Inspectors failing parts due to Reference Dimensions 2

[Sparweb]

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.

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

 

[drawoh]

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

 

[Belanger]

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

 

[3DDave]

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.

 

[jassco]

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

 

[WKTaylor]

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]

 

[jassco]

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

 

[drawoh]

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

 

[Sparweb]

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.


STF

 

[SwinnyGG]

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.

 

[3DDave]

"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.

 

[Vindicit]

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.

 

[3DDave]

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.

 

[drawoh]

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.[ ]

--
JHG

 

[3DDave]

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

 

[KoachCSR]

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.

 

[Sparweb]

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

 

[CWB1]

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

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.

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.

 

[Nescius]

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".

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.

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.

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.

 

[gabimo]

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--"