Knurling in the lathe with a standard tool exerts substantial strain on the front mandrel bearings, all the loading is lateral at 90 degrees to the lathe axiz - and this can lead to damage in light lathes if the work is un-supported, and is exceptionally bad if working some distance from the chuck. The standard tool holds a pair of knurls which are usually allowed to 'float' on a pivot to align with the work. I have a large tool with 3 pairs on a rotating holder, but I have hardly ever used it. Either of these designs are fine for the larger lathe (6" C/H and up) where the bearings are usually much more substantial. The loading can be reduced somewhat by using narrower cross-section knurling wheels 3/16" or 1/4" wide instead of the usual 3/8", and using tailstock support also reduces the strain on the bearings. However, the best option for users of light lathes is to use a clamp (or 'straddle') knurling tool instead. In this design the loading is retained within the tool itself as a compression exerted between the two clamp arms.
Knurling is not a cutting process, it is a method of squeezing the metal hard enough to cause plastic movements of metal into peaks and troughs. In the process flakes of metal and metal dust are produced which need to be removed if the process is to work well. The choice is to either flood the work with coolant or to run dry so the dust is simply thrown away from the work. Dabbing the work with a little cutting oil is not much help as this quickly turns to sludge, the particles are continually squished between the knurls and the surface finish suffers as a result.
Selection of commercial knurls (left 3 pairs) and home-made knurls.
There are a two primary patterns of knurling wheel available, straight and diamond, and all come in a variety of sizes. The wheels can be made from either HSS or hardened carbon steel, and I don't think it makes much difference which ones you use. It's unlikely you'll wear either out (unless you accidentally try to knurl a hardened shaft!) Straight knurls are easy to make from silver steel if you have a rotary table or dividing head, it is simply a matter of machining grooves with an endmill, parting off and hardening. This is a useful way of producing extra fine pitch wheels or narrow wheels that are otherwise difficult to get hold of.
Cutting knurls on a 5/8" silver steel rod.
Three knurls made from hardened silver steel.
The problem of getting the same pattern on a variety of diameters is not one I have solved. Some diameters produce a perfect knurl every time, on others I get a persistant 'doubling', sometimes resulting in a perfect diamond pattern of half the pitch it should be, at other times I get just random patterning. I have read that you should start knurling by applying the wheels to the outer edge of the work and rotate by hand increasing pressure until the correct pattern appears. Only then switching on the motor (fairly slow - about 1/3 normal turning speed). Well, all I can say is that this sometimes works - and sometimes not. I think the theory is that, if the circumference cannot be divided as an even factor of the pitch of the knurl, then slippage will occur so the teeth of the knurl sort of 'slip into' the next groove. Increasing the pressure encourages this slip but in my experience it's not until the work has been compressed to a smaller diameter does any significant change in the patterning occur. Sometimes reducing the diameter is the only way to get a good matching pattern and this I will do if a match is important (10 or 20 thou usually works).
Once the knurl is complete it's a simple matter to chamfer the sides or use a parting tool tidy up the knurled area. A good trick is to use a straight knurl and then use a sharp threading tool to cut grooves at regular intervals across the knurled surface, this gives an unusual and pleasing square pattern. don't do it the other way around because the knurling will squeeze metal into the machined grooves which doesn't look so good.
On narrow brass components it's possible to get away with a single knurl wheel (straight pattern) in a holder, the strains involved being that much less. This is useful for items like telescope lens mounts which may be of a larger diamter than can be accomodated by the clamp knurling tool. For an exhibition finish I have actually cut a straight knurl using a headstock dividing attachment and engraving tool, doesn't take as long as you might think because the choice of a convenient number of divisions is up to the worker. Done this way, the depth of cut can be controlled so that the peaks are narrow polished flats which really looks good.
Special concave knurl for a decorative finish.
Specialised knurls can be home-made to knurl convex surfaces. Take a length of 5/8" diameter silver steel and set it up in the dividing head or a chuck mounted on a rotary table. Use the front corner of an endmill (in this example, 1/4" diameter) to form notches around the sliver steel rod, thus producing a concave row of teeth. Back on the lathe, centre, drill, ream 1/4" diameter and part off the wheel. Harden and temper as normal. On the workpiece, form a ring on the surface 1/4" wide and 5/32" high (i.e., 5/16" larger diameter than the main body of the workpiece). Chamfer the edges of the ring and then bring the concave knurl into play. Press the knurl against the ring and and attractive knurled sculptured ring will be formed. This looks very nice on your home made tools and instruments.
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(c) Chris Heapy 1996.
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