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Why did the tap stick?

Not a water tap, that is

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Chris Gill23/09/2018 19:06:58
43 forum posts
17 photos

Wow, thanks all. As ever, some real food for thought.

Just to respond to one or two - the work was in the lathe chuck and hadn't been moved since drilling and the tap was lined up in the tailstock. Although I said it was a taper I was wrong - it was a general purpose tap (shorter lead). I lubricated with 3-in-1 (always seemed to work in the past) and I always reverse at least once per turn (depends on tap size)

As for scaffolding ... I once had a Landrover serviced and some idiot used the wrong torque on the wheel nuts. I couldn't shift them so I went back and complained. They couldn't shift then even when warmed up with an oxy torch so they sent me to the nearest lorry service place. They simply attached a socket and bar, a long length of scaffolding and then jumped up and swung on it until the nuts came loose.

Neil Wyatt23/09/2018 19:09:35
17729 forum posts
697 photos
77 articles
Posted by JasonB on 23/09/2018 08:19:06:

About 11mm in is where the taper on your taper tap(you did say so) will end and is where the tap starts to take off the most material, try going in with your second tap and then back in with the taper so that it will then have a bit less to cut.

It should not be a problem with a 3/8" tap but if it is a bit worn, gummy steel or you are not that strong the load may be a bit much.

Dare I suggest a serial tap might be a solution to this situation devil


Marcus Bowman23/09/2018 19:14:57
162 forum posts

I tend to think SillyOldDuffer has a point.

Chris stated he thought the steel might be EN8. But suppose it was, say, EN24 or EN24T. All three may well work harden to some extent, and certainly may be sticky enough during tapping to cause some problems. I have used EN24T a little, and have cut sizeable threads using the lathe. It machines fine. But I would not be surprised if it was not so easily tapped. Cutting in a lathe is a rather different action, because the chips are quickly separated from the work, and there is more power applied. When tapping, especially on the tapered section, the same section of thread is being repeatedly cut just a little deeper, so if there is any suspicion of dullness in the tap, there will be much repeated rubbing, at least to some extent, and that may well induce a degree of work hardening. By the time you have the whole of the taper section (typically 6 threads or so) in contact with that rubbed surface, you are bound to have a fair degree of resistance. EN24T comes hardened and tempered, but it is usually further hardened by heat once it has been machined. EN8 is not hardened as supplied, but, again, it is hardened by heat.

SillyOldDuffer's question is interesting, though. How can we prove, or disprove, this?

Marcus Bowman23/09/2018 19:18:53
162 forum posts

Serial taps are a good idea, but I would offer the observation that coated serial taps I have bought recently are definitely not as sharp as uncoated HSS taps. In fact, they cut as though they are slightly dull. One might argue that the coating does dull the cutting edges. So that might give rise to the same problem.

My coated serial taps have been consigned to the wish-I-hadn't-bothered bin. My uncoated serial taps continue to give sterling service.

JasonB23/09/2018 19:36:39
17856 forum posts
1954 photos
1 articles
Posted by Neil Wyatt on 23/09/2018 19:09:35:
Posted by JasonB on 23/09/2018 08:19:06:

About 11mm in is where the taper on your taper tap(you did say so) will end and is where the tap starts to take off the most material, try going in with your second tap and then back in with the taper so that it will then have a bit less to cut.

It should not be a problem with a 3/8" tap but if it is a bit worn, gummy steel or you are not that strong the load may be a bit much.

Dare I suggest a serial tap might be a solution to this situation devil


Or more spinachsmile p

Andrew Johnston23/09/2018 20:05:50
5411 forum posts
627 photos
Posted by Chris Gill on 23/09/2018 19:06:58:

Although I said it was a taper I was wrong - it was a general purpose tap (shorter lead).

Hmmm, therein lies the problem I suspect. Unless the tap is spiral flute or point then a shorter than standard taper just makes things more difficult. Does the set just have one tap per thread size?

For tapping lubrication ideally you need something with high pressure additives, not a light oil.


Nigel McBurney 124/09/2018 09:37:01
699 forum posts
3 photos

Tapping en8 with a good 3/8 unf HSS tap should be an easy one pass job, using lubricant, no need to stop and reverse the tap, unless the tap is not only blunt but worn on the od so that the diameter at the cutting edge is very slightly smaller than the remaining odds making a wedging effect which will jam solid, in recent years tap quality can vary and some have very little clearance on diameter for performance hss ground thread taps are required.Some time ago I bought a new tap which just not cut just jam up, the supplier changed the tap and the replacement worked ok, About 25 years ago I bought a bargain price set of model engineer taps and dies, cheap rubbish, carbon cut thread and some of those will not cut. Gone are the days when any tap with a British manufacturers name on it ,carbon or hss was expected to give a good performance and long life. I recently made over 30 nuts for a full size traction engine ,1.250 ins whit, an old slightly blunt tap but not worn was given a grind on the cutting edged ,used on the lathe to cut these nuts at one pass ,the engine owner was amazed hat I did not have to cut a bit and reverse,I just said if industry cut threads like that engineering would grind to a halt.

IanT24/09/2018 11:10:12
1508 forum posts
142 photos

Chris, invest in a small tin of Trefolex cutting paste (also known as the 'green stuff' ) - it will probably last you forever.

I have decanted a little into a small glass jar. Restaurants will very kindly give you one (with Jam in it) if you order a Cream Tea from them. I dip my taps in it before use and wipe it off afterwards together with any swarf produced. It also helps to save wear if you smear a little on your hacksaw blades before use. I normally use cutting oil when drilling but there are occasions when the green stuff is easier to use.

It does have a particular smell that some folk don't seem to like - but it tends to remind me of Pete & Norm, my very patient instructors from evening classes many years ago. There may be more modern products around today but it was standard issue in those classes, works very well for my uses and it is most definitely very much better than using spit or 3-in-1 oil.



Edited By IanT on 24/09/2018 11:10:37

Tim Stevens24/09/2018 17:57:23
1159 forum posts

Further comment on S.O. D's remarks about small shops finding problems in metal supplies that big ones don't:

1. There is bound to be a fairly rigorous quality system in force at the big workshop (eg Mercedes, Vickers), and their steel orders must run into double figures; this means that suppliers will not be inclined to try it on, or palm-off dodgy steel, lest they lose the whole relationship. Guess who is likely to find the dodgy stuff in his delivery?

2. Many suppliers of large quantities are sniffy about supplying 'about 2 feet of the stuff we had last week' or even '1m of EN36 x 25mm dia in the as drawn condition'. So, Joe Hobby and Fred the Weld are both limited to suppliers who must rely to a certain extent on job-lots, ex-ministry stuff, and even good-looking (reject) stuff from the scrap yard. So, it is not surprising that the big firms find no problems, and that some of the stuff rejected en route to the big boys finds its way into the hands of Joe & Fred.

3. Another guess for you - exactly where in this deal does e-bay (etc) fit?

Not new thoughts, I'm sure - just a reminder.


Nimble24/09/2018 20:30:07
38 forum posts
2 photos

Rather than “not done it yet’s” theory that the hole is not straight could it be that the tap is not quite straight. In that case the tap will eventually bind. My thoughts for a cure would be to re-dill and fit a Helicoil.

Nimble Neil.

Howard Lewis25/09/2018 08:35:07
3149 forum posts
2 photos

F W I W,

Back off every half turn, and use a good tapping lubricant, (Rocol STD, Trefolex etc) every so often, remove the tap and clean with an old toothbrush, and then carry on. If the Tap sticks, and does not want to come out, wriggle it to and fro, but don't use great force, for fear of breaking it.

Failing to do these things cost me a M10 tap!


not done it yet25/09/2018 09:03:58
4505 forum posts
16 photos


Not read my second post?

There are more than a single scenario with these problems.

A crooked hole - pilot drill wanders and subsequent drill follows.

A hole that is belled at the start due to excess pressure on a flimsy base. Tap not started straight.

A hole that is drilled straight but no longer perpendicular due to a flimsy machine where the drill pressure deviates the hole-angle from perpendicular but the tap is presented perpendicular when tapping.

Simple poor starting by hand - not threading down the line of the hole.

Not all taps, held in a tailstock, are straight - with the tail stock quill extended excessively and seriously worn it can result in a thread being started off-line. Lousy practice, but can happen. The tap only needs to start a thread crooked and it will continue on that line. It will not correct itself, once started crooked.

In this thread starter, the OP did not give any useful procedural info - only that the tap stuck part-way through the hole. Nothing unusual in that - it happens so often.

Later, the suggestions are refuted until an apparent suitable suggestion fits the bill. Surprising that the taper tap was mistaken for a another, after so much use over the years, and not checked/noticed when the problem occured. Might have happened here, might not. An 8.6mm drill might not cut an 8.6mm hole. Even an 8.7mm drill might only cut an 8.6mm hole (was the diameter verified?).

I might have even questioned whether the set of taps were really only thread chasers, but I did not. One threading, with an unused tap from a set that has otherwise apparently been brilliant over the last umpteen years, seems out of place. That the OP had not attempted even a second threading, before posting, was a bit mystifying as that would be my first check, had I had this problem.

Analysing the whole thread just makes me wonder....

Ian S C25/09/2018 11:55:23
7468 forum posts
230 photos

Out of interest, I had a look on line about tapping drills for 3/8" UNF, the sizes ranged from 8,7 mm to 9.2 for steel and stainless steel, another site said T, 23/64", and U. Another site just said 5/16".

Ian S C

Chris Gill25/09/2018 23:07:10
43 forum posts
17 photos

Sorry if I didn't reply to some of the suggestions - I had a couple of days away. Not a nice jaunt though - crammed into a corner trying to do a decent job of plastering around a load pipes without sticking my foot in it or plastering myself!

As always, something I thought was a simple job turns out to have many more aspects to it and I'm really grateful for the wealth of knowledge people are willing to share.

I used a light oil as a lube so that it wouldn't gum up but I never thought about how it handles pressure. OK, lesson learned, I'll add something better to my shopping list.

Given my uncertainty about the tap, I've got a decent set on the shopping list as well. I only had the one but all the others have worked well for years.

I drilled the hole with 8 then 8.6 (Dormer precision drill) and later 8.7 so I'd expect the hole to be very close to 8.6 / 8.7. I don't think my tap was off-axis but I'm perfectly capable of fouling it up! I've seen a drunken thread before, courtesy of yours truly.


Ian S C26/09/2018 12:25:53
7468 forum posts
230 photos

Chris, you could safely go up to 9 mm for steel, some say 9.2 mm for more difficult steels, that's about 50%.

Ian S C

XD 35126/09/2018 14:25:19
1419 forum posts
1 photos

Sounds like a dodgy tap then .

Chris , a quick tip with drilling if i may - don't drill with so many fine steps . I pick a pilot drill that is a little larger than the chisel piont on the finishing drill bit i wish to use and drill a pilot hole , i then drill with the finishing size drill .

If you drill with fine steps - 0.1 mm in your case you will wear out drill bits faster as the load and heat is concentrated at the ends of the cutting surface and not over the entire length of them . You also run the risk of the hole drifting off centre as each time you drill out a hole the drill bit can wander a little .

I would have drilled that hole with a 4mm bit first then gone right to 8.7mm and with anything under 1/2 or 13mm i do in two steps , if it were a big hole say 25 mm or bigger i do three steps - 4mm , 8mm then 25 mm etc.

I use neat Soluable oil for tapping and machining , if not that then tap magic of if desperate trefolex ( horrible smelly stuff ) The company that makes trefolex also do a product called CTD ( cutting , tapping ,drilling ) it is a spray on sort of thing that works ok and on alloy wd40 or similar - i never use motor oil or the like as it is not designed to work efficiently as a cutting fluid or tapping fluid / compound .

John McNamara26/09/2018 16:10:47
1311 forum posts
113 photos


Hi All

I am working on a new CNC Mill made from Mineral castings, (10-12% Epoxy and Graded aggregate).

It requires about 250 steel inserts of various types.

Many are tapped M6 and M8. I broke an M8 tap today, annoying, I don't break taps very often. The material is the problem it appears to have hard spots. Because of the number I used the lathe to power tap the thread, it has clutch reverse so reversing out is not a problem. Had I not already machined the inserts I would have used leaded steel. It was too late for that. A 30mm thread is required for design reasons not strength so I decided to ream the holes out to 7mm yes this oversize for M8 it should be 6.7mm. The result was no more broken taps and clean threads.

I will do a write up of the mill separately



Rail mount to be cast in place. Note cams to position rail prior to drilling. They will also be left in position.

insert and rail.jpg

Edited By John McNamara on 26/09/2018 16:17:09

Neil Wyatt26/09/2018 17:29:17
17729 forum posts
697 photos
77 articles

John, it would be excellent to see the whole build written up for MEW.

Do email me if you are interested.



Andrew Johnston26/09/2018 18:42:37
5411 forum posts
627 photos
Posted by John McNamara on 26/09/2018 16:10:47:

A 30mm thread is required for design reasons not strength so I decided to ream the holes out to 7mm yes this oversize for M8 it should be 6.7mm. The result was no more broken taps and clean threads.

No wonder you're breaking taps, 6.7mm is far too small. For M8 I'd drill 7mm, or even 7.1mm, to start with.


Howard Lewis26/09/2018 19:32:40
3149 forum posts
2 photos

Using the rule of thumb for Metric Coarse, Tapping drill = (Nominal Dia - Pitch) The smallest tapping drill should be 6.75 (* - 1.25). So 6.8 would be the smallest. Ordinary oil is not really suitable for the pressures involved in tapping.

7mm drill will decrease engagement, but not enough to worry about; many folk advise it to avoid just your problem.

But DO use a proper tapping lubricant. Works out a lot cheaper, than buying new taps and the wasted time, effort and material of having to remake the piece when the tap breaks in the last holes (as it always does).

Since the tapping will throw up a burr, either chamfer, counterbore, or overdrill the start of the hole..


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