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How to Cut Metric Threads on an Inch Lathe
Guide · Updated October 28, 2026
An inch leadscrew can cut metric pitches: a 127-tooth transposing gear converts exactly, because 127 ÷ 5 = 25.4. Without one, approximate gearing works for short threads. Either way, keep the half nut engaged and reverse the spindle between passes.
Check pitch diameter
Measure with three wires or a thread mic proved on gage blocks. The free Gage Block Calculator builds the stack.
Step by step
- Find the needed pitch: For example, 1.5 mm.
- Install transposing gears: 127-tooth gear with a 100 or 120 tooth gear per the lathe chart.
- Set the gearbox: Set to the TPI chart entry listed for the metric pitch.
- Engage the half nut: Engage once and never release until done.
- Cut and reverse: At the end of each pass, withdraw the tool, reverse the spindle to return.
- Infeed: With compound at 29.5°, take decreasing passes.
- Check: Measure with a pitch gage and nut.
Worked example
A 1.5 mm pitch equals 25.4 ÷ 1.5 = 16.93 TPI. Setting 17 TPI gives a 25.4 ÷ 17 = 1.494 mm pitch: error 0.006 mm per thread, or 0.06 mm (0.0024 in) over 10 threads; fine for a short thread with a nut, not for a long one. The 127-tooth gear makes it exact because 127 ÷ 5 = 25.4.
Why the half nut stays engaged
The thread dial is based on inch pitches, so it cannot pick up a metric lead. Disengaging loses the lead.
Common problems
A cross-threaded result means the half nut was released. Accumulating pitch error means approximate gearing. A drunken thread means backlash in gears.
Tips
- Conversion: inch and metric conversion.
- Threads: cut threads on a lathe.
- Pitch: thread pitch gages.
Worked example: M10 × 1.5 on an 8 TPI leadscrew
Most inch lathes use a 127-tooth transposing gear because 127 mm = 5 in exactly. An M10 × 1.5 thread has 16.93 threads per inch (25.4 ÷ 1.5). Keep the half nut engaged and reverse the spindle between passes, because the threading dial cannot be used for metric pitches.
Thread depth for M10 × 1.5 external: about 0.92 mm (0.613 × P). Check the pitch diameter with thread wires or a mating nut.
Fuller worked example: 1.5 mm pitch on an 8 TPI leadscrew
An 8 TPI leadscrew has a pitch of 25.4 ÷ 8 = 3.175 mm. The gear ratio needed is 1.5 ÷ 3.175 = 0.4724, which is exactly 60/127 because 1.5 × 8 ÷ 25.4 = 12 ÷ 25.4 = 60 ÷ 127. The 127-tooth gear makes the inch-to-metric conversion exact.
Keep the half-nuts engaged for the whole job and reverse the spindle to return, since the thread dial does not track metric pitches. Check pitch over 10 threads (15.0 mm) with a rule or the mating part.
FAQ
Can an inch lathe cut metric threads?
Yes, with a 127-tooth transposing gear or approximate gearing.
Why 127 teeth?
Because 127 ÷ 5 = 25.4.
Can I use the thread dial?
No, keep the half nut engaged.
How accurate is 17 TPI for 1.5 mm?
About 0.006 mm per thread error.
How do I return the carriage?
Reverse the spindle.
Related guides and calculators
- How to Cut Threads on a Lathe
- Thread Pitch Gage Basics
- Inch and Metric Conversion for Machinists
- How to Measure Threads with Three Wires
- Leadscrew Backlash Basics
- How to Cut a Taper with the Compound
- All free machinist calculators
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