Duane,

As Ray Admire has pointed out, the correct way to report on a profile call out would be to apply +/- half of the full tolerance zone to the nominal surface measurement but allow me to reemphasize his reply. We are of course discussing a bilateral profile here. As you know there are two types of profile call outs effecting surface measurements: Profile of a surface and Profile of a line. A profile tolerance is just that, a tolerance. Much like a 10mm diameter has a +/- 0.5mm hole size. We report the actual size of the diameter as say 10.3mm and call it good being within the +/- tolerance zone. When we look at our facial “profile” in the mirror we are looking at the side of our face. Now, if you consider a surface profile as a side view or cut view if you will, this will all become very clear. What you are measuring is the amount of surface variation above and below the nominal relative to a known datum surface. Ok, two types of variation: Bilateral and Unilateral. A print with proper GD&T will have a single pointer “arrow” pointing to the surface indicating a bilateral tolerance or two sided, equal spaced zone (Profile 1.0mm = nominal +/- 0.5mm). Two pointers, one above and one below the surface would indicate a unilateral tolerance or one sided unequally disposed zone. Likewise, proper GD&T will show dashed lines “Phantom lines” above or below the surface to clearly indicate the zone. A dashed line below is to mean the surface may not vary above nominal only below to the max and vise versa for a dashed line above the surface. We here at Lapeer Metal deal primarily with sheet metal stampings for the Automotive Industry. I’m sure you are familiar with the term body line. For example: Given - A part lies flat to the world with a known surface nominal to be at the “Z” axis 100mm body line, with an applied bilateral profile tolerance of 1.0mm. The interpretation is nominal equals 100mm, +/- 0.5mm or 0.5mm on each side of the true surface (99.5 to 100.5). If your actual measurement is 100.6mm then you are out of spec. + 0.10mm. At 99.4 actual you would be out of spec. – 0.10mm. The same rules apply to profile of a line except your only concern is the edge of the surface (matching the door to the rear quarter panel) not the entire surface itself. In any case, unless otherwise specified always measure normal to the basic surface profile. Assure that your part is properly aligned to the datum structure. Like true position call outs, it’s virtually impossible to accurately measure these features with height gage and surface plate. Leave those tools for the quick flatness check to report the highs and lows of a surface and record the maximum range (absolute value) between readings. Remember, profiles are not absolute and they will be positive or negative from the norm or true print specification nominal for the surface to be measured.

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