Why did the tap stick?

[JasonBModerator @jasonb]

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

Neil

Or more spinach

[Anonymous]

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.

Andrew

[Nigel McBurney 1Participant @nigelmcburney1]

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.

[IanTParticipant @iant]

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.

Regards,

IanT

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

[Tim StevensParticipant @timstevens64731]

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.

Tim

[NimbleParticipant @nimble]

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 LewisParticipant @howardlewis46836]

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!

Howard

[not done it yetParticipant @notdoneityet]

Nimble,

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 CParticipant @iansc]

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 GillParticipant @chrisgill22114]

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.

Chris

[Ian S CParticipant @iansc]

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

[I.M. OUTAHEREParticipant @i-m-outahere]

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 McNamaraParticipant @johnmcnamara74883]

 

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

Regards
John

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

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

[Neil WyattModerator @neilwyatt]

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

Do email me if you are interested.

neil.wyatt@mytimemedia.com

Thanks,

Neil

[Anonymous]

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.

Andrew

[Howard LewisParticipant @howardlewis46836]

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..

Howard

[I.M. OUTAHEREParticipant @i-m-outahere]

Chris may not have known about the dia – pitch formula and there are a few duff tapping charts out there that seem to err towards fuller engagement ( 75 % or greater) for some reason . For most of what we do something around 60% is more than adequate and the only time i really worry about anything better than this is for head bolts in alloy castings .

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