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I keep hearing folks say that the primary datum used to control a hole must be at right angles to the axis of the hole. Not true! Pick your datums based usually on function. If some of the holes happen to be at an angle other than 90°, so be it. As long as the BASIC linear and angular dimensions used to locate the hole eventually get back to the datum, no problem. In this example, the 8.5mm hole is not perpendicular to any of the datums referenced. The Ø0.4mm positional tolerance is located by the 20mm, 30mm and 45° BASIC dimensions.
This tip is in accordance with ASME Y14.5M-1994, Y14.5-2009 and Y14.5-2018.
This tip was originally released in April 2000.