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Threads to bottom of hole

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Wolfie28/10/2010 14:07:26
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502 forum posts
Ohh heres another (probably stupid) question.
 
Taps are tapered so you can start your hole off. So what happens when you can't go all the way through, When your hole has a bottom so to speak. The tap will stop when it hits the bottom but it won't have made a thread all the way down cos of the taper?
 
Am I missing something?
KWIL28/10/2010 14:14:15
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There used to be full bottoming taps but a lot of bottoming taps still have the centre ground on them. The taper tap you refer to is for starting, not for finishing!
 
After the taper tap there is a second cut tap, the third is the bottoming tap.  If you need a certain depth of thread you must ensure you can have a deep enough hole.
Andrew Johnston28/10/2010 14:40:22
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As KWIL says, but you can grind the point off the bottom of a bottoming tap. That should get you within a couple of threads of the bottom of the hole. Or you could use a spiral flute tap. They're really intended for machine tapping of blind holes, but used by hand to finish off they should get you within one thread or so of the bottom.
 
The only way to get a thread right to the bottom of the hole is to thread mill, so I understand. It's on my list of things to try on the CNC mill, when I can locate a thread mill that doesn't cost several hundred pounds.
 
Regards,
 
Andrew
KWIL28/10/2010 15:29:27
3681 forum posts
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M6 thread mills start at around £58+vat         www.cutweltools.net
Tony Pratt 128/10/2010 17:35:06
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It really is a waste of time and effort to tap a hole right to the very bottom, you will gain very little out of the exercise and there is a very real possibility of tap breakage. Where possible drill the hole deeper than the depth of thread reqd. ie. if you want a tapped hole 12mm deep go to a depth of 15mm with the tapping drill
Tony
mick28/10/2010 17:42:54
421 forum posts
49 photos
You can always flatten off the bottom of the hole, by either using a flat bottom drill or D bit and use a plug tap, this will give a couple of extra threads.     The point ground on the end of the tap is supposed to contact the V of the drilled hole to help prevent breakage.
KWIL28/10/2010 18:25:36
3681 forum posts
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The point on the end of the tap is to support the end of the tap blank during grinding, if it touches the bottom you can still break the tap.
Andrew Johnston28/10/2010 21:41:56
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KWIL: Thanks for the link. I couldn't get it to work, but did find the company website via Google. Looks interesting, although everything apart from M6 is rather more expensive. Breaking a £60 tool is one thing, breaking a £200 tool is another! Apart from anything else it wouldn't leave anything in the kitty for beer to drown one's sorrows. I've been looking for a single point thread mill, slow I know, but easier to envisage what is happening. They just don't seem to be available in the UK and the US makers don't do international. I expect I'll end up buying a multitooth thread mill from Maritool in Chicago. I bought a tension/compression tapping head for the CNC mill from them a while ago and have been very pleased with it.
 
I must admit I'd always thought that the point on the end of a bottoming tap was to do with the manufacturing. I don't see how it's going to stop you breaking the tap.
 
As Tony says, ideally the best solution is to drill the tapping hole deeper than the thread needed, but sometimes the design simply doesn't allow that. So being able to thread nearly to the bottom can be useful sometimes.
 
Regards,
 
Andrew

Bogstandard29/10/2010 05:23:38
263 forum posts
Wolfie,
 
To answer your question.
 
The normal way to cut a thread to the bottom of a blind hole is to use the second cut tap of your tapping set, thread as far as you possibly can down the hole.
 
With the plug or bottoming tap, very gently grind the end off completely square, making sure you don't over heat the tap by continually quenching it in water. You grind off all the lead in taper and point, if it has them. Carefully deburr all ground edges. I use a small diamond file for finishing mine off, but try not to touch the main cutting edges of the tap.
 
Then by feeding down the previously tapped second cut hole, using plenty of good quality cutting lube, you should be able to finish the threading right to the bottom of the hole. You might have to retract the tap fully a couple of times to clear out swarf at the bottom of the hole before getting to final and full depth.
 
As you have said, normally supplied taps won't cut all the way to the bottom, but by doing what I have suggested, will get you as far as possible by hand tapping.
 
Bogs
 
Ian S C29/10/2010 10:14:09
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7468 forum posts
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Originaly the tips were center drilled (some still are),but it was found thatthe taps tended to crack during heat treatment, a male center over came this problem, once the tap is completed there is no need for the center, so grind it off. With a drilled hole the thread shoul go the full depth off the hole, I have'nt found a need to do moe than grind off the tip just clear of the first full thread on the tap. Ian S C

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