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?"

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

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

Quando Omni Flunkus Moritati

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

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.

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

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
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You are correct, I believe the longest part is under 15" with most key dimensions well under 1"

don't mylars include a datum square so you can verify the mylar is 100% full size ?

Quando Omni Flunkus Moritati

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.

Does anyone else have a written procedure in place for monitoring and maintaining Mylars for accuracy?

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


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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.

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

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.

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

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"

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"

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

https://www.google.ca/search?q=controlled+environm...

Hit number 2: http://www.airqualitycontrols.com/pdf/Environmenta...

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

"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." ... exactly, and will's given you the out "stored iaw ASME Y14.31".

but still i think the digital file, if there is one, could be your true master, and the mylars are "just" copies of the master data in a form that can be used more readily.

Quando Omni Flunkus Moritati

On a somewhat related note, back around '96 I was working for Boeing doing structures work on the 757-300 stretch. The 757 was originally a paper design but the modified areas of the structure were designed using CATIA V4. One of the largest tasks for engineering was building a digital model set of the existing structures using the paper drawings and mylar flat pattern masters for formed sheet metal parts. In order to expedite the work Boeing developed a process that scanned the mylar flat patterns and created a 2D CAD wireframe of the outline plus the bend lines, mold lines, tooling holes, etc. The 2D wireframe was imported into a custom CATIA V4 application where you would identify particular wireframe elements as part outline, bend line, etc. and input values like bend radius, bend direction, bend angle, material thickness, etc. Then the CATIA V4 application would create a solid model of the formed part.

It was amazing how well the process actually worked and how accurate the 3D models produced from the scanned mylar masters were. There were tolerance standards that each solid model had to be checked against, and except for very simple parts each model did require a bit of manual clean-up work. But given the state of CAD technology in 1996 the process did significantly reduce the amount of labor required to build the baseline 757 digital model database. The process would never have produced satisfactory results unless those 15 year-old mylar masters were drawn with high accuracy and were well maintained.

Attached is part of a supplier quality standard for sheet metal parts including tolerances for those produced by photo transfer from (mylar) master templates. There is a brief description of validating the dimensional accuracy of the templates.

Best regards,
Terry

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