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

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

Yup.

Go carbide.

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

Hi 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

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&#8221 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

Sid,

 

Not quite sure why you want a chart for dies. The stress on them is negligible compared with that on taps. Just use nominal diameter. You can use a diameter slightly less than nominal if necessary for very lightly stressed uses, but there is little need normally.

 

It can be helpful to turn a bevel or a short length of reduced diameter to help get the die started. Personally I dislike cutting male threads with dies; I often cut them most of the way by screwcutting and finish with a die to get the correct form.

 

Interestingly, one of the things which gets overlooked when people discuss tapping drill sizes for ISO metric threads is that the tap has a “diameter” (i.e. the smallest circle it will fit in) which is larger than nominal; there is a significant relief space at the bottom of the female thread.

 

David

Posted by David Littlewood on 26/01/2012 16:31:52:

Two reasons (for using lower than nominal part dia):

 

1. For larger tap sizes it considerably reduces the effort (some of us are rather frailer than we were ).

3. For smaller sizes it reduces the likelihood of breaking the part (obviously it’s not the die that’s going to give).

It would, however, be nice to have some idea of how much thread engagement I’ll end up with.