Profile Tolerance on Sheet Metal Parts 1

[ScottHaselden]

I am dealing with a situation that I have never seen in my 27 years of working with and using GD&T, and I have been unable to find any info about this anywhere. I am dealing with sheet metal parts that have been formed on a brake press, and parts formed by metal stamping processes. The company that I just went to work for uses laser-cut profile gages to check the profile of all of their parts, due to the number of parts that they produce (14000 part numbers and over 100 metal forming presses in the facility). Basically, these are flat panel pieces of sheet metal, which has had the profile shape of the bent part laser-cut out of it. They slide the profile gage over one end, and run the gage to the other end. I realize that this is incorrect GD&T due to the fact that the print calls for profile of a surface, and this method only checks profile of a line but this is an argument for a different day (they are still on the ASME Y14.5 - 1994 standard here, and don't even have a clue of what GD&T even is. They just use it haphazardly), and I have been unable to convince Engineering that the flat gage only checks one line at a time (basically, where the gage is located along the part).

My understanding of GD&T tolerance zones is that if the part falls within the tolerance envelope of the nominal value (in our case, this means an equilateral bilateral tolerance zone. All tolerances are equilateral and bilateral, unless otherwise specified at this facility) - so if it meets this zone in every location, it is acceptable. I'll include a graphic from Dimensioning and Tolerancing Handbook by Paul Drake, published by McGraw-Hill, which emphasizes this. It is an older standard publication, but it still has a lot of useful information when it comes to advanced GD&T Concepts.

Now, I know that in cylindrical positional tolerance with MMC, the smaller the pin, the more bonus tolerance that you have. Also the larger the hole, the larger the bonus tolerance you have.

So, my question is this: Does the material thickness (which is usually the thinnest it can be, per steel gage specification tolerance) provide an automatic MMC bonus tolerance, similar to the way a circular MMC would be on a cylindrical pin? If a smaller pin and a larger hole allow for bonus location tolerance, does the thinner material create an unexpected MMC bonus in a folded sheet metal part, per the bilateral tolerance zone of the GD&T?

I also know that another factor in this equation are the datum references. The planar datums are only one side of the sheet metal part surface so it would seem that the datum origin for the profile would have to be constricted somehow, in order for my question to be valid, and currently, they are not. Previous Engineers that did some design work on the gages are incredulous that I am even asking this question, since their design control was to simply apply an arbitrary percentage of tolerance to the datum reference and the other dimensions get a different arbitrary tolerance. The following is an excerpt from the current procedure (in this case the 50% and 60% values were arbitrarily decided upon. I was told "you got to just pick a number and go with it"):

"Profile gages should mimic the actual profile of CAD without any profile tolerance built-in. Drop thru
fixtures should mimic the profile tolerance of the print. Therefore a drop thru should be designed using
CAD plus the appropriate profile tolerance as the cut line. For any surface that is a datum use 50% of
the profile tolerance associated with that surface. Ex 1mm profile or +/-.5 x 50% = +/- .25mm profile for
the datum surface. For all other surfaces use 60% of the profile tolerance associated with the surfaces.
If holes are going to be in the fixture to be used as locating pins the holes should be made a virtual
condition (minimum hole size minus positional tolerance) however if the holes are datums exclude the
positional tolerance when determining the hole size."

I don't like arbitrary. I want to design these gages correctly, with good GD&T profile tolerances, and so, I am asking for advice from other Engineering Professionals to make sure I get the designs correct. I can't find anything online regarding this situation or anything similar, and nothing in the GD&T Standard that specifies whether the material thickness provides bonus tolerance or not in this situation. The fear here is that if we use full profile tolerance, some parts that check good on the gage, may come out to be out of tolerance. But currently, we are having a lot rejections of parts that don't fit the profile gages that, when put on a CMM turn out to be way in tolerance. This is hitting us with a massive cost issue, as we do not have the manpower (or CMM's) to verify all produced parts on 14,000 part numbers on the CMM.

So, is my understanding correct, and the extra space afforded by the thinner material still an acceptable value per the GD&T, or should I need to constrain the profile gage to the datum, somehow, or is it a case where the arbitrary tolerance created by my predecessors the best way to go? Any insights this forum could give would be greatly appreciated.

 

[3DDave]

Profile tolerance doesn't use material condition so there is no additional tolerance for the form of the part controlled by a profile tolerance.

Most/many sheet metal parts are forced into place by fasteners or other restraints and this should be reflected by the drawing showing where those forces are applied and and how large those forces are allowed to be. It doesn't sound like that is on the drawing.

It sounds like a case of sick-factory, where no one knows what they are doing nor do they mostly care. That's not a dimensioning and tolerancing problem. That's a management problem. Don't worry about the massive cost - get a resume in good shape. I worked with a sick-factory and they never wanted to change, no matter the cost. Rather than doing reasonable checks and instituting reasonable procedures, they handed power grinders to final assemblers to make parts fit. Everyone, and I mean everyone, in that factory was of the opinion that a grinder was easier to use than to do a better job.

 

[ScottHaselden]

This is True 3DDave. Unfortunately, most of my 27 year career has been spent in factories similar to this one. I did find that ASME has a Standard that directly answers my question: Y14.43 - 2011.

That standard is very clear when it states that the profile gage MUST be made to the Maximum Material Boundary (meaning the maximum boundary related to the material thickness, combined with the profile tolerance). It also states that the profile tolerance is unilaterally IN, from there. and it also states that the gage MUST Isolate to the required part datums.

Attached is a Word Document, where I copied and pasted the relevant sections and figures from the spec. Just in case anyone else ever runs into this issue.