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Q: Dear Gary,
By definition, the primary datum plane requires a minimum of 3 points of contact. Let us assume that the flatness tolerance on a cube-shaped part is rather loose--let's say .100 inches. Many times, the part would be placed on the datum simulator and quality control would proceed without a question. However, since the flatness tolerance is so loose, it would be feasible to have a noticeable high spot on the cube’s primary datum plane that is directly below the part’s center of mass, causing the part to possess more than one “stable” position — a “teeter-totter” situation, if you will.
The quality engineer might place the part on the datum simulator to achieve 3 points of contact. However, he might place it down a second time and achieve 3 different points. If the part can acquire stability in multiple orientations, how would you determine which 3 points of contact to use to define your datum feature plane? If the answer is to use the secondary and tertiary datum planes to assist, what if those are not defined?
Thanks Gary!
A: 1. The standard states that if the part wobbles it may be adjusted to an optimum position, if necessary, to simulate the datum. 2. The drawing could also state datum targets. 3. Tighten up the flatness tolerance. 4. Level the surface in three places to the inspection table to simulate the assembly condition. 5. Let the CMM level the surface with best fit condition.
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