Machining Tungsten

When lead shot weights used by coarse fishermen were made illegal, I was invited totour a factory researching a material using tungsten powder embedded in plastic. I think it was one of the companies which were previously part of the Royal Ordnance group. They were also involved in making the bodies for tungsten darts, and tungsten penetrators for armour-piercing artillery shells. I saw tungsten items being turned and tapped with conventional machine tools, and when I enquired about the machinability of the metal I seem to remember being told that they never used pure tungsten, but a tungsten-nickel-iron alloy with a density of around 17. They gave me a set of tungsten darts with bodies slim enough to get three in the treble 20, but not being a player I gave them to a pal who was. The tungsten matrix material wasnt a success, and neither has it been a success as a substitute for lead shotgun shot.

Just noticed Andrew's post The brother has machined Tungsten but not for a few years and it was done on a Bohringer lathe that had a heating source to raise the temp up to approx. 200c The Carbide tools had 0 deg top rake and 8-10 deg side rake and quite a large nose radius. After turning the parts [ some of which started at 70mm Ø] they went for finish grinding. Were for parts of "fishing tools " [Oil]. Alll drilling was done using Titex Carbide drills [ which Titex had modified]. Hope this has been of some use.

Should have added that the Tungsten was not very tolerant of any stress risers Careful design and surface finish was paramount.

My tungsten alloy bar arrived last Thursday:

I've had a go at machining it this evening. It machines very well, no issues with facing and turning. The finish looks a little rough, but I was using a 0.2mm radius insert at 0.1mm/rev. So no doubt I could improve on the finish:

I measured the surface roughness at six points around the circumference; the average was 2.09µm Ra. Surprisingly that's comparable to what I achieve on other metals so not bad.

Sadly drilling and tapping proved to be more problematic. I managed to get a M6 tapped hole produced, but it wasn't easy. Once I'd started a hole using a carbide slotdrill (removing the broken HSS drill in the process) HSS drills opened up the hole without a problem. Tapping proved difficult, although my M6 taps are far from sharp. More research is needed and I feel a tooling order coming on.

Parting off was simple at 4 thou/rev feed and a conservative 260rpm, giving a good finish:

The centre tab was easily removed by filing.

The next exercise is to use the hydraulic copy unit to produce a hemisphere.

Andrew

Andrew might be worth investing in one of the colour coded taps, maybe a red ring ?

Yep, I was reading about the issues last night. The problem seems to be that tungsten has a high Young's modulus so it tends to grip the tap and contract rather than flow slightly. From what I can see a blue or green ring tap may be better as these are intended for materials that close up slightly. The question is do I go for blue, intended for tough stainless steels and tool steels, or green which is intended for tough non-ferrous materials, which of course includes tungsten. Ideally taps would be two flute and have extra relief ground. Or should I just use red, as they are intended for tough steels. Decisions, decisions!

Andrew

More progress, forwards and backwards. I've had another go at drilling and tapping tungsten alloy, and drilling small holes. I still don't seem to have the tapping sorted. Even with spiral flute and spiral point ring taps I managed to break one. And that was with a thread engagement of about 30%. Although it was my fault, I think I put a bending load on the tap when trying to back it off. This stuff seems to grip the tap like a boa constrictor. The spiral point tap seemed best, even then it looks like the black finish has worn away after one hole.

On the plus side I found that slow helix carbide drills cut the stuff like butter, beautiful. Still at £11 for the 3.6mm drill I'd have been might upset if it hadn't worked. I also had a go at drilling 1.2mm holes, as I will need these for the 18swg hoops that stop the balls flying off if the governor springs break. Here's the result, two 1.2mm holes 6mm deep, no problem:

The drills are 4-facet so I didn't centre first, just straight in at 2500rpm, using WD40 as a lubricant.

Andrew

Looking promising. What density is this stuff?

Cheers,

Rod

can you not drill all the way through, counterbore the back and glue in a nut. Tapping this stuff looks like self imposed purgatory

Drilling blind to say 5mm dia and silver soldering in a more user friendly metal sleeve that can then be tapped M4 would be another option, this would mean your outer ball half looks solid. I've had no problem silver soldering tungsten TIG electrodes.

Edited By JasonB on 03/09/2017 07:33:01

Do you know anyone who could try thread milling this material? I suspect that would be the way it would be done in an industrial production process.

Martin C

Based on the reported composition I've calculated the density as 17.92gm/cm³. So a bit less than pure tungsten, but much easier to machine.

I'm not beaten yet on the tapping, although I do have some alternative plans sketched out, just in case. As long as I have enough thread to hold the blank while I form the hemisphere I'll be happy. To avoid loss of density due to the tapped holes I plan to buy some tungsten/copper alloy from fleabay and make some short studs. If I use a Coventry diehead I can tweak the thread OD to be a snug fit in whatever tapped hole I manage to produce.

I've considered thread milling. Although there are a number of local machine shops in my area I expect their prices for a relatively small PITA job would be out of my budget. Anyway if I thread mill I want to do it myself. I've bought some single point cutters to have a go, but I'd like to start with larger threads in a more benign material. Thread mills for M4 cost around £80; that's a lot of wasted drinking vouchers if I break the cutter.

I now need to trim the hedge in the front garden before the rain arrives, but I'll be doing more experimenting this afternoon/evening.

Andrew

The box hedge in the front garden has now had a haircut. Unfortunately the electric cable for the hedge trimmer is now shorter than it was.

I've been beavering away in the workshop this afternoon making a practice governor ball in tungsten. I still haven't sorted the tapping issue, but everything else was straightforward:

The ball weighs 55gms versus the theoretical weight of 65gms. Obviously there are internal voids for the screw threads and the screws themselves are not tungsten. And my kitchen scales only resolve 5gms. All in all not a bad result.

Andrew

This stuff was used for mass balance in some aircraft components:-
Heavy metal alloys are pseudo-alloys of tungsten with a nickel-iron or nickel-copper matrix. They are produced by powder metaland sintering processes. The most outstanding property of heavy metal alloys is the high density of 17-19 g/cm³.

It's not heavy metal, it's rock'n'roll.

Hmmm, this is going to be embarrassing.

Out of idle curiosity this afternoon I drilled a couple more 3.6mm holes, about 50% thread engagement for M4, in my remaining bar and tried tapping with an old set of Dormer HSS straight flute hand taps. Worked just fine! I used all three taps in sequence, and cutting fluid definitely helped. For one hole I used Rocol RTD and the other Ferrofluid, a thin oil intended for tapping tough materials like stainless steel. It's difficult to know which was best, but the oil is less messy. The taps needed noticably more torque than normal materials, but no worse than austenitic stainless steels. So much for the fancy sooper dooper taps. Sometimes you just have to reverse up and try a different path rather than press on regardless with ever more involved, and expensive, 'solutions'.

The next question is do I stick with M4 or change to M3; the original design specified 6BA? Ultimately it'll depend on the leaf spring design. But I'm inclined to stick with M4 as during manufacture the thread takes all the cutting torque generated when using the hydraulic copy unit to form the hemispheres.

I'm going to treat myself to a beer tonight.

Andrew

You might think about generating your sphere, using a cheap & cheerful carbide tipped hole saw from that well known internet auction site. It should put less load on your holding thread, perhaps?

Cheers, Tim

Thanks, the balls are actually two hemispheres screwed together so that the leaf spring can be held between them. Would that make a difference to the idea of using a hole saw?

Andrew

'WD40' and 'lubricant' in the same sentence. There's a contradiction in terms!

I would have though conventional turning with the copy attachment would put less load onto the screw than a hole saw. As you can set the depth of cut on the attachment you will have one cutting edge taking off whatever depth of cut you want rather than large "cutting" edges in contact  with less control over cut depth. Also not sure how well carbide would cut tungsten allow with no real sharp edge, more likely to try and grind it's way through.

Being hemispheres would not affect the use of a saw

Edited By JasonB on 05/09/2017 09:40:51