Drill/Tap/Hex-Bolt set for mounting fets on heatsinks - diyAudio
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Old 26th August 2013, 03:00 AM   #1
The Dastardly Dad of Three
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Default Drill/Tap/Hex-Bolt set for mounting fets on heatsinks

Hi chaps - searched the forums but am unfamiliar with this topic so don't know where to go. I need to tap some Al heatsinks to mount output mosfets for an amp.

Can anyone suggest an online source where I can buy matching drill-bits, taps and compatible hext-bolts like i see in all the beautiful solid state builds in the forums ?

Metric sizes are preferable but as I'll want to buy them all in a set I don't mind inches either.

If anyone can point me to a source for sets and suggest appropriate sizes (in US terminology) for my application, I'd be very grateful. I believe mcmaster sells them but again, I dont know anything about the sizes.

Thanks again in advance! Cheers,
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Old 26th August 2013, 07:12 AM   #2
Enzo is offline Enzo  United States
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McMaster will have metric or imperial both, so take your choice.

Pick a screw size. Taps come in the same sizes 3mm screw, 3mm tap, #8 screw, #8 tap.

You need to drill a clearance hole, I forget the official term for it. The hole is large enough for the body of the screw, but leaves material where the threads will be. I don't have to do this, so I forget what number drill goes with what screw size, but McMaster ought to have that information in their catalog.

One alternative might be "self-tapping" screws. You drill the hole, but no need to tap it. The tip of the screw is beveled and will cut its own threads when first driven into a fresh hole. They work very well.

There are some amps using sheet metal screws into the aluminum as well.


And just drilling a full size hole for the screw and putting a nut on the end is not a bad solution.
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Old 26th August 2013, 09:12 AM   #3
The Dastardly Dad of Three
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Thanks Enzo. I can't use self-taps or nut solution this time so will go for the M3 solution. From what I can see that will mean a 2.5mm drill bit - if anyone knows different pls let me know ?

Also one other question - what screw length do folks typically use for this ?
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Old 26th August 2013, 09:36 AM   #4
Mihkus is offline Mihkus  Estonia
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The measurements on bolts are taken from the top of the thread...
You really dont need to tap the holes...
For m2,5 you would need 2mm drill or in ideal case 2,05mm one
m2 is pretty small,2,5 is ideal for fastening any type of device, such as chips and transistors.
Its mostly in use, ive never seen m2 being used in such case.
Try to find m2,5 8mm that have a little cut in them, add a bit oil and screw them in, there you go.
Ive found that m3 is sometimes too big, it can crack the device package.

If I was you, i would probably choose: ( search ebay )

M2.5 X 0.45 pitch HSS 2H Tap & Tapping Size Drill Bit 2.05mm HSS

Socket Head Cap Screws M2.5 x 8mm

Im not sure how long will the chinese "HSS" hold but who knows...

Last edited by Mihkus; 26th August 2013 at 09:46 AM.
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Old 26th August 2013, 10:44 AM   #5
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Quote:
Originally Posted by Enzo View Post
McMaster will have metric or imperial both, so take your choice.

Pick a screw size. Taps come in the same sizes 3mm screw, 3mm tap, #8 screw, #8 tap.

You need to drill a clearance hole, I forget the official term for it. The hole is large enough for the body of the screw, but leaves material where the threads will be. I don't have to do this, so I forget what number drill goes with what screw size, but McMaster ought to have that information in their catalog.

One alternative might be "self-tapping" screws. You drill the hole, but no need to tap it. The tip of the screw is beveled and will cut its own threads when first driven into a fresh hole. They work very well.

There are some amps using sheet metal screws into the aluminum as well.


And just drilling a full size hole for the screw and putting a nut on the end is not a bad solution.
Wrong. If you drill a 3mm hole, the 3mm tap will fall into the hole.

There is no rule of thumb as the ratio changes as the bolts get bigger, but for am M3 tap you can use a 2.5mm drill.

You can buy a tap set which might have for example all three M3 taps (taper, second cut and blind) and the correct drill bit.
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Old 26th August 2013, 10:55 AM   #6
AndrewT is offline AndrewT  Scotland
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Staying with metric makes for very simple tapping and screwing into aluminium.

The relatively coarser thread of the M cf. the UNF (the more common bolt thread) gives good grip in aluminium. UNC is the coarse equivalent in imperial threads.

The thread angle in Metric it very simple to determine the pilot drill size.
Just subtract the pitch from the thread diameter for the pilot drill size.

eg M3 0.5pitch requires a 2.5mm drill.
M6 1mm pitch requires a 5mm drill.

Tapping into steel uses a very slightly bigger pilot drill. This reduces the thread engagement, but leaves sufficient to give the "strength" expected of the threaded steel fixing. I cannot recall a "rule" for tapping drill size into steel.
In the UNF & UNC sizes there are drill and tapping look up tables for the different materials.

Thread forming taps are very different from thread cutting taps. See the tap manufacturer's data.
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Old 26th August 2013, 11:56 AM   #7
The Dastardly Dad of Three
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Thanks for the help so far.. As you might have guessed I can't recall having bought screws even once my stash comes from a few hours in front of the tv unscrewing bits from equipment headed for the trash heap. You can't imagine the number of screws and bits that come off a VCR or a mini photocopy machine.
It's just that they don't come with size labels

From what I've read so far that self tap screw thing sounds like a treat - but all the ones I have and the ones I can google are the very coarse type with very tapered threads and look almost like wood screws instead of the cylindrical metal type screws with hex heads.

Are there screws with the little slit or notch at the end like that ? Three step taps into a soft material is a bit too much of a fine touch for me..
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Old 26th August 2013, 01:03 PM   #8
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I have always used 3mm bolts with a 2.5mm pilot hole for soft ali (heatsinks).

Manual 3mm tap, using a leisurely one full turn forwards and one half turn back, stopping to clear out swarf half way through.

I've never broken a tap yet (daft think to say ) and have been doing this for many years.

Steele is different and needs care and greater precision.
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Old 26th August 2013, 02:08 PM   #9
Enzo is offline Enzo  United States
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Katie, what did I get wrong? I said 3mm screw, 3mm tap. I never mentioned 3mm hole with 3mm tap. PLus I mentioned the hole must leave room for the body of the screw but not the threads. What terms should I have used?
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Old 26th August 2013, 02:24 PM   #10
AndrewT is offline AndrewT  Scotland
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Enzo,
I have gone back and read your post2.
I can see nothing "wrong" with it.

I do not agree with all the advice on self tappers for clamping up transistors to heatsinks.
I would never do that. But that is just my opinion.
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