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Bad parallelism and good thickness, what can be common...

Hi all.
Situation: I have a round thin part, and on each side I measure 6 points on 3 radii exactly opposite each other. Next, I measure the thickness of this part between them. As well as its flatness and parallelism between the planes built from these points.
Please help me understand how this can happen and why. I understand the difference between flatness and parallelism, but here I can’t explain why, with such an even thickness over the entire surface of the parts, we have such poor parallelism. I would be very grateful if this was depicted visually, so that the explanation would be just for dummies.

Parents Reply Children
  • Your results show a tolerance of 0.2 for flatness and 0.02 for parallelism if these are the true tolerances on the drawing then the drawing is in error as flatness should never exceed a parallelism tolerance and you should get clarification as to what the customer wants or needs.
  • Not necessarily. Point to point distance can be good but parallelism, profile, & straightness can be out. This is related to the idea of extreme form variation that deals with bending, thickness & fit. Moreover if the form of any datum is out of whack it will negatively affect those gd&t call outs. Best practice is to check form of all datums before you dig into adding more features & gd&t to the program.
  • Indeed. Form tolerances must be smaller than gd&t characteristics that depend on them. Otherwise you don't know what is really out. Once again likely another bummer from an MBA engineer who was dozing during that one semester long gd&t class while taking 5 humanities classes instead. Apparently engineering bosses thought engineers are better when versed well in poetry and not so in gd&t.