Die cut thread in aluminium

I am finding a die cut thread in aluminium coming out udersized.

The die is 1/4x40 ME and has cut perfecly good threads in brass and phospor bronze. The o/d of the cut trhread is close to 6.35mm.

I turned an aluminium bar to 6.35mm and used my normal die holder. The resulting thread was a very sloppy fit and when measered the o/d was 6.06mm. A second attempt with the die opened as far as my diie holder would allow gave 6.16mm, still a sloppy fit. Is this normal?

Are you sure the die is cutting a full profile thread? It sounds as if the upper part of the thread is being torn away by the die. That can be a problem with some of the softer aluminium alloys. I assume you mean aluminium alloy rather than pure aluminium, and if so which alloy?

Andrew

You could bore the diestock to about 0.1mm larger and try that. Even if the die cracked, it would still work. As already mentioned, there are many different grades of aluminium and some take threads better than others. Make sure you use cutting oil. Measured male threads are commonly smaller than the theoretical, and the only way to be sure of the size is to singlepoint on a lathe, using the mating part as a gauge.

Edited By old mart on 27/06/2023 16:26:48

Edited By old mart on 27/06/2023 16:30:07

Does the die still cut Brass? Phosphor Bronze is tough stuff, so maybe the die had a hard time and is now blunt.

Otherwise, I'm with Andrew in suspecting the Aluminium. Pure Aluminium is very soft and doesn't machine well. Many Aluminium alloys cut poorly because they're race-tuned to be extruded, bent or stamped, not cut. The difference is marked. Whilst being cut, especially threading, the softer alloys tear, deform, smear and stick to the cutter rather than cut. Horrible.

If a cutter is choked with Aluminium that won't come off when poked, it dissolves in Caustic Soda solution. (H&S warning: read the label!)

Dave

I agree Andrew is probably right. It is an alloy - not sure what but probably H30. For the second test I used a smear of Trefolex but what would you recommend - WD40?

I have ducked the issue by switching to brass for the part and the thread cut perfectly so the die is probably fine.

Roger, A lot depends on the quality of your die, less expensive cut thread carbon dies tend to be lax tolerances, better to use ground thread dies and personally I use HSS.

Regarding lubricant try methylated spirit or paraffin, meths works well, bit smelly though !

Some years ago i purchased some ME taps & dies from a well known supplier, thread quality was poor IMO so contacted them and they did me a swop for HSS ground thread -- far better, there was of course a price adjustment.

Thanks for all the comments.

The die is marked Apex so probably run of the mill. I do not often need to use taps and dies on aluminium so will just stick with what I have got. Will definitely experiment with lubricants if I need to do this again.

I think the die threading Brass correctly proves the Aluminium rod is the problem.

Most likely it's probably a soft alloy rather than H30. Any tendency of the Aluminium to tear will be worsened by a fine thread like 40tpi. Andrew suggested tearing in the first answer and I think he's right. Cure: buy an Aluminium alloy known to be machinable rather than hoping what's in the junk box will do!

For threading I recommend a designer Tapping Fluid rather than a plain Cutting Fluid or what's to hand. I use CT90, because it's what my local emporium stocks. It works well.

Some sort of fluid is essential when threading Aluminium. Almost anything is better than nothing, but meths doesn't lubricate and it evaporates quickly. WD40, Paraffin and other light oils work well as Cutting Fluids on Aluminium, but neither handles high-pressure well, so less suitable for threading. Never used Trefolex, but should be suitable. Not keen on old-wives methods like Bacon fat - although they work moderately well they're a stinky bio-hazard.

No cutting fluid will fix a fundamental problem like the wrong alloy.

Final thought, failure to cut a decent thread may have prevented a worse problem - the fixing failing under load. If the alloy is torn by the die, it means the thread will be weak and liable to strip. Not good in a braking system or for holding on aircraft wings!

Dave

I have an aluminium tap of the thread forming type. I don't recall seeing the equivalent thread forming die for aluminium. The point is that the thread forming tap does not cut, it pushes the aluminium into the correct thread form to form the thread profile, as cutting threads in low alloyed aluminium with a tap can result in poor thread forms and low strength threads. Tri-lobe thread forming screws work using a similar process but can be used on steel. I imagine most external threads in aluminium of mass produced parts are rolled rather than cut. It is a common process used for the majority of fasteners that are available off the shelf. The concave ends visible on many machine screws is an obvious result of this process.

I think I have seen thread rolling boxes for use in lathe tailstocks but it is a bit of a specialised need and you would have to be making a lot of parts to justify the cost of buying one.

Martin C

Should have said a tap for aluminium at the beginning. What I put makes it sound like the tap is made of aluminium.

Edited By Martin Connelly on 28/06/2023 11:08:22