Thread Cutting

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

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.

Types of screw threads

Screw thread nomenclature

Lathe setting for thread cutting

Types of gear trains Simple gear train

Compound gear train

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)

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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