Tapping pure aluminium

I tried to tap some M2.5 x 0.45 holes in an aluminium heatsink today and it didn't go well. I'm assuming that the material is pure (ish?) aluminium because it didn't drill or tap like 'ali' at all - it was horribly gummy. Despite copious wetting with WD40 it just didn't seem right - I didn't get the 'crack' which you normally feel when you reverse the tap half a turn. I managed two threads then had to resort to caustic soda to clean the flutes because the swarf was so solidly impacted. Then broke the tap on the third hole of course.

I'd welcome any advice. I'm wondering if it might go better with a spiral point tap or if I'm being over-ambitious in trying to through tap 8mm at this size. I'm pretty sure I've done it in brass, but a different animal obviously.

Robin.

Edited By Robin Graham on 28/01/2023 22:36:05

Pure aluminium is a gummy material, is it possible to drill a clearance hole to avoid tapping the full thickness of the material?
Mike

I wonder if a thread forming tap might be the way to go, along with a high pressure lubricant; I've never used one that small though, but have used thread forming screws.

Alternatively, drill part way through with a clearance drill and tap a shorter hole.

Bill

It's just a heatsink - just drill clearance size through and use a bolt, lockwasher, and nut.

Or drill a hole and use a self tapping screw.

I would try water & dishwashing soap mix.

Thread forming taps that don't have flutes work well in soft materials.

You don't say what size hole you drilled, I usually go with 2.1mm but i it is not heavily loaded then upping the size to 2.2mm would still give enough bite.

Use a tapping fluid rather than WD40 which is OK for turning/milling but not as good for tapping.

8mm deep in one go won't help, as the size goes down even if breaking off the chips by back turning they can clog the flutes so remove the tap every so often before they become clogged. Spiral flute or spiral point will help here.

Also don't use coated taps if they are clogging and the swarf sticking to them.

Edited By JasonB on 29/01/2023 07:18:27

In a mass part production environment most soft metals are thread formed not cut. The main problem is getting a reasonably accurately sized hole as with small thread pitches there is little to deform. I would drill a smaller hole first then open it out to the required size, I do this for my small cutting taps to get a better sized hole and therefore better fit thread.

Folded heatsinks are very soft and gummy, probably near pure aluminium Extruded heatsinks are aluminium alloy, albeit not the nicest material to machine.

I've machined and tapped a lot of heatsinks for work. I tend to drill for about 50% thread engagement. An M2.5 by 8mm hole is doable with ordinary uncoated hand taps. But it's not great design, so I'd change the design to shorten the thread depth. I usually tap dry as I don't want to contaminate the heatsink and electronics.

Andrew

If you get a tough one which needs threaded then I make the hole a bit bigger, ream it out a bit

Means less thread engagement but tapping is far easier and it always seems to hold fine

Adapt the system for the job kinda thing

I learnt a long time ago "Do not try tapping pure aluminium". I found it to be a mug's game. There are plenty of other ways to solve the problem, drill clearance and use nuts and bolts is probably the simplest.

Andrew.

Slightly OT because not a hobby solution, one of the neatest solutios I've seen in heatsink fastenings is tha ADVEL RivScrew.
These look like a pop rivet with threads on the shank the mandel is hexagonal and as as it is drawn through forms the central hole into a hex and expands the threads into the heatsink. This gives fast industrial fixing but you ann unscrew the installed fastner with an allen key and refit it. They are also use on some platics and composites as structural fastners.

https://rhf.co.uk/product/avdel-stanley/speed-fasteners/rivscrew/ (random supplier)

Thread forming screws are also an option. Taptite is a major brand. While I've used a lot at M3.5 they are available in smaller sizes. Down to a surprising, to me at least, M1 x0.25

https://taptite.com/images/REMINC-Catalog-English-web-quality-20210119.pdf

Robert G8RPI.

Heatsinks are usually extrusions sliced to length, of alloys that are of acceptable or good machinability.

Other solutions might be indicated in your situation, but normally I'd think a spiral point - or better still, spiral flute - tap with suitable tapping lube like Rocol RTD or suchlike would work. I've not found WD40 much help when tapping sticky metals - but most heatsink profiles I've come across haven't been too difficult that way.

As Andrew suggests above, I'd try to shorten the tapping depth - 3+ diameters is a long way, and 17.78 turns is plenty enough for something to go wrong...

QUOTE Spiral Point NO this will fill your hole with swarf or better spiral flute - Yes much better

It's a through hole Kwil

Horrible stuff as everyone says - a larger hole than that usually dictated and use paraffin (kerosene) rather than WD40 - much better than anything on ali at any time. Plus Gas is good too but difficult to find these days I would think.

If it's as gummy as you say though you may well have to think of an alternative

Best - Tug

+1 for Roberts suggestion of Taptite screws.

These are tri-lobular in shape, essentially the same as thread forming taps. I could be convinced that they actually work better than proper thread forming tap in gummy alloy. Use as is or, if something really secure is wanted without undue tightness, run a normal screw into the hole.

Last time I bought some a small box was usefully cheaper than a thread forming tap or a set of decent quality normal taps. Given the likelihood of breaking taps in gummy alloy I figured that finding storage space for the 40(?) odd screws I wasn't going to use for that job was a fair trade off. Over the years they have slowly gotten used, mostly on an "as I have them" basis.

Clive

I've not tried this in an engineering sense, more in a ' bodge-it' sense. I have done this with other soft metal~

Why not try just screwing the fastener into the tapping hole direct?

Thanks to all for replies - forgot to say I'd drilled 2.1mm.

Clearance drilling for a screw / nut would be difficult because of the geometry of the heatsink, otherwise I would have just done that:

My reason for posting this query was, at least partly, that I wanted to understand why I f..f..fouled this up. I find it difficult to say 'well, that didn't work, I'll try something else' without trying to understand why it didn't work.

From replies it sound's like it's doable, but in this material not as straightforward as tapping normally is. That's reassuring. I'll carry on trying, taking heed of advice - I don't like giving up! The heatsinks actually have some M3 tapped holes (not in the right place obv) as supplied, so it's possible.

For practical purposes it sounds like Taptite is the way to go. Didn't know of them, thanks Robert.

Robin

I was thinking very thin fluid. I scrapped a old Dell laptop yesterday to get the disk magnet, it had a sizebe heatsink, will try a few fluids on that, starting with WD40, then Fuchs, then try other.

Try using Methylated Spirit as a lubricant, you need to add frequently with a brush, we used to get L34 aluminium, an aircraft grade material, very soft and gummy, horrible stuff to machine but meths worked well as a lube.

What Elf 'n' safety would say now ????

John