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Avoid Over
Constraint in Your Designs.
Often designers use self-aligning features in assemblies. For the cover shown below on the left, two countersunk holes are being used for location. Placing the part on datum feature A will arrest 3 of the part's 6 degrees of freedom. Tightening a flathead screw through one of the countersunk holes will arrest two more degrees of freedom. Tightening a second flathead screw would cause over constraint and very often this second screwhead will not seat properly. To avoid this situation, a countersunk slot may be used in place of the second hole as is shown in the figure on the right. The countersink around the hole may then be used as a secondary datum feature. The countersink on the width of the slot may serve as the tertiary datum feature to provide exact constraint during inspection and in the assembly.
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