Knurling speed

I have just watched "The Repair Shop" on B.B.C. T.V. Their watch maker was knurling a large piece of brass to make a replacement tuning dial ring. He seemed to do it at a very high speed. I read in M.E. many years ago to do it in back-gear. I just took it as gospel, and have done it so, ever since. What is the correct speed to do it?

Maurice

Hi Maurice,

I tend to do my knurling jobs at a somewhat lower speed than for turning but it seems you can use the same speeds as you would for turning. I found this and this on the web that may be of help.

Thor

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 have never calculated a diameter to fit a knurl. It finds its own happy medium, every time. Anyway, diameter of the job increases as the knurls throw up displaced metal, but the effective diameter at which the tool sits decreases as you wind the cross slide in.

It's sort of like freeform hobbing a wormgear with a tap. You might end up with an extra tooth on the gear, but not usually an extra half a tooth.

I always use back gear, more out of tradition and doing as I was trained than by any scientific study of the subject. But my old straight plunge knurling tool puts a lot of strain on headstock bearings, if you don't put the fixed steady between chuck and knurl area, so best to take it easy on the poor old girls. Also hardened steel knurl rollers spinning on hardened steel pins will last longer at lower rpm. (And remember if you are knurling a 1" diam or so job, the knurl wheels are turning at probably double spindle speed or so.)

Edited By Hopper on 03/04/2019 07:48:56

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

I use a Marlco type knurling tool I made as a Hemingway kit. Saves on putting pressure on the bearings.

Some knurls will produce poor tracking if the diameter of the workpiece is not in relation to the pitch of the knurl. For many jobs an ad hoc set up will produce acceptable results, but for critical work I always check the pitch of the knurl against the diameter of the work and adjust the diameter accordingly, it might only be a few thou but really does make a difference to finish.

I'll dig out the calculation and see what the maths gurus have to say about it.

Mick

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

This is a myth. Do you choose the distance between teeth on the PCD at the top, middle or bottom of the knurl form? Three different distances, three different diameters. The 'correct' diameter would depend on how aggressively you start the knurl and to what extent the material deforms rather than cuts; in practice you just need to start with confidence and the knurl falls into step.

The reason doing a calculation works is because it gives the operator the confidence to make a good start, rather than pussy-footing which is the best way to get a bastard knurl.

Someone can do a calculation, make a knurl, and be happy with the finish - but don't ask them count how many teeth they get as it often won't agree with the calculation.

Neil

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

Simple maths shows the suggestion that stock diameter must be an exact multiple of pitch is pretty much irrelevant for anything other than coarse knurling on small work. Considering the example of 1" nominal diameter stock knurled at at tooth pitch of 0.060" cited by Plasma one tooth pitch error is equivalent to a variation in diameter of approximately 0.020" which is not a lot especially if you follow the suggestion to add 0.010" to accommodate error. Might as well just start with 1" Ø and be done with it. For larger work the theoretical diameter error becomes correspondingly less. In a practical world you will be significantly below 1/2" diameter before the issue becomes relevant. Even if doing the coarse knurling often used on thumbscrews.

With displacement knurling things find a balance with material re-distributing to produce a pitch circle diameter corresponding to an integer number of knurls. This effect is measurable. If you try to push too hard material will be shaved off to alter the effective PCD.

Easily demonstrated by leaning on a Marlco knurling tool. I've just done a batch of 20 medium knurls on 16 mm nominal Ø 303 stainless steel stock using a proper Marlco tool without paying major attention to getting things exactly the same. The OD variations were sufficient to be noticeable when doing the final tapping operation on a pillar drill with the work held in a lever operated 5C collet chuck. Perhaps 15° to 20° variation in lever travel angle from smallest to largest. Taking a quick measurement actual OD variation was between approximately 16.35 and 16.45 mm. Its surprisingly hard to get really accurate and repeatable results when measuring over knurls. As experimentation with a 10 ths thou reading micrometer will soon show. Not that super accuracy has any real world relevance here.

Clive

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.

Well that's debunked the theory for form knurling. What about cut knurling?

Andrew

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.

So, as I always thought no need to calculate, did the late GHT write an article about knurling?

Next point for discussion, should the face of the knurl be parallel to the work or at an angle, either leading or trailing?

John

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

Parallel or you will get a ditch at the end where you stop moving the tool one direction before moving it back the other way.

Could well be.

I have a natural inability to create a nice knurl.

Maths wouldn't help that.

AtB

Daniel

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

Surely with a double or treble wheel the pattern depends on each wheel being in sync (which is certainly not designed in), and this is a separate concern to whether the pitch 'matches' the diameter? And doesn't this tend to show that worrying about the exact diameter is somewhat, er, nerdy?

Cheers, Tim

Edited By Tim Stevens on 03/04/2019 14:58:28