DefinitionKnurling 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.
Explained
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
Form (roll) knurling presses a wheel into the part and plastically deforms the surface. The wheel axis is parallel to the part axis, no material is removed, and the diameter rises, which is why straight form knurls are used for press fits.
Cut knurling tilts the wheel to the part so its teeth cut the pattern, removing material as chips. It is less common and 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.
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.
Key takeaways
- Knurling creates a textured pattern of ridges and grooves on a round part, usually on a lathe.
- Form (roll) knurling displaces metal and raises the diameter; cut knurling removes chips and keeps the diameter closer to the blank.
- Diamond knurling is the most common grip pattern; straight knurling can enlarge a diameter for a press fit.
- Pitch selection depends on diameter: coarse for large parts, fine for small parts, and medium for most work.
- Drawings should specify pattern, pitch, length, blank diameter and finished diameter, plus deburring or chamfer requirements.
Example
References
- US Army TC 9-524, Fundamentals of Machine Tools, Chapter 7: Lathes — US Army / Carnegie Mellon University
- US Patent 5,946,991, Background — Google Patents