Thread Cutting on a Lathe Step by Step: Settings, RPM Selection, Tools and Dimension Control

Thread cutting on a lathe is one of the most important machining operations. Success depends on correct machine setup, proper tooling, reasonable spindle speed, and continuous dimensional control. Below is a step-by-step guide to help you produce accurate threads with good surface finish and proper fit.

1. Choose the thread type and parameters

• Metric (M) – pitch in mm (e.g., M12x1.75)
• Imperial (UNC/UNF/BSP) – threads per inch (TPI)
• External or internal
• Right-hand or left-hand

2. Prepare the workpiece

Turn the base diameter before threading:

• External thread – nominal diameter
• Internal thread – correct tapping drill size

3. Select the threading tool

• Indexable threading inserts
• HSS threading tools
• Internal threading boring bars

4. Set the lathe gearbox and leadscrew feed

Set the correct pitch using the gearbox chart and ensure proper half-nut engagement.

5. Choose RPM and use lubrication

Threading uses lower speeds for safety and accuracy. Use cutting oil or coolant.

6. Set the compound slide to 29–30°

This reduces tool load and improves surface finish.

7. Make multiple passes

Start with deeper cuts, finish with light passes.

8. Check dimensions

• Nuts or mating parts
• Thread ring gauges (GO/NO-GO)
• Three-wire method
• Plug gauges (internal threads)

Summary

Correct pitch setup, speed control, tooling, and measurement are key to successful threading. For rigid and precise performance, explore CORMAK lathes.