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  #1  
Old 30th January 2018, 02:47 PM
colin.hepburn colin.hepburn is offline
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Default help tapping screw threads

Hi All
Hoping for a beginners how to guide to tapping screw threads in heatsinks
Screw size M3 how do I go about it

I donít have the necessary tool so what do I need to get hand tools only and cheap of eBay and how do I go about tapping the threads
Hoping you engineerís guys will chime in thanks i.e. bob Philip etc,
Thanks Colin
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Old 30th January 2018, 02:59 PM
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Greg. Greg. is offline
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Default Re: help tapping screw threads

Use this chart.
http://www.shender4.com/metric_thread_chart.htm

I would recommend buy your tap and die set from Screwfix or Tool Station. Reasonable quality for occasional hobbie use at a nice price.

After drilling the hole, use some oil and start cutting the thread using the tapered tap. As tap starts to bite, Rotate clockwise a quarter turn at a time, then turn back a quarter, followed by the next quarter turn cut. Keep doing this until cut is complete.
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Old 30th January 2018, 03:12 PM
bob orbell bob orbell is offline
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Default Re: help tapping screw threads

Hi Colin, its easy, first you need to know the pitch of the thread you are doing, in your case M3, course is .5mm, other pitches also exist, so, you deduct the pitch from the size of thread you are cutting, in your case 3mm - .5mm = 2.5mm, this is the drill size you need, the same rule apply's to all metric threads, if you are threading a bar, you simply use 3mm and a 3mm die. and so on. If you are cutting a thread in Aluminium, then you should use paraffin as a cutting lubricant, and go steady with the tap, half a turn then back to break the chips off, and continue like that, keep the tap straight, it will break very easily. also, if you are tapping a blind hole. you start with a taper tap and then change to a plug, or bottoming tap. Any other questions, just ask. Hope this helps you. BOB
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Old 30th January 2018, 03:36 PM
snowman_al snowman_al is offline
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Default Re: help tapping screw threads

First you need some 2.5mm drills.
A set of 3mm / M3 taps (3 normally for aluminium, a starter, a middle and a finish) and a tap wrench that fits the square end of the tap. Old boys often hammer a suitable nut onto the square and use a spanner to turn it, but the tap wrench is the proper tool.

Mark out the holes and drill them using the 2.5mm bit. De-burr the hole.

Everything depends on keeping the drill and then the taps vertical to the work piece.

The starter (or no 1) is the tap with the 'pointiest' lead to the thread. Gently place it in the hole and keeping it vertical start to turn it. Just a little until it bites the metal. Then go half a turn further and check again. Then another half turn and yep check again. Once you are sure it is true go another turn gently. You will feel the tap cutting and screwing in by now. After a couple of turns unscrew the tap and clear the swarf. Make sure you pick up the original thread and do a couple more turns, back off and repeat. Clear the thread from time to time. Aluminium is easy, so shouldn't need much lubricating, but light oil (or washing up liquid) helps.
If it binds at any time back out and check why. You do not need half a tap stuck in aluminium, you cannot drill it out...

Once you have threaded all the way through, blow the chippings out and do the same with the middle or no. 2 tap. Then the finish no 3. Job done.

If the holes do not go all the way through you need special 'bottom' taps that can cut almost to the bottom, but you need to feel carefully not to force the tap too far.

Best practice on a few scraps first in any case.
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Old 30th January 2018, 03:47 PM
colin.hepburn colin.hepburn is offline
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Default Re: help tapping screw threads

That was quick replies just made me a coffee
Thank guys
Yes two things I never done at school was metalwork and woodwork come to think about it did not do electronics ether

Yes the plan is to mount transistors to heatsinks and using angle aluminium strip on heat sinks making the sides of the case

I havenít fully though it through yet but you get the idea with all your tip and advice it should be fine will also have a look on YouTube for a good sensibly video on this I do find seeing something being done helps me a lot
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Old 30th January 2018, 07:32 PM
JohnG JohnG is offline
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Default Re: help tapping screw threads

All the advice you need about sizing is supplied.
Here is a little tip to avoid frustration when fastening. If you can get the screws with a Head to receive a Allen Key or Torx, there is much less chance of slipping and shearing the edges of the screw head as the screwdriver easily slips out of its seat when tightening pozi or phillips.
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Old 30th January 2018, 08:02 PM
bob orbell bob orbell is offline
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Default Re: help tapping screw threads

Only if you use a pozi on a Philips screw, or vice verse, some people don't even know there is a difference . BOB
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Old 30th January 2018, 08:39 PM
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Default Re: help tapping screw threads

Yep, and considering the application, an Allen or Torx screw is a bit over the top. They donít need the ability to torque to that degree.
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Old 31st January 2018, 09:05 AM
John Caswell John Caswell is offline
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Default Re: help tapping screw threads

Hi all,
JohnG's and Bob's comments are very relevant, the original Philips head screws used to suffer from "cam out" due to their sloping sides, and very often worn screwdrivers. "Pozidriv" heads were supposed to overcome this, due to parallel side, and did to some degree. "Allen" "Torx" and "Bristol Spline" are considerably better but as usual you must use decent screwdrivers.
It is interesting to note that motor industry wholeheartedly took up "Torx" heads for a lot of their fixings due I suspect to less danger of the driver slipping out.

John
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Old 31st January 2018, 09:27 AM
bob orbell bob orbell is offline
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Default Re: help tapping screw threads

John, to my understanding, Torx bolts replaced Allen head bolts, because of the greater contact area of the Torx, I have never yet had a Torx bolt that rounds off, they always undo, even rusty ones. BOB
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