Knurling is a machining process that presses or cuts a regular pattern of straight, angled or diamond-shaped ridges into the surface of a part, usually a round part turned on a lathe. It adds grip to handles, knobs and thumbscrews, improves appearance, and a straight knurl can enlarge a diameter for a press fit.
In short: knurling is the textured grip on a tool handle or a thumbscrew, and it is made by hardened wheels, not by cutting a thread.
How Does Knurling Work?
A knurling tool holds one or more hardened wheels, called knurls, with teeth on their rims. The part spins in the lathe while the tool is forced against it and fed along its length. The teeth are rolled into the surface and leave a matching pattern of ridges and grooves.
On a manual lathe the job runs slowly with a relatively coarse feed. One US Army machining manual gives 60–80 rpm and a feed of 0.015–0.030 in (0.4–0.8 mm) per revolution. The tool is pressed in about 0.010 in (0.25 mm) to start, checked to confirm the pattern is tracking cleanly, then fed across and back until the pattern is fully formed. Knurling past that point wears the ridges down, and the finished knurl is brushed clean and deburred.
The pressure involved is high, so the part must be held firmly in a chuck or between centers, with the tailstock or a steady rest supporting long work.
Form Knurling vs Cut Knurling
There are two ways to make a knurl, and they behave differently.
| Form (roll) knurling | Cut knurling | |
|---|---|---|
| How it works | Wheel pressed into the part, plastically deforming the surface | Wheel tilted to the part so its teeth cut the pattern |
| Material removed | None, metal is displaced into ridges | Removed as chips |
| Wheel axis | Parallel to the part axis | Tilted to the part axis |
| Effect on diameter | Raises it, which is why straight form knurls are used for press fits | Material is removed rather than pushed up |
| Tool setup | Wheel pressed straight in | Wheel set at a tilt angle, with a clearance angle so its edge can cut |
| How common | The typical method | Less common, needs a special holder |
Knurl Patterns and Pitches
- Diamond: crossing right- and left-hand ridges. It is the most common pattern and the usual choice for grips.
- Straight: ridges parallel to the axis. Besides grip, it is used to raise a diameter for a press fit.
- Diagonal (helical): ridges at an angle in one direction only.
Pitch is the spacing of the teeth. Knurls come in coarse, medium and fine pitches (the Army manual lists diamond knurls in pitches 14, 21 and 33). Medium is used most often; coarse suits large diameters and fine suits small ones.
How to Specify a Knurl on a Drawing
- Call out the pattern, pitch and length of the knurled band.
- For form knurling, give the blank diameter before knurling as well as the finished diameter, since the ridges rise above the original surface.
- For a press fit, specify the straight knurl and the target diameter over the ridges.
- Note if the knurled band must be deburred or chamfered at its ends, since knurling throws up burrs.
For turned parts with knurled grips or press-fit knurls, the relevant service is CNC machining, including CNC turning of shafts, pins and other round parts.
Frequently Asked Questions
What is knurling used for?
Knurling puts a textured grip on handles, knobs, thumbscrews and adjusters, and gives parts a finished look. Straight knurls are also used to raise a shaft's diameter slightly, so it can be pressed into a hole and held by friction.
What is the difference between straight and diamond knurling?
A straight knurl has ridges running parallel to the part's axis and is also used to enlarge a diameter for press fits. A diamond knurl crosses two sets of angled ridges into a diamond grid. Diamond is the most common pattern.
Does knurling remove material?
Form knurling does not: the wheels displace metal into ridges, so the diameter grows. Cut knurling does remove material, as chips, because the wheel is tilted so its teeth cut the pattern rather than press it.
Related Services at NICE Rapid
- CNC Machining — in-house CNC turning for shafts, bushings, pins and other rotational parts, in aluminum, steel, stainless, brass and engineering plastics.
- Rapid Prototyping — machined prototypes in 3–5 days, from a single part.
- Request a Quote — send a STEP file and a drawing showing the knurl callout.
Sources
- US Army TC 9-524, Fundamentals of Machine Tools, Chapter 7: Lathes (hosted by Carnegie Mellon University) — knurling impresses a diamond or straight pattern with hardened wheels to improve appearance and grip; straight knurling increases diameter for a press fit; 60–80 rpm, 0.015–0.030 in/rev feed; ~0.010 in initial depth; diamond most common, medium pitch used most; pitches 14, 21, 33; coarse for large and fine for small diameters; excessive knurling ruins the work; remove burrs; chuck or centers, steady rest for long work — uhv.cheme.cmu.edu
- US Patent 5,946,991, Background — form (roll) knurling plastically deforms the surface with a wheel whose axis is parallel to the workpiece; cut knurling tilts the wheel so it cuts the pattern by removing metal chips; helical knurl wheel teeth — patents.google.com
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