Yet another knurling question.

I normally get along OK with knurling, either scissor or bump, but have never needed to produce a knurl longer than about 1.5 times the width of the knurling wheel.

However, in off-forum discussions with another member, the possibility of making continuously knurled bar came up. This is my first (disastrous) attempt:

It's obviously horrible in many ways, but I'm not ready to throw in the towel yet. I have some ideas myself but would be interested to hear any suggestions for improvement. Bar spending £100's on a fancy cut-knurling tool of course!

Some data:

- 1/2" EN3B.

- Cheap and floppy scissor tool with good quality 3/4" diameter x 1/4" width 75 tooth wheels.

- Full depth wound in at start of cut, lathe running at 65rpm, power feed from right to left in the picture at a tedious 0.01mm/rev.

- The periodicity, most evident near the head stock end, is about 1.5 - 2 times the wheel width.

Some of the problems I think I can address but at the moment I don't understand the mechanism behind the tearing of the ridges which results in them becoming whiskers pointing against the direction of tool travel. So that may a place to start understanding what's going on. Or maybe not.

Any advice/discussion would be welcome.

Robin.

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Edited By Robin Graham on 19/12/2021 00:36:22

Did you have tailstock support?

pgk

Yes.

Robin

Are the knurls bevelled edge or plain ?

Helps if they are bevelled when when asked to travel sideways.

Emgee

You may be going in too deep, first the wheels settle into a pattern that fits around the work and then as they dig in deeper there is only room for 1 less "ridge" so it gets sheered off, think of a gear as you reduce OD you reduce the number of teeth. Also like a gear there is some of the knurl above the PCD and some below so the actual depth below stock diameter is not the depth of the wheels teeth due to metal geting pushed up.

Some form of tapping/cutting fluid will help as does using a small wire brush to remove swarf

Edited By JasonB on 19/12/2021 07:17:47

For longer knurl lengths, I normally start at the chuck end and finish at the tailstock end. I normally use coolant to wash away any micro bits of what ever. At the start I wind back and forth for about a roller width to see that I am happy with the depth or definition of the knurl then cut down the length. I normally run quite a bit faster than you did, around the 200 rpm range on steel that diameter. Sometimes the knurl does not quite work out due to the pitch of the form not matching the diameter. A bit like gears. If the diameter is too big the top of the teeth become very pointy. If the diameter is too small the top is very wide. I normally just hand feed the carriage when knurling.

Hi Robin,

Three observations:

As JasonB has said, plenty of cutting fluid should be used - although it is a forming process, knurling produces small swarf fragments that will be pressed in to the pattern unless washed away. (I use flood coolant at work, but as I do not have this facility in my home workshop I dribble a constant trickle of cutting fluid from a washing-up liquid bottle)

Feed rate - I feed at a "comfortable threading rate" (about 1mm per rev.)

I differ from JasonB regarding matching diameter/circumfrence to the knurl pitch - this is unnecessary, in over 50 years of using lathes (including a series of systematic trials and experiments) I have never found the need to adjust a diameter to match a knurl pitch. If unconvinced on this point, consider the case of the "rope knurl", an example is shown in the link below:

Illustration of rope knurl tool and pattern

The form tool has a larger diameter at its edges than in its middle (so the pattern must have a smaller pitch in the middle compared to the edges), the finished knurl pattern has a smaller diameter at the edges than in the middle (so the pattern must have a larger pitch in the middle compared to the edges). Despite this paradox, the tool works....

All the best,

Ian

Ian, I also don't bother to calculate pitch but it was an observation of what can happen if going too deep.

I also think Robin has gone too deep.

Tony

When I have to do long knurls I get the knurls to get the correct pattern (timed together) which is not necessarily full depth and then traverse the length of the knurl then repeat a bit deeper till full pattern is achieved and use plenty of coolant to wash away debris. Having said that it’s not the easiest of jobs. Out of interest I have the lathe spinning at around 350 to 500 rpm depending on dia.

Logic [or is it instinct ?] would suggest that producing long lengths of straight knurl might be more demanding than producing diamond pattern.

MichaelG.

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P.S. __ If you didn’t see it in another recent thread; have a look at this Dorian tool in action:

**LINK**

FWIW, I almost always use power feed, when knurling, but at the minimum feed rate so that the tool does not have to work too hard, and the knurl already formed can support over the maximum length of the wheel; just a few thou compared to the width of the wheel width.

What is being formed is approximately a spur gear so the tendency for the wheel to swivel from the forming / cutting forces has to be resisted by the knurl just formed . So for a 1/4" wide wheel, the feed per rev needs to be as small a proportion of that as possible. Possibly 0.004" / rev, with a low rotation speed.

If the clamp type tool is sloppy, that may contribute to the problem in that one wheel is not following the same path as the other (It should, since effectively two gears are being forced into close mesh.

Depth should only be sufficient to form a full height knurl. If in any doubt, take more than one "cut" so that the later cuts produce the full depth.

Howard

Thanks for replies. One of the ideas I had was that I was going too deep, and as that was reinforced by JasonB's and Tony's comments I had a go with a lighter cut and two passes:

That was going right (tailstock) to left for the first pass, then cranking a quarter turn on the tool before reversing power feed and running back.

Still a long way to go, but a definite improvement.

Another thing I had thought about was the use of flood coolant, which has also been mentioned in comments. Although 'pressure' knurling is regarded a a deformation process, that's clearly not the whole story as significant amounts of swarf are produced. I'm sure that trapped swarf is responsible for some of the deficiencies in the knurl pictured above. Not only does it cause scarring, but I suspect it builds up and deflect the wheel as as well. Flood coolant is the next thing I shall try.

On speeds/feeds, the reason I go so slowly (65 rpm) is that someone somewhere once told me that it was good to go slow and feed hard so that the pattern was well enough established on the first turn of the work to force the wheel to track. My belief (at the moment!) is that the established pattern on the work dominates the 'natural pitch' of the wheel, so the wheel slips to accommodate. But maybe that's wrong? Certainly it seems folk here are going a lot faster and getting good results.

Emgee - the wheels have only a very slight bevel. That's something I could change and shall bear in mind - thank you for pointing that out.

MichaelG - can you unpack your logic (or unpick your intuitions) about the relative difficulties of making long diamond and straight knurls? My intuition was exactly the opposite to yours - but I have no logic. I did look at the Dorian tools, but £s....

I'm unlikely to get back to this in a practical way before the new year (by which time Santa might possibly have dropped a Marlco kit down the chimney, who knows?) - but I'm not giving up on this.

Hope you all have as good a festive season as possible in the circumstances - stay safe!

Robin.

The swarf produced when attempting a long straight knurl is mostly due to the knurl teeth not being an involute or, for that matter, any other recognised efficient gear tooth pattern.

Besides deformation of the workpiece displacing material to form the 'umps and 'ollows of the knurl the actual shape of the spaces will be scraped to give enough space for free rotation of the knurl. As the knurl depth changes so does the shape. Hence the formation of swarf as the knurl deepens.

With the normal cross pattern the effect is masked because one normally never feeds the knurls to full depth so there is a bit of clearance in the equivalent of a gullet between the knurl teeth that the pyramids making up the knurl can deform into. The general insensitivity of knurls of reasonable pitch on work of mutually reasonable diameter is due to the material being shifted up or down during the process so that the final "pitch diameter" permits an integer number of knurls around a diameter at half knurl height.

But there must still be a little space in the knurl gullet above the peak. Force the knurl deep enough to more than eliminate that clearance and plenty of thin, needle like, swarf results.

Clive

Regarding Michael's comment the human eye can easily see slight deviations in a straight knurl but it would be a lot harder to tell if the helix of a diamond knurl was slightly shifting along the length of a workpiece.

If you want it really straight then get out a Vee tool and a means to index in the mill, I've done that a few times before I got a set of straight wheels

That’s certainly part of my logic, Jason

But I also suspect [‘though cannot prove] that the knurls themselves become self-aligning when they are producing the helix .. Straight knurls however do not receive the same stabilising influence.

i.e. the helix is a ‘natural’ form to be produced by any tool traversing in the Z axis of a rotating workpiece, and the straight line is a very special case.

Successful straight knurling over a length greater than the width of the knurls probably has similar probability of success to that of balancing a pencil on its point.

I would posit that the pattern in Robin’s first picture is effectively a sawtooth effect caused by bursts of instability.

MichaelG.

Thanks for further comments.

Clive - your explanation of how knurling actually works and how swarf forms makes sense to me. I think I now understand the process better - thanks.

JasonB/MichaelG - I too had wondered if if it was an 'eye' thing with long straight vs diamond knurls. Despite having vowed not to get distracted from my primary objective of producing straight knurls I succumbed to the temptation of trying my current technique with diamond pattern wheels:

Length of knurl is about 2" so ~8 times wheel width. It's obviously much cleaner than previous examples, but one wheel is cutting deeper than the other - so yet  another mystery to be investigated.

Robin

Edited By Robin Graham on 22/12/2021 00:43:52

Edited By Robin Graham on 22/12/2021 00:57:40

If you are using a Clamp type knurling tool, and one wheel is cutting deeper than the other, it would seem that one of the carriers for the wheels is not pivoting easily on the holder. The purpose of the pivots is for the knurling wheels to centralise themselves about the work.

A clamp type holder should be positioned so that the line between the centres of the wheels is on the centreline of the work., and the cutting / forming pressure applied by bringing the wheels into contact with the work.

In the case where a single,"push" type knurl is being used, excessive pressure should be avoided wherever possible. a ) because it will cause slender work to flex, and

b ) because of the extra load imposed on the Headstock bearings.

If more than one pass is needed to give a full depth knurl, the wheels should engage with the earlier pass, in the manner of two gears going into mesh.. Only about 0.010" "interference" is usually needed. to produce a satisfactory knurl. Probably because some of the material is forced outwards by the process.

For this reason,knurling is often used to recover a slightly loose fit of of a shaft into a housing.

Despite concerns over needing an exact ratio between the diameters of the wheel(s ) and of the work, it does not yet seem to have caused me a problem.

Howard

Hi Robin, I started knurling a long piece of 3/4 inch brass and it did just like this example. This is a new to me knurling tool I used, so just loaded more pressure on and forced it to cut deeper, then it started to cut better. If you carefully on the bar left over, you can see the double cut marks. I did another piece and used the same depth setting that I had with the scissor type knurling tool and all worked like it should have. Iam making some knurled locking rings.

A (possibly final) update on progress with this.

First, thanks for your contribution Neil and apologies for not responding. This project started because a fellow forum member was looking for continuous knurled bar to slice up in much the same way as you have done.Your work looks enviably good.

After some experimentation I managed to make this (at least sort-of) work with the cheapo floppy scissor tool:

Those are details of 250mm lengths.

The pictures are not flattering to either my machining or photography skills - the bars look better in real life - but they do show that the diamond knurls are reasonably crisp and even, and the straight ones are - well, at least straight. And not ragged as in the in photo in my opening post.

I think the main reasons for the improvement were using flood coolant (there were visible amounts of swarf carried away) and not attempting full depth in a single pass. I ended up putting 'a bit of a cut' on at the tailstock end at 200rpm, traversing to the headstock at 0.2mm/rev, then putting 'a bit more cut' on on and reversing to the tailstock. Quite possibly I could have gone faster, but it was working and I'm not on piecework.

The devil in the detail is 'a bit of a cut' perhaps - I don't know what that means, and neither do you .Just what felt right after many trials. Which isn't helpful!

Anyhow, thanks to all for advice.

One day I shall make a Marlco type tool perhaps and see how that goes. I do like a crisp knurl I have to admit.

Robin.

Edited By Robin Graham on 08/02/2022 00:46:49

Edited By Robin Graham on 08/02/2022 00:57:49