Parallelism Is an Individual Tolerance (#251)
(In accordance with the ASME Y14.5-2009 standard)
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Here is an incomplete redraw of a print of a weldment sent to me recently. The person making the drawing said he wanted to "locate the orientation" of the fins. He applied parallelism as shown on the first drawing. According to section 6.2 of the current Y14.5 standard, parallelism does not control location to the datum axis or to the other fins in the pattern. So, the part shown would be in accordance with the print. I know, it doesn’t look like the print but, since there is no control for location, there is no limit to the amount the fins may float around relative to the datum axis. I am certain your supplier would not make a part this bad. But, at what point would a part be unacceptable?
The solution is once the datum features are identified, other features must be located. If you want to locate the center of the fins, use position. Or, if you need to control the location of the surface of the fins, use profile of a surface as shown on the second drawing. Since the profile tolerance applies to the entire feature, the orientation is also controlled. If you need the parallelism to be better than the profile tolerance, you should add a refinement of orientation. Because the 4X is included with the profile tolerance, it is controlling the four surfaces to the datum axis and to each other in the pattern.
