Slide 003: Threading, Parting, and Knurling¶

Slide Visual¶

Threading, Parting, and Knurling

Slide Overview¶

This slide covers three specialized lathe operations: single-point threading, parting (cut-off), and knurling—each with unique setup requirements and techniques.

Instruction Notes¶

Single-Point Threading¶

Threading on the lathe uses a ground or inserted threading tool synchronized with the lead screw to cut precise screw threads.

Setup: 1. Grind or select a 60° threading tool (for Unified/ISO threads). Verify the angle with a center gauge (fishtail gauge). 2. Set the tool exactly on center height. Misalignment causes incorrect thread profile. 3. Set the compound rest to 29° (or 29.5°) to feed along one thread flank. This provides a shearing cut rather than plowing with the full tool width. 4. Set the gear train (or electronic gearbox) for the desired pitch (e.g., 20 TPI, 13 TPI). 5. Reduce spindle speed: 1/4 to 1/3 of normal turning speed. Threading requires careful observation and precise engagement.

Cutting procedure: 1. Engage the half-nut lever at the correct threading dial graduation. 2. Take a light pass (0.005-0.010" on the compound). Retract with the cross slide at the end of the thread length. 3. Disengage the half-nut. Return the carriage to the start position. 4. Advance the compound for the next pass. Re-engage at the same threading dial graduation. 5. Repeat with decreasing depth per pass (spring passes at final depth: no additional compound advance, just re-cutting to clean up). 6. Check thread fit with a nut (external) or bolt (internal) or thread ring/plug gauge.

Thread depth: Total compound infeed ≈ 0.750 / TPI (for 60° threads). Example: 20 TPI → 0.750/20 = 0.0375" total depth.

Parting (Cut-Off)¶

Parting separates a finished part from the bar stock using a narrow blade.

Critical requirements: - Tool exactly on center height (±0.005") - Tool perpendicular to the workpiece axis—any angle causes binding - Slowest practical speed (50-60% of normal turning speed) - Steady, continuous feed—never stop feeding during a parting cut (causes rubbing and work hardening) - Use cutting fluid liberally - Minimize workpiece overhang from chuck

Common problems: - Tool chattering: speed too high, tool below center, or excessive overhang - Tool breaking: tool deflecting into the cut, jamming on chips, or hitting the center - Workpiece grabbing: tool not perpendicular, creating a wedging action

Knurling¶

Knurling impresses a diamond or straight-line pattern onto the workpiece surface for grip. Unlike other operations, knurling displaces material rather than removing it.

Procedure: 1. Mount the knurling tool in the tool post. Align the wheels parallel to the workpiece axis. 2. Set speed to 60-100 RPM (slow). 3. Force the knurling wheels into the workpiece with the cross slide. Apply significant pressure—0.030-0.050" initial penetration. 4. Engage power feed (0.010-0.020 IPR). 5. Apply cutting oil to prevent tearing. 6. Check the pattern after one pass. A good knurl has fully formed, sharp diamond points with no double tracking. 7. Make additional passes if the pattern is not fully developed.

Double tracking (misaligned second pass) is the most common knurling defect. It occurs when the knurl wheels do not track in the same grooves on subsequent passes. Solutions: use a scissor-type knurling tool, ensure exact starting pressure, or use a bump knurler.

Key Talking Points¶

Threading requires practice—start with coarse pitches (13 TPI or coarser) on aluminum
The threading dial must be read correctly—engaging at the wrong mark creates crossed threads
Parting is unforgiving—proper setup prevents 90% of problems
Knurling generates extreme forces—ensure workpiece and setup can handle it

Learning Objectives (Concept Check)¶

Can students describe the threading compound angle and why it is used?
Can students list the critical requirements for successful parting?
Do students understand why knurling displaces rather than removes material?

Last Updated: 2026-03-19