Drilling & Tapping

Hello, I need to drill & tap an M4 0.7 x 15mm hole in the ends of a few high tensile 8.8 steel bolts.
I've been using a 3.2mm drill followed by a HSS tap from a cheap box set
with limited success, it's taking approx 1/2 hr each hole.
Can anyone recommend what's the best drill & tap combination to be using???
Ideally with a pointer to a UK supplier.
Thank you.
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Would it be stupid for me to ask why you are tapping 15mm when probably 6mm will do the job equally as well with such a small size.
best taps you could use for the job would be machine taps ..make titex
all the best.mark
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Cheap taps are about as much use as chocolate teapots. OK for cleaning out existing threads and not much else. Buy a brand new quality make and it'll transform what you're trying to do. J&L Online
http://www.jlindustrial.co.uk/cgi/insrhm
have what you need at a good price. The Hertel taps on page 114 are only 2.74 each in that size. You'll either need a plug tap to start each hole and a bottoming one to finish it or you could maybe get away with just a second tap depending on how close to the bottom of the hole you need the threads to go.
I'd also use tapping fluid on steel if you have any. Turn the tap a bit and then back off to break the chip before going in deeper.
Probably best to double drill the holes. A 3mm followed by a 3.3mm at a slower speed will give you a better tolerance on the size than single drilling. A stub drill will be more rigid and less likely to wander than a jobber length one. Page 10 is what you want for those. A couple of each size is only about two quid. Start each hole with a centre drill (page 90) to give the stub drill a clean chamfer to work in.
--
Dave Baker



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...and you'd have to go a bloody long way to get a better tutorial than that! Thanks, Dave.
--
Chris Edwards (in deepest Dorset) "....there *must* be an easier way!"

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Absolutely right Chris, save for one minor typo, which I point out only to protect the innocent! Where Dave says a 'plug' tap to start the hole I suspect he meant to use the word 'taper'.
Mike
--
Mike Whittome

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You're quite right. Clearly I should have said taper or 1st tap. Senility creeping in I'm afraid.
To the OP. In soft materials like aluminium and especially with larger tap sizes you can manage perfectly well with just a 2nd tap unless the threads have to go right to the bottom of a blind hole. In hard materials it's best to start the hole with a taper tap which centralises better, requires less torque to turn it and is less likely to break. Breaking taps in the job is probably the biggest problem with small sizes but you avoid that with good quality sharp new taps and tapping fluid.
Starting a tap true in a hole is another story. I usually do this by holding the tap in the machine chuck I've just drilled with and turning the spindle by hand keeping light pressure on the quill or tailstock until the first few threads are cut. After that you can put a tap wrench on and finish manually.
--
Dave Baker



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wrote:

Problem is that the Septics call Plug taps Bottoming taps and don't know what a second tap is...
This is applied in the J&L catalog(ue) because J&L are from that neck of the woods.
Mark Rand RTFM
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On Mon, 01 Dec 2008 23:19:31 +0000, Mark Rand

Complicated further because AIUI they call a second tap a plug tap, or thereabouts, in the US, and that nomenclature sometimes creeps into the J&L catalogue.
Tim
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Annoying !
--
Boo

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wrote:

That's what I meant :-)
Mark Rand RTFM
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I have to say I hadn't spotted that J&L sometimes call 2nd taps plug taps even in the UK catalogue. Very confusing for the novice. On page 114 they call the 2nd tap a plug tap in the Hertel section but in the value tools section they call the general purpose tap a "second lead" tap. On page 115 they then use the terms taper, second and bottoming but if you look at the metric fine listing they call the general purpose tap a bottoming tap which clearly it isn't. Someone there needs a good kick up the arse.
To the OP again. In the UK we refer to taps as follows.
1st (or taper) tap - Has a long taper lead-in for starting holes, especially useful in tough materials and small tap sizes.
2nd tap - general purpose tap with some lead-in. Can be used for both starting and finishing holes most of the time and is what you'll get in a tap and die set with only one tap per size.
Bottoming (or plug) tap - No lead-in and with full diameter threads right to the end for tapping to the bottom of blind holes but can't be used for starting holes. It's pretty rare you need a blind hole with a thread right to the bottom and you can't just make the hole a bit deeper so that a 2nd tap will do.
In America they call a 2nd tap a plug tap for reasons unknown.
As a general point there shouldn't be a difference on tolerance on the full size threads on a tap so for a through hole you can use either a 1st or a 2nd equally well as long as you run it all the way through the hole.
--
Dave Baker



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wrote:

Interesting, I was reading something on the web the other day that said this was generally the wrong thing to do. The article indicated that it was better to spot or centre to not much more than the point width on the drill end. The reasoning was that when drilling into a large cone the extreme edges of the drill make initial contact leading to chipped corners and possible eccentricity.
I must admit that until I read that I have always centred to slightly greater than tap diameter to give a nice cone edge to the tapped hole.
Richard
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Richard Edwards wrote:

I recently heard some US research types had done some research though, and found that double drilling actually makes worse holes than single drilling (though they always used center or spotting drills first).
Maybe partly for the same reason, but I'm not entirely clear about the details. or why or how the holes were worse.
However, there is no doubt that a good set of taps is worth while, especially in things like M4 where the cost is small - the difference is astonishing, in M4 with good taps you can tap then extract the tap by spinning the wrench with one finger after the chips are cleared.
That might not seem like much, but if you consider that you are applying the force at an angle to the hole, and it's a tap with sharp edges, it implies something good about the surface of the cut thread. Also, they go in easier :)
And I also would use 3.3mm, not 3.2mm, for tapping M4 in hard stuff. If you haven't got a 3.3 handy, try 1/8".
-- Peter Fairbrother
(been very lergied, just managed to get up for more than a mimute today)
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I find that hard to believe. Single drilling can lead to quite a variation in hole size, especially at high speeds when a single drilling invariably cuts oversize. I always predrill at normal speed and then second drill at about half that if I want a really good size tolerance.

A 1/8" is even smaller than a 3.2mm though.
--
Dave Baker



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Dave Baker wrote:

Single drilling at low speed presumably takes about the same or perhaps even less time than fast/slow double drilling - so unless you are impatient and want to single drill fast, time isn't a concern.
The question then for hole size accuracy is whether a slow drill makes a better hole in a pre-existing hole of say 7/8 it's diameter than in raw material. I can't see why it should?
For location accuracy, how self-centering is a large drill going into a hole of 7/8 it's diameter, as opposed to raw material?
For roundness, and straightness ... I don't know ... ?
Whatever, some modern advice seems to be not to double drill at all, ever, as it's never going to give a better hole. Which surprised me.

Yeh, duh, sorry, brain lergied (been in bed for a few days with flu, just got up yesterday) -
- and 3.3 is a minimum: though I wouldn't go as far as 3.5 unless it was of low-strength and importance, I'd be happy with 3.4 in most cases. 3.2 is too small, even for plastic.
So OP, the advice is better taps. use lube, get everything straight (eg tap in the lathe) as even a slight squintness, especially in a deep tapping, can increase the force required beyond the breaking strength of the tap, - and use a bigger hole. HTH.
-- Peter Fairbrother
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Swarf is though, especially in tough materials. Single drilling in blind holes can lead to swarf bunching up and ruining the hole size and finish. The second drill doesn't have to deal with that.

Maybe it's just habit and perhaps I should run some tests one day to see if I can get as good a finish and size with slow single drilling.
--
Dave Baker



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S' what reamers are for :-)
No I don't generally ream holes for tapping!
Mark Rand RTFM
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Start each hole with a centre drill (page 90) to

My sentence wasn't trying to say that the chamfer 'needed' to be bigger than the drill. I usually centre drill to about half the drill diameter. However for threads when you usually want a chamfer on the finished article anyway I sometimes do centre drill big enough for that at the start to save a second op later. I can't say I find it leads to chipped drill bits although I suppose that's a possibility in very tough materials. As for eccentricity I find the more material the centre drill has removed the better the twist drill centralises.
--
Dave Baker



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I agree with most of the other comments about tap quality, cutting fluids etc; however, I am surprised that Peter F is the only one so far to mention tapping size. I would go further, and recommend 3.5mm* - gives 65% thread engagement, more than enough for use in high tensile steel.
Your taps will be far less likely to break, and the effort involved will be less.
*Many people forget that an M4 tap has a diameter greater than 4.0mm. From memory it is about 4.15mm or 4.2mm - the close fit standard clearance hole for M4 is 4.3mm - and this must be taken into account.
David
--
David Littlewood

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David Littlewood wrote:

> I would go further, and recommend 3.5mm* -

I second that. 100% thread engagement (TE) is overkill for steel, let alone HT steel. 65% is quite enough, and that requires a 3.5mm tapping drill. Even for 100%, 3.2mm is too small - it should be 3.3mm.
Several years ago Tubal Cain published (ISBN 0-85242-866-9) the results of some experiments he made using a nut thickness of one bolt diameter. At 100% TE, the ratio of shear strength (of the threads) to tensile strength (of the bolt) was 3:1 and at 65% TE the shear/tensile ratio was still 2.5:1, BUT the torque required to cut the thread meant that the taps were less than half as likely to break.
A further point is why 15mm depth? A standard M4 nut, which presumably you would agree has adequate strength, is 2.9 to 3.2mm thick, which is only about 4-5 thread depths. So a tapping depth of 4mm is much more than you need - the threads are 2.5 times as strong as the bolt, which will break long before the threads shear.
--
Regards, Gary Wooding
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