CNC / machinist calculator
Knurling Tracking Diameter Calculator
A knurl only forms a clean, closed pattern when the circumference holds a whole number of teeth. Enter your target diameter and the knurl pitch and this calculator finds the nearest tracking diameter, where the pattern closes, and the tooth count it produces. You can subtract an optional growth allowance to get a starting blank size. Sizing the blank to a tracking diameter is what stops the doubled or drifting pattern that comes from an off-size blank.
- Tooth count
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- Starting blank (closing - growth)
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The tracking diameter is the standard, sourceable part. Diameter growth is empirical and varies by knurl and material - measure it and enter it as the allowance.
How it works
As a knurl wheel rolls onto a rotating blank, each tooth presses a groove. For the grooves to line up after a full turn rather than overlapping into a doubled or smeared pattern, the circumference must be an exact whole number of teeth. The tracking diameter is the nearest diameter that satisfies that, found by rounding pi times the target diameter divided by the circular pitch to a whole tooth count, then converting back to a diameter.
Circular pitch is the distance from one tooth to the next around the blank, equal to one divided by the teeth per inch for an inch knurl. This tracking math is the agreed, standard part of knurling and is what the pattern-closing rule depends on.
Diameter growth is the other half of knurling and is deliberately not computed here. As the knurl displaces metal, the diameter grows, so the blank is turned undersize, but how much is empirical and the sources disagree by a wide margin, from a fixed few thousandths to a fraction of the tooth pitch. Treat growth as a measured value for your knurl and material, enter it as the allowance, and verify on a test piece.
Worked example
For a 0.5 in target with a 20 TPI knurl, the nearest tracking diameter is 0.4934 in at 31 teeth, where the pattern closes cleanly.
Frequently asked questions
What is the tracking diameter for knurling?
It is the blank diameter at which the knurl pattern closes cleanly, where the circumference holds a whole number of teeth. It equals the tooth count times the circular pitch divided by pi, with the tooth count rounded from your target diameter.
Why does my knurl pattern double or drift?
Because the blank is not at a tracking diameter, so the teeth do not line up after a full revolution and start cutting between the previous grooves. Sizing the blank to the nearest tracking diameter fixes a doubled or wandering pattern.
How much does a knurl grow the diameter?
It grows as the teeth displace metal, but the amount is empirical and material dependent, and published values disagree widely. There is no single standard formula, so measure the growth for your knurl and material on a test piece rather than trusting one number.
What is circular pitch on a knurl?
Circular pitch is the spacing from one tooth to the next measured around the blank circumference. For an inch knurl rated in teeth per inch it is simply one divided by the teeth per inch, so a 20 TPI knurl has a 0.05 inch circular pitch.
Will the same knurl track on the same blank at another shop?
Not necessarily. Even nominally identical knurls vary between makers, and Machinery's Handbook notes the tracking diameter is approximate, so confirm the closing diameter with your specific wheels rather than assuming it transfers.
Related calculators
Sources
Every formula on this page is shown and sourced. See how we verify.
These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.