Knurling that tracks

Why a knurl double tracks

A knurl wheel presses or cuts its teeth into the work as it turns. After one revolution the wheel’s teeth come back around and have to land in the grooves they made on the first pass.

That only happens when the work’s circumference holds a whole number of teeth. If it holds 34.5, the second time round each tooth lands half a tooth off and starts a second, finer pattern between the first. That’s double tracking, and no amount of pressure fixes it.

A few thousandths on the diameter decides it, because the circumference is π times the diameter. So the fix starts at the turning cut before the knurl, not at the knurl.

Find the diameter that tracks

Knurl makers print a tracking value for each wheel: how much diameter one tooth on the work takes. Divide the blank diameter by it to get the teeth that fit, round to a whole number, and multiply back. That is the diameter to turn.

Dorian Tool’s example: a 1.138″ blank with a .0330″ wheel holds 34.5 teeth. Turn to 34 × .0330 = 1.122″, or 35 × .0330 = 1.155″, and either one tracks.

Form knurling pushes metal up as well as down, so the part grows. If the print gives the knurled diameter, take the growth off first to find the blank, then round the teeth. At 20 TPI in soft steel a straight knurl grows the diameter about .020″.

Diametral pitch wheels (ASME B94.6: 64, 96, 128 and 160 DP) are made so common blanks track as they are: a 96 DP straight knurl puts 96 teeth on every inch of blank diameter.

Form or cut, and how to run it

Form (rolled) knurling squeezes the pattern in without chips. It can run right up to a shoulder and leaves a clean finish, but it grows the part and needs pressure, so coarse pitches on small or thin work push hard.

Cut knurling makes chips, so the part doesn’t grow and it takes less force. It suits big and hard work and long knurls, but it can’t reach a shoulder: in the middle of a part it needs an undercut to run out into.

Start at the low end of the makers’ speeds and work up. Get to depth in as few turns as you can, at least 75% of the tooth depth so it tracks, and use plenty of coolant so chips aren’t rolled into the teeth. Stainless work-hardens, so don’t let the wheel rub.