Is there a separate standard for dimensioning and tolerancing sheet metal work?

apestate

Quite Involved in Discussions
#1
Hello forum

I recall touching on this subject in previous threads, hopefully it is worthwhile to bring it up again.

In my travels, many people have made it sound like there's a separate standard for sheet metal dimensioning and tolerancing, and I don't think it exists!

For example, a sheet metal part is bent at 90 degree angles in a brake press to form a channel. The print for this piece calls out a width of 5.000" +/- .030. Because the part wasn't hit hard enough in the brake press, the dimension is 5.000 near the bend lines, but 5.045 out at the top.

Some inspectors are trying to tell me that's ok because the angle tolerance on the 90 degree angle is +/- 1 degree. They say that it's acceptable to measure the part at the bend line, and NOT to measure overall lengths.

Is there any truth to this at all? Is there a separate standard or convention for sheetmetal dimensioning and tolerancing?
 
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CarolX

Super Moderator
Super Moderator
#2
Hello forum

I recall touching on this subject in previous threads, hopefully it is worthwhile to bring it up again.

In my travels, many people have made it sound like there's a separate standard for sheet metal dimensioning and tolerancing, and I don't think it exists!

For example, a sheet metal part is bent at 90 degree angles in a brake press to form a channel. The print for this piece calls out a width of 5.000" +/- .030. Because the part wasn't hit hard enough in the brake press, the dimension is 5.000 near the bend lines, but 5.045 out at the top.

Some inspectors are trying to tell me that's ok because the angle tolerance on the 90 degree angle is +/- 1 degree. They say that it's acceptable to measure the part at the bend line, and NOT to measure overall lengths.

Is there any truth to this at all? Is there a separate standard or convention for sheetmetal dimensioning and tolerancing?
Yes, there is truth to this - but as far as a "standard" for sheet metal - I have never found one, and I've been at it for over 12 years.

As a general rule - you measure at the bend line. But there can be exceptions to this depending on your customers application. For example, we have channels we make that are about 1" in width with 1/2" flange on each side. Parts can not be overbent - as the part would not fit over it's mating part.

One of the difficulties are print that are drawn incorrectly. We have customers that will dimension hole locations across 4 bend lines, tolerance +-.010, but will give us a tolerance of +-1 degree on bends. :lmao: :lmao: :lmao:
 

Jim Wynne

Staff member
Admin
#3
Some inspectors are trying to tell me that's ok because the angle tolerance on the 90 degree angle is +/- 1 degree. They say that it's acceptable to measure the part at the bend line, and NOT to measure overall lengths.
Carol is on the front lines in this type of work, so she's an excellent source. But there's always (or nearly always) a problem with dopey engineers and CAD people who don't account for conflicting block tolerances such as you've described. Two words: contract review. These things need to be understood and agreed upon before the work is accepted.
 

Coury Ferguson

Moderator here to help
Staff member
Super Moderator
#4
Carol is on the front lines in this type of work, so she's an excellent source. But there's always (or nearly always) a problem with dopey engineers and CAD people who don't account for conflicting block tolerances such as you've described. Two words: contract review. These things need to be understood and agreed upon before the work is accepted.
:thanx: Jim

Let's not forget Design Review. But Contract Review is where it all needs to be hammered out, before the tender and acceptance of the Contract.
 
#6
You're right, Coury. An awful lot of problems could be avoided if design people would get input from suppliers early in the process.
Absolutley. The intended use of the product is what really matters in the end.

I spent many years inspecting prototypes and initial samples. During those years, I came across many utterly unusable products... ...that were within all given specifications.

So: How will the product in question be used?

/Claes
 

CarolX

Super Moderator
Super Moderator
#7
But there's always (or nearly always) a problem with dopey engineers and CAD people who don't account for conflicting block tolerances such as you've described.
Jim,

My experience has been that it is a lack of knowledge about sheet metal that create this situation. Our company spends a lot of time educating our customers.

We had a very refreshing experience of late - a new customer who never used sheet metal components - asked us for assitance in developing their drawings. They still overkilled it (true position requirements, for example, that are unnecessary), but refreshing none the less.
 

Jim Wynne

Staff member
Admin
#8
Jim,

My experience has been that it is a lack of knowledge about sheet metal that create this situation. Our company spends a lot of time educating our customers.
I took the liberty of adjusting your statement:D Having worked in sheet metal, machining, plating and stamping job shops, I can say that the ignorance isn't limited to sheet metal. Many design engineers have become highly competent at being ignorant in multiple disciplines.

CarolX said:
We have customers that will dimension hole locations across 4 bend lines...
This sort of thing is a perfect example; I'm sure you're all too familiar with the phenomenon of hole locations being toleranced +/- .005" when both the x and y datums are sheared edges. Some of this is due to lack of familiarity with the state of the art as you suggest, but often it's just dumb.
 

apestate

Quite Involved in Discussions
#9
Thank you all for the review of this question...

Learning the necessity of proper and adequate contract review has been one of the most interesting lessons this forum has taught me.
 
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