Thread Turning-Everything you need to know

Thread turning is a lathe operation used to produce external or internal screw threads on a rotating workpiece. A single-point cutting tool moves longitudinally along the workpiece in synchronization with its rotation, following a helical path. This operation allows precise control over thread size, pitch, and form, and is commonly used for making threaded shafts, bolts, and nuts.

Thread Turning (Screw Cutting) on a Lathe

1. What is Thread Turning?

Thread turning, also called screw cutting, is a lathe operation used to produce helical grooves (threads) on the external or internal surface of a cylindrical workpiece using a single-point cutting tool.

2. Purpose of Thread Turning

To produce fastening elements (bolts, screws)
To allow assembly and disassembly
To transmit motion or force (lead screws)
To obtain precise mating parts

3. Principle of Thread Turning

The workpiece rotates in the spindle.
The threading tool is fed longitudinally, parallel to the axis.
The lead screw drives the carriage.
A fixed relationship between spindle speed and tool feed produces the correct pitch.

4. Types of Threads (Lathe Cutting)

a) Based on Location

External threads – cut on outer surface
Internal threads – cut inside a hole

b) Based on Form

V-thread (Metric, Unified)
Square thread
Acme thread
Buttress thread
Knuckle thread

c) Based on Direction

Right-hand thread (most common)
Left-hand thread

5. Thread Turning Tools

Single-Point Thread Cutting Tool
- Ground to thread profile
- HSS or carbide
Insert Threading Tool
- Indexable carbide insert
- Used in CNC lathes

Tool angles (Metric V-thread):

Included angle: 60°
Clearance angle: 5°–7°
Small nose radius for accuracy

6. Thread Turning Setup

Work holding: 3-jaw chuck, 4-jaw chuck, or between centers
Tool setting:
- Tool tip at center height
- Tool aligned perpendicular to work axis
Gear train / Quick-change gearbox: Set to required pitch
Compound rest: Set at 29° (for 60° threads)

7. Methods of Thread Cutting

a) Straight In-Feed Method

Tool fed radially inward
Used for coarse or short threads

b) Compound Slide Method (Most common)

Tool fed at an angle (29°)
Produces better finish and tool life

c) CNC Threading

Controlled by G-codes (G76, G92)
High accuracy and repeatability

8. Steps in Thread Turning (Manual Lathe)

Prepare workpiece to correct diameter.
Set threading tool at center height.
Adjust gearbox for required pitch.
Engage half-nut lever.
Start lathe at low speed.
Feed tool gradually using compound slide.
Retract tool at thread end.
Repeat passes until full depth achieved.
Check thread using gauge or nut.

9. Thread Dimensions (Metric Thread)

Pitch (P): Distance between threads
Major diameter: Outer diameter
Minor diameter: Root diameter
Depth of thread: ≈ 0.613 × P (for 60° thread)

10. Cutting Parameters

Cutting speed: Low (⅓ of turning speed)
Feed: Equal to thread pitch
Depth of cut: Gradual, multiple passes
Coolant: Improves finish and tool life

11. Common Defects & Remedies

Defect	Cause	Remedy
Incorrect pitch	Gearbox setting wrong	Reset gearbox
Poor surface finish	Blunt tool	Sharpen tool
Double threading	Half-nut disengaged	Keep half-nut engaged
Tool breakage	Excess depth of cut	Reduce in-feed