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Verifying "calibrating" MYLAR Drawings 1

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robbsapp

robbsapp

Aerospace
Jul 21, 2008
7
We had a write up the other day from a customer who supplies their data to us, in part as un-DIM'ed MYLAR Drawings.

The write up was because we do not have a process in place to verify the accuracy of these customer supplied mylar drawings.

The tolerance on the parts in question is 0.03" generally some hole locations may be a close as 0.015" if particularly fine.

My question is: How do you verify a Mylar Drawing with out having access to the Master?

Thanks
 
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My reply to the customer would be something along the lines of "How the hell do you expect US to verify the accuracy of YOUR undimensioned drawing?"
 
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isn't the mylar the master ? aren't mylar drwgs full size lofts of the finished part ?

my reply (as the customer) would be along the lines "Why the hellare you questioning the accuracy of the master ?"

Quando Omni Flunkus Moritati
 
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aren't mylar drwgs full size masters of the finished part ? that's why they're undimensioned.

Quando Omni Flunkus Moritati
 
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If you're getting the actual mylars, the customer is insane for letting them get out of his physical possession and a stable environment.

If you're getting prints on paper, they're probably off by a couple of percent in at least one directon anyway, depending on the humidity history of the paper, and the print process.



Mike Halloran
Pembroke Pines, FL, USA
 
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These are copies of the original mylars not the originals of course.

I think the issue is the stability of the inviroment, But I have to assume unles we are doing inspections on the face of the sun the mylar can't move enough to cause any real tolaerance concerns on a +/- .03" product.

I believe MYLAR is stable to something like 9.5 X 10–6 in/in/°F.
Being the the melting point is around 500F we would be running very close to terminal for this to be an issue.
 
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if there is a digital file behind the mylar, then could that be used with CMM to verify that the produced part conforms ?

if there is a digital file, couldn't a dim'ned drawing be produced ??

if it's a flat piece, is comparing it the mylar that difficult ?

does the part fit ?

Quando Omni Flunkus Moritati
 
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You didn't state how big the base dimension is, i.e., how far away are the holes from the datum? for a 25-in separation, the error is 0.2 mil/ºF, so a 20ºF delta is 1/4 of your error allocation.

TTFN
faq731-376
7ofakss

Need help writing a question or understanding a reply? forum1529
 
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You are correct, I believe the longest part is under 15" with most key dimensions well under 1"
 
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don't mylars include a datum square so you can verify the mylar is 100% full size ?

Quando Omni Flunkus Moritati
 
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The tick marks for the Drawing location A,B,C..1,2,3 around the periphery are "Normally" exactly 10.00" for this very reason.
Not sure what they do with smaller drawings.
A paper copy of a mylar will be off just a smiggen (engineering term)due to copier error, as Mike said.
 
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Does anyone else have a written procedure in place for monitoring and maintaining Mylars for accuracy?
 
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I don't, it is probably filed alongside the original specs for Stonehenge and Isambard Kingdom Brunel's working drawings for his Atmospheric Railway.

Sad bastard that I am I have the SAE handbook for 1976 and there is nothing pertinent in there, my guess is that you need to talk to procurement in Defense or the Aircraft Industry, since 20 or thirty years is outside the retention period in automotive, I think.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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I would hold on to the SAE hanbook - 'cause (as my mom always said) "you never know"
I have a DAC STD hardware Approved Equivalent parts book that was last up dated in the mid 90's and a color/Finish spec book from the 80's both of which I use often enough that they have not been filed away.
 
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the mylar should have a verification grid on it ... if that's good, then the drwg's good ... no?

i suspect it's a digital file printed on mylar, yes? the mylar would be the official master ('cause it'd be the one that's signed) but the electronic data behind it is just as much a master (so if the original mylar gets damaged a reprint can be substituted).

Quando Omni Flunkus Moritati
 
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Yes - I'm sure it's a reprint of digital data on mylar or in some cases a reprint of older original extant drawings.

I assume if pressed by the customer our procedure should be something like:
- Store Mylars in a temerature controled area. away from ...dirt ...heat ...etc...
- Document delta between ambiant temp at time of inspection to documented temp on Mylar (when noted)
- Verify Dimential check block when available.

 
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yes, mylar's should be taken care of, though if there is a digital file that could be mastered (just like any other drawing) then printed on mylar as needed. anyone using a mylar to inspect a part should check the reference grid first.

Quando Omni Flunkus Moritati
 
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Guys....

ASME Y14.31 Undimensioned Drawings has a LOT of good info on this subject.

AS I understand this subject, IF the customer data is provided by un-dimensioned drawing to a fabricator, then the customer is obligated to provide a verified mylar that meets dimensional accuracy at X hours after unrolling, at y-temperature and at z-humidity. that mylar MUST have a calibration scale/grid for incidental dimensional confirmation of stability. AND this should be so stated in contract/procurement documents.

NOTE. The vendor also has obligations to roll, store, transport and use thes drawing is a suitable environment to ensure they remain within dimensional accuracy limits.

Trust, but verify!

OR, as my wise departed-Grandpa used to say: "to trust is to bust; to bust is hell-to-pay: so, no trust, no-bust, no-hell-to-pay."

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

Unfortunately, in science what You 'believe' is irrelevant – "Orion"
 
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wktaylor Thanks -

Mylars are maintained in a controled enviroment
ie: indoors, in air conditioned area.
And (for the most part) used in the same controled enviroment.

I'm thinking the customer has a spot on his audit sheet that says;
PROCESS FOR CONTROLING AND VERIFYING MYLARS: and just needs to have something in writing so he can check it off.

Like I stated earlier there is no inviromentel situation that could occur where the ambiant temperature would be high (or low) enough to effect the mylar in any meaningful way when our tolerance is of +/-0.015"


 
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Rob,
"...inviromentel situation..." I think I know why your google searches come up dry. [wink]


Hit number 2:
Despite the fact that the mylars are sensitive to temperature and humidity, the customer may be concerned more with the "controlled access" to the drawings, rather than the "controlled environment". Depends on the wording of the contract. If there is a written contract, but I assume there is one. Somehow I doubt that a duplicate in your library needs to be audited for shrinkage on a regular basis, which would be a very onerous task on you, except for that check you need to make the moment before actually using the drawing for reference.

STF
 
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