Sigh, practicing a skill I would rather not need

So far 7BA on a steel through hole. The tips have a cross cut in them, all you do is draw a cross over the hole centre and align the tip and work crosses with each other and give it a quick PSST and the busted tap comes out as sparks. The Plasma jet is about 1mm dia and so far the thread has survived untouched.

Managed to complete the second part without any mishaps  The “backing” is 30mm square, so hopefully adequately rigid.

The D bit technique is one I got from James Harvey’s Machine Shop Trade Secrets. It’s a good book of short tips and tricks. I misremembered it slightly - he uses a ball nose, which presumably is a better geometry. He also likes to make the cutters hexagonal rather than round for some reason.

The important bit tho I’m sure is having a rigid setup and pecking a thou at a time:

Mods - if there is any sort of copyright issue then feel free to remove the pictures- I think it’s ok for educational sharing.

I’ve used this truck a couple of times on taps, but also to rescue a watch case where I snapped a tiny carbide drill in 316 stainless. In that case the cutter was bigger and I sleeved the hole, but the D bit did the job, with the tiny drill shattering as the pecking happened.

Dave

Edited By Dave S on 18/04/2022 14:44:00

I've learned quite a lot out of this saga, only sorry that it has to be at the expense of progress. But it does strike me that there is the makings of a magazine article in the art of tap removal/carbide drilling here.

I look forward to it.

Rgds Simon

Thanks Dave. Another possible use for the recently acquired plasma cutter.🙂

I have broken a couple of taps in the last couple of years (none for a long time before that). One grabbed deep in brass (possibly not quite threaded straight enough?) and was dissolved away, with alum. The other (caused by not resetting my electric screwdriver to the appropriate torque setting🙁 ) was bypassed with a fresh hole drilled close by.

Once, I was told that many Taps fail in bending,

So wherever possible use the Tap in a tapping fixture, in a drill chuck in the mill or lathe (Only under power if you are very brave ), supported by a spring loaded support, such as a "Tap Tru", or in guide, held against the workpiece.

If in doubt, the hole can be a tiny bit oversize, (It only decreases the % engagement slightly ) and use plenty of lubricant, and back off frequently to break the swarf into small pieces. Withdrawing frequently to clear the sly can be time well spent..

Lubricant,? Trefolex, Rocol RTD or even bacon fat.

With the tap supported, an suitably sized tap Wrench should allow you to feel how much torque is being applied. If the tap can be felt "springing" it is time to back off a little, clear the swarf, relubricate and try again, more gently.

For the lathe, I made a Tailstock sliding tap holder, similar to a Tailstock sliding Die holder, using ER collets, to hold and support the tap, whilst allowing it to follow the drilling, without any side forces..

HTH

Howard