approaches can be made to work if the drafter is competent.

A conventional claim about GD&T is that positional tolerances provide 50% more allowance than plus/minus. All of this allowance is functional. I looked into this and it is only sort of true.

My notes on Positional Tolerances.

--
JHG

RE: Dimensioning for Interchangeability

Paul,

Admittedly I haven't had a chance to finish reading your dense presentation yet, but here are a few of my initial thoughts.

1) It seems the concepts of size/MMC size, bilateral/unilateral tolerancing, and the envelope principle/rule#1 are conflated with geometric tolerances applied at MMC and the virtual condition generated from such a tolerance. These are separate, but not mutually exclusive, concepts and being precise about which one is being referred to or utilized is important.

2) I'm not sure I agree that "Product Development has lost sight of [interchangeable parts] as a design objective." Every Engineer and Designer wants parts to fit and assemble reliably and this is usually one of the top considerations, and if you're making more than one part thats design for interchangeability. I'd argue that this is one of the top considerations in creating and maintaining standards like Y14.5. In fact, I'd say the focus on ergonomics, ease of assembly, and throughput has only increased as the years go on - especially as factories turn to automation where high assembly forces don't just result in a complaint from assembly line workers but a rejection by a robot which is programmed to not exceed a certain maximum force, or damage by an indiscriminate robot with installation/applied forces that are too high.

3) The presentation seems to be focuses on MMC as the only way to achieve design goals. There are plenty of situations where RFS might be desirable or even required. Press fits come to mind where RFS actually best represents design intent, and certain controls like profile require RFS. MMC is often the most robust way to ensure assembly/design requirements and interchangeability, but its certainly not the only one. GD&T standards are often compared to toolboxes, they provide the tools to use and aren't necessarily there to tell you which ones to use.

4) You said ISO "doesn't have the tools for dimensioning for interchangeability" because the envelope requirement is not default? I'm no ISO expert, but can certainly be enforced with the Envelope symbol. Additionally the claim in ISO is that default enforcement actually can cause good parts to be rejected which is also true in ASME unless zero (insert geometric tolerance - typically position) @ MMC is utilized. There are also some nuances there due to the difference in the way size is evaluated in ISO vs ASME.

5) There is the claim that "there is no getting around the fact that there can be no tolerances added beyond that [rule#1 envelope of perfect form at MMC] or there becomes a conflict with this rock solid requirement." There are several exceptions to this rule even without the Independency symbol (I) which are right there in 2.7.1 after the portion you quoted - namely straightness, flatness, bars/stock, parts subject to free state variation (or more precisely when the free state modifier is used in Y14.5-2018), and one that it omits but it mentioned elsewhere (but is again mentioned in 2018) as when average diameter is used. I'm not sure how you can reconcile this with your claim.

6) Your definition of the MMC part as the ideal or "strongest" part ignores something as simple as assembly requirements - it would stand to reason that in most cases your "MMC part" would result in the highest installation forces and most difficult assembly (as well as highest imparted stresses due to press fit if that is the design condition). So on each end you have a less desirable condition depending on what you are considering, taking both into account a bias toward nominal could be considered most "desirable".

7) There are several references to the verbiage "true geometric form" contained in the standard. To my knowledge this isn't directly defined other than the context it is used in the quoted section, I haven't given it much thought till now but I would probably consider this in the same vein as True Profile. Not something that suggests "ideal" meaning "best" form, but that defines the "ideal" as in "theoretical" geometry of the feature.

RE: Dimensioning for Interchangeability

Paul Finch,

Most of my design experience has been machined and sheet metal parts. In CAD, I prefer to model to what I call nominal size.

Eg. I want to slide a ⌀40mm shaft accurately through a hole. Let's use an RC5 fit as per ANSI B4.1‑1967 (R1987). This means that the shaft is ⌀40e7, and the hole is ⌀40H8. I am working from the 26th edition of Machinery's Handbook.

Dimensions:  Hole       Shaft
Nominal       40          40
Maximum       40.041      39.949 Ⓜ
Median        30.0205     39.9365
Minimum       40 Ⓜ        39.924
 

In SolidWorks, if I model at nominal size, I can call up the ISO tolerances in the dimension box. Now, I can change the nominal size, and be confident that my tolerance specifications continue to be correct. I can make sure the final drawing at the shop has the actual inspection numbers. The drawing's primary purpose is to tell everyone what we want from the fabricator. I care about the fabricators and inspectors, and I will make sure my drawings are clear to them. I don't care much about anyone else.

I have since sent parts out for rapid prototyping. Nominal size modelling is a mistake. If I want something rapid prototyped, or other process directly from the 3D model, median size modelling is the right way. If my 3D model is at MMC, half of the rapid prototyped variation will be outside MMC, and wrong. Median modelling gives me maximum allowance.

I am sure other people here can add to this.

--
JHG

RE: Dimensioning for Interchangeability

How would a casting be treated, with tool shops cutting the tool to the 3D model, at this point how does this work? If the tool shop cuts to MMC per the model, that will be bad situation for the tool, especially with die tolerances. Now if the die tolerances are out above MMC from your model, the tool will need to be welded on to get it back to tolerance that is less than your MMC. Interesting presentation.

RE: Dimensioning for Interchangeability

(OP)
Fit calculations should be considered pre-product manufacturing data. Those calculations don't work by the way with purchased pins. End product verification is only concerned with MMC of the pin in relation to the datum reference frame of the assembly.

RE: Dimensioning for Interchangeability

(OP)
With castings you are normally only concerned with machined surfaces that mate and fit with other parts within an assembly. Some companies do not even dimension cast surfaces and default to the model STP with a general note specifying a loose profile tolerance per ASME Y14.5-2009, para. 8.3.1.1 to what is termed the "true profile" --- an undefined term which is generally taken to mean "nominal"---another undefined term within the standard.

RE: Dimensioning for Interchangeability

Casting patterns aren't often made to nominals anyway, which is why shrink rulers exist (aside from slipping one to an unwary newbie for hilarious results.)

If they can do that they can look at the rest of the process variation for similar parts and, because they have experience with the amount of shrink and sink, know where to put vents, gates, riser, and otherwise manage the process to make the dies and patterns that turn out compliant parts.

FDM parts should probably use a path along the LMC surface as the squeeze-out will add part volume and, unless that is very well dialed in, will add in. Unless the part will be machined after, and then the MMC boundary with the CNC path on the median surface path. Same with SLS (selective laser sintering.)

RE: Dimensioning for Interchangeability

Quote (SDETERS)


How would a casting be treated...

A foundry would be similar to a rapid prototyping shop. A model at median size allows maximum variation of functional parts. On the other hand, a batch of castings is expensive. I would expect my foundry to read my drawings carefully. The 3D Prototyping shop I dealt with did not. Also, I would make a point of talking to the foundry to see how we need to communicate. I would expect all sorts of drafting tips, most or all of which, I would follow.

--
JHG

RE: Dimensioning for Interchangeability

Quote (3DDave)



FDM parts should probably use a path along the LMC surface as the squeeze-out will add part volume and, unless that is very well dialed in, will add in...

Are you sure about that? Is there any reason people designing FDM machines cannot make them account for their nozzle diameter when they construct their model? And of course, is any reason why other people making FDM machines won't bother to do this? This is why we need to prepare drawings with tolerances.

--
JHG

RE: Dimensioning for Interchangeability

I haven't read every page and bullet of this. I have read some though so here are my thoughts:


Quote (Paul Finch)

The American standard ASME Y14.5 nowhere uses the terms “ideal” nor “nominal”.
Have a look at 1.3.56 and 1.6 in the 2009 version.

Quote:

If ASME Y14.5, paragraph 2.7.2(1) states that the MMC boundary is the “true geometric
form represented by the drawing,” then there is no getting around the fact that there can...
It doesn't say that.

Quote:

Therefore, this statement can only mean one thing all tolerances must be applied in
the direction from the boundary at MMC to LMC.
Why do you think it means that? I certainly don't glean that from that statement that is actually in 2.7.1(a), not 2.7.2(1).

Are you saying that there's a real difference between
.500+/-.005
.495-.505
.505 +.000/-.010
.495 +.010/-.000
?
The last question is regarding page 13. What's the difference between your scenario and specifying a mean dimension and equally disposing the tolerance zone? 1.000 +.000/-.010 is the exact same thing as .995 +/-.005. Is this what your argument is, that all dimensions should be at the MMC limit of size and the tolerance be unilateral?

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: Dimensioning for Interchangeability

Paul,

You have posted a very dense (and honestly, potentially conflicted) document with the intent of sparking discussion and when several members myself included provide multiple responses to many aspects of said "presentation" you chose only those tiny portions to respond to? I'm trying to keep an open mind but if my frustration is palpable in my ensuing replies, I apologize.

Coupled with my previous bullet points (1) about the intermixing/conflated terminology and (5) stating rule #1 cannot be broken it is somewhat concerning that you claim "true profile" is an undefined term however its right there in the introduction to profile para 8.2 as "A true profile is a profile defined by basic radii, basic angular dimensions, basic coordinate dimensions, basic size dimensions, undimensioned drawings, formulas, or mathematical data, including design models." Y14.5-2018 goes on to add this directly to the definitions section. Despite this dubious claim about true profile being undefined (probably one of the least ambiguous ideas in Y14.5, at least until you dig into borderline cases and extension of concept) your entire presentation centers around a term mentioned once in the standard without any sort of actual definition for "true geometric form." We can of course deduce meaning from context so I'm not suggesting there is much ambiguity but your insistence on undefined terms is somewhat....ironic.

While we're at it, despite what you stated nominal is actually also defined (albeit not well, but a definition is provided) in Y14.5-2009 para 1.3.56 as "the designation used for purposes of general identification." I wouldn't consider that lack of specific definition as terribly consequential, as nominal is relatively subjective and could refer to whatever the designer deems as appropriate for the "purposes of general identification" including a value not actually contained within the tolerance bounds, common on press fits and threads among others. If one wanted to refer to the middle of the tolerance zone, one might utilize "median" as drawoh pointed out as the mathematically precise term.

RE: Dimensioning for Interchangeability

drawoh,

Since the FDM machine has no idea which is the outside of the part then it cannot manage that calculation - that's why it should be up to the MANUFACTURING engineer to change their version of the model to account for their particular process. It's possible the path will be entirely inside the LMC surface.

The point is - the design engineer should not have to care for one second about the manufacturing model requirements.

RE: Dimensioning for Interchangeability

(OP)
Which came first, the chicken or the egg; the flea or the dog? On the one hand you have boundary conditions, but on the other hand boundary conditions are defined by their tolerances, or are they? Tolerances are variables that have to be tied to something. So right off the bat tolerances can't come first. "Nominal" is no boundary condition. So where do you hang the tolerances on? It is my belief that you start with the "boundary of interchangeability" --- a term that I coined --- to represent the boundary condition known as "Maximum Material Condition". The 1.3.56 reference applies to like "two by four" or number six screw and has no meaningful purpose to hang tolerances on. So what do you hang tolerances on? Yes, there is "confliction". But does not that "confliction" stem from the standard itself?

RE: Dimensioning for Interchangeability

Paul,

I don't really think we need more terms to describe concepts which already have names - such as MMC and Virtual Condition. You have not convinced me this adds any real value.

There are multiple ways to specify a tolerance as I'm sure you're familiar with - unilateral, unequal bilateral, equal bilateral (symmetric), and limit. The irony is that if you specify a unilateral tolerance from your MMC size, your MMC size could be considered nominal. These are just conventions - all else being equal (ie: your MMC and LMC size remain the same as in JP's example on 18 Oct 21 20:18) they are identical and can be used interchangeably. Sure as you noted you have to start somewhere, but the result will be the same.

If you want to advocate for one method over the other based on reasons for drafting conventions, to prevent errors with later revisions, from a CAD/modeling standpoint, or to facilitate ease of use by manufacturing these are all potentially valid viewpoints. But to claim choosing one of these methods over the other has some profound impact, I'm just not buying it.

RE: Dimensioning for Interchangeability

(OP)
Keep it simple. Dimensioning for interchangeability is the subject. End item drawings should maintain the boundary of interchangeability even when tolerances change. Nominal dimensioning makes the MMC/VC boundary a moving target and possibly destroys interchangeability, forcing a new part number change for each assembly level up until interchangeability is reestablished. Since the results will not be the "same" are you "buying" that?

RE: Dimensioning for Interchangeability

Paul,

The standard does not define a "nominal dimensioning" method. What I assume you are advocating for is either limit dimensioning or unilateral dimensioning. While I would agree that these may be slightly more robust from a fitment or interchangeability standpoint as they may "suggest" to whoever is viewing/editing the drawing that there is a limit which should not be exceeded, it is not foolproof. Heres what they both look like in solidworks.



Theres nothing stopping me from putting a nonzero value on the minus side.

Additionally, while rarer there are cases where LMC is your functional target, as well as the size from which your VC is calculated. MMC is not the only solution.

Finally, your size tolerance is not the only thing which establishes your VC - it is the culmination of both your size tolerance and geometric tolerance. And while it may not impact the VC calculation, Datum Reference Frame and Datum Feature boundary condition(s) will have an impact on validation and your tolerance zones wrt your datum features. There are any number of ways which someone can unwittingly deviate from your design targets when modifying a drawing. At the end of the day its up to the designer and responsible engineer to be extremely careful about editing, reviewing, and checking drawings.

If this was the main thrust of your presentation, it seems like it could have been much shorter.

RE: Dimensioning for Interchangeability

Paul Finch,

As per my terminology and using SolidWorks, I can model something at nominal size, use that feature to control the size of the mating feature, and I can apply ISO tolerances. The ISO tolerances are based on the fit that I want. This will be maintained even when I significantly change the size of the feature. I expect that the other intelligent CADs do this too.

When I am done designing, it is good practise to clarify dimensions for reading by the fabricator and inspectors. They need the numbers. They don't need the ISO codes.

If my feature is an accurate diameter for a machine shop, probably I will show limit dimensioning. This shows off the actual numbers the feature must conform to. I don't care about MMC. Everything in the range is good. If the fabricator can easily do my tolerances, they may be inclined to get as close to MMC as possible, but I don't care. Consider that I don't want to tell the fabricator what my part is for. An obvious failure mode is that the connection will be loose, and it will rattle. Alternately, the joint can bind due to thermal expansion, making MMC the failure mode. Note how the ANSI RC5 "medium running" fit allows clearances of 0.05mm to 0.12mm. The MMC condition can be as much of a failure as the LMC condition.

Look at my example, above. I can specify my forty millimetre diameter as ⌀39.95/39.93 which is convenient for inspection, or I can go ⌀40-0.05-0.07. That latter specification seems to bother some people, but it shows off the nominal model size in addition to the tolerances. If my fabricator works from the CAD model, this may be a safer specification. Don't assume your round part is going into a lathe. Joe Osborn might do it on a CNC mill.

Forget diameters. Let's control a complex outline with a profile tolerance. All the comments above still apply. The MMC condition affects fit and access. The LMC condition may affect structural rigidity and strength, or it may be providing enough "meat" to permit blind tapped holes on the side, or perhaps spot faces on the front. If I am specifying holes in castings that I will later machine out accurately, the LMC condition is my big failure mode.

--
JHG

RE: Dimensioning for Interchangeability

(OP)
I'm the one who said "keep it simple." However, when trying to explain an overall topic you have to cover all bases otherwise you get, but what about this, and what about that? Parts fit together on a MMC basis. LMC is only for controlling thin walls or whatever and has nothing to do with the boundary of interchangeability. When the standard says that the MMC boundary is "perfect" and is the true geometric form represented by the drawing, then there is beauty in its simplicity. This is the "go" condition of the Taylor Principle. The mid-range or LMC cannot be a basis for applying tolerances because they are imperfect and vary from part to part and frankly are a phantom in terms of Rule 1 definition.

RE: Dimensioning for Interchangeability

Paul Finch,

The MMC condition determines that parts can be assembled. If I need an accurate sliding fit, LMC determines whether or not my parts rattle. Maybe I have a bunch of parts that assemble next to each other and whose edges must line up. When I apply tolerances to a drawing, I define an acceptable range of dimensions for which I will accept the fabricated part. I am solving multiple problems only one of which is interchangeability.

--
JHG

RE: Dimensioning for Interchangeability

Paul,

As drawoh and I have both mentioned, design considerations are usually much more complex than just getting them to fit. Simplifying it to say thats the only consideration is disingenuous. Interchangeability is only one piece of the puzzle, as engineers and designers we have to consider all aspects.

Quote (Paul Finch, 19 Oct 21 17:16)

When the standard says that the MMC boundary is "perfect" and is the true geometric form represented by the drawing, then there is beauty in its simplicity.

I'm not sure what higher meaning you're gleaning from rule #1. Your boundary of perfect form is at MMC if your feature is at MMC/RFS, it is at LMC if your feature (geometric tolerance) is specified LMC. It can be overridden by the Independency (I) symbol or any number of the other exceptions I noted previously - something as simple as a flatness or straightness tolerance. Thats it.

Also this envelope is not your only interface consideration. A hole of size 5.00+.01/-.00 with position .01@MMC to |A|B| must satisfy both a dia 5 boundary not constrained to any datum features (your rule #1 envelope) and a dia 4.99 boundary constrained to |A|B| (your VC). These boundaries are not necessarily, and not required to be, the same size.

Quote (Paul Finch, 19 Oct 21 17:16)

The mid-range or LMC cannot be a basis for applying tolerances because they are imperfect and vary from part to part.

I'm again not really sure what you're trying to say here. Parts have variation, a drawing is exact no matter what we consider nominal or what dimensioning scheme we use.

RE: Dimensioning for Interchangeability

Quote (Paul Finch)

Keep it simple.

OK.

In simple terms, what it appears that you advocating is that without exception, MMC represents the condition in which a group of parts are most likely to have best interchangeability. This simply isn't true.

On top of that, it also appears (granted I have not absorbed your entire presentation; it's very dense, and it is a work day...) that you are advocating, basically, that all tolerances should be unilateral and MMC should never be exceeded. While this may make sense in certain scenarios, in the real world it makes manufacturing things more complicated than it needs to be.

Say I'm machining a surface (or trimming a part or milling a pocket. Whatever. Doesn't much matter) that needs to be 50mm tall, and I know that if this surface is ANY taller than 50mm, my parts will not fit. 50.000001mm is a failure.

I know to a certainty that if I make 1,000 of these parts and I've commanded my milling machine to give me 50mm height, I'm going to get some that are exactly 50mm but I'm also going to get some that are 50.01 and some that are 49.99 and so on.

If the drawing that's been handed to me by the engineer states in no uncertain terms that I can NEVER exceed 50mm, what am I to do? If I'm a smart machinist, I'm going to shift the target of my programming toward the center of the tolerance, however far away from the nominal (this is what that word means) target that may be, such that I produce the highest possible percentage of good parts. This in turn means that the center of my statistical distribution of finished dimensions is going to shift away from the nominal target, and toward the center of the tolerance. Depending on the machining operation, it may move a lot.

Point is this: when we set a value and add a tolerance, we are telling the machinist/fabricator/foundry/whomever two things: where we want the center of the statistical distribution of feature sizes to land, and how many of the inevitable outliers they get to count as good. If we, as designers, are smart and understand how are parts are going to be made (which we should) this may mean the tolerance value is not symmetric. But whether it is or is not, it is critical that we set what the target is. What you are doing, in effect, by setting MMC as the upper limit on a dimension and providing a unilateral tolerance is allowing whomever is next in the chain of custody to decide on their own what the true target value is for the manufacturing process. You've effectively just removed information from the game of telephone that we are already playing. There is no value added; you wind up with your drawings being less useful and carrying less meaning to the guy who has to interpret them.

RE: Dimensioning for Interchangeability

Quote (SwinnyyGG, 19 Oct 21 22:00)

when we set a value and add a tolerance, we are telling the machinist/fabricator/foundry/whomever two things: where we want the center of the statistical distribution of feature sizes to land, and how many of the inevitable outliers they get to count as good.

While this is a common assumption, this is actually not the case - and its been codified in Y14.5-2018 para 4.1 fundamental rules:

Quote (ASME Y14.5-2018)

(q) UOS by a drawing/model note or reference to a separate document, the as-designed dimension value does not establish a functional or manufacturing target

Its up to manufacturing/quality to determine their targets and from there based on process reliability determine their upper and lower process limits (at least for a shop which knows what they are doing) inside of the drawing spec limits to ensure 100% acceptance based on sample size and measurement uncertainty. Unless absolutely required by design, it would be generally inadvisable for this to be specified on the drawing.

RE: Dimensioning for Interchangeability

You can quote ASME Y14 to the guy turning wheels on the Bridgeport or punching keys on the Mazak all you want - that guy does not care. What ASME Y14 says about nominal dimension values and what they mean in the real world are different things.

What that line actually does is give the guy turning wheels or punching keys the freedom to adjust his process away from the nominal value such that he provides the highest percentage of 'good' parts; it does not change the definition of what is or is not good, and it does not affect what the designer's original intent was. As designers, we are responsible for providing values that accomplish our design intent, independently of processing; ASME Y14 just gives the manufacturing guy an out when we make mistakes. Which, as humans, we do, whether we admit it or not.

RE: Dimensioning for Interchangeability

I'm so glad we agree that whats in the standard should be largely ignored in favor of whatever the general consensus is, and should only be consulted when someone makes a mistake. I've seen that play out, it always ends well.

Quote (SwinnyGG, 19 Oct 21 23:03)

What that line actually does is give the guy turning wheels or punching keys the freedom to adjust his process away from the nominal value such that he provides the highest percentage of 'good' parts

Correct, this is exactly what I said. This would be beneficial to manufacturing and would allow them to adjust their process and targets without engineering imposing undue limitations - especially important when the engineering is done by a different party without intimate knowledge of the manufacturing process. This seems like a good thing, wouldn't you agree? You even said it yourself, design intent should be established independent of processing, establishing nominal values as manufacturing targets isn't exactly independent is it? This makes even less sense when you realize a nominal value, though less commonly, is not required to be within the tolerance zone - ie: .500 -.005/-.010

Quote (SwinnyGG, 19 Oct 21 23:03)

it does not change the definition of what is or is not good, and it does not affect what the designer's original intent was

I didn't say it did, on either point.

Quote (SwinnyGG, 19 Oct 21 23:03)

ASME Y14 just gives the manufacturing guy an out when we make mistakes

It does a lot more than that.

RE: Dimensioning for Interchangeability

Quote (chez311 )

I'm so glad we agree that whats in the standard should be largely ignored in favor of whatever the general consensus is,

Oh please. You know exactly what I'm saying.

Yes, it is true that in a perfect world, Y14 would would cover every possible scenario and prescribe exactly how to communicate design intent perfectly, and in turn manufacturing staff would interpret drawings exactly, according to what's prescribed in the standard, without error and without ambiguity.

We both know that those conditions are rarely true.

Quote (chez311)

This seems like a good thing, wouldn't you agree? You even said it yourself, design intent should be established independent of processing, establishing nominal values as manufacturing targets isn't exactly independent is it?

Of course it's a good thing; I never said it wasn't. As a designer, one of my goals should always be to make manufacturing's job of making good parts as simple as possible.

There's a very subtle difference between what I actually said and how you're interpreting it. If I put a hole dimension on a drawing at .500 with a tolerance, I am telling manufacturing what my ideal part looks like; my ideal part has a .500 hole. If they have to use a .495 drill bit to get the highest possible percentage of holes at exactly .500 (if we're operating on the assumption that my tolerance is symmetric and the process yields a normal distribution of sizes, than the most parts within tolerance and the most parts exactly at nominal are the same result), I do not care. I have communicated what I want, they have made a process adjustment to deliver it. It's not complicated. I am not dictating to them what drill bit size to use.

What OP is advocating is providing a window in which they can cut my hole; but by removing one of those three numbers, I am removing a critical piece of information. That's bad.

The point I'm really getting at is that, as you've stated (and I agree with you), the current standard design practice and what OP is advocating for eventually wind up in the same place; the difference is that the current design standard also is sensible in cases where the guy drawing the drawing and the guy reading the drawing are not experts on the interpretation of Y14; there is more room for error, and that is critical.

RE: Dimensioning for Interchangeability

SwinnyGG,

I'm not suggesting that the standard can or should cover every possible scenario, or that we live in a perfect world. I am suggesting that it benefits everyone to have a better understanding of the standard to which a drawing conforms.

Quote (SwinnyGG, 20 Oct 21 14:13)

If I put a hole dimension on a drawing at .500 with a tolerance, I am telling manufacturing what my ideal part looks like; my ideal part has a .500 hole.

That pretty much how I interpreted it, and thats also similar to what OP is saying about MMC being the "ideal" part. Its just not the case. That may be what you assume to be the case, and it may also be the same assumption of whoever is reading your drawings - but its not what the standard says. And thats all it is, an assumption or generally held belief - but it is not universal, and above all is NOT unambiguous. Hoping that someone else has the same assumptions as the person who developed the drawing is not a good basis for robust design practice. If you want nominals to be manufacturing targets, you should state it as such clearly on a note or accompanying internal standard. The opposite could apply as well, if you have multiple unequal bilateral/unilateral tolerances (or nominals outside the tolerance zone) and do not wish or more likely do not care that they be used for manufacturing targets then you could also clearly state as much on a note - though except for possibly providing a cost advantage there is less incentive for this on the design side. In this case it greatly benefits the people making the part to know the standard well.

Quote (SwinnyGG, 20 Oct 21 14:13)

by removing one of those three numbers, I am removing a critical piece of information. That's bad.

I disagree. I don't think anyone would argue that limit tolerancing is a bad design practice by just providing your upper and lower limit on the drawing. In fact, it might even be better to remove any ambiguity or assumptions one might make about a nominal value.

RE: Dimensioning for Interchangeability

Quote (chez311)

I am suggesting that it benefits everyone to have a better understanding of the standard to which a drawing conforms.

I agree that it benefits everyone if we all understand the standards at a high level; but unfortunately we live in the real world, where that isn't the case, and where 'go read Y14' is not an answer that's going to be taken well if someone askes you what something means (even if 'go read Y14' is what needs to actually happen).

Quote (chez311)

but its not what the standard says

Uh.... what? You're conflating a 'manufacturing target' with an actual definition of the part.

By the logic you're following, no dimensions of any kind mean anything, because they all represent a 'manufacturing target' which is disallowed by the standard.

The entire point of Y14 is to clearly define how designers communicate to manufacturing two things:

1) what perfection looks like
2) what deviations from perfection are allowed

If you look at a drawing and don't consider that the drawing shows the ideal final result, than none of the applied tolerances mean anything.

RE: Dimensioning for Interchangeability

Quote:

If you look at a drawing and don't consider that the drawing shows the ideal final result, than none of the applied tolerances mean anything.

If this is the case, then if you look at a drawing and don't consider that someone may not interpret it the way you think they should, and you blow off the standard that governs how a drawing should, and shouldn't be interpreted, then the standard means nothing and we're back to doing things the way my great-grandfather did them.

Quote:

...where 'go read Y14' is not an answer that's going to be taken well if someone askes you what something means...

If someone asks me what something means, "go read Y14.5" is definitely not a suitable answer. I don't think chez ever said that though. The answer to this is actually in Y14.5 but some have chosen to ignore it. It's a fundamental rule, in fact. This argument is already settled, it's just a matter of accepting that the answer isn't what one thought it would be, or even should be.

Quote:

You can quote ASME Y14 to the guy turning wheels on the Bridgeport or punching keys on the Mazak all you want - that guy does not care.

He should. If he makes a living reading prints then he should know, or at least care about, how to interpret those prints correctly. That's kind of like saying that truck drivers don't care about the rules of the road, they just want to transport stuff.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: Dimensioning for Interchangeability

Quote (powerhound)

It's a fundamental rule, in fact.

Which was added in 2018.

Do you think there has been a dramatic change in how drawings were created and interpreted within the last three years?

RE: Dimensioning for Interchangeability

I agree with John wholeheartedly.

Quote (SwinnyGG, 21 Oct 21 16:18)

By the logic you're following, no dimensions of any kind mean anything, because they all represent a 'manufacturing target' which is disallowed by the standard.

Thats very clearly not what I said. Not even remotely.

RE: Dimensioning for Interchangeability

You two are completely misunderstanding what I'm saying- I'm not blaming you for that, clearly I haven't clarified exactly enough.

I am not blowing off the entire Y14 standard. I have lived by it, advocated for it, and argued against people in my supply chain who have a 'GD&T is a waste of time attitude' for more than a decade.

The disagreement here is that you seem to be interpreting one minor change, added in the 2018 edition and not previously codified, as a fundamental and standard-altering change in approach; my interpretation is that that added subphrase serves only to aid in the ability of a Y14 compliant manufacturer to alter their processes as necessary without blowback from an engineer or designer. I do not believe it was intended to create a fundamental shift in the way drawings are created or interpreted. The interpretation you are advocating is exactly that.

RE: Dimensioning for Interchangeability

Quote (chez311)

Thats very clearly not what I said. Not even remotely.

I agree, you didn't say that- but that's the conclusion one reaches if your line of thinking is followed to the end.

RE: Dimensioning for Interchangeability

Quote (SwinnyGG, 22 Oct 21 14:08)

that added subphrase serves only to aid in the ability of a Y14 compliant manufacturer to alter their processes as necessary without blowback from an engineer or designer

Thats literally all I said from the beginning. Go back to my initial reply to you (19 Oct 21 22:44) - you said a nominal value tells manufacturing "where we want the center of the statistical distribution of feature sizes to land" and I said no, its up to manufacturing/quality to determine their own targets/process limits as long as its within the drawing spec limits. Maybe we have been talking past each other somewhat, but thats all I said from the beginning.

Quote (SwinnyGG, 22 Oct 21 14:08)

I do not believe it was intended to create a fundamental shift in the way drawings are created or interpreted

I don't either, besides from releasing manufacturing from any perceived manufacturing targets suggested by a nominal value. I didn't mean to give the impression that it did.

Quote (SwinnyGG, 22 Oct 21 14:09)

that's the conclusion one reaches if your line of thinking is followed to the end

Its not at all, though its difficult to explain without getting into the weeds of Y14.5 and Y14.5.1 minutiae - a whole other can of worms really. And I don't think it adds much to this particular discussion.

RE: Dimensioning for Interchangeability

Quote (chez311)

you said a nominal value tells manufacturing "where we want the center of the statistical distribution of feature sizes to land" and I said no, its up to manufacturing/quality to determine their own targets/process limits as long as its within the drawing spec limits. Maybe we have been talking past each other somewhat, but thats all I said from the beginning.

We are talking past each other, I think.

This is a gross oversimplification, but in simple terms:

We put a value on a drawing, and then tolerance that value with a bilateral tolerance (most of the time). That nominal value tells manufacturing what we think the ideal dimension is. So for a hole that's 4.000 +/- .010, if manufacturing and materials are perfect, every single hole will be exactly 4.000. We obviously know that's not going to happen, so the tolerance allows any hole between 4.010 and 3.990. So we have communicated to the manufacturer what we believe is ideal, and what we believe is allowable.

If the manufacturing guy wants to set his boring bar to cut a 4.009 diameter, and then run 24,000 parts such that tool wear at the end of the run results in holes that are 3.991, I have no issue with that. My tolerance has given 'permission' for that approach, but it has also not dictated that that approach is what must be done. I do not care. The adjustment made to the manufacturing process is still a black box to me, which is fine.

But maybe he's going to cut those holes with an EDM which is capable of extreme accuracy. Now he's going to command that EDM to cut a 4.001 hole, or whatever, and he's going to make a bunch of parts that are bang on 4.000.

By providing that nominal value, I'm telling the manufacturing guy what is ideal, and if he has means and methods to make a lot of parts that are ideal, he can do it. If he doesn't, the tolerance means he doesn't have to.

If I spec that hole as MAX 4.010 MIN 3.990, and he's using a process that is capable of way more than .020 accuracy at that diameter, where does he start? He doesn't know, because I have removed information from the drawing. All I have done by removing the nominal value is reduce the amount of communicated information, and make the job of interpreting the drawing a tiny bit more difficult. Multiply this by tens or hundreds of features and it adds up.

More complicated features make this whole interaction... more complicated, but the principle remains the same.

RE: Dimensioning for Interchangeability

Hi, SwinnyGG:

Sorry! I have to disagree with you.

3.990/4.010 is identical to 4.000+/-.010.

Both of them means as follows:

LSL: 3.990
USL: 4.010

Best regards,

Alex

RE: Dimensioning for Interchangeability

First off, not to be nitpicky (I guess I have been this whole time really) and I'm not sure if you meant that as a literal example of a drawing specification but MAX and MIN should not be used together like that. The only thing necessary for a limit dimension is the limits 4.010/3.990 would suffice. MAX and MIN have special meaning and should only be used with single limit tolerances.

I can understand arguing for using bilateral/symmetric tolerancing from an ease of readability standpoint (ie: since 99% of time a machinist will try and target the middle of the tolerance zone, so providing this value means less math is necessary 99% of the time even though they are NOT required to use this value), but it does not communicate what our "ideal" part looks like. At least not by Y14.5 definitions.

Quote (SwinnyGG, 22 Oct 21 16:44)

That nominal value tells manufacturing what we think the ideal dimension is.

Lets define what we mean by "ideal." Are you using it to mean something as simply theoretically exact, or do you mean it as "perfection or excellence" as in best satisfies design intent? If the former, we agree. If the latter, thats often subjective and also not always the case. First there are so many variables going into most designs that I doubt one could confidently say one dimension reflects the most perfect part taking into account all these variables. Secondly, I imagine theres plenty of times a drawing and corresponding model is adjusted to some value other than what the designer/engineer might consider "perfect" a simple case being the engineer desires a fit at MMC but the company's internal standard/standard practice requires all models be at the median of the tolerance zone.

I think you're right in that many people will read it the way you are saying, and that many shops reading a .500+.003/-.007 will likely target that .500 value even when a target of .498 would provide them more working tolerance. What I am saying is that they really should just be concerning themselves with getting the most parts within the tolerance zone - and that clause frees them to do exactly that. They can use the stated nominal as a suggestion if they like, but without specific instruction to do so it just (potentially) makes their job harder.

RE: Dimensioning for Interchangeability

Quote (chez311)

If the latter, thats often subjective and also not always the case

It's the latter.

With that said, in this type of argument, anyone is going to come up with an edge case which 'breaks' the rule. I acknowledge that there are unique cases where certain approaches need to change; that's the entire reason why Y14 is not contained on a single page.

But we're talking about basic, fundamental practices which apply most of the time.

Quote (chez311)

What I am saying is that they really should just be concerning themselves with getting the most parts within the tolerance zone - and that clause frees them to do exactly that.

Agree with this 100%.

Quote (chez311)

They can use the stated nominal as a suggestion if they like, but without specific instruction to do so it just (potentially) makes their job harder.

And disagree with this completely. I cannot imagine a scenario what a nominal value with a symmetric bilateral tolerance is more difficult to understand than any of the alternatives discussed in this thread.

While 'we've always done it that way' is almost never a valid reason to stay the course, 'after century or so of trying different things this method won out and became the standard' is, at least, informative.

If, as you seem to be saying, nominal values with symmetric bilateral tolerances make life more difficult, how did they 'win' against the alternatives after a century of humans drawing things on paper to hand to someone else to tell them what to make?

RE: Dimensioning for Interchangeability

I didn't come up with an edge case. Theres typically many variables which designs have to balance - to say that one dimension perfectly satisfies them all is unlikely. Hence why I said this could be subjective - heck you've got someone just a few comments up (OP) that claims the part at the MMC limit is always the "ideal" part.

My other example was also not an edge case, I'm almost certain theres some companies out there that have an internal requirement to model at the median value so regardless of what the design target might be, or what the engineer/designer imagines is "ideal", those models and therefore the drawings (unless they manually edit the drawing value to not follow the model - which is usually a bad idea for a whole host of other reasons) will always be at the middle of the tolerance zone. Hence why I said "not always the case" - the specified nominal won't even line up with what the engineer/designer views as "ideal" in this totally plausible case.

Quote (SwinnyGG, 22 Oct 21 19:49)

If, as you seem to be saying, nominal values with symmetric bilateral tolerances make life more difficult

Again, I did not say that. I'm trying not to get frustrated, but you've claimed several times that I've said something when I clearly didn't and/or cherrypicked my statements without context. I clearly provided an example right before that statement that was unequal bilateral NOT symmetric and said that it POSSIBLY might make their life harder - not necessarily from a readability standpoint but that they're missing out on the wider available tolerance if they target the specified nominal. Obviously its not desirable for every process to target nominal and thats another reason why I said POSSIBLY. I clearly don't think that a symmetric/equal bilateral tolerance would usually make their job harder - I even said as much just above that!

Quote (me, 22 Oct 21 18:34)

since 99% of time a machinist will try and target the middle of the tolerance zone, so providing this value means less math is necessary 99% of the time

RE: Dimensioning for Interchangeability

Dude. This:

Quote (chez311)

a simple case being the engineer desires a fit at MMC but the company's internal standard/standard practice requires all models be at the median of the tolerance zone.

Is an edge case.

Quote (chez311)

claimed several times that I've said something when I clearly didn't and/or cherrypicked my statements without context

I haven't done that. I've followed your logic to the end, in an attempt to illustrate why I believe you're not correct.

At this point I don't believe this conversation is productive, either personally or for the thread. At the end of the day neither of us agrees with OP, we're just arguing with each other about minutiae. I'm moving on. Good luck out there.

RE: Dimensioning for Interchangeability

Quote (SwinnyGG, 25 Oct 21 03:20)

At this point I don't believe this conversation is productive, either personally or for the thread.

My thoughts exactly. Best of luck.

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