Thread Relief and Chamfers
Threads up to a shoulder or in a blind hole present the same problem. How close to the shoulder or bottom can a full thread be produced? This depends on the size and pitch of the thread and the material threaded.
External threads can usually be produced within 1-1/2 threads from a shoulder and internal threads tapped to within 2-1/2 threads from a hole bottom. The number varies with the pitch of the thread and by the machinability of the material.
When a mating part must be seated against a shoulder or hole bottom, a thread relief (recess or undercut) should be shown on the part. This provides clearance for the threading tools. This is shown in Figures 3 and 4.
When drawing a recess or undercut, it is necessary that its diameter be minimum .015″ larger than the major diameter of the thread in Figure 4 and minimum .015” less than the minor diameter of thread in Figure 3.
Since threading often produces starting burrs, these can be minimized by specifying a 45° countersink or chamfer which is .015″ minimum larger than the major diameter on internal threads (see Figure 4) and .015″ minimum smaller than the minor diameter on external threads (see Figure 3). End of thread at undercut or recess should also be chamfered at 45°.
When threads are rolled, the angle on the first and last threads may approximate a 45° angle. Exact angles can not be produced due to the displacement of the metal forming the thread.