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.
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