Thread Relief: When a Threaded Part Needs an Undercut

A thread relief, also called an undercut or recess, is a small groove at the end of a thread, and it's often left off the drawing.

Do You Really Need a Thread Relief, and How Big Should It Be?

You need one when a mating part has to seat against a shoulder or the bottom of a hole. If nothing bottoms out, you can often leave it off. When you do draw one, size it from a guide or standard, not a guess.

An r/Machinists user designing an aluminum part with internal 5/8-24 UNEF threads asked whether a 0.022 in relief was enough, or whether one was needed at all, since the mating part couldn't bottom out. One reply explained that the relief keeps the threading tool off the bottom face and lets a bolt bottom out if needed. Since nothing would bottom out, the part would be fine.

Why a thread relief exists

Threading tools can't cut a full thread right up to a shoulder. The Precision Machined Products Association designer's guide says external threads can usually be produced within 1-1/2 threads of a shoulder, and internal threads tapped to within 2-1/2 threads of a hole bottom, depending on pitch and material. Without a relief, the mating part stops on those partial threads instead of the face.

When to leave it off

In a Practical Machinist thread about a shaft on a new CNC lathe, one reply said to make it per the print. Another said the part is slightly stronger without a relief, and that the call belongs to the designer, not the machinist.

Sizing and drawing a thread relief

• Follow a guide. PMPA says the relief diameter should be at least 0.015 in larger than the thread's major diameter for an internal thread, and at least 0.015 in smaller than the minor diameter for an external thread.
• Or cite a standard. In a separate r/Machinists drawing critique, one reply suggested picking a thread runout type from DIN 76.
• Add chamfers. PMPA recommends a 45° chamfer at the start of the thread to keep burrs down, and a 45° chamfer at the end of the thread where it meets the relief.
• Don't write "undercut as needed." A reply in that critique called it kind of meaningless, because the machinist can't know how many full threads you need.

Related: roll taps vs cut taps, thread milling vs tapping and tapping blind holes in titanium.

At DigiForge

Our CNC service covers 3-, 4- and 5-axis milling and turning, with ISO 2768-m as the listed standard tolerance. Draw thread reliefs with their size and position so they're part of the quote. If a thread runs into a shoulder and you're not sure it needs a relief, ask us when you quote.