Knurling

[Vic]

On Vic Said:

Conclusion after 35 minutes of video: Diameter does not make a difference but using heavy pressure on the knurl wheels at the start does, when it comes to avoiding a double knurl pattern.

Note his warning about keeping brushes and especially fingers away from the pinch point between knurl wheel and job. I had  a cutting oil brush catch a few bristles there and instantly the whole lot went through the knurl wheel/job interface. Luckily only the bristles not the wooden handle. It made me realise how dangerous my (until then) habit of dabbing a finger on the revolving job occasionally to feel how close to a  point the knurls were. Never again.

[Graham MeekParticipant @grahammeek88282]

I did see a 4″ / 100mm steel rule pass beneath the knurls and the rotating work, while I was in the Training school during my apprenticeship. The rule would have made a good single sided rasp for curved work afterwards, but as a rule it was of course useless.

I only lubricate my knurls with oil from a dispenser. Bits of swarf from a swarf brush can ruin the knurling. Especially if that swarf is harder than the material being knurled.

Regards

Gray,

[Grindstone CowboyParticipant @grindstonecowboy]

“I had a cutting oil brush catch a few bristles there and instantly the whole lot went through the knurl wheel/job interface.”

Been there, done that – as you say, it does focus the mind somewhat! 🙂

Rob

[jaCK Hobson]

On jaCK Hobson Said:

You tube (and now, common sense) tells me knurling tools have a pitch and so work best on certain diameters. Small diameter changes might make a difference? I can’t knurl.

Been in this game 50 plus years and in theory maybe certain size wheels work on certain size parts but in practice it doesn’t seem to matter.

Tony

[Clive FosterParticipant @clivefoster55965]

If you knurl deep enough to more than bottom out the tips of the knurl in the valleys of a forming type knurling wheel thin, very sharp, slivers of metal will be produced as the knurling wheels cut (scrape?) excess metal from the sides of the formed knurls to make room for the wheel.

Presumably if you do your calculations correctly and start with exactly the right size of material a knurl with full depth valleys and full height peaks is produced immediately before this cutting / scraping process starts.

If you are a production line manufacturer wishing to reliably produce knurling of a specified shape and depth, whether full or partial, its clearly essential to do the maths and not only start with the correct size material but also finish with the correct depth feed on the knurls. Hence the reference given earlier by Graham Meek with its details of 5 type off knurl.

In our case where we just want a nice looking knurl or three any deviation of the starting stock from the nominally correct diameter will merely offset the centre line of the knurl slightly with no obviously visible effect. Unless the knurl is driven deep enough to reach its full depth there will be sufficient space inside the knurling wheel to accommodate the slightly higher peaks needed to accommodate deviations from the theoretically correct starting diameter. In general for any practicable combination of knurl pitch and workpiece size the difference in diameter between dead right and half way to needing either one more or one less “knurl tooth” to cover the diameter is very small. Which is why the “drive it in hard and it will sort it out school” approach is valid. There just isn’t much physical variation to cope with.

My observations suggest that double knurling is more an operator technique thing than a size thing. Folk getting double knurls still tend to suffer even when starting from dead nuts size because they don’t manage to get enough force on one of the knurls for both to start cleanly.

These days I mostly use a Marlco scissor style tool. An industrial piece of kit that is beyond reproach.

I found the once common, inexpensive, rotating head style with three knurls a total pain to use with successful results a matter of pure luck. Presumably then knurls need to be presented to the work with almost exactly equal force if good results are to be produced. Logically setting the thing dead on centre would meet this criteria but I never found it reliable.

Interestingly I have  Pratt & Whitney branded single size knurling tool with a similar style of pivot but offset so one knurling wheel is lower than the other. In case the upper knurl is presented to the work at centre height. The pressure exerted as knurling begins bringing the other knurl into contact a tiny bit later. This tool is absolutely reliable. I imagine the lower knurl pretty much always works on material partially formed into a knurl by the upper one giving it a easier time of things so slip is quickly eliminated.

I also have a hand held “nut-cracker” style tool which works well. That has three knurling wheels arrange in a triangular pattern. I imagine these generate a similar operating on a partial knurl effect significantly reducing the squeeze needed to form a good pattern. Too gentle a squeeze produces the familiar misalignment but the hand generated forces needed for decent results are clearly considerably less than those produced by a machine.

Clive

[Michael GilliganParticipant @michaelgilligan61133]

Excellent logical analysis, Clive

… much better than my earlier ‘throw-away comment’

Hopefully both sides in this debate can now accept that there is an element of truth in the other’s version.

MichaelG.

[bernard towersParticipant @bernardtowers37738]

I cant imagine in this day and age that knurling done commercially is done with scissor knurls more likely with cut knurling by Quick tools. reliable results with perfect finish every time and no change in dia.

[Graham MeekParticipant @grahammeek88282]

This is what one would expect Bernard, but there are still firms out there who do not want to invest in such “Expensive tooling, (their words not mine), and who will buy standard commercial quality bar on which to Knurl. Rather than buy in a bar which is more ductile and would assist in the Knurl forming process. When the difference in the cost of the bars is only pence. The time wasted on knurl forming standard bar and getting a good finish, far out strips the few pence in additional cost for the material.

I have heard it time and again “we have always done it this way”, so nothing changes.

Regards

Gray,

 

 

[Clive FosterParticipant @clivefoster55965]

Picking up on the very valid points raised by Bernard and Gray about firms being unwilling to invest in expensive tooling the economics depend greatly on how much knurling work you are going to do. For mass production and all CNC shops the emphasis must be on speed and repeatability so cutting knurls operating on correctly sized stock is the only way. Even at the lowest end with a small firm operating older CNC machines relatively cheaply purchased used the volume of work makes a more relaxed attitude to tooling investment essential.

An old style manual machine shop has to be more careful. The number of jobs done per week being vastly less and if knurling is relatively rare job it’s far harder to justify the costs. A couple or three years back friend took a quick look a  setting up with cutting knurls and rapidly figured it was likely to be a “£1,000 job” making further investigation irrelevant.

Although old fashioned the Marlco style tool is completely reliable and easy to set up on an ordinary manual lathe so it has its place. Mine cost me £30 and paid for itself within a couple of weeks doing a couple of mates jobs I’d never have taken on before.

Clive

[HopperParticipant @hopper]

I never worked on CNC machines. (Punch cards were rumoured to be the way of the future when I was an apprentice but I never saw any.) but if you search “CNC knurling” on YouTube there seems to be plenty of CNC setters-up using conventional form knurl tools in swivel head holders plunged straight in the same as old time manual lathists did in industry. No mention of special diameters than I saw. Just plunge straight in to depth, run along the length once, withdraw and the job’s good. (As the actress said to the bishop.)

There is also an interesting video of a purpose made knurling machine for knurling the bar on a barbell weights set. The wheels are about 8 inch diameter by 4 inch wide and they rotate.The bar, about 1 inch diameter is fed by hand between the wheels which then move inwards and apply the knurl.

Then there is another showing a knurling done on CNC lathe with a screwcutting tool, essentially cutting a (very) multi-start (very) coarse lead RH thread, followed by the same in LH format, resulting in a cut diamond pattern with no increase in daimeter.

Plus of course Quick cut knurl tools also used on CNC. So there are always many ways to skin a cat.

Interesting they (American) refer to a Mazak CNC machine as a “May-zak”. I ran a Mazak manual lathe back in the day and it was always referred to as the “Muhzak” (or maybe “M’zak” in broadest Strine). What is the UK pronunciation?

 

[bernard towers]

On bernard towers Said:

I cant imagine in this day and age that knurling done commercially is done with scissor knurls more likely with cut knurling by Quick tools. reliable results with perfect finish every time and no change in dia.

I agree. The samples I’ve seen from these tools is excellent. Sadly outside my pocket money range these days.

[jaCK HobsonParticipant @jackhobson50760]

I really cranked on my RDG knurling tool on this stainless and it made a big difference! As good a knurl as I ever done. However, I don’t think the tool is really up to the job – it gets bent out of shape (as can be seen by the knurl at an angle). Need something much stiffer.

 

 

[Graham MeekParticipant @grahammeek88282]

This is the classic scissors type problem. Thrust from the helical knurls sends each arm in different directions. There is usually far too much play at the pivot point.

 

 

This is one of the reasons I designed the tool above. The pivot points are Hardened dowels and are as close to the work as possible. The other reason was to keep my hands away from the chuck while adjusting the knurls.

I am currently in the process of making a scaled down version for my Compact 5. The results of which I shall post on the Compact 5 thread I started some while back.

Regards

Gray,

[HopperParticipant @hopper]

The solution for the flexible cheap knurling tools is to a add a flat plate to either side to hold the two arms in line. It is easy enough to do, but you will probably have to make or buy longer bolts or pivot pins to extend out through the plates. See the hemingway kits tool for the general idea https://www.hemingwaykits.com/HK1110

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