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General Surface Profile Callout

dl.melton
We have had a difference of opinion here at the shop that I work at and I'd like to get the opinion of some people that have been in this field for a while, because I know there are people out there that have a better understanding of GD&T than I do.
Let's say you have a CAD part model with a general profile tolerance ("Unless otherwise specified, all part features are subject to a surface profile tolerance zone of .020 with respect to the primary datum A, secondary datum B, and the tertiary datum C") and a specified thickness tolerance of +/-.010. You have a wall on this part that the model clearly shows does not have a constant thickness from one side to the other. Do you tolerance both sides of the wall according to the general profile tolerance, or do you hold one side to the general profile tolerance and then the other side to the thickness callout in spite of the fact that the thickness is not constant? Obviously if you hold both sides to the general profile tolerance, you have a potential for the thickness to run as high as +.020 if both sides of the wall were at top tolerance, but I don't see how you can hold it to a thickness tolerance when you don't have a constant thickness to hold.

Opinions? If you can back up your opinion with a specific aspect of the GD&T standard instead of "here's what I think" that would be even better...
  • What kind of part, stamped or milled?

    If it is a stamped part (sheet metal), one side is "master" (even though CAD is master, one side is more master than the other, re the stock thickness tolerance).

    The only time I've seen this in a model is for extrustions. The 'master' side of the CAD model is the side that the extrusion punch is on, since it determines the 'straight' wall, the other side of stock 'thins' due to the forming process. So, the 'punch' side of stock is profile 0.020
  • One other thing.... since the CAD is master (assumption) and since it shows the thinning (taper), the stock thickness tolerance applies to the CAD thickness, not some 'general' part thickness.

    So, if the stock is (nominally) 0.060" thick, with a thickness tolerance of +/-0.010, and the design (cad) shows it tapering from 0.060" at the base to 0.040" at the edge, the +/-0.010 applies to the 0.040" at the edge and to the 0.060" at the base, if ya follow.
  • My general rule: Unless it is specifically called out otherwise, non-constant thickness surfaces get the profile tolerance on both sides.
  • What kind of part, stamped or milled?

    If it is a stamped part (sheet metal), one side is "master" (even though CAD is master, one side is more master than the other, re the stock thickness tolerance).

    The only time I've seen this in a model is for extrustions. The 'master' side of the CAD model is the side that the extrusion punch is on, since it determines the 'straight' wall, the other side of stock 'thins' due to the forming process. So, the 'punch' side of stock is profile 0.020

    We primarily deal with milled parts. I've never dealt with punched parts. We do have some parts that are cut from extrusions, but they usually go through milling operations.
  • My general rule: Unless it is specifically called out otherwise, non-constant thickness surfaces get the profile tolerance on both sides.

    I have seen a few parts come through (usually older parts that are print-based with a CAD for reference purposes only) that do specify that the surface profile tolerance only applies to one side and the thickness applies to the other, but it's a pretty rare callout. But I have inspectors that I work with that, in my opinion, have basically taken that callout as an industry standard in spite of the fact that you don't see it very often. I tend to agree with you...if it's not specifically called out otherwise, I believe the surface profile tolerance applies to both sides on a non-constant thickness. But I'm not so egotistical as to believe that my opinion is the end-all and be-all of the issue, so I wanted to get some input on the subject from some people other than the ones I currently work with Slight smile.
  • Pretty much, for a milled part, I would say the profile applies to "both sides" of stock, but if the profile is 0.020 (total) and the thickness is +/-0.010, then the thickness is a refinement tolerance, meaning you can't be -0.009" on both sides of stock because you would then be 0.018" thin.
  • Pretty much, for a milled part, I would say the profile applies to "both sides" of stock, but if the profile is 0.020 (total) and the thickness is +/-0.010, then the thickness is a refinement tolerance, meaning you can't be -0.009" on both sides of stock because you would then be 0.018" thin.

    Bingo.

    You still have to meet both criteria simultaneously. So, you still have the predicament of which side is the "right" side.

    Like Matt, though, I work in automotive stamping, so generally we know that one side is the actual master, regardless of CAD thickness. And, we work with tailor rolled blanks, which can have variable thickness (which is shown in CAD, but doesn't exactly match it).
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