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Avoid Over Constraint in Your Designs
(In accordance with Y14.5M-1994 standard)

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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.

Back to Tips Tip added Jun 2001