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How to Cut Internal Threads on a Lathe

Guide · Updated October 28, 2026

Internal threading on a lathe uses a small boring-bar style threading tool inside a bored hole. The steps are the same as for external threads, but the tool is out of sight and the compound moves the other way.

Check the minor diameter

Bore the minor diameter with a bore gage set from gage blocks. Check it with a mic you've verified against gage blocks; the free Gage Block Calculator builds the stack.

Step by step

Bore to minor diameter: Bore the hole to the thread minor diameter and chamfer the mouth.
Cut a relief: Cut a relief groove at the bottom of blind threads so the tool has room to stop.
Set the tool: Set the tool on center and square it with a center gage held against the bore face.
Set the compound: Swing the compound 29–30° the opposite way from external threading so it feeds the tool outward.
Engage: Set the gearbox to the pitch, and use the threading dial or leave the half-nuts engaged.
Take passes: Feed the compound outward a little each pass, backing the cross slide out to return.
Check it: Try the plug gage or mating part near the end and take spring passes until it fits.

Worked example

1-8 UNC nut thread: pitch P = 0.125 in. Bore to about 0.875 in (the 7/8 in tap drill size). Basic internal thread depth is about 0.541 × P ≈ 0.068 in, so with the compound at 29.5° expect about 0.068 / cos 29.5° ≈ 0.078 in of compound travel. Take 0.010 in passes early, drop to 0.002–0.003 in near size, then check with a 1-8 plug gage.

Blind holes

Mark the depth on the carriage or use a stop, and run slowly. Some machinists thread away from the chuck with the tool upside down and the spindle in reverse, so the tool runs out of the hole instead of toward the bottom.

Bar rigidity

Use the largest bar that clears the minor diameter, with the shortest stickout. Chatter on internal threads is usually stickout.

Tips

External threads first? See cutting threads on a lathe.
Boring bars: how to use a boring bar.
Check fit with thread gages.
Identify pitch with a thread pitch gage.

FAQ

What size do I bore before internal threading?

The thread minor diameter, usually close to the tap drill size.

Which way does the compound go for internal threads?

The opposite way from external threading, so the tool feeds outward.

How deep is an internal thread?

About 0.541 times the pitch for UN threads; finish to a plug gage.

How do I thread a blind hole on a lathe?

Cut a relief groove and use a carriage stop, or thread away from the chuck in reverse.

How do I check an internal thread?

With a go/no-go plug thread gage or the mating part.

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