Tapping Drill for M14 x 2mm tap

[Andrew TinsleyParticipant @andrewtinsley63637]

I am somewhat nonplussed. The recommended drill is 12 mm so I drilled my round stock in the chuck 12mm expecting this to be give me a bit of a struggle when tapping from the tailstock, which it did! Not a trace of a cut with a brand new Presto taper tap The material is simply mild steel which turns and saws as one would expect, that is easily!

I then enlarged the hole to 1/2″ and still could not get a thread going. In desperation I enlarged the hole to 13mm still no joy! tried taper, second tap and plug tap, all without success.

I am suspecting that the taps are the problem, although I have always had excellent results from Presto taps. Just as a cross check. I tried tapping an M16 hole with Presto taps from the same supplier. Using a 14mm drill, I had no problem getting the tap started.

Any suggestions welcome as I can’t work out what is going on, unless the new presto M14 taps are faulty.

Andrew.

[Andrew Tinsley]

On 19 July 2024 at 13:28 Andrew Tinsley Said:

I am somewhat nonplussed. The recommended drill is 12 mm ..

Not in my book.  13mm is more like it.  Depends on the pitch, p=2, 12.7mm; p=1.5, 13.00mm; p=1.25, 13.2mm.

Unless there’s good reason not to, I follow Tubal Cain’s guidelines and drill tap holes a shade bigger than industry.

Be interesting to put a microscope on the Presto Tap to see how much damage has been done by attempting to force it into a too small hole.  Hopefully not much, though the later refusals to start are worrying. At least the Presto survived an accident that might have snapped a cheaper tap.

Could the taps be counterfeit?

Dave

 

[JasonBModerator @jasonb]

Usual rule of thumb is dia less pitch so the 12mm would have been right for the OP’s M14 x 2 though lightly loaded you can go larger.

 

Have you been sent a set of left hand taps.

[Andrew TinsleyParticipant @andrewtinsley63637]

I usually use Tubal Cain’s recommendations but M14 x 2mm isn’t on any of his tables. I think Dave must be suffering from the heat, as just about any list of tapping drills for Metric coarse gives a 12mm tapping drill for M14. Although I normally go larger.

I don’t think they are left handed taps, but I will check, could be a good explanation for the problem!

Andrew.

[HopperParticipant @hopper]

Yes, sounds like duff taps. 12mm is recommended tap drill on my chart and near enough using the ISO formula of thread depth = .541 x pitch, on which the OD minus pitch rule of thumb is roughly based.

Easy check is drill the hole then poke the taper tap into the hole. It should go in to right about where the first notch in the tap is. That first feint notch is the root of the thread. It should ideally just enter the drilled hole and that is all.

(If you have one of those drill gauges with all the marked holes in it, its a lazy way of picking a tapping size drill without looking up charts etc.)

Have a look at the tap with a magnifying glass, and compare it with a known good one. Maybe the flutes were not ground at the right angle, or the teeth were not backed off for clearance. (Or as you say, maybe LH thread??)

And measure the tap to make sure it is 14mm and not a mislabelled larger tap. It happens. And try screwing the tap into a 14mm nut if you have one.

[Michael GilliganParticipant @michaelgilligan61133]

A handy tabulation [just one of many]

https://www.carbidedepot.com/formulas-tap-metric.htm

MichaelG.

[Andrew TinsleyParticipant @andrewtinsley63637]

Well Jason has hit it on the head! The taps are left handed ones! The Presto box makes no mention of this, so I must assume that error occurred at the factory. Just checked the taps for cutting ability and as usual they are excellent.

I suppose I should have noticed the problem, after all the pitch is 2mm, didn’t even cross my mind that the taps could be left handed. That is what I like about this forum, there is usually someone alert enough to suss out the real problem!

Thanks, everyone,

Andrew.

[HOWARDTParticipant @howardt]

I have always started with metric thread diameter minus pitch to give the tapping drill size.  While this mostly works, smaller diameters in harder materials will benefit from slightly bigger diameter.  On larger threads always run through the typical three tap set, smaller threads will benifit also but sometimes the lead on the tap is too much for the drill depth.  Essentially you must make sure you have started the thread square to the hole otherwise it may become tight or strip the thread.

[HopperParticipant @hopper]

All you have to do now is make some 14mm LH thread bolts!

[Andrew Tinsley]

On Andrew Tinsley Said:

… I think Dave must be suffering from the heat, as just about any list of tapping drills for Metric coarse gives a 12mm tapping drill for M14. …

Can’t blame the heat as I’m in the West Country where it’s cooler!

Maybe gremlins temporarily changed the page in my book: just checked again, and of course it says 12mm.   More likely, being uncomfortably unwell at the moment is doing my head in.

Sorry for the confusion.

Dave

 

[Andrew TinsleyParticipant @andrewtinsley63637]

No problem Dave. I am suffering from severe sinus problems so I know how you feel. If I had been normal, I might well have spotted the left hand tap problem, but I didn’t!

 

Andrew.

[JasonBModerator @jasonb]

Sounds more like too much west country cider😜

[larry phelan 1Participant @larryphelan1]

Far be it from me to barge in where Angles fear to thread [excuse the pun ], but as it happens, I was asked recently to make some sleeves with a 14mm thread, and checking my Zeus  book, it gives a drill size of 12mm. I simply started the thread in the lathe, then finished it with a tap wrench, since the machine is not strong enough to handle taps that size. Worked fine, so drill size was not your problem.

The thing is, if you Wanted left hand taps, you might have difficulty getting them ! So easy to be caught out, but the chances of this happening are slim, but then, the Titanic only sank once !

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