Duane,

Obviously there seems to be some confusion here. Sorry to have lost you George, I’ll try to explain again. But first, how can one compare a profile tolerance to that of True Position? True Position is the square root of the sum of the deviation squares, multiplied by 2 (X squared + Y squared, square root of that times 2), better known as the Pythagorean theorem. Where in you calculate the hypotenuse and record the absolute value as your total deviation from nominal. Now then, back to the profile call out. It was said, “This is not what the call out asks for.” I repeat…a profile call out is in fact the “tolerance” zone. The profile tolerance specifies the uniform boundary along the true profile within which the elements of the surface must lie. We do not report the tolerance zone we repot the Elements! I can’t speak for the engineers of other companies but ours want to know the actual not the (+/-) range/distance between high and low. How else are they expected to make corrections to the surface without knowing the direction the surface is out of tolerance? It appears that some may be confusing flatness with profile. I suggest you pick up a copy of Dimensioning and Tolerancing from the American Society of Mechanical Engineers (An ASME Standard). Page 157 of chapter 6, paragraph 6.2 “Form tolerances control straightness, flatness, circularity and cylindricity. Orientation tolerances control angularity, parallelism and perpendicularity. A profile tolerance may control form, orientation, size and location depending on how it is applied. Since, to a certain degree, the limits of size control form and parallelism, and tolerances of location control orientation, the extent of these controls should be considered before specifying form and orientation tolerances.” Therefore, a form or orientation tolerance (the profile) specifies a zone within which the considered feature (the true surface profile / the nominal / the print location for the surface to be measured), its line elements, its axis, or its center plane must be contained. From page 167 of the same book I referred to earlier, chapter 6, paragraph 5, and sub paragraph 3 and I quote “Explanation of a Profile Tolerance: The tolerance value represents the distance between two boundaries equally or unequally disposed about the true profile or entirely disposed on one side of the true profile. Profile tolerances apply normal (perpendicular) to the true profile at all points along the profile. The boundaries of the tolerance zone follow the geometric shape of the true profile. The actual surface or line element must be within the specified tolerance zone, and all variations from the true profile must blend. With profile tolerancing, the true profile may be defined by the following: basic radii, basic angular dimensions, basic coordinate dimensions, basic size dimensions, undimensioned drawings or formulas. In any event, it is the true profile (actual surface or line elements / the nominal) you want to report, not the tolerance zone.

Geoff
Sr. CMM Programmer
Lapeer Metal Stamping
LMS Metrology Lab
Lapeer, MI