What are thread reliefs and undercuts?

Also called an undercut, it belongs as much to a ground or turned surface ending against a shoulder as to a thread. Without it, a thread leaves a stretch of incomplete thread, the run-out, and grinding or turning leaves a rounded corner. On an external thread the bottom of the relief lies just below the minor diameter; on an internal thread just above the nominal diameter. That lets the tool run out and a nut, bearing or hub seat flat against the shoulder.

What do thread reliefs and undercuts depend on?

A single-point threading tool needs room to leave the cut; the pitch, the tool nose and the machine's retract move decide how much. A tap leaves incomplete turns in a blind hole, depending on its chamfer form, so the hole has to go deeper than the usable thread. Thread milling gets closer to a shoulder, but a run-out remains there too. A relief is also a notch: on a shaft under alternating load it raises the stress concentration, which is why drawings then often specify a form with a larger radius.

What does the standard say about thread reliefs and undercuts?

For ISO metric threads, DIN 76-1 sets run-outs and undercuts, with forms for external and internal threads and sizes per pitch. DIN 509 sets undercuts at a shoulder on a turned or ground shaft, in forms E, F, G and H. The drawing often gives only a reference to the standard with form and size; the other dimensions then come from the standard. If the relief is fully dimensioned, the drawn dimensions apply.

How do thread reliefs and undercuts work in practice?

A relief is usually a short grooving operation, with a grooving tool or a form insert that cuts the standard shape in one go. It gets awkward when it is missing: a full thread right up to a shoulder cannot be cut with a single-point tool, and the customer then has to choose between a relief, an allowed run-out or a different design. Settle that before you quote. For an internal thread in a blind hole, a relief needs an internal grooving tool and room in the bore.

What are thread reliefs and undercuts often confused with?

A thread relief is not the run-out: the run-out is the stretch of incomplete thread left when there is no relief, and DIN 76-1 gives sizes for both. Nor is a thread relief the same as a DIN 509 undercut at a shoulder: the first is there for thread run-out, the second for grinding, turning and flat seating.