Thread cutting is a lathe operation that produces helical threads on cylindrical workpieces using a tool with the same shape as the thread. There are different types of screw threads and thread nomenclature. Lathe setting for thread cutting involves introducing a translating gear, where the gear ratio equals the pitch of the workpiece divided by the pitch of the lead screw, which can be expressed as 5 times the number of threads per inch divided by 127, where 5 is the progression and the number of threads per inch is the reciprocal of the pitch. Lathe speeds are measured in revolutions per minute and can be changed using stepped pulleys, gear levers, changing belts on belt-driven lathes, or moving speed levers on

1. Thread Cutting
Radha.R, Assistant Professor(Sr), VIT University, Chennai

2. Thread cutting
• A thread is a helical shaped groove formed on cylindrical
surface of workpiece.
• Thread cutting is an operation performed on lathe to produce
threads by using a tool whose shape will be same as that of
thread.

3. Types of screw threads

4. Screw thread nomenclature

6. Lathe setting for thread cutting

7. Types of gear trains
Simple gear train
Compound gear train

8. Important terms
The cutting of metric threads on a lathe with a lead screw is carried
out by introducing translating gear of 127 teeth,
we know that,
Gear ratio = Driver teeth/ Driven teeth
= Pitch of the workpiece/Pitch of the lead screw
= pitch of workpiece/pitch of lead screw
= pitch of workpiece/(1/n x 127/5)
= 5pn/127
where 5= progression
P=pitch = 1/n
where n= no.of threads per inch (tpi)

9. 9
Setting Speeds on a Lathe
• Speeds measured in revolutions per minute
– Changed by stepped pulleys or gear levers
• Belt-driven lathe
– Various speeds obtained by changing flat belt and
back gear drive
• Geared-head lathe
– Speeds changed by moving speed levers into proper
positions according to rev/min chart fastened to
headstock