As a general rule, undercuts are used at inside corners or intersections as an option for the part producer. At times, an undercut will be designed into a part for function. Drawing #20 illustrates a few typical examples where undercuts can be used to advantage.
Figure 1 illustrates how an undercut or a recess between a thread and a shoulder will enable seating securely with a mating piece. Undercuts in these examples are usually two to three threads wide and below the depth of the thread minor diameter.
Undercuts are properly used at the junction of bodies of different diameters which are to be ground. This is illustrated in Figure 3.
Figure 2 illustrates how a requirement for a sharp internal corner or a corner with a minimum radius can be met with an undercut. This is especially necessary where the small diameter is to be ground all the way to the shoulder. If the small diameter and the shoulder require grinding, the undercut shown in “D” is recommended, but is the most costly.
For ease of production, all undercuts should be dimensioned by specifying the maximum width and depth, and, if possible, the type of undercut should be left to the descretion of the part producer.