Another broken tap thread

Am over hauling some braking units for my ME society for some passenger trolleys they bought. Basically they are two opposed pistons in a cylinder. This is silver soldered onto a 2" x 2" plate which is drilled with four 6mm holes, one at each corner.

 

These needed mounting onto riser blocks to get them at the right height. So I cut each block, milled square and tidy, then spotted through the 6mm holes and drilled with a 5mm for the core.

 

I have now broken two 6mm taps so two of the holes in four blocks have the broken taps in them. I am sure the other three 6mm bolts will hold each block securely but as they are going back to my club I will feel somewhat ashamed about the standard of my work.

 

I want to get the broken taps out but there is nothing to get hold of. The holes are through,not blind so I can get at either end. All my drills just skate over the broken end and don't bite to start drilling a hole so I could use a stud extractor. The blasted things won't even file flat!!

 

I need help please

 

John

Have done similar!

I managed to put one end of the broken tap against a lump of HSS and hit the other end with another piece which shattered the remains and was able to remove the bits!

Dont forget the safety glasses! Good Luck

 

Alan

What material are you tapping into John? 6 mm taps are not something I would usually "cringe" over when tapping and breaking two in a row suggests that something is a problem. Perhaps material; perhaps dull taps?

 

Tubal Cain (in "Drills Taps & Dies") suggests 5.2 - 5.3 mm as tapping drill to give 75% - 65% thread engagement. 5 mm would give 90% or more according to his charts which is pretty high (although I've seen it listed in other sources) and might be a problem in some situations.

 

(Not much help in getting the broken taps out, I know, but once they're out, you still have to tap the holes.

John, you may be able to 'punch it through' leoosing the teeth on the tap or tearing the thread through the job, either way, I think you might end up with a larger size thread!

 

Alan

John, didn't realise it was that thick!!!

Local engineering shop and ask them to spark erode them out? (keeps the same hole size)

 

Alan

John, Sounds like a decent procedure to me. And I know 5mm is given as a standard recommendation by many sources. I've used it myself in the past although I wasn't particularly comfortable with it (good for industry with sensitive tapping heads but more problematic for hand tapping). Now I tend to follow the TC recommendations for all tapping.

 

I take it you were free-hand tapping (no guidance). I've broken my share of taps in the past (though not generally taps that big) but now I use a guided tap wrench wherever possible (i.e. most of the time) to avoid any side-loads.. Also, I try to go through the full sequence of taps - not just the plug or bottoming. Except short, through holes of course where I can do it all on the taper tap.

 

Probably a lot of what I do is overkill but at least I rarely break a tap these days (I don't tap anything real teensy- weeny either).

 

A few possibilities, some of which have already been mentioned:

 

1) Hit the tap with a centre punch, or similar, to try and turn it such that it comes out of the hole

 

2) Similar to 1, but aim to shatter the remains of the tap

 

3) Weld a rod to the top of the tap, which then gives something to turn it with

 

4) Use a carbide stub drill or ball nose cutter to drill into the tap. The idea is to get the cutter and work red hot, that'll allow you to drill a hole large enough to break up the remains of the tap. Go easy to start with or you'll chip the cutter. You'll also need a rigid setup, use the mill, not a drill press.

 

5) Spark eroder, although I assume that is a non-starter

 

The subject of tapping size drills has been covered recently, with some controversy I might add. Personally I agree with Sid; I use 5.3mm as a normal tapping drill for M6 in aluminium, steel and cast iron. I go up to 5.4mm for stainless steel.

 

Regards,

 

Andrew

Either carbide tipped or cobalt drills will successfully cut into a HSS or carbon steel tap.

 

I prefer the cobalt as they seem to keep their edge better and being solid cobalt, have less tendency to 'lose' the tip, If a carbide tip 'bites' it tends to shatter the tip.

That said, I have had success in the past by reshaping the tip on a standard masonry drill bit and have removed hardened steel bolts.

 

regards

 

CB

Yup.

Go carbide.

I have a nice selection of needle roller bearings in the right "tapping sizes" for broken taps in a thru hole location.

Insert needle roller from back side, cover with rag and whack with BFH. Job requires plenty of rigid support. Don't use the back of a silly 3" vise. Best to use the side of the Costa Concordia whilst it's in a suitable position.

 

If it doesn't come out on the first whack all that proves is that the BFH isn't big enough.

 

Tried carbide drills and end mills but all you do is shag cutting tools worth 5 times more than the job and still have a broken tap and mangled elongated hole.

 

John S.

If you have enough of a stub to work with then select a nut big enough to MIG/MAG weld right through, start welding from the centre directly on the remains of the tap fill up the hole in the nut with weld and then while it's still glowing hot apply a socket and hopefuly unscrew.

 

* Start the weld directly on the remains of the tap ensure a good "bite".

 

* Dont use too big a nut otherwise you may end up welding that onto your "parent" surface.

 

* MIG/MAG ensures the heat is as localised as it can be.

 

I have used this routine for donkeys years and have had good success removing broken screws studs and occasionally taps mainly from alloy motorcycle engines.

 

Good luck you will need it.

 

Regards

 

Paul

Thanks for all the help guys

 

There just isn't anything protruding above the hole to get a grip on or weld something to. Just a tiny jagged point. So I'm going to use the punch out method with a punch from the underside. I've got some more of this plate to put under the workpiece when I get to work with the BFH. If that doesn't work I'll look to buy a 5mm cobalt drill but I'll have to come at it from the underside due to the jagged broken top which skews any drill off centre

 

Having now broken four 6mm taps recently I try to be gentle and thought my method would be OK. Seems that a larger core would have helped my cause a lot.

 

And to answer some of the other points:

 

Drills - my two sets are 1.0 - 13.0mm rising in 0.5mm OR an imperial set rising by 64ths to 1/2"

Tap - HSS first (tapered) tap. Bought in a 3 piece set from ebay supposedly 1st, 2nd and plug but they and a replacement set were all 1st type so I gave up chasing the seller

 

Onward and upward!

 

John

Although I have a 0-10mm drill set rising 0.1mm (plus a collection of larger drills) and the same imperial set as yourself as well as a letter set, I keep sets of tap drills separately in a drilled wooden block. Although I now almost exclusively work in Metric I keep one drill block for metric and one for BA, next job is one for a new metric fine set. This keeps the drills sharp and readily at hand.

 

By the way, Alan Munday (Engineers Utilities) quotes 5.1mm for 0BA. I used his recommended sizes to collect my tap drills and have only ever broken a very small tap (10BA).

 

In your efforts to remove taps, if all else fails, I have heard of this method of removing stuck/broken taps but never tried it so I can't comment on it's effectiveness but here goes. Make a hollow drill (rather like a hole saw) and use it to cut out the section containing the tap on the milling machine. A plug can then be silver soldered or welded in and the surface dressed. A new hole can then be tapped. Hollow drill made from silver steel or carbon tool steel drilled out, end filed to coarse tooth form then hardened and tempered and used with plenty of coolant, slow speed and feed, and frequent withdrawal for chip clearance. Might be worth a try on some scrap first .

 

Best regards

 

Terry

To save a bit of sweat on similar jobs in future, and maybe when finishing the two salvaged holes, drill half way through if you can on a thick bit of steel like that to clearance and only tap half way or less, you can use the enlarged bit to guide the tap. Ian S C

John There many ways of digging out broken taps. But before I begin are your holes ‘through’ or ‘blind holes’?
Method 1 The chemical method. This depends on the fact that hardened steels are more readily attacked by chemicals than mild steel. I generally use this method for taps smaller than 3.2 mm. Get hold of a solution of Ferric Chloride (the stuff used to etch Printed circuit boards). Clean out the cutting oil with isopropanol then drip in the Ferric Chloride. If it is a open ended hole stop up the bottom with something. This method is slow.
Method 2 Make a hollow cutter. As you are using 6mm I would make the hole down the centre 6.32 mm (1/4” with an outside diameter 7.5mm. Now cut out a larger hole. Plug the hole In 15mm I would silver solder in the plug – drill and re-tap-
Method 3 This is similar to method 4 Make a short-ish stub of some tough old steel. Drill a shallow-ish hole in one end say 3/8” deep. This will have the same diameter as the core of the tap. Now file 2, 3 or 4 short prongs (The number you cut will depend on the number of lands in the tap). It is a good idea to make them fit closely into the lands of the tap. They do not want to be more than ¼” long. Try to unscrew the tap.
Method 4 This work on larger sizes of taps 6mm up. Buy or make a tap extractor. You could look on ‘Flea bay’ for them you want a 6mm. I bought a few at a car boot sale – the geezer flogging them did not know what they were and I did not tell him.
Method 5 get yourself a thin rat tailed diamond file (I think AC Euro do them as I bought a set from them) use one to file away part of the core of the tap.
Method 6 The ultimate make a new pad.
The picture shows a selection of tap extractors in the top left is a 3/8” tap in the 3/8”extractor. In the bottom left is one in pieces –it also has a bit missing –the sleeve- I will make one after I have made a round tuit (or bought one)

Oh the smallest is for 1/8" (3,2mm - 5BA)

.
David just looking at them I have figured out how to make them (Not the tuits). Do you want a short article?

 

John

 

With so many broken taps it is obvious that the tapping drill size is too small. 5.2mm which gives a thread depth of 65% is adequate for all but the most demanding requirements. In your case, with the lenght of the hole being tapped being 2-1/2 times its diameter, more than adequate.

 

See here for a list that quotes the thread depth achieved with three adjacent drill sizes. The mid one being the preferred one for almost all cases.

 

Harold