Snapping taps

[AjohnwParticipant @ajohnw51620]

I don't think there is all that much theory in this area. What standards have done is to establish fits that in some cases in my view are worse than they need be. The fact that things generally produce holes larger than their diameter seems to have gone – pure "shaft" based tolerancing. So an M6 tap for instance is highly unlikely to be exactly 6mm dia it will be over and a matching die will be under. Standards tend to do this sort of thing – or parts manufacturers tend to make use of any tolerance that is quoted and want and get limits when the standards are established which will always make them looser than they think they need be to ease manufacturing problems.

On thread pitches BSW was aimed at ductile materials – wrought iron mainly. Cast iron too for different reasons. BSF was added later for steels. UN is pretty similar but forgets the difficulties of having sharp corners on cutting tools. Threads are often rolled these days as well.

When I mentioned metric fine to an instructor he more or less exploded and said even metric coarse is too fine really but what do you expect from a bunch of watchmakers, they are not engineers. Bit biased really but I understand they did make use of imperial threads in the past. For some reason even the extended range doesn't offer coarser pitches but some table may do some where or the other. He was probably concerned about mild steel.

I built a kit car once John. It called for lots of self tappers. I used trilobal, wonderful things but do need a min thickness of metal to hold really tight. Metric would be good for that.

John

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[John FieldingParticipant @johnfielding34086]

A classic example involved the Brussels Parliament deeming that only metric threads would be legal tender. This caused a huge argument and much taking of sides. The NATO standards for all radio frequency connectors all use imperial threads. When NATO sat down and worked out how much it would cost to replace every rf connector and mating connector it ran into billions of $. The fact that no equivalent metric thread connectors existed and would have to be designed and certified apparently was unknown to the Brussels politicians! Even the Russian equivalent rf connectors are direct copies of the NATO pattern and was due to the fact that during WW2 a lot of American and British radio and radar equipment was given to the Russians. As they became the accepted pattern throughout the world there was no sane reason to change.

Aircraft bolts etc are usually imperial for the simple reason they are stronger.

The very first metric fastener system used in Britain was the BA, introduced in 1884, which is a true metric system, but due to the average mechanical engineer not being au fait with the metric system they were specified in imperial dimensions. The largest BA thread is the 0BA being 6mm in diameter and a thread pitch of 1mm, the same as a metric coarse M6 today. The difference was the metric thread angle is 60-degrees whereas the BA is 47 1/2-degrees. BA threads were based on a Swiss pattern adopted earlier by instrument makers where small diameter fasteners were needed.

[John FieldingParticipant @johnfielding34086]

I bought a Taiwanese mill/drill machine a while back, about 25 years actually. It has metric dials and it is claimed to be a metric machine. Well, it isn't metric at all.

Every single screw is imperial including the lead screws are 10tpi. One rev of the handle is supposed to give 2.5mm movement. It gives 2.54mm ie 0.1 inch. Thinking something was odd I measured the lead screw pitch and sure enough it is exactly 10tpi.

The table locking screws are UNC and the others are Whitworth.  Actually a good choice as metric threads in cast iron isn't a good idea as the pitch and thread depth is not suitable for these applications.

 

 

 

Edited By John Fielding on 25/03/2016 08:07:43

[john duffus 1Participant @johnduffus1]

I see lots here about how to use taps but as far as I can see nothing about removing taps that are broken in a job. Years ago in M.E. there was a mention in a letter that one should try ALUM. I was presented with a job with a broken 4BA HSS tap & submerged the offending part in a saturated solution of ALUM- bit of heat now & then & bingo- the tap more or less crumbled to a black dust after about 3 days. Give it time & a bit of heat- it works. The job was in mild steel & had no more than a grey finish on it. I was told by a chemist who used the same pub that there was likely something like a nickel-iron cell formed- if memory serves!

[David ColwillParticipant @davidcolwill19261]

If anyone wants to guarantee not to break a tap, this is the way forward.

Some years ago I had a job where I needed to tap 1100 m3 holes through 15mm steel bushes with an ID of 6mm. since I was very tight for time and figured I was going to break taps I ordered 5 sets of Lyndon M3 taps.

The holes were drilled 2.5mm, I used a battery drill and did the whole lot with 1 tap.

Regards.

David.

[MWParticipant @mw27036]

Posted by john duffus 1 on 02/04/2016 00:50:51:

I see lots here about how to use taps but as far as I can see nothing about removing taps that are broken in a job. Years ago in M.E. there was a mention in a letter that one should try ALUM. I was presented with a job with a broken 4BA HSS tap & submerged the offending part in a saturated solution of ALUM- bit of heat now & then & bingo- the tap more or less crumbled to a black dust after about 3 days. Give it time & a bit of heat- it works. The job was in mild steel & had no more than a grey finish on it. I was told by a chemist who used the same pub that there was likely something like a nickel-iron cell formed- if memory serves!

That sounds very interesting, John, i'd like to know where you could buy that if you don't mind me asking?

I've tapped a few holes since writing this and i'm pretty sure it was because the starter drill i was using was 0.1mm too small, doesnt sound much but makes a bit of a difference.

Michael W

[peak4Participant @peak4]

Posted by Michael Walters on 02/04/2016 11:09:18:

Posted by john duffus 1 on 02/04/2016 00:50:51:

I see lots here about how to use taps but as far as I can see nothing about removing taps that are broken in a job. Years ago in M.E. there was a mention in a letter that one should try ALUM. I was presented with a job with a broken 4BA HSS tap & submerged the offending part in a saturated solution of ALUM- bit of heat now & then & bingo- the tap more or less crumbled to a black dust after about 3 days. Give it time & a bit of heat- it works. The job was in mild steel & had no more than a grey finish on it. I was told by a chemist who used the same pub that there was likely something like a nickel-iron cell formed- if memory serves!

That sounds very interesting, John, i'd like to know where you could buy that if you don't mind me asking?

I've tapped a few holes since writing this and i'm pretty sure it was because the starter drill i was using was 0.1mm too small, doesnt sound much but makes a bit of a difference.

Michael W

Here You Go, alternatively try your local Indian/Asian supermarket for Fatakdi powder.

[MWParticipant @mw27036]

Thanks Peak, i presume by a solution he means just mixed with water? I find it perplexing how it just disintegrated the HSS after 3 days of being submerged in this powder and then heating it til it crumbles. Amazing.

Michael W

[Ian S CParticipant @iansc]

The Alum, or it may be found as Aluminium Sulfate is used in a saturated solution, and works best warm. aluminium Sulfate may be found in (usually) 1kg bags in the garden shop. the stuff you get in the Super Market is more refined, and dearer, and the Chemist will have even more refined and sterilized medical grade.

John Duffus 1, yes I think the process is electrolytic, it works fastest in brass, and I have found it a bit slower in aluminium. Wouldn't it be nice if there was a similar method of getting broken taps out of ferrous metals.

Ian S C

[mark costello 1Participant @markcostello1]

It also is sold in the canning supplies of the local grocers. I tried it once and it would not dissolve the tap I had after keeping it warm (or hotter) on a wood stove for a week. I had heard it would not work on some taps and I guess I had one of them.

[KWILParticipant @kwil]

Micheal W,

The action is electrolytic, two dissimilar metals forming a cell. If you cannot help with added heat, it still works but a lot slower. Removed a broken tap from a copper boiler took days at ambient temperature and covered it with cling film to reduce the water evaporation, black powder removed from time to time with a "pick" and finally blown out with an air line.

[MWParticipant @mw27036]

Posted by KWIL on 02/04/2016 19:54:22:

Micheal W,

The action is electrolytic, two dissimilar metals forming a cell. If you cannot help with added heat, it still works but a lot slower. Removed a broken tap from a copper boiler took days at ambient temperature and covered it with cling film to reduce the water evaporation, black powder removed from time to time with a "pick" and finally blown out with an air line.

Sounds like a black art to me but i'll give it a shot one day.

Michael W

[Anna 1Participant @anna1]

Hello Michael.

Re your original question regarding broken taps, Can I ask were all the taps from the same manufacturer?

The reason I ask.is that I have recently tapped over 100, 5BA blind holes ( with reccomended drill size) in cast iron with the same set of carbon taps from a British manufacturer and the taps are still very usable.

I subsequently bought some 5BA HSS grnd taps for another job from the same manufacturer and they didn't seem sharp at all in fact were virtually unusable

Do you think you may have bought a duff Batch.?

Kind regards

Anna

[MWParticipant @mw27036]

Hi Anna,

It is a good point as they were not all from the same manufacturer, i do have some taps where the grooves along the pitch look as though theyve been smoothed over and not sharp, they get tight very quickly and i'm wary of using them.

I have changed from using HSS to M4 carbon taps from tracy's and haven't had a problem since, the shanks of some small HSS taps look so thin for how brittle a material HSS can be when the neck is too thin, this rule is often obeyed with turning tools, you wouldn't grind a neck too thin for a parting tool so why do it for a tap? it's not enough to support the forces involved. So from now on i generally go 0.1 over the stated size for tapping if i'm using those particular taps, the moment it starts to snag, i back the whole thing out and get another one.

Michael W

[mechman48Participant @mechman48]

Posted by peak4 on 02/04/2016 11:34:28:

Posted by Michael Walters on 02/04/2016 11:09:18:

Posted by john duffus 1 on 02/04/2016 00:50:51:

I see lots here about how to use taps but as far as I can see nothing about removing taps that are broken in a job. Years ago in M.E. there was a mention in a letter that one should try ALUM. I was presented with a job with a broken 4BA HSS tap & submerged the offending part in a saturated solution of ALUM- bit of heat now & then & bingo- the tap more or less crumbled to a black dust after about 3 days. Give it time & a bit of heat- it works. The job was in mild steel & had no more than a grey finish on it. I was told by a chemist who used the same pub that there was likely something like a nickel-iron cell formed- if memory serves!

That sounds very interesting, John, i'd like to know where you could buy that if you don't mind me asking?

I've tapped a few holes since writing this and i'm pretty sure it was because the starter drill i was using was 0.1mm too small, doesnt sound much but makes a bit of a difference.

Michael W

Here You Go, alternatively try your local Indian/Asian supermarket for Fatakdi powder.

Having a loose adjustment screw on one of my Dickson tool holders I decided to clean out the thread & tap a deeper thread, I used a set of 5 mm HSS taps to start the process as there only seemed to be aprrox 3 threads of engagement so tapping deeper was a logical progression.

I only managed to get a turn on the 2nd tap when guess what ! yeah there is a broken tap in the hole, admittedly it''s not the first time I've broken a tap but it's annoying when its an expensive HSS one. I poured a few drops of some citric acid pickling solution into the hole & left it overnight, this morning it looks as though there is some reaction with the tap as it has started turning black, question is; has any one used this method of removing broken taps?. I don't have access to an EDM machine nor have the right size solid carbide drill ( expensive ).

Having looked back through previous posts I came upon the above article describing the use of Alum so have ordered a sachet ( 100 gms ) via link & will try that, so the next questions are..

What ration of alum powder to water is best ?.

How often does it need to be heated ?

Has /does Citric acid work as well ?

Any other information would be very much appreciated.

TIA

George.

[JasonBModerator @jasonb]

Alum is only really suitable for when the tap is stuck in non ferrous metal as it does not know the difference between a steel tap and a steel tool holder so will attack both. Same with most of the other acids

Toolholders are hardened so that is what is the likely cause of breakage

[Brian HutchingsParticipant @brianhutchings]

I can vouch for the efficacy of Alum solution to remove broken taps. Even when the tap is in steel the solution works to loosen the broken tap so that the core of it falls out, there is no need to disolve it completely but it may be necessary to retap to a larger size if the thread is at all critical. In copper or brass there are no such worries.

Brian

[Andrew TinsleyParticipant @andrewtinsley63637]

I am probably an exception to the rule. The taps I have broken have always been HSS. I have never broken a carbon steel one (asking for trouble next time I use one!).

Like Anna, I find that the carbon steel variety are sharper and cut more easily. My HSS taps are of reputable manufacture and most of my carbon steel ones are from Tracy Tools.

Andrew.

[Nick Clarke 3Participant @nickclarke3]

This what Citric acid does to steel and as Jason has noted it can't tell the difference between toolholder and tap.

[Bob StevensonParticipant @bobstevenson13909]

To remove the tap from the tool holder;……use a small stone on a Dremel to level up the broken tap then drill down gthe centre with a carbide drill,…..then slightly larger bits until just the outside of the tap is left in gthe hole then carefully use a scriber to break the remaining 'shell' of the tap and remove bits from hole.

[Roderick JenkinsParticipant @roderickjenkins93242]

One advantage of carbon steel taps over HSS is that they it can be softened by heating up the whole thing to say 400C (hotter than "blue" but not as hot as "cherry red" . The offending tap can then be drilled out.

Having said that, I have great confidence in spiral flute taps which I now use confidently down to M2. The evacuation of the swarf makes them much less likely to sieze

Rod

Edited By Roderick Jenkins on 21/04/2021 15:33:19

[mechman48Participant @mechman48]

Thanks for the tip Bob, have done this method waaaaay back in the past & remember seeing the carbide drill glowing , have also used carbide tap removal drills as in pic..

**LINK**

**LINK**.

Both exhorbitant price; for the price of one of the Walton extractors I can buy a new toollholder. Thanks again.

George.

[old martParticipant @oldmart]

There is no substitute for having a number of different taps in the same size. Unfortunately this is too expensive for most people. I have some Densimet which is a heavy sintered tungsten alloy. It is useful for making heavy, stiff holders for carbide inserts. It can be turned and milled, and even threaded for holding the inserts. I had to tap some 2.5mm threads in it, so I bought a couple of Europa tools green band taps. They are premium quality, but I had great difficulty tapping the densimet. There are several types of tap available in any size, and I chose the wrong grade. An old unidentified tap from the box of 2.5's tapped the densimet much better.

[bernard towersParticipant @bernardtowers37738]

I’m with Rod spiral point or spiral flute both types have front geometry that is different to conventional 1 2. 3 taps. The angles at the front of the tap are such that the cutting load is greatly reduced, that’s why industry uses them ( less breakages)

[mechman48Participant @mechman48]

Posted by Bob Stevenson on 21/04/2021 15:30:08:

To remove the tap from the tool holder;……use a small stone on a Dremel to level up the broken tap then drill down gthe centre with a carbide drill,…..then slightly larger bits until just the outside of the tap is left in gthe hole then carefully use a scriber to break the remaining 'shell' of the tap and remove bits from hole.

Probably the best way to go, have drilled out studs etc in similar fashion. I do have spiral flute /spiral point taps but didn't want to risk snapping these, as said it's probably 'cause the holder is case hardened & I was a bit over exhuberant with the tap, one of those instances when you know it's going to happen but seem powerless to stop giving it that 'bit extra' & all I wanted was to put a couple of extra threads into the holder.

I'll let the citric acid do it's stuff untill I get the Alum then try that; more a case of lets see what happens, it's not that I 'need' the holder as I've got 18 in all with various tools set up.

George.

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