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.