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JMF11 6th June 2020 09:39 AM

Tap aluminum heatsink

I need to tap two M3 holes in aluminum heatsinks. I have 3 pass 3mm taps.

I don't succeed to have nice threads. They finish "too big hole" and the screw slips in them.

I had the advice to use only the last finishing one, as aluminum is a rather soft metal. This is a bit better, but still not very nice.

I have difficulties to start and stay vertical...

How do you do ?

Any tips to tap heatsinks ?


KatieandDad 6th June 2020 09:45 AM

There are several ways to do this.

1. Tap from the rear. This results in the thread at the working surface being better formed.

2. Fit a suitable metal plate on the face of the heatsink so that the loose hole is taken up in the scrap material.

3. Use a drill press.

Kjeldsen 6th June 2020 09:46 AM

It seems you drill the hole to big. I just go very slow to begin with en make sure it's more or less straight. If you have a drill bench that can go very slow then this could be an option.

JMF11 6th June 2020 10:08 AM

Thanks for the tips.

I like the scrap metal plate idea.

How do you use a drill press to do the job? I would be afraid:
- either to press too hard
- or to break the tap ?

I drill at 2.5mm for M3, which looks like the "official" value.


edbarx 6th June 2020 10:10 AM

Taps require a very steady and accurate hand control which is difficult to achieve with taps in the range of a few millimeters in diameter. My advice is to use a drill stand and work very slowly by manually rotating the chuck with your hand. If your holes came out to be too big, use small bolts through the heatsink.

JMF11 6th June 2020 10:15 AM

OK, understood.

I take the opporttunity to ask if there is a better tool tu cut heatsink than a Hack Saw (I don't find that they cut thick aluminum so easily).


Sadface 6th June 2020 10:22 AM

2.5mm is too large for M3

You need to use 2mm.
It took me a while to figure that one out. Once I changed to 2mm I stopped having issues with laxity.
3mm for m4

I second using a drill press

I know its not considered good practice but........
I have also had great success using a cordless drill driver for taping m3 and m4 holes. I find that the freedom from having to provide rotation allows me to focus my limited brain power on jamming the tap as hard into the hole as possible till it bites and then things are easy.

Kjeldsen 6th June 2020 10:22 AM

2.5 mm is good, but also measure the final hole. It can easily be bigger. I simply don't see how the final tapped hole can be to big. I have done this a lot, and never experienced this

KatieandDad 6th June 2020 10:31 AM

2.5mm is the correct drill for an M3 tap.

If you use a smaller drill you run the risk of snapping the tap.

coolnose 6th June 2020 10:35 AM

Stupid question, but I suppose your are lubricating as needed and turn the tap back half a turn out every 1 or 1 1/2 turn ?

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