Knurling Aluminium / Indexing Parts for Milling

All, my son wants to make a replica Lightsaber. He’s got an old, broken plastic one, and we’re going to strip the electronics and light tube from it, and graft them onto a machined aluminium handle.

We've got a lathe (ML7), and milling machine (SX2P). This morning we’ve been experimenting with machining features like tapers, grooves and milled flutes. He wants some diamond knurling on the handle...I mistakenly thought that cutting a 2mm pitch spiral in 2 directions would do it. After some puzzlement it became obvious that it can’t work because it’s effectively one line per handed spiral, which will cross once per diameter, not giving a diamond pattern.

So then to the knurling tool, gradually tightened onto the o/d. It didn’t really work, giving a mushy finish with no real definition.

We got some nice longitudinal features by intersecting a ball nosed milling cutter with a taper, but we need, say, 6 or 8 around the diameter. These features were also on a piece that was overhanging the vice. I suppose we could secure a nut to the end of the part, and index like that, but that would give a big overhang and not much grip area.

So:

1) Best way to get a great diamond knurling finish on aluminium?

2) Best way of indexing an overhung part on the mill (without an indexing chuck/turntable)?

Thanks!

2. 3D print a thin octagon and fix to end of part, use a square against each edge as you position it in the vice.

Hexagonal collet block in the mill vice will give you 6 positions or a square one rotating the work half an index would give you 8. For support use a machinists Jack under the overhang with a clamp on top of the work if you don't need to go all the way to the end. If you do you will have to move the clamp mid cut.

Why not bite the bullet and get a rotary table and tailstock though, once you have it you will find other uses for it.for parts for your engines.

Paul

Have never had a problem producing a diamond knurl with a clamp type tool. Usual advice is to finish with a cut about 0.010" deep, so that the points are just on the surface.. It often works to apply the cut in stages and to gently traverse the knurling tool along the work.

At one time I wondered if there needed to be an integer ratio between the diameter of work and the knurl. Thw worry was that if not an integer ratio, as the work rotated, the knurling wheels would be out of phase with the peaks, and obliterate what had just been produced.

Maybe, I've been lucky but this has not happened; so far!

Depends on how many facets you want on the part being indexed on the mill.

One way is to hold the part in a collet, in a block, (Like a Stevenson Collet block ) but this would only give four or six facets. Another way might be to find a changewheel with the required number of teeth and clamp this to the work in some way, and then use a spring loaded pin engaging with the teeth to index tom provide the required number of facets.

Food for thought?

Howard

Joe Pie's knurling lesson

For what it’s worth [i.e. not much] ... the original props used a Graflex flash-gun handle.

These are pricey now, for that reason !

Here are some photos of what is claimed to be a good replica: **LINK**

https://www.graflexshop.com/cart/index.php?route=product/product&product_id=50

MichaelG.

Thanks all, plenty of ideas there.

Re. The torch - I didn’t know that, but I think he’s wanting to design his own, hence some knurling. BTW one of my favourite characters is IG-88 (and IG-11). Their heads were based on combustion chambers from a Rolls-Royce Derwent jet engine. I enquired about one at an aircraft scrapyard a while ago, and the owner’s first response was “ah, another Star Wars fan?” The price reflected demand I think.

I’ve always fancied a rotary table to play with, but have always been confused by the choice available and the widely varying costs and reported issues with the affordable ones. If anyone can recommend a decent one for a small mill, I might indulge (birthday next month). What I don’t want is something I have to immediately strip down and modify.

Thanks all.

Ok, it appears that he doesn’t actually want a knurling effect, rather a large diamond grid lightly engraved on the surface of the tube. so After much messing about with felt tip pens and cardboard tubes:



...what we’re after in engineering terms - I think - are effectively two intersecting twin start threads, one left-hand, one right-hand. The starts offset 90 degrees.

So I think I was on the right lines with the 2mm pitch thread.

Thinking is - no power, use the hand wheel and light cuts.

1) Turn the first thread as normal, right to left.

2) Then do the second thread, right to left, but turning the chuck the opposite direction with the tool upside down.

3) Then what? The next normal thread (ie the other start for step 1) needs to start exactly 180 degrees from the first. How do you do this while accounting for backlash etc?

Assuming this is done ok, the final spiral would be as per 2), turned opposite direction and starting 180 degrees from 2).

I think from the tube experiments that if the start points are not exactly 90 degrees apart, we’ll get asymmetric diamonds.

Any ideas?

You will need a lot more than a 2mm pitch thread, depending on the diameter and number of starts the pitch will be in the region of 1 to 2 times the diameter of the grip.

To do it you will need to workout the changewheels and to save straining the machine you turn the LEADSCREW by hand and that drives the spindle. Multi starts are best done with the topslide set along the lathe axis and if say using a 3 start thread then move the topslide 1/3rd of the pitch for each pass, 1/2 pitch for 2 start.

Edited By JasonB on 24/05/2021 07:10:37

80mm pitch with pairs 22.5deg apart so move topslide 1/16th of pitch

40mm pitch, pair spacing the same but shallower pitch puts them closer together so something like 40deg offset so 1/9th pitch or 4.45mm topslide movement would give similar pair spacing to the 80mm pitch sketch

Edited By JasonB on 24/05/2021 07:41:56

This is what you want I presume.

This was an example for another thread from July 2018. Not easy on a lathe, you need a pitch equal to the circumference to get 45° helix angles. It is an example of helical milling which used to be done with a gear driven head mounted on a milling table but is now relatively easy to do with CNC if you have it.

On a lathe as you said driving the leadscrew would be best if you can set up suitable gearing, seems unlikely to me but you may find a way.

For a hand grip of about Ø25mm you would need a pitch of about 78.5mm. You then need to be able to index the part around by a suitable amount to get evenly spaced grooves. So let's say use 72mm pitch then you can divide this easily by 3 to give 24 positions around the circumference by just moving the carriage further along the leadscrew. Use similar sums if you have a machine with an inch leadscrew. This will be close to but not actually 45° but you could use a wooden dowel and marker pen to see if it works.

Martin C

I assumed a 3mm pitch leadscrew, hence the divide by 3 above.

Edited By Martin Connelly on 24/05/2021 18:13:47

Thanks both.

he’s looking for a very coarse pattern, perhaps only two full diamonds around the circumference, ie 2 node points. I think that’s what a twin start, left and right hand spiral gives.

having equal angles doesn’t seem important, so elongated diamonds would be OK.

I think a 3mm pitch is the biggest available as standard on the ML7, which would have been ok to try. I don’t think this would have needed the leadscrew turning.

Anyway, getting the start points seem tricky, and also inverting the tool. Perhaps it’s a process too far at this stage.

You should just be able to reverse the leadscrew, I'm sure Myfords can do that so tool stays same way up but just cuts from the chuck to the tailstock for LH threads.

3mm pitch will still be way too little, compare the 3mm or 1/8" pitch of your leadscrew to the pitch you have drawn on the cardboard tubes to see how much longer the pitch needs to be. You just need to play with what change gears you have to get the longest pitch you can and try it with a sharpie in the toolpost like my video.