Undercut
Glossary · Part Features
At a glance
- Category /
- Part Features
- Also called /
- relief, thread relief
- Related /
- 2 terms
- Source /
- Written in the shop
An undercut is a narrow groove machined at the base of a shoulder or the end of a thread so the mating part or the cutting tool can run out cleanly.
Also called: relief · thread relief
In practice
Sometimes called a relief. At a shoulder it guarantees the mating component seats flat instead of riding up on a leftover radius; at the end of a thread it gives the threading tool somewhere to stop.
Why it matters
An undercut removes material at exactly the point where stress concentrates, so it trades assembly reliability against fatigue strength. On highly loaded shafts the size of the undercut is a real engineering decision, not a machining convenience.
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