Knurling forms a pattern by pressure, so it has its own speed rules and a tracking constraint that turning does not. This calculator gives the spindle RPM from a sensible surface speed, estimates infeed and passes, and checks that the diamond will actually close on your diameter.
How it works
The spindle speed uses the standard surface-speed relationship, the same one used for turning:
RPM = (SFM × 12) / (π × diameter)
circumference = π × diameter
teeth around = circumference × knurl TPI
passes = ceil(total form depth / infeed per pass)Knurling runs slow — roughly 100 to 150 SFM for steel — because it displaces metal
rather than shearing it. The pattern only closes cleanly when teeth around is a
whole number; otherwise the second revolution lands its teeth between the first
set and you get a doubled, muddy diamond.
The tracking check — the most important number
The tracking calculation is what separates a clean, sharp knurl from a doubled or smeared pattern. Multiply the workpiece circumference by the wheel’s TPI. If the result is very close to a whole number, the pattern closes perfectly on the first revolution. If it is not — say the result is 31.4 — the wheel’s teeth on the second pass land at 0.4-pitch intervals between the first set, producing two overlapping rows of shallow diamonds.
The tool reports teeth around and the nearest clean diameters: the blank
diameter you would need to pre-turn slightly to land on exactly 31 or 32 teeth.
Adjusting by a few thousandths of an inch is usually enough to fix tracking
without affecting the fit of the finished part.
Pattern types
| Pattern | Wheel setup | Common use |
|---|---|---|
| Straight | Single wheel, parallel to axis | Grip surfaces, knobs |
| Diamond | Two angled wheels (or one diamond wheel) | Handles, adjustment rings |
| Diagonal | Single angled wheel | Decorative, angled grip |
Straight knurling relaxes the tracking constraint slightly because the pattern repeats along the axis rather than forming a closed diamond. Diamond knurling requires the tightest tracking, since the diagonal rows must mesh on both wheels.
Surface speed and material
| Material | Recommended SFM | Notes |
|---|---|---|
| Mild steel | 100–150 | Most common; use cutting oil |
| Aluminium | 150–250 | Softer; use lower pressure to avoid tearing |
| Brass | 120–180 | Good tracking; minimal oil needed |
| Stainless steel | 50–100 | Hard; use heavy pressure, slow down |
| Delrin / plastic | 200–400 | No oil; sharp wheel; minimal infeed |
Example and tips
Knurling a 0.500 inch diameter at 120 SFM gives RPM = (120 × 12) / (π × 0.5) ≈ 917 RPM. With a 20 TPI wheel the circumference is 1.571 inch, so teeth around is 1.571 × 20 = 31.4 — not a whole number, so it double-tracks. Turning the blank to about 0.493 inch lands on 31 teeth exactly. Apply firm, steady infeed and flood with cutting oil; light passes that never reach full depth cause flaking.