Advice on how best to convert a 3/16x40 Plug Tap into a Taper Tap

I happen to have three 3/16"x40 plug taps which are all very well but perhaps one more than I really need. What I would like is a taper version of the same thread, achieved perhaps by grinding the end of one of the plugs to give a leading taper which would then make life a lot easier. Initial thoughts are to put one of the plug taps into a hand held pin vice and offer it up to the grinding wheel but before doing so I have wondered if anyone may be able to suggest better ways of doing this.

You could indeed get a taper using the method you mentioned, but it is not quite as simple as that. The tapered flutes also need some relief behind the cutting edge, that is harder to achieve. If you are tapping something relatively soft, you may get away with just the taper, though. Since the length is good and you have three, it would be worth using one as a guinea pig. Maybe using an electric drill rather than the pin vise would get a better match of the three flutes, though.

Edited By old mart on 11/07/2022 15:50:31

Edited By old mart on 11/07/2022 15:52:56

The proper way to grind reliefs on taps:

Andrew

In the absence of a T&C grinder I could set up a small grinding arrangement on the top slide using a Dremel. If this becomes an option at what angle should the top slide be set over to achieve the taper lead?

As Old Mart says turning a taper won’t really work, best bet is to buy a couple from somewhere like Tracy Tools. Life is too short to p++s about trying to modify them without the correct equipment.

I'd give it a go with a diamond file. You'd have nice control of the taper & clearance that way.

Martin.

Edited By blowlamp on 11/07/2022 19:00:38

Would''t it be better to use the face of thicker wheel rather than the side of a wheel as pictured ?

Emgee

The pin vice idea is one that I've used. It's a bit tricky because, as Old Mart says, you gotta put some clearance behind the tapered cutting edge, and without grinding the leading edge off the taper of the following land !

IIRC I was doing cheap M4 taps, using the side of the wheel, and got a failure rate of 1 in 3.

+1, @ £2 + p&p from Tracy Tools for a properly made item, I wouldn't bother unless I couldn't wait for the next day's post..

OTOH if you can resolve it with a few minutes' work on an expendable item right now...

No idea what you're trying to say, but the reliefs on the tap are being ground on the periphery of the wheel, as shown in the Clarkson manual.

Andrew

Life is tooo short, even with a Clarkson, or Brierley. Andrew is right in his method ! Noel.

The first time a bodgey ground tap snaps off and ruins a job you just spent 2 days machining, you will wish you spent the threepence ha'penny to buy the correct tap.

Another solution?

**LINK**

Carbon steel only two quid a throw.

Andrews method is correct, in fact it is very difficult if not impossible to do any other way on a Clarkson.

i just cannot believe this is being discussed, just go and buy the right tool for the job, After all, what does a new 'proper' tap cost?.

I think gentlemen, that some of us have too much time on their hands.

I'm a Yorkshireman, so not used to spending money willy nilly, but even I would just go out and buy one, lifes too short as it is.

I would not even go as far as buying a taper, I've manages for 35yrs with just a plug tap for 3/16" and 1/4" x 40 for all my steam fittings etc, it's such a fine thread very little effort is needed to turn the tap.

Well, despite all the doom and gloom and with absolutely nothing to lose I gave it a go using the Dremel with the tap indexed in the lathe and have achieved a useful compromise, the test nut running quite happily up the modified tap.

That's just a plug tap with a chamfer on the end. Won't start cutting until that first tooth hits the job. Not what I thought you were asking about, ie gradual taper over the first half inch or more like a proper taper tap. But if it works for you, go for it!

You could get the same effect as this one by using your unground plug tap and chamfering the edge of your hole instead.

Edited By Hopper on 12/07/2022 10:42:17

The dremel illustrated could be set at a much shallower angle for more lead in. Starting this way at least makes all the flutes match, a job much harder freehand with a pin vise. Then some relief could be stoned behind the leading edge of each flute leaving only a tiny bit of original taper. The tap would probably work, but test first before committing it to an actual workpiece.