Knurling is the process of cutting a regular raised pattern in the handle of a tool. A knurled tool is easier to grip and more comfortable to use for extended periods compared to a smooth handle. Knurling is performed on a lathe with a specialized cutting tool called a knurl, which has a pair of toothed cutting wheels. The tooth depth of a knurl determines how deeply the pattern will be impressed on the handle and is defined by the knurl's teeth per inch, or TPI. The tooth depth can be calculated from the knurl's TPI by using a few simple formulas.
Measure a cutting wheel across the blank diameter of the wheel with a ruler. The blank diameter is the portion of the wheel within the ring of cutting teeth and is measured from the lowest notch of the teeth.
Multiply the blank diameter by pi (3.14) to calculate the blank circumference of the cutting wheel. For example, a cutting wheel with a blank diameter of 2 inches will have a blank circumference of 6.28 inches.
Mark one cutting tooth with a marker or piece of chalk. Count the cutting teeth along the circumference of the wheel until you return to the marked tooth.
Divide the number of cutting teeth on the wheel by the blank circumference of the wheel in inches to find the number of teeth per inch (TPI). For example, if a wheel with a blank circumference of 6.28 inches has 157 teeth, it has 25 TPI.
Divide 1.000 by the TPI of the knurl to calculate the knurl's circular pitch. For example, a knurl that has 25 TPI will have a circular pitch of 0.04.
Multiply the circular pitch by the percentage affecting the depth of the knurl to find the tooth depth. The percentage is a constant that varies, depending on the type of knurl. A straight tooth or diagonal knurl has a constant of 35 percent. A diamond male knurl has a constant of 40 percent. A diamond female knurl has a constant of 25 percent. For example, a straight tooth knurl with a circular pitch of 0.04 has a tooth depth of 0.014 inches.
If you are measuring a small knurl, it may be more precise to measure the blank diameter in millimeters then convert the measurement to inches.