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Old 26th Jun 2021, 8:46 pm   #21
Philips210
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Default Re: Tapping a hole in metal: What size hole?

There's a useful comarison chart for metric, imperial and number drills here: https://www.custompartnet.com/drill-...more%20rows%20

Regards,
Symon
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Old 26th Jun 2021, 10:12 pm   #22
kalee20
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Default Re: Tapping a hole in metal: What size hole?

Quote:
Originally Posted by Alistair D View Post
An old mechanical engineering friend gave me this tip many years ago. I have used it many times so I can vouch that it works.

Find a nut that matches the thread you want to tap. Then find a drill that only just passes through the nut. Quite logical really.

Quote:
Originally Posted by McMurdo View Post
That's a top tap tip
Absolutely!

Definitely one to remember, and 'Why didn't I think of that?"

Thanks Alastair!
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Old 26th Jun 2021, 10:59 pm   #23
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Default Re: Tapping a hole in metal: What size hole?

Blimey, taps and dies. Takes me back to O Level Engineering Workshop Theory and Practice in 1970

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Old 27th Jun 2021, 9:43 am   #24
60 oldjohn
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Default Re: Tapping a hole in metal: What size hole?

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Blimey, taps and dies. Takes me back to O Level Engineering Workshop Theory and Practice in 1970

Aub

IIRC the rule was 1 turn in half a turn out, this is after entry has been established, this is to break the swath and give a better finish to the thread.

John.
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Old 27th Jun 2021, 4:56 pm   #25
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Default Re: Tapping a hole in metal: What size hole?

Also note the difference between taper-taps and bottoming-taps.

For thinnish stuff where the tap will emerge from the other side a taper-tap is what you need - its tapered-end helps get it started.

If it's for a thicker workpiece - or anything where the hole-to-be-tapped is closed-ended - you go in first with a taper-tap then follow with a bottoming-tap so the thread is cut parallel right to the bottom of the hole.

[I've seen - even in a production environment - a taper-tap-only used in blind holes; when fitting a stud to said holes this can be kinda OK as the tapered-ness helps lock the stud into the workpiece - but it still seems all-wrong to me. nevertheless I've flown in aircraft whose engines have involved such ad-hoc kludgery and none of them have crashed...]
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Old 27th Jun 2021, 5:33 pm   #26
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Default Re: Tapping a hole in metal: What size hole?

Strictly a taper tap should never cut to the full depth of the thread, it is only for starting. You should never only use a taper tap.

The second-cut tap is also tapered but not as much, and will cut the full depth of the thread. You can often start with a second-cut tap and on thin materials, through holes, it's all you need. If you only get one tap for a particular thread, that's the one to get.

The bottoming or plug tap is parallel over its full length. You use this after the second cut tap in blind holes to tap the full thread to the bottom of the hole.

Incidentally, for what we do here the thread engagement is not too critical. It will be strong enough (aircraft are another matter!). So if you don't have the exact drill, say it's between 6.7 and 6.8mm, use the _larger_ drill. Trying to tap too small a hole is a good way to break a tap off in the workpiece, and removing it can be a right pain! The larger hole will give slightly less thread engagement, but will be strong enough on a vintage radio.

Taps are hard, and thus they are brittle. Never force one, it will break. The rule of 'one turn in, half a turn back' has already been mentioned, but if you find the tap starts to bind after less than a full turn, turn it back to clear the swarf and then go forwards again. In some 'easy' materials like machining brass, you can actually turn the tap several turns in at a time without problems. Go by feel.

It is essential that the tap is aligned along the hole. If it is at an angle it will jam.

Finally, I will warn you that a lot of DIY drills have far too much play in their bearings. The hole comes out considerably larger than the drill bit you use. So you might want to start will too small a bit and go up 0.1mm at a time until the hole is the right size.
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Old 27th Jun 2021, 7:10 pm   #27
kalee20
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Default Re: Tapping a hole in metal: What size hole?

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Originally Posted by TonyDuell View Post
Strictly a taper tap should never cut to the full depth of the thread, it is only for starting. You should never only use a taper tap.
Well, I've often used just a taper tap - and I've never found that it doesn't cut sufficient metal that a screw or bolt doesn't go in easily!

I've also, once, used the same tap to thread a couple of blind holes in a piece of brass, almost to the bottom. Technique: use the taper tap to start the threads, having marked the tap to the depth of the holes so I don't try screwing it in with the tip binding on the end of the hole; then use angle grinder to grind off the tapered part of the tap, thus turning it into a plug tap.
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Old 27th Jun 2021, 9:11 pm   #28
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Default Re: Tapping a hole in metal: What size hole?

Quote:
Originally Posted by Aub View Post
Blimey, taps and dies. Takes me back to O Level Engineering Workshop Theory and Practice in 1970

Aub
Yep. Have that (as in EWTP O Level).

On the original question, every size tap has a defined drill hole size. The old Zeuss tables contained that, but internet searches will find it easily. If there's a size needed for some particular requirement, you will be able to buy it one way or another, even if it's not something you can source via the average retail outlet.

It's interseting about the number drill sizes. I'm not sure where it came from originally, but I do (did) have a set that were my Dad's. It's (was) missing a couple, fairly sure that I broke in my Youth, but was wondering what to do with them (though they may have gone to a tool sale).
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Old 27th Jun 2021, 9:16 pm   #29
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Default Re: Tapping a hole in metal: What size hole?

Also of vintage-relevance: in days-gone-by suppliers of engineers' requisites gave away rotary-calculator ready-reckoner-thingies that let shop-floor-types easily identify the tapping-size and clearance-size drills to use for a whole range of threads.

I guess that there must be 'collectibles' forums for these sorts of things just as there are for slide-rules?
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Old 27th Jun 2021, 10:08 pm   #30
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Default Re: Tapping a hole in metal: What size hole?

Just a little note about "guestimating" the drill size to tap a hole.
o
As a 16 year old apprentice, I had to machine a large nut with a 4 inch Whitworth thread. Rather than screwcut the thread, my tiny mind decided it would easier to tap it. I drilled the hole to the size I thought the tap would need, put a centre on the tap, applied a spanner to the tap shank, started the lathe at a slow speed and followed it in with the tailstock centre. The lathe did it work, but I had badly miscalculated the hole size.

A 4 inch tap explodes quite spectacularly under these conditions.
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Old 27th Jun 2021, 10:38 pm   #31
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Default Re: Tapping a hole in metal: What size hole?

For making tapped holes in thin metal, there are also non-cutting 'friction drills' which melt and swage a hole through the metal, using the material from the hole made to make a thickened ring around the hole. When tapped you get more threads worth of contact and a stronger fixing.

There are combined friction drills and taps, but they need a smart machine to have the power to melt through, and the finesse to stop before the tap runs into the end of its thread.

David
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Old 28th Jun 2021, 12:26 am   #32
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Default Re: Tapping a hole in metal: What size hole?

I believe that the number of number drills was orignally the gauge of the wire stock that the drills were made from. Before the UK adopted the British Standard Wire Gauge to create order from chaos, there were many different wire gauges in use, and the number drill sizes correspond with one of them.
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Old 28th Jun 2021, 8:17 am   #33
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Default Re: Tapping a hole in metal: What size hole?

Gents, you have all more than answered my original question but it has generated such a wealth of good, detailed information that maybe it could be made a 'sticky' in this section?
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