Tolerances - Undimensioned Drawings 2

[MechDesignR]

I am designing several new brake formed parts that are defined on undimensioned drawings with a +/- .030” general tolerance. I am not sure if the interpretation of this general tolerance is the same across the aircraft industry (seems like it should be). I need to know the correct interpretation since I have hole patterns used to attach purchased parts and I need to determine the required tolerances to ensure interchangeability. I don’t want to add dimensions to hold things tighter than the general tolerance unless actually needed. Additionally, we don't use GD&T (even though we should).

First, considering just a flat part, is my understanding that tolerances do not build-up, in other words, the tolerance between two holes would still be +/- .030” and the tolerance on the length of a part would be +/- .030”. If that is the case, then an undimensioned drawing is actually holding tighter tolerances than a dimensioned drawing with 2 place dimensions (because it would be like having every possible relationship dimensioned, really over-dimensioned). Is that right?

Second, what is the interpretation when bends come into play – is there an additional tolerance on the location of each bend? I assume you would have to at least add the angular tolerance (+/- 2 degrees) to determine the tolerance on the length of a part that has several 90 degree bends?

Third, Is the flat pattern inspected before forming and the final part inspected again after forming? Does anyone have first article inspection experience with sheet metal parts? I have heard several conflicting opinions - does anyone work at a company that has an official document to clarify the interpretation of undimensioned part tolerances?

Thanks,
MechDesignR

Atlanta

 

[IRstuff]

Clearly, regardless of what answer you eventually wind up with, your company is missing even a rudimentally drawing standard, which is now encumbent on you to develop. Had your company had such a standard, this question would be unnecessary.


TTFN

FAQ731-376

 

[mfgenggear]

Handcalc hit it on the nail. fixtures where made to insp & for manufacturing. that where coordinated to a master pattern. all holes need to be exact when assembling. the tolerancing needs to be to allow assembly. this mostly from experience.

some of you may not like this but it was how it was done once & it worked.

Design engineers would draw their designs it was upto the
manufacturing engineer to coordinated with all departments to make it work.

secondly one lone engineer can not make or change company policy. it takes the management to implement change.
busting the op chops is not helping.

 

[KENAT]

mfgenggear, once upon a time most 'engineering' components were hand made & fettled to fit as need be with little or no interchangeability (hammer to fit, paint to match etc). It 'worked' however whether it was particularly efficient is another matter.

There are certainly cases where match drilling, or simulating this with fixtures, is perhaps the only way to do things, I don't think anyone is arguing that point.

However, to have parts that were done this way and then by the sounds of it wanting to be able to make new parts to fit them without match drilling sounds tricky.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[mfgenggear]

Kenat

all of you spend your time helping engineers in need help.
that you all spend your valuable time helping others.
all of you are much appreciated.

however Kenat the last comment I do not concur.
during world war 2 on all sides. aircraft & military equipment was mass produce.

the reason it could be mass produced is because all the tooling was coordinated to master plasters. all parts could be interchanged & would all ways fit (assy).

so to say it was hammered in place is a demeaning to these people who designed & made this equipment in the past.

the technology is ten times better but in the wrong hands it can make a lot of junk.
does that means it is better ? would I ever hand draw again, The answer would be NO. I like my cad system.
I enjoy the close tolerances CNC equipment can hold.
because it is reliable in the proper hands.

what we are not getting from the OP is what is the function of this part and how it is assembled.
can it be machined from a cnc laser or punch in the flat pattern, in other words how close does it need to be to assemble correctly.

I dislike making these type of tools, how ever one has to weight the quantity & quality required.
In the OP case it might make sense for this part.
Or it may not. I just added 2 cents

With out knowing fit form or function & cost it’s hard to say.

 

[ewh]

That was all well and good back in the day, makes sense and perhaps is OK in the OP's situation; however, many of today's major aircraft companies require all subcontractors to establish procedures that ensure the accuracy of any "manufacturing aids" and that they be tracable back to the original authority dataset.
I'm not claiming that the companies themselves follow standards of this type, I don't know. As an employee of a subcontractor to those companies, I do know that unverified mylars and other aids are subject to audit findings against us.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[KENAT]

I wasn't particularly thinking of WWII, the concept of interchangeability dates back to at least the American Civil War.

That said, there were problems of interchangeability during WWII, which supposedly was actually one of the prompts for the development of GD&T etc. So don't try telling me they always fit. I've worked on some moderately old stuff and worked with folks that worked on even older stuff and they confirmed that hammer to fit, paint to match etc. happened.

Heck, I had a structures prof at uni not so long ago talking about the 'hammer to fit' mentality on the shop floor of a major aircraft company at least as recently as the 1980's so don't tell me I'm demeaning the memory of Rosie the Riveter and her buds.

We had parts at my last employer that were basically made off the tooling and that was how we knew they'd work. It was a problem when the tooling wore out though as even when we got specialist tooling folks in they had trouble working out how the tooling had been made with respect to tolerances etc.

However apologies to the OP, this is way off topic.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[taglia]

In our case, talking about some years ago, the dimensionless drawings were used without specific tolerances, we used 0,2 mm as general tolerance. The drawings were on Mylar, and had two crosses that were supposed to be inside a tolerance of +- 0,1 mm for a 1000 mm distance. These mylars were to be used in workshops under controlled temperature and humidity environment (21ºC and 55% aprox.).
They were read with high precision boring machines (SYP 7A) with optical viewers to center the crosses (0,35mm line width).
When requeried, coordination and guide holes (crosses) were added for the single sheet metal parts, in the drawings.
Afterwards, machined templates were made to transport those coordination-holes to other assembly jigs. So, the SYP operator first read the position of the holes in the mylar, and then drilled them in the template.
Then the master template was used to transfer the holes for different purposes.
Also the guide-holes for the molds of sheet metal parts were taken from these master templates.
After conforming and calibrated the sheet metal part to the mold , these guide-holes were drilled to the part with the mold guide-holes. These guide-holes were previously coordinated with the master template.
If the part should perform in an assembly, the assembly jig was already set up with the master template.
The guide-holes were for the single parts and the coordination-holes were for the assemblies. Some holes were also guide and coordination ones.
Simple? more or less, it demanded some tuning up.
Just my two cents.

 

[madvlad]

One thing I could contribute that hasn't been mentioned is that the general policy at DeHavilland Canada, according to their sheet metal design manual, was such that features for which alignment was critical were simply not shown on the scale "undimensioned" drawing, and were shown on the NHA instead.

Ie a row of rivets may be shown marked off as ref, but at the part fabrication stage, these holes aren't drilled.

Concentric holes through multiple parts were drilled on installation, always. At most, and where possible, one part was pilot drilled to use as a template.

This general practice resolves the OP's issue with 'interchangeable parts'.

This general practice is not compatible with the modern practice of trimming the extra 0.010" of unnecessary edge distance to save 0.00001 lb. Not compatible with CATIA either, you have to use your brain to maintain adequate ED.

And if you look at a 707 or something, they didn't always get the edge distance right...

 

[mfgenggear]

madvlad

good point I had forgotten about the pilot holes on the most outer sheet (part). thus setting location.

MfgEnggear

 

[dgapilot]

Some thaughts on this discussion:
For sheet metal parts, the dimensions of the flat plate will vary based on the method of forming. A press brake will have a slightly different bend allowance than a punch press or a bench brake. Most parts that are formed and have holes for locating something will be drawn as the finished part and the flat plate drawing will be a function of the tooling involved.

Most of the existing manufacturers using process as described can't make parts that are interchangable. When I ran a shop that did primarily structural repairs, I would pay extra for undrilled parts for Cessna, Piper and Beech. Anything with pd holes was almost guarenteed not to fit.

Companies that have taken the concept of GD&T to hart make much better parts. In the homebuilt industry for example, Vands will make parts that actually are interchangable, and all the holes line up. They are all lasar drilled on CNC, and it works. Funny how the homebuilt industry can produce better products than the established manufacturers, but it's true.

 

[ewh]

Even some "progressive" companies that did take GD&T to heart had trouble making parts line up. A recent failure in the VLJ sector of the market never could get the wing structures to properly align to the airframe and would require parts be fabricated sans the holes that were defined on the drawings (and in the models) so that they could be drilled on assy.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]