Knurling, Parting, and Drilling on the Lathe

Knurling, Parting, and Drilling on the Lathe

This lab brings together three finishing and sizing operations that appear on nearly every production drawing: knurling for grip, parting to separate a finished part, and drilling/boring to create a precise through-hole. Each operation has its own setup rules, speeds, and safety requirements. Mastering all three on a single part is the benchmark for this lesson.

Knurling

Knurling presses a hardened wheel into the workpiece surface to form a raised pattern. A diamond (crossed) knurl is the most common and gives a grip surface for handles, knobs, and adjustment screws. The knurl tool holds two diagonal wheels that intersect to create the diamond grid.

Starting diameter is critical. If the OD is not at the correct size, the two wheel patterns will not align and you will get a double or ragged track instead of a clean diamond. Use this formula:

> Starting OD = (Number of teeth on knurl wheel) ÷ Pitch

Pitch for a knurl is expressed in teeth per inch (TPI). For a 96-pitch wheel with 32 teeth: Starting OD = 32 ÷ 96 = 0.333 in. This gives a diameter where the knurl pattern divides evenly around the circumference. Turn the OD to that calculated value before applying the knurl.

Set spindle speed low — roughly one-quarter of the normal turning speed for the material. Apply heavy cutting oil (sulfurized or general-purpose) to the wheels before and during the cut. Engage both wheels firmly against the work in a single plunge: 0.005–0.010 in of infeed is typical. Feed the carriage slowly from right to left across the knurl zone, then return without disengaging. Check the pattern after the first pass. If tracking (a double image) appears, increase infeed slightly and continue; do not back the wheels off and re-engage mid-knurl. The finished knurl must show complete, uniform diamonds over at least 80% of the knurled length.

Safety: Knurling generates chip fragments and cutting oil mist. Wear safety glasses or a face shield. Never wipe a spinning workpiece with a rag near the knurling tool. Keep hands and files clear of the work while the spindle is turning.

Parting

Parting (cutoff) separates the finished section from bar stock. The operation uses a narrow blade — 1/16 in (0.0625 in) is standard — which is the most vulnerable tool on the lathe. A parting blade that digs in or deflects can shatter, sending fragments at high speed.

Set up for parting:

• Speed: Reduce to roughly half of normal turning speed for the material. Many machinists use 30–50% of the facing speed.
• Tool height: The blade tip must be exactly on center. A blade even slightly above center will rub and chatter; below center is worse and can pull the blade under the work.
• Overhang: Minimize overhang on both the toolholder and the workpiece. Part as close to the chuck as the knurled length and chuck jaws allow.
• Feed: Slow, steady, manual cross-slide feed. Never power-feed a parting cut unless you have significant experience.
• Cutting oil: Apply generously and continuously. The narrow kerf traps heat quickly.

As the blade approaches the center, reduce feed further. The slug will fall when only a thin web remains — support it or allow clearance so it does not jam the blade. Measure the parted length with a 6 in digital caliper. The tolerance is ±0.010 in.

Safety: Never part under power with the tailstock in contact with the work unless setup is confirmed safe. Wear a face shield for parting. Never reach across a spinning parting blade.

Center Drilling, Drilling, and Boring

A through-hole centered within 0.010 in TIR (total indicator reading) requires a three-step sequence:

1. Center drill: A combined drill and countersink seats the following drill precisely on the axis. Apply at low speed; the small pilot diameter breaks easily if fed too hard.

2. Jobber drill: Drill progressively to within about 1/32 in of final diameter. Use cutting oil and peck-drill (retract every 1/4–1/2 in) to clear chips, especially in steel. Check that the drill enters straight — a drifting drill creates a bell-mouthed hole that the boring bar must clean up.

3. Boring bar: The boring bar removes the final material in one or two finish passes to hit the print diameter (±0.003 in). Boring corrects any drill wander and produces a truly round, on-axis hole. Take a light spring pass at the same setting before measuring — tool pressure can cause the bar to deflect slightly on the cutting pass.

Measure the finished bore with a telescoping gauge set to a snug fit, then transfer to an outside micrometer. Check runout by inserting a precision test bar (or a clean-ground drill shank close to bore size) and sweeping a dial indicator around the OD of the bar; the reading must not exceed 0.010 in TIR.

Safety: Always remove the chuck key before starting the spindle — a flying chuck key is a fatal hazard. Use chip guards or a shield when drilling. Lock out / tag out the lathe before changing drill bits or adjusting the toolpost.

Summary

All three operations — knurling, parting, and boring — demand correct speed, proper tool height, continuous lubrication, and disciplined measurement. A part passes this lab only when the knurl is uniform, the parted length is within ±0.010 in, and the bore is within ±0.003 in diameter and 0.010 in TIR runout. Set up carefully, measure before you remove the part from the chuck, and apply LOTO before any tooling change.