I dont think speed is too critical. Clearance of chips and dwarf is though so turning at lower speeds would help in being able to keep the knurls and job clean with either coolant or an air jet.

The most important thing is work diameter, you cant just knurl any size workpiece with a given set of knurls. You have to calculate the various diameters your knurl will track correctly on. Otherwise the pattern will come out badly.

Mick

I was going to start this post suggesting that knurling doesn't produce swarf, coolant would help the metal "move" under the influence of the knurls. In reality I find that debris is created and needs washing away.

Could this be due to not bothering to calculate the diameter of the work piece to suit the pitch of the knurl?

I've never bothered to calculate the diameter and always get an entirely satisfactory knurl…..

As for speed, on a small lathe I would reduce speed quite a lot to make sure there is sufficient torque, the forces in knurling seem quite high.

I have a larger lathe, plenty of power, I always reduce the speed to knurl because it seems kinder to the knurls.

John

Allegedly. A proper diameter to knurl must be a multiple of the distance between the teeth of the knurling tool divided by Pi.

So for a 1 inch shaft to be knurled, with a wheel with a .060 inch tooth spacing.

0.060 ÷ 3.14159 = 0.019

By juggling some numbers 52 x 0.019 = 0.988

Because of possible inaccuracy in measuring the spacing add 0.010 to the calculated diameter and do a trial pattern. If it tracks correctly happy days, if not turn down a couple of thou and try again. Once you have the proper diameter record, machine and knurl. Simples.

I know that a calculation should be an exact process and this involves some guestimation, but that's just to account for mistakes in measuring the tooth spacing.

Mick

Two reasons to use back-gear:

1. To reduce speed for the job. For improved control as when screw-cutting or to suit the cutting speed of a large drill.
2. To increase torque when asking the lathe to work harder than normal, as in knurling.

Engaging back-gear for knurling is solid advice on a lathe with a smallish motor or, in the good old days, when arthritic knees were painfully working a treadle! The additional 'turning power' available whilst knurling helps when the lathe only has a ⅓ or ½HP motor. But, in this case, reduced speed is a side-effect of using back-gear, not why the gear is used.

Provided the lathe has enough torque, knurling can be done at higher speeds. However, I tend to knurl slowly because it saves metal if the cut can be stopped accurately. If the metal under the excess knurl is going to be removed anyway, I knurl a lot faster.

Back-gear was once seen as essential on a lathe. I think mainly because of the limitations imposed by belt drive and small motors. With a big motor and a VFD I've not been inconvenienced by the absence of back-gear on my 2-speed belt lathe – so far!

Dave

There you are then. I knew the maths gurus wouldcwither debunk or support the allegation.

I was quoting from my American machinists guide rather than from formal ejucashun. So maybe our continental cousins get a bit anal about their knurling.

Neil I see you're reasoning clearly, it does depend on where you propose to measure. Another myth gone.

Carry on regardless but definitely at lower rather than higher speed.

Might be an American thing. I have an American book of pro shop tips or some such thing that talks about taking a cut over the surface to be knurled if you get a double pattern first time around, to "match" the diameter to the knurl. But I have never ever had to do that. Perhaps because I use the method Neil pointed to of having "confidence to make a good start" ie go in hard at the start and keep going in hard as you move the knurling tool back and forth along the job. Double pattern if it occurs is quickly subsumed by the correct single pattern as it deepens.

I use mineral oil to lube knurling tool rather than coolant. Lubricates the metal as it is formed by the rollers, and the pins in the rollers too. Metal from the root of the knurl pattern is pushed and flows up to the crests, while in contact with the roller surface, so there is considerable friction. Coolant is,as the name suggests, a good cooling agent but not a great lubricant. Straight mineral oil is good for screwcutting too. Neat cutting oil too.

And how would you accurately measure the pitch on a knurling roller? Along the curve? Or in a straight line? And how does that relate to the curve in the opposite direction of the job surface? All too hard for me I'm afraid.

Do you suppose that the 'myth' arose because when the first knurl went wrong, the surface was machined off and the next attempt was a success there is a tendency to believe it was because the second diameter was better suited.

It's kind of like you always find things in the last place you look because most normal mortals stop looking at that point rather than the thing kind of knows whitch end of the list of places you start at.

regards Martin

PS I don't calculate either but do sometimes get a duff knurl

Could well be.

I imagine that cut knurling works better if you do the maths and start off at a diameter that gives an integer number of knurls. Obviously with cut knurling its the finish diameter the matters. It is a form cutting tool after all so depth of cut is a fundamental parameter. No doubt that the small amount of excess material will cut away just fine but starting at the right diameter minimises swarf. Cut knurling strikes me as being very prone to swarf re-cutting if you don't do it right. Which can't be good for the finish.

Theres nothing intrinsically wrong with the theory that circumference should be an integer number of knurl point. Its just that in practice the effects of any such errors are small and will be lost in the "noise" of the fairly ill-controlled form knurling process. Slight double patterning when starting form knurling is almost inevitable but, unless circumference error is exactly 1/2 pitch so that two patterns are identical, the deeper one will have more grip and win out over the other. Playing around with a Marlco tool easily demonstrates this.

Double patterns can persist if you don't have the machine or tool rigidity needed to make the forming process work properly. Such as with lightweight older Model Engineers machines or hand squeezed "nutcracker" type three wheel tools. I have a "nutcracker" tool which is very good but does have limitations. Needs a brutal squeeze to clear a double pattern if one does get established.

Knurl parallel to work is the correct way. Good knurls have a slight end chamfer to act as a lead during longitudinal movement. That said "nutcracker" users will know that tilting the tool from side to side makes the job go easier. How much is a real cut effect and how much due to different leverage on the tool I know not.

Clive

Edited By Clive Foster on 03/04/2019 14:04:37

There is a lot of miss-information about knurling. Many references state that it’s rather like cutting gear teeth and that the circumference of the item to be knurled must be an integral number of knurls, otherwise they won’t line up and you get cross knurling. This is total nonsense; it’s nothing like cutting gear teeth; each dimple is formed by deforming or moulding the metal, not by cutting it. Yes, you do get some powdery swarf, but nowhere near enough to fill the dimples.

Once the pattern has been established, as the workpiece revolves, each “hill” of the knurling wheel slips into the next “valley” of the workpiece, provided that the valley is deep enough to exert sufficient force to change the speed of the knurling wheel. If the next valley isn’t deep enough, then the next hill tries to make its own valley and you get a cross knurl. The secret is to make the first set of valleys as deep as you can by exerting as much initial force as possible. You can then deepen the pattern until you get a really nice, deep, knurl.

John Stevenson once proved, by knurling a taper, that calculation for formed knurling is not required for adequate engineers