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Inspectors failing parts due to Reference Dimensions
2

Inspectors failing parts due to Reference Dimensions

Inspectors failing parts due to Reference Dimensions

(OP)
Have I forgotten something important about dimensioning? Am I not reading ASME Y14.5 correctly?

I have an inspector who won't accept a part because it doesn't conform to a reference dimension on a drawing.
This has happened many times to my colleagues and they have been revising drawings to suit.
Now it's one of my drawings, and it seems someone has already removed some reference dimensions but apparently not enough, yet.
FYI it is a machined part with a sequence of holes to be drilled/csk that are centered on pre-existing holes in the part.
The new holes are fully dimensioned linearly, but I included reference data about the existing holes in case the fabricator wants to just pick up on them, or to check that the new holes line up.
The part would fit & function just fine no matter how the machining job is set up.

Quote:


1.3.24 Dimension, Reference
dimension, reference: a dimension, usually without a tolerance, that
is used for informational purposes only.

NOTE: A reference dimension is a repeat of a dimension or is
derived from other values shown on the drawing or on related
drawings. It is considered auxiliary information and does not govern
production or inspection operations. See Figs. 1-19 and 1-20.
Where a basic dimension is repeated on a drawing, it need not be
identified as reference. For information on how to indicate a reference
dimension, see para 1.7.6.

It's not the first time I've tried to get this inspector to stop inspecting references. Before I go squeal to his boss I just want to be sure I'm on solid ground.
(Yeah, I guess someone is going to say: "don't put reference dimensions on drawings..." But I use them when I sincerely believe they will help a fabricator set up or locate a tool.)

STF

RE: Inspectors failing parts due to Reference Dimensions

SparWeb,

On a drafting board, you need to be very careful at applying reference dimensions. If the part is modified in a way that affects the reference dimension, this must be recognized and updated.

We don't work on drafting boards anymore.

If you apply a reference dimension to a drawing generated by 3D CAD, it should be correct, and it should stay correct when the model is updated. If I measure a part to verify a reference dimension, the part should match closely, unless it is at the wrong end of a tolerance stack. If it does not match, I would assume something is wrong, and I would investigate. I have put reference dimensions on some drawings because they are a convenient quick and dirty check on some less convenient, real dimensions.

How is your inspector working out the tolerances of your reference dimensions? If he is using the decimal accuracy called up on your drawing notes, he is an idiot, and you have cause for commplaint.

--
JHG

RE: Inspectors failing parts due to Reference Dimensions

SparWeb -- you are completely correct. The standard says it in black-and-white: To be used "for informational purposes ONLY."

My emphasis added. But you should emphasize what the word "only" means to the inspector!

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Inspectors failing parts due to Reference Dimensions

What tolerance does he think applies to them? There is an interesting bottom to every worm bucket. In a similar situation I asked an inspector where he got an interpretation and he said that's what he thought the engineer meant. So I said he should call the engineer to clarify. Suddenly he didn't want clarity. Since my job did not involve firing inspectors, I just went back to the NC programmer who kept getting his parts rejected and told him why. And yeah, just because there is a chain of basic dimensions, it doesn't mean measuring between the last two features in the chain when datum references say otherwise. Not that I'm still bitter.

RE: Inspectors failing parts due to Reference Dimensions

Hi, SparWeb:

You are on a solid ground with this issue. Reference dimensions are for information only, and there should be ignored during manufacturing and inspection processes. If I were you, I would send him a copy of the quote from Y14.5.

Best regards,

Alex

RE: Inspectors failing parts due to Reference Dimensions

Decades ago an old gray haired designer and tooling guy cornered me on this topic.

Their opinion was that referenced dimensions have NO place on a drawing... except to confuse everyone. Let the drawing as-dimensioned stand on its own.

IF any dimension/feature ABSOLUTELY HAD-TO-BE highlighted or repeated or referenced for clarity [VERY rare], then a special flag note(s) needed to be associated with that dimension/set-of-dimensions/features/etc. Examples of how they recommended I Flag-Note these specific dimensions are like this.

[X> ENGINEERING REFERENCE [ONLY].
[X> REFERENCE ONLY [NO INSPECTION REQUIREMENT]
[X> REFERENCE ONLY [REQUESTED BY MECHANICAL]
[X> REFERENCE ONLY [TOOLING/COORDINATION CRITICAL xxxxxxxx]

NOTE.
This same concept applied for dimensions or features that had to be coordinated ACROSS one-or-more drawing sheets [same drawing number] or other drawing numbers [different elements, but dimensional coordination mandatory].

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

RE: Inspectors failing parts due to Reference Dimensions

Hi, WKTaylor:

Well, it does not matter how many special flags you use, people are still going to use them as long as they are on prints.

The only way to stop them is not to provide reference dimensions, which what I have been doing.

I put a big watermark on one of my prints to show it is a preliminary and not for production. But our vendor still uses it as a production print. (LOL)

Best regards,

Alex

RE: Inspectors failing parts due to Reference Dimensions

WKTaylor,

I think you are about my age. An old, gray haired designer and tooling guy from many years ago would have been working on a drafting board. Reference dimensions would have been highly unreliable, and best not done. In modern CAD, reference dimensions are nominally accurate.

I am trying to understand what the problem is here. It sounds like the existing holes are sloppier than your existing machining tolerances. Should the machine shop hit the new specified dimensions, or centre on the existing features?

--
JHG

RE: Inspectors failing parts due to Reference Dimensions

(OP)
Drawoh,
...I am trying to understand what the problem is here. It sounds like the existing holes are sloppier than your existing machining tolerances...

Very good guess but not true in this case. Existing holes were machined by the part supplier with just a few thousandths of an inch tolerance on position and diameter. Now that we have the basic part, to install it I'm specifying new sloppy holes that can wander 0.010 in any direction. The fact that I want them concentric with the originals needed to be communicated, and it's clear given the way I dimensioned the drawing, but while I was there I provided the diameter of the existing holes in case the machinist wanted to start by picking up the existing hole center. In my thinking it's a better way to get the center than dragging a feeler across the side and the end of the part.

Since many who've responded are in the aviation business, FYI this is a simple length of "brownline" seat track, and we're putting the mounting holes on-center with the milled notches. The notches are existing (and conform to a military standard in size and position tolerance) whereas the new mounting holes have to pick up on existing holes in the floor structure which may have variable positions. The stress analyst (guess who) let me specify sloppy holes so that some misalignment on the underlying floor won't prevent the screws from going in. This is a fairly common installation for aircraft, and this drawing is actually a copy of a copy that's been duplicated many times. Every time, inspection finds a new way to snag these drawings. sadeyes

STF

RE: Inspectors failing parts due to Reference Dimensions

I'm not an aerospace guy. And normally I'm pretty nice, especially to techs who make the world go 'round..

But I've taken a hard line on this stance in the past- reference dimensions are VERY clearly defined in the standard as reference dimensions and nothing more.

Stand your ground. People who read your drawing, which tightly conforms to the appropriate standard, without understanding the standard are not (or should not be) your problem- the inspector needs his interpretation corrected.

ESPECIALLY in a case like this, where it appears to me that you added the reference dimensions to save the machinists and inspectors time and make their jobs easier when you could have gone on without giving them a second thought.

RE: Inspectors failing parts due to Reference Dimensions

"inspection finds a new way to snag these drawings."

Usually that's the job of the checkers. Same note used for 10 years? Nope, decided it needed to be reworded so it's more clear in spite of zero problems with procurement, fab, inspection, customer, field service.

RE: Inspectors failing parts due to Reference Dimensions

I would say to stand your ground also. You are correct in your assumptions. Just think of all the implied dimensions that the inspector is not inspecting that he should be doing. Ask him what he is doing with those. Also I am starting to realize more and more that I shouldn't have any tolerance block in my title block and just dimension everything out individually. If done properly there are no questions. The only questions come from the people who dont understand the drawing and GD&T. Then its easy to see who is not capable of doing their job properly.

RE: Inspectors failing parts due to Reference Dimensions

TACOM issued an edict that every angle should be directly toleranced and no default tolerance should be used.

So my company removed the angle tolerance from the title block and never applied a direct tolerance to any angles. No one questioned it.

RE: Inspectors failing parts due to Reference Dimensions

3DDave,

How about ASME Y14.5-2009 paragraph 2.1.1.3? For you ASME Y14.5M-1994 guys, that is paragraph 2.1.1.2. smile

--
JHG

RE: Inspectors failing parts due to Reference Dimensions

I meant any non-basic angle, implied or not. Did not get a tolerance.

RE: Inspectors failing parts due to Reference Dimensions

I've had a similar issue in the past with respect to being requested to have design drawings revised by the field installer ONLY to update reference dimensions for their QC buy-off. It becomes a more difficult issue to push back on when someone else in the company has acquiesced to their request previously...regardless, I found that enough push back and standard-quoting would get them to come around. I agree with the previous responses, SparWeb - you are correct in your stance and are completely justified in pushing back on such a wasteful request.

RE: Inspectors failing parts due to Reference Dimensions

(OP)
To put a fitting end to this story:
I called the inspector and discussed the problems. Seems that there was more than just the reference dim putting the brakes on. There was also a previous change notice stating that certain reference dimensions should be removed, but it was not fully implemented by the last person to revise the drawing. In the inspector's mind, that meant the previous drawing was wrong, and that his previous insistence to remove the ref's was accepted by engineering as correct practice. By not fully implementing the CN to his liking, the drawing actually had TWO outstanding issues.
Once I sorted out what his hold-ups were, I was able to methodically break them down and overcome them one at a time. Ironically, when I looked at the CN I found that it referred to removing other ref dimensions but not the specific hole ref diameter that I mentioned before. He had just extrapolated that the rest of the reference dimensions should be removed. Once it was clear that even the CN didn't say that, when looked at carefully, I was able to break down the rest of the walls.

All in all, 3 hours wasted for all the people running paperwork and phone calls around.
For a dimension that I expected would save a machinist 60 seconds per piece.

STF

RE: Inspectors failing parts due to Reference Dimensions

From my perspective you have a compounding series of issues and bad assumptions.

Quote:

Have I forgotten something important about dimensioning? Am I not reading ASME Y14.5 correctly?

Yes and no, but the second is irrelevant. Tradesmen are typically not governed by ASME or other society standard but rather by company standard which may/may not dictate otherwise, so ASME's definition is irrelevant. What does your company's drafting standard say? Many apply identical tolerances to reference dimensions as they do standard dimensions and if its on the print it MUST be correct on the part. Typically reference dims on a part print signify suggested inspection areas that you dont want included in a critical to quality metric. Regardless, the inspection team isn't simply inspecting via your print's dimensions but the design as a whole which may/may not include dimensions duplicated via a reference or others calculated from the provided dims.

Quote:

Existing holes were machined by the part supplier with just a few thousandths of an inch tolerance on position and diameter. Now that we have the basic part, to install it I'm specifying new sloppy holes that can wander 0.010 in any direction. The fact that I want them concentric with the originals needed to be communicated, and it's clear given the way I dimensioned the drawing, but while I was there I provided the diameter of the existing holes in case the machinist wanted to start by picking up the existing hole center. In my thinking it's a better way to get the center than dragging a feeler across the side and the end of the part.

The shop should have a purchase-level print and already know the info you are adding as a reference on your modification-level. If they decide to combine the info via a shop markup or manufacturing print that is their responsibility, design should NOT be involved in this effort nor should design prints ever devolve to include manufacturing details. As a general rule of thumb, keep design engineers out of the shop and manufacturing engineers out of the design office as very few have the experience necessary to properly fill the other's role.

RE: Inspectors failing parts due to Reference Dimensions


Quote (CWB1)

Tradesmen are typically not governed by ASME or other society standard but rather by company standard which may/may not dictate otherwise, so ASME's definition is irrelevant.

Nonsense. Nobody gets to ignore fundamental rules about drawing conventions because they are "not governed by ASME".

Quote (CWB1)

What does your company's drafting standard say? Many apply identical tolerances to reference dimensions as they do standard dimensions and if its on the print it MUST be correct on the part.

More nonsense. If that's the company policy, they need some outside help.

Quote (CWB1)

Typically reference dims on a part print signify suggested inspection areas that you dont want included in a critical to quality metric.

What is a "suggested inspection area"? That is another arbitrary interpretation of a reference dimension.

Quote (CWB1)

Regardless, the inspection team isn't simply inspecting via your print's dimensions but the design as a whole which may/may not include dimensions duplicated via a reference or others calculated from the provided dims.

Please tell me the inspectors aren't calculating a dimension not shown on a print, then measuring it and applying the default tolerance to it.


RE: Inspectors failing parts due to Reference Dimensions

Nescius,

I agree with almost everything what you are saying, however, remember that Y14.5 is a voluntary standard.
If you invoke it, then you have to follow it “entirely” (unless otherwise specified) if not, I guess, the company standard take precedence over the ASME ""YOUR quote"" "--fundamental rules about drawing conventions--"

RE: Inspectors failing parts due to Reference Dimensions

(OP)
Hi CWB1,
Some of the things you're asking about are not always so explicitly defined that one can rub a guy's nose in the letter of the law.
Our company standard doesn't define reference dimensions because it refers to the ASME Y14 instead - our designers should be familiar with that stuff FIRST before getting into the peculiarities of our company drawings (styles and preferred practices based on the things we typically work on and put on drawings).

My last comments may have sounded a bit prejudiced but rest assured the inspector and I had a respectful and collegial discussion about how to interpret this drawing. I purposefully kept the tone light while still driving home my argument because I need to have his ears open the NEXT time the issue comes up. I can tell that it will come up again, and that's what was getting my goat when I posted on Thursday.

Quote:

...The shop should have a purchase-level print and already know the info you are adding as a reference on your modification-level. If they decide to combine the info via a shop markup or manufacturing print that is their responsibility, design should NOT be involved in this effort nor should design prints ever devolve to include manufacturing details. As a general rule of thumb, keep design engineers out of the shop and manufacturing engineers out of the design office as very few have the experience necessary to properly fill the other's role...

Here, I can't support what you're saying at all. Many of the assumptions built into your statement are false in this case and/or are poor as a general rule. Except perhaps "...design prints ever devolve to include manufacturing details..." and I see where you're going with that, but I rather insist that designers get into the shop and understand the tools that are used. That must be understood before designing any part. We can't all design like we have a 3D printer on our desk.

STF

RE: Inspectors failing parts due to Reference Dimensions

I agree also with "As a general rule of thumb, keep design engineers out of the shop and manufacturing engineers out of the design office as very few have the experience necessary to properly fill the other's role. "

Make a clear difference and distinction between PRODUCT drawing and a PROCESS drawing.

Copy-paste from a different thread

They are simply two different drawings: One is a PRODUCT drawing (goals to achieve and not instructions on how to make the part) and the other is a PROCESS drawing (the PROCESS requirements are the instructions *for one particular supplier's chosen method* to achieve those goals that were stated on the PRODUCT drawing.)

The PRODUCT drawing is the legal requirement and obligation of the supplier. The PROCESS drawing is the chosen method for a given supplier, but another supplier could chose a different process to achieve the same PRODUCT requirements

Product drawing is controlled by Design.
Process drawing is controlled by Manufacturing.



RE: Inspectors failing parts due to Reference Dimensions

Quote (gabimot)


Nescius,

I agree with almost everything what you are saying, however, remember that Y14.5 is a voluntary standard.

...

I am not sure I understand "voluntary" in the context of ASME Y14.5. The standard explains what all the symbols and notes on your drawing mean. You can prepare your drawing any way you damn well please.

I believe there is an old joke about somebody going to a French restaurant and ordering in French to impress his lady friend.

--
JHG

RE: Inspectors failing parts due to Reference Dimensions

Gabimot,

Regarding standards, I agree. However, there has to be some unspoken common ground. This goes far beyond line styles, or colors, extensions of principles, or any number of adaptive quirks that an entity might institute to make their lives easier. Company policy or not, the things that CWB1 described cannot be reconciled. It boils down to the inspectors demanding the part satisfy more than a fully-defining set of the dimensions: the ones shown AND the ones they are inventing. bugeyed

Edited: reworded for clarity.

RE: Inspectors failing parts due to Reference Dimensions

Re: Quote drawoh: "The standard explains what all the symbols and notes on your drawing mean"

But the meaning can be driven by your own company standard and not by ISO or ASME.

It is not a building code/standard or an electrical code....................It is not a MANDATORY standard.

RE: Inspectors failing parts due to Reference Dimensions

2
I have been following this thread - please let me add my two cents.

To avoid any confusion with being "voluntary" or not, our drawings state: Interpret per Y14.5-2009. We take great effort to "stick with standard". We consider it a "dictionary' of how to "read" the drawing symbology. Now you can put anything on a drawing, and you can make up symbols all you want, but if there is no common understanding from a "dictionary" you are fooling yourself.

Certified Sr. GD&T Professional

RE: Inspectors failing parts due to Reference Dimensions

Nescius,

Quote:

Nonsense. Nobody gets to ignore fundamental rules about drawing conventions because they are "not governed by ASME".

Fundamental rules? That's a rather comical notion given the ambiguities present in many society "standards." In this case there's a decent example - do reference dims have a tolerance or not? Per ASME either is allowable. Beyond that, tradesmen typically arent given access to society standards and are often trained on the job with minimal excess, so freely distributable company standards are typically created to resolve any ambiguities and confusion in these matters. Admittedly, often they are little more than a detailed rewrite of a society standard with clarification added, however they help drive standard process and avoid confusion. Not bashing anyone in the least but had the OP had a company standard that covered interpretation of reference dims he could've resolved this in ~1 minute if he was even called.

Quote:

What is a "suggested inspection area"? That is another arbitrary interpretation of a reference dimension.

Its one which designers feel inspection should focus on due to non-critical quality concerns, often the most likely failed non-critical dim. Like anything else production-related on a design print, its purely a suggestion.

Quote:

Please tell me the inspectors aren't calculating a dimension not shown on a print, then measuring it and applying the default tolerance to it.

Please tell me you don't believe parts are only inspected via dimensions explicitly called out on a print? In reality inspectors calculate many of the infinite number of inspection dims and points using a combination of the given dims and GD&T.


Sparweb,

Quote:

Here, I can't support what you're saying at all. Many of the assumptions built into your statement are false in this case and/or are poor as a general rule. Except perhaps "...design prints ever devolve to include manufacturing details..." and I see where you're going with that, but I rather insist that designers get into the shop and understand the tools that are used. That must be understood before designing any part. We can't all design like we have a 3D printer on our desk.

Glad to hear you resolved the issue but out of curiosity what assumptions of mine are wrong in that paragraph? Do shop supervisors and manufacturing engineers not have access to a concise structure of prints starting with a purchase level and ending with the highest locally modified level? If not then I would highly encourage improving standards, print structure, and process to eliminate future confusion. Arm the shop with proper tools and everyone becomes more efficient. As someone that grew up on and spent the first decade of his working adult life on the shop floor before attending college I can attest that you can never communicate too clearly nor be too organized. While I also wholly support putting design engineers to work in the shop (esp when assembling prototypes), when it comes to setting up production in the skilled trades there is no replacement for experience. Unless they've attended trade school or spent years of their life in the shop, a design engineer will never be remotely knowledgable enough to spec many manufacturing details so there really is need to keep them, their process, and their prints separate. Many will disagree but IMHO that is well beyond arrogant and usually drives production costs unnecessarily high.

RE: Inspectors failing parts due to Reference Dimensions

CWB1,

You can defend the position with a shield of "internal policy", but that's not much of a defense when you're dealing with the outside world. The policy didn't specify a special symbol or similar device to communicate your requirement; you hijacked an existing concept.

If a reference dimension has a tolerance, and the part is fully defined with non-reference dimensions, the part is over-defined. The part could satisfy all non-reference dimension tolerances (as it should) but still fail to meet the bogus tolerance on a reference dimension.

Perhaps your drawings don't fully define the part with non-reference dimensions...the references are "needed". In that case, you've simply applied an existing symbology to some dimensions and deemed them "reference" dimensions.

Quote (CWB1)


Quote (Nescius)

Please tell me the inspectors aren't calculating a dimension not shown on a print, then measuring it and applying the default tolerance to it.

Please tell me you don't believe parts are only inspected via dimensions explicitly called out on a print? In reality inspectors calculate many of the infinite number of inspection dims and points using a combination of the given dims and GD&T.

You didn't answer my question. Of course, inspectors may calculate any number of dimensions to, let's say, build a path to get where they need to go. The point is, NONE of these calculated dimensions are judged against a tolerance.

RE: Inspectors failing parts due to Reference Dimensions

Nescius,

The problem is that everyone needs to know how to interpret stuff on the drawing. When you specify that the drawing is to be interpreted as per ASME Y14.5-2009, you provide a reference that everybody can acquire and look at. There is nothing to stop Dominion Consolidated Widgets Incorporated from creating their own dimensioning and tolerancing standard, and calling that up. They are doing a great deal of work that, on the whole, will confuse everyone they work with.

--
JHG

RE: Inspectors failing parts due to Reference Dimensions

Drawoh,

I agree 100%. To be clear, when I say:

Quote (Nescius)

If a reference dimension has a tolerance, and the part is fully defined with non-reference dimensions, the part is over-defined. The part could satisfy all non-reference dimension tolerances (as it should) but still fail to meet the bogus tolerance on a reference dimension.

Perhaps your drawings don't fully define the part with non-reference dimensions...the references are "needed". In that case, you've simply applied an existing symbology to some dimensions and deemed them "reference" dimensions.

I'm highlighting the insanity of that policy, not trying to rationalize it.

RE: Inspectors failing parts due to Reference Dimensions

(OP)
Hi CWB1

Quote:

Glad to hear you resolved the issue but out of curiosity what assumptions of mine are wrong in that paragraph?

This one:

Quote:

...keep design engineers out of the shop and manufacturing engineers out of the design office...

I think you have grossly oversimplified it to the point of being useless advice. Perhaps the tasks done by design engineers should not be seen as equivalent to the tasks done by production engineers, but there should ALWAYS be a conversation between them. If a design engineer cannot draw a part that can be economically manufactured, when a different design would both work and be easy to fabricate, then NO AMOUNT OF PRODUCTION ENGINEERING CAN FIX IT. The design engineer must have a means of fabrication in his/her head at all times while designing, otherwise you end up with bags on the sides of boxes and fasteners inside cavities with 1" diameter access holes!

Quote:

Do shop supervisors and manufacturing engineers not have access to a concise structure of prints starting with a purchase level and ending with the highest locally modified level?

This is one of the problem assumptions. No. This shop makes what appears on the engineering drawings, and engineering is not involved in preparing production drawings, if any - in the particular shop I am dealing with. My company has other facilities that DO make production drawings based on customer drawings, but that's not the case in the shop I am sending this current batch of drawings to. There is a "Production Planner" at work in this shop who does a lot of interpretation between engineering and the shop, but the department as a whole discourages making any subordinate drawings at all (and the opposite of the other facilities we have - it's quite the split-personality problem).

Quote:

As someone that grew up on and spent the first decade of his working adult life on the shop floor before attending college I can attest that you can never communicate too clearly nor be too organized. While I also wholly support putting design engineers to work in the shop (esp when assembling prototypes), when it comes to setting up production in the skilled trades there is no replacement for experience.
Unless they've attended trade school or spent years of their life in the shop, a design engineer will never be remotely knowledgable enough to spec many manufacturing details so there really is need to keep them, their process, and their prints separate. Many will disagree but IMHO that is well beyond arrogant and usually drives production costs unnecessarily high.

Like you, I spent just enough time in the shop to be truly dangerous, then learned that my calling is the design office! Since it is an asset to my experience, why shouldn't I encourage the junior designers to learn in similar fashion? I can't hold their hand every step of the way - they have to learn enough to find their own solutions, but that knowledge is not just in the engineering office. It's also downstairs in the minds of the technicians. Furthermore, I think your second and third statements are contradictory. Anyway, this doesn't really pertain to the inspection problems, which is what I see as the root of this particular problem.

STF

RE: Inspectors failing parts due to Reference Dimensions

Quote:

You can defend the position with a shield of "internal policy", but that's not much of a defense when you're dealing with the outside world. The policy didn't specify a special symbol or similar device to communicate your requirement; you hijacked an existing concept.

Reread the post. I'm not defending tolerancing reference dims as internal policy, I'm saying every company should have internal standards that clarify the various "ors" and "mays" within society standards, the "or" in the Y14.5 quote above being just one example. The use of tolerance on reference dims needs no defense as its not uncommon.

As for the outside world, I authorize POs exceeding $10M annually, grew up in the family job shop, worked in several others, and have spent the last decade in large corp design working internationally. Not sure where this other "outside world" you speak of is but I'm often called on to bridge the communication gap between design and manufacturing, often due to lack of process and standards.

Quote:

If a reference dimension has a tolerance, and the part is fully defined with non-reference dimensions, the part is over-defined. The part could satisfy all non-reference dimension tolerances (as it should) but still fail to meet the bogus tolerance on a reference dimension.

No, if something is on a print it MUST be correct or the print isn't valid, every person who signed off on said print has failed, and the shop should kick it back for simple stupidity. I can duplicate or show excess reference dims ad-infinitum so long as any applied tolerances are correct. The fact that many people screw this up is likely why so many don't apply tolerances to reference dims, but given the past few decades of CAD availability there's really no excuse for not being able to do this work correctly.

Quote:

You didn't answer my question. Of course, inspectors may calculate any number of dimensions to, let's say, build a path to get where they need to go. The point is, NONE of these calculated dimensions are judged against a tolerance.

It was a silly question, obviously no. Regardless, youre incorrect again. Every point and dimension measured is viewed within its acceptable tolerance, hence the main purpose of GD&T - to establish tolerance/variance for the infinite number of inspection points via MMC and LMC regardless of the surface's complexity. Not every inspection point is explicitly called out on a print nor does standard tolerance apply to calculated/extrapolated dims.

RE: Inspectors failing parts due to Reference Dimensions

Quote (CWB1)

I can duplicate or show excess reference dims ad-infinitum so long as any applied tolerances are correct. The fact that many people screw this up is likely why so many don't apply tolerances to reference dims

OK, good luck. I'm not doubting your experience or your value at your current position, or anything like that. What I am saying is that you have a really, really non-standard notion of what "reference" dimensions are. Take it or leave it; I'm just some dude on the internet.

RE: Inspectors failing parts due to Reference Dimensions

SparWeb, I agree with most of your last post and think we're on the same page without realizing it. My point isn't that design engineers should stay out of the shop entirely nor even that they shouldn't be involved in manufacturing related discussion when necessary, simply that they need to be very careful to give the manufacturing team reign to make and inspect the part their way while refusing to enable their laziness. On the design side we own the design, but everything beyond that needs to be the shop's responsibility. I'll gladly work with them occasionally on minor design/print tweaks and review deviations but if they want changes to accommodate their process or add non-design reference info its on them. Rather than your specific reference diameters, I'd have ensured the shop had access to the print hierarchy from purchase-level up to your print (without the references) and called it done.

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