Diametral Pitch Knurling & Conversions
Determining Centerline with Straddle Holders
Approx. Increase in Knurled Diameter
Surface Speed to RPM Conversions
Numbers of Teeth on Stock Knurls
The Problem of Straight Form Knurling in a 2 Die Straddle Holder
Convex Knurls Speed Rotor-Shaft Production for Power-System Manufacturer
When to Roll Form, When to Cut
Tips to Upgrade Knurling Practices
General Knurling Information
Knurling is widely used in industry for many applications. Some of these include decorative and "grip" surfaces, repair of undersized shafts and oversize bores, and driving serrations and splines. The word "knurling" applies to both the method of production, and the rolled section on the part. It is usually produced by forcing a knurling die into the surface of a rotating part, displacing material from the original diameter. Another method is “cut knurling” which uses special cut knurl holders, and cut type knurl wheels.
There are two basic methods of specifying tooth to tooth spacing, or PITCH: CIRCULAR PITCH and DIAMETRAL PITCH. See page 6 for additional information.
The CIRCULAR PITCH system has been used for many years and is based on the distance from tooth to tooth, and is measured as TPI (Teeth per Inch) or in the similar Metric Pitch and are expressed as the following:
- TEETH per INCH or TPI. The TPI refers to the number of Teeth per (linear) Inch measured on the circumference of the work blank diameter. For straight knurl wheels the approximate TPI, may be measured on the outside diameter of the knurling for reference purposes. For diagonal knurl wheels, the *TPI is measured perpendicular to the teeth or helix angle. The circular pitch measurement in inches, is derived from 1 inch divided by the number of Teeth per Inch with the exception of 14, 21, and 33 TPI.
- METRIC PITCH. For knurls listed as a Metric Pitch, the pitch on straight knurls is the approximate tooth to tooth distance at the outside diameter of the work part measured in millimeters. For diagonal and diamond wheels, the Metric Pitch is measured perpendicular to the teeth or helix angle.
- DIAMETRAL PITCH or DP. This system is fully explained in American Standard ANSI/ASME B94.6-1984. There are only four standard pitches are used 64, 96, 128, & 160 DP. Diametral Pitch dies are designed to permit accurate tracking on standard fractional sized blanks up to 1-inch diameter in increments of 1/64”, 1/96”, 1/128”, & 1/160 inch, making blank diameter selection easier.
General Knurl Terms
Due to the many variables involved in any knurling operation (speeds, feeds, coolant, hardness of work piece, condition of pins, etc.), determining proper blank diameters for circular pitch dies is a bit more difficult, and usually involves some amount of experimentation. The tips along with the formulas on the following pages should help, but if problems persist, feel free to call us and we'll be glad to offer our advice.
*Exceptions: All 14, 21, & 33 TPI wheels on all diagonal or diamond knurls.
NORMAL CIRCULAR PITCH
APPROXIMATE TOOTH DEPTH OF ACCU-TRAK KNURLS
90° FORM KNURLS: Pn (Normal Circular Pitch) or P (Circular Pitch) are for knurls listed by TPI (Teeth Per Inch) and all Metric Pitch knurls. The tooth depth formed on the workpiece is approximately 42% of the Normal Circular Pitch.
80° FORM KNURLS: Pn (Normal Circular Pitch) for knurls listed by DP (Diametral Pitch). The tooth depth formed on the workpiece is approximately 48% of the Normal Circular Pitch.
70° FORM KNURLS: Pn (Normal Circular Pitch) for knurls 50 TPI and finer. The tooth depth formed on the workpiece is approximately 55% of the Normal Circular Pitch.
COARSE, MEDIUM, and FINE Knurls verses TPI (Teeth per inch) Knurls
In the middle of the 20th century, some knurl manufacturers (Armstrong and Williams) sold only 3 different pitches of diagonal knurling wheels as standard: Coarse, Medium and Fine.
They measured the pitch of their tools by the “Transverse TPI” (TTPI) as shown below. 1/transverse circular pitch = TTPI.
The 3 pitches were:
So, for example, a 14 TTPI wheel is a slightly “finer” pitch, not coarser, than a 16 NTPI wheel, which has caused much confusion over the years. Except for these 3 TPI’s, Accu-Trak, and all other current knurl manufacturers, produce their diagonal and diamond knurling wheels to the “Normal TPI”.
(TPI) = 1/normal circular pitch.
There is no exact definition for coarse, medium, and fine pitch knurling. If that is the callout on a drawing, you have some leeway on either side of what is shown above.
Metric knurls throughout the world are produced to the “normal circular pitch” and are measured in millimeters (mm).