Knurling puts a regular pattern of ridges onto a cylindrical surface so a hand or a mating part can grip it. It is one of the oldest lathe operations and it is still used because nothing else gives grip that cheaply. The one thing that catches people out is that the common form does not cut material away, it displaces it, so the part gets bigger.
Formed and cut knurls are different operations
A formed knurl is pressed into the surface by hardened wheels, displacing material sideways and upwards. It is fast, needs no chip clearance, and increases the diameter by roughly half the tooth depth. A cut knurl uses a tool that removes material at an angle, producing a cleaner pattern with no diameter growth and no bulging at the ends, at a slower rate.
The patterns and what they are for
Straight knurls run parallel to the axis and resist rotation in a bore. Diagonal or helical knurls run at an angle. Diamond knurls cross two helices and are the familiar pattern on adjusting knobs and handles, giving grip in every direction. Pitch is coarse, medium or fine, and coarse patterns grip better while fine ones look neater.
Getting the starting diameter right
For a formed knurl the pattern only tracks correctly if the circumference divides evenly by the knurl pitch, otherwise the wheel skips and produces a doubled or ragged pattern. Machinists adjust the pre knurl diameter to suit. On the drawing, dimension the diameter before knurling and state the finished diameter as a reference, or state the knurl specification and let the shop compute the blank.
Press fit knurls
A straight knurl on a shaft that is pressed into a plastic or soft metal boss is a standard way to stop rotation and pull out, and it is what most knurled inserts rely on. The interference comes from the raised material, so the knurl specification and the hole size have to be considered together rather than separately.
Questions people ask about knurling
Does knurling remove material?
A formed knurl does not; it displaces material and increases the diameter. A cut knurl removes material and leaves the diameter essentially unchanged.
Which knurl pattern gives the best grip?
A coarse diamond pattern for a hand grip in any direction, or a straight knurl where the load is purely rotational, such as an insert pressed into a bore.
Why did my knurl come out doubled?
Almost always because the circumference does not divide evenly by the knurl pitch, so the wheel does not track its own pattern. Adjusting the pre knurl diameter fixes it.
How should knurling be shown on a drawing?
Specify the pattern and pitch, the axial extent of the knurled area, and the diameter before knurling, with the finished diameter as a reference if it matters.