Profile Tolerance on Sheet Metal Parts
Profile Tolerance on Sheet Metal Parts
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.