Interpretation of Tolerances - Undimensioned Drawings

[MechDesignR]

I am working for an aircraft company that uses “old school” undimensioned drawings to define brake formed sheet metal parts (the same way things have been done for many decades). These undimensioned drawings go by different names at different companies: PCM, EMD, Mylar, Undimensioned Master, etc. Typically, these “undimensioned” drawings define a flat pattern, show bend centerlines, specify bend angle, direction and radius. They only contain dimensions for features that need to be held tighter than the general tolerance of +/- .030”. For example, full size fastener holes would have a diameter dimension. Please only reply if you are very familar with this type of drawing.

Where I currently work, undimensioned drawings have a general tolerance of +/- .030” with a note that states “no tolerance accumulation is implied in any series of features”. Some people understand this to mean that the length of the part (scaled off of the mylar or measured in cad) would have a tolerance of +/- .030” and similarly, the distance between the centers of two holes would be +/- .030”. Other people interpret this to mean that one edge of the part is held to +/- .030” and that the location of a hole is held to +/- .030” and that the length of the part would have a total tolerance of +/- .060” and similarly, the distance between the centers of two holes would be +/- .060”. The correct interpretation is important since I am designing new sheet metal parts that will mate with purchased parts (things can’t be drilled at instl) – I have to ensure interchangeability by calculating the tolerance needed for my hole pattern. Sometimes a general tolerance of +/- .030 will work for the hole pattern (due to floating nutplates and large holes) and no dimensions are needed. While other times a tighter tolerance is required.

Apparently, the interpretation of undimensioned drawing tolerances varies from company to company. I would like to hear what other people in Aerospace have to say – what is your understanding of the general tolerances on undimensioned parts? What statement is used on the drawing or elsewhere to clarify things? Obviously, GD&T could be used to express exactly what is needed, but that is not an option.

Thanks!

 

[fsincox]

MechDesignR,
I have a very similar situation to yours. I believe it is safe to say ASME recomends not to do that and not many here will recommend it either, including myself. I, also believe, many "small companies" do not follow these recommendations. Your general tolerances practices are pretty standard, I have seen title block variations like "machined parts" which effectively leaves the door wide open for fabrications, in my opinion. They are relying on "custommary practices".
There is no standard to cover this in our country, unless your company has it's own. Your additional note appears to imply just what it says, I have not seen that one before.
Frank

 

[MikeHalloran]

You got a huge response in Aircraft Engineering.

Please do not double post.


Mike Halloran
Pembroke Pines, FL, USA

 

[ewh]

or better, post a link there pointing to the continuation here.

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

 

[fcsuper]

First, there is no such thing as "no tolerance accumulation is implied in any series of features". This is an illogical statement that admits the drawing is dimensioned incorrectly and/or double dimensioned. There cannot be ambiguity on a drawing. If it is dimensioned so that accumulated tolerance happens, then adding a note that said it does not happen doesn't stop it from happening. It creates ambiguity and can unintentionally put your company at risk should parts come in one way instead of the other.

It sounds like you have ambiguity, so therefore your drawings are in violation of ASME Y14.5, which states (paraphrasing) that anything called out on a drawing has to be stated so clearly that it can only have one interpretation.

Now, there are rules for undimensioned drawings such as the ones you described in the ASME standards. I forget which one it is right now, but I recommend looking it up to see how ASME controls these sort of documents.

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

http://groups.yahoo.com/group/solidworks & http://twitter.com/fcsuper