What drill bit size to use?

[Lo_Tse]

Hi there,

A real quick question from a newbie. What size drill bit should one use to make holes (on a heat sink) that are suitable to be tapped for M3 screws? It seems that I can only find drill bit in imperial scale? Would a 1/8 (0.125) inch drill bit work for the M3 (0.118) ? Thanks!

Lo_tse

[madtecchy]

Hi Maybe this will help.

Ian

[nigelwright7557]

Thanks for that, copied it to my pc.

Dont forget to oil the hole the tap will go down to ease the cutting.
I broke two taps before finding that out !

[jordheis]

Hi,
for a 3mm x .5 pitch fastener;
as noted use a 2.5 mm drill, or a close equivalent is a #40 drill.

The Fractional Drill Chart reference

Drill/Tap chart reference

[Lo_Tse]

Originally Posted by madtecchy

Hi Maybe this will help.

Ian

Thanks! That's useful. Here in Canada, it seems that I can only find drill bit in Imperial size (e.g. Home Depot) but not metric. Using you chart, may be I should try 7/64 inch (2.75 mm) drill bit for the M3 (3 mm) screws.

[jordheis]

Sure - You can give 7/64 a try on a scrap piece to test how it goes with a tap and fastener. Aluminum being soft be careful to not over torque the M3 fasteners and strip out the hole.
Otherwise go down a fraction and test that too. That's how I would proceed on my stuff.

Remember the tap lube as nigelwright7557 noted. Have a spare tap....

[Conrad Hoffman]

Heat sinks are usually made out of a gummy aluminum that likes to break taps and machines poorly. If you're going to do much of this, it pays to order drills and taps from a machine tool supplier. For taps that cut threads in blind holes, order spiral flute taps (not spiral point) as those will send the chips back out the top of the hole. Spiral point taps will send the chips forward, but they have to have a place to go. Those are the only two styles one should mess with. For something a bit nicer, order a Balax forming tap. You'll need a very specific (and larger) drill size, but the tap will form the threads by deforming the aluminum. No chips at all and very strong threads. The taps are much stronger than cutting taps and last near to forever. In any case be sure to use oil for both drilling and tapping. Vegetable or animal oils work best so if you don't have a commercial tapping fluid, try a bit of canola oil or bacon grease!

[Lo_Tse]

Thank you very much for the link. Very comperhensive information!

Originally Posted by jordheis

Hi,
for a 3mm x .5 pitch fastener;
as noted use a 2.5 mm drill, or a close equivalent is a #40 drill.

The Fractional Drill Chart reference

Drill/Tap chart reference

[Lo_Tse]

Thank you for the info and advices. I do have a set of taps purchased from a hardware store, but have not tried it out yet.

Your point on using the spiral flute taps is interesting. I thought, like drill bits, taps would pull the chips back out on top! Good to know. I guess I just have to get a scrap piece to try things out.

Once again, thank you very much for all the info and advices. Much appreciated.

Lo_tse

Originally Posted by Conrad Hoffman

Heat sinks are usually made out of a gummy aluminum that likes to break taps and machines poorly. If you're going to do much of this, it pays to order drills and taps from a machine tool supplier. For taps that cut threads in blind holes, order spiral flute taps (not spiral point) as those will send the chips back out the top of the hole. Spiral point taps will send the chips forward, but they have to have a place to go. Those are the only two styles one should mess with. For something a bit nicer, order a Balax forming tap. You'll need a very specific (and larger) drill size, but the tap will form the threads by deforming the aluminum. No chips at all and very strong threads. The taps are much stronger than cutting taps and last near to forever. In any case be sure to use oil for both drilling and tapping. Vegetable or animal oils work best so if you don't have a commercial tapping fluid, try a bit of canola oil or bacon grease!

[AndrewT]

Metric threads have been made easy for us since they adopted a 60degree thread face angle.

Take the screw thread size and subtract the pitch.

eg M3 0.5mm pitch needs a 2.5mm drill.

In hard materials (steel) you can drill slightly bigger than 2.5mm to make for easier tapping and this gives slightly less engagement.
In softer materials (aluminium) you can drill slightly smaller than 2.5mm to ensure full engagement of the male and female threads.
The extreme form of this are the thread forming taps, rather than the thread cutting taps.

Note "slightly".
The total engagement of that M3 0.5mm pitch is only 0.25mm. Take 10% off that for 0.225mm engagement results in the required hole size being 2.55mm diameter. 80% engagement requires a 2.6mm diameter hole.

Poor drilling technique and poor sharpening of the drill point both result in oversize holes.
Be careful you leave sufficient engagement to give a "strong" thread.