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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 constrain 3 of the part’s 6 degrees of freedom. Tightening a flathead screw through one of the countersunk holes will constrain 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. If there is any ambiguity, the true geometric counterpart (1994 & 2018)/theoretical datum feature simulator (2009) should be explicitly defined.
NOTE: The intent of this Tip is to highlight possible datum reference frames constraint issues and optimum datum feature selection. It is not intended to provide recommendations for complete datum feature dimensioning.
This tip is in accordance with ASME Y14.5M-1994, Y14.5-2009 and Y14.5-2018.
This tip was originally released in June 2001.