Author Topic: Tapping expertise...  (Read 740 times)

Offline Allen Smithee

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Tapping expertise...
« on: January 20, 2020, 10:53:51 PM »
I'm doing a simple job for a colleague which involves making a boss to fit the mainshaft of his unimat 3. The boss is simple enough, being a mild steel round 30mm long which has a hole tapped M14x1 (8mm deep) at one end, a hole tapped M10 through the remainder and a 10mm cross hole near the other end. Very simple stuff. I did all except the M14 thread in a lazy hour yesterday - I didn't have any M14 taps, but as I also own a unimat 3 I was happy to buy them as they might come in handy for my own use. RDG had a 3-tap set that looked very reasonable, so I ordered it and it arrived today:



They look nice enough - only carbon steel but they'd do for what I want, or so I thought until I examined the business ends:



The taper and second taps look fine, but the plug tap has almost as much taper as the second. I'm looking to tap what is essentially a blind hole only 8mm deep and this tap won't get a thread deep enough. I would expect the plug tap to have full thread dia for all but perhaps one thread. Am I wrong in this? Is a true "bottoming" tap different to a plug tap?

Being that I actually need to get this job done...

The taper tap is no use to me at all - not for a shallow hole. The second tap probably isn't either.  I'm thinking about using this "plug" tap as the starting tap and maybe grinding the end off the second tap until I get to the full-thread part so that it becomes a true bottoming tap. Would this work, or is it  a silly thing to do?

All thoughts appreciated,

AS
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Online Vixen

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Re: Tapping expertise...
« Reply #1 on: January 20, 2020, 11:19:50 PM »
Hello Allen,

That should work. BUT... I would turn a 2mm wide relief slot/ groove/ recess at the bottom of the 8mm deep boss thread. That will ensure all the tapped threads are full and complete rather than haing an undefined last few turns. You need ensure the boss contacts the wide spindle flange at the base of the 14 x 1 spindle thread and not let the boss grind to a halt somewhere along it's internal thread

Does that make sense?

It may be better to cut off the tapered end of the tap using a dremel cut off disk rather than the bench grinder as there is less risk of overheating and drawing the temper from the tap

Mike
« Last Edit: January 20, 2020, 11:24:14 PM by Vixen »
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Offline Allen Smithee

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Re: Tapping expertise...
« Reply #2 on: January 20, 2020, 11:29:26 PM »
Thanks for that - and a good thought on using the dremmel! I'd like to cut an undercut, but the hole is only 8mm deep and there simply isn't enough material. This boss won't sit on the spindle seating flange - it will sit on the boss of the gear it is clamping to the spindle to form a drive chain to the lead screw (part of a home-made screw-cutting attachment for the unimat). Hence the shallow thread. I will avoid bottoming the tap by using the travel stop on the tailstock quill.

Thanks,

AS
« Last Edit: January 20, 2020, 11:34:37 PM by Allen Smithee »
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Online Jo

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Re: Tapping expertise...
« Reply #3 on: January 21, 2020, 07:35:38 AM »
I just checked my M14 by 1 taps. I brought a taper and a second which look like your second and plug. My second has 4mm of taper on the end.

If I was to shorten a tap I would use a Dremel cut off blade like Mike says then grind the flutes on a grinder using some sort of jig to hold the shank of the tap firmly to give an even grind on each flute (I have a cutter grinder  :-X ). The aim would be to have only one thread tapered as the lead.

Jo
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Offline Jasonb

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Re: Tapping expertise...
« Reply #4 on: January 21, 2020, 01:16:54 PM »
At M14 I'd be getting the angle grinder out rather than spending ages wearing out several Dremel discs but bench grinder is what I tend to use to shorten smaller taps and take the pointed ends off.

Offline Allen Smithee

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Re: Tapping expertise...
« Reply #5 on: January 21, 2020, 01:47:27 PM »
The part in question is as per the attached drawing (note the annoying lack of dimensions!). After some thought it has dawned that the M10 thread serves no function beyond the cross-hole (whch accepts a shaft with a flat on it). That means that there is no reason why I can't extend the 13mm bore (tapping sixe for M14x1) all the way up to the cross hole. The unimat's spindle will only have about 7mm of threaded part protruding beyond the gear, so with the hole running full-depth I should easily be able to get over 7mm of full-thread with that "plug" tap.

That sounds like a much simpler option to me...

 :facepalm:

Thanks for the comments, even though I'm not using the advice it's still filed away. Every day is a school day in this place!

AS
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Offline derekwarner

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Re: Tapping expertise...
« Reply #6 on: January 21, 2020, 08:16:38 PM »
Alan....without understanding the application spacing too well, is there sufficient space to install an M14 Locknut against the block body?

In anyone language, having a threaded engagement of 0.57:1 is fraught with danger  :killcomputer: [even if Loctite welded]

[Oh...and just on the subject, if the M10 is for a HPGS, an M10 Locknut would not be out of place]

Derek
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Offline Allen Smithee

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Re: Tapping expertise...
« Reply #7 on: January 21, 2020, 09:46:18 PM »
The application is just a hand-crank handle for a unimat 3, so it wont see much speed or load, and only needs to be hand-tight. It drives the spindle and a set of change gears which are mounted on a backplate so that they also drive the leadscrew at the desired ratio to screwcut a given thread. the unimat has no intrinsic screw-cutting capability largely because the drive from the spindle to the leadscrew is a non-toothed belt!

AS
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Offline Zephyrin

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Re: Tapping expertise...
« Reply #8 on: January 22, 2020, 09:21:47 AM »
looking at the picture of the taps, 8mm deep suggests that there is enough depth to have 2 turns of effective cutting, and this would help for centring second tap, then  the third, with its pointed end grinded off.