Thread: How to cut a BSPT tapered tread?

Do a google search,let your fingers be your friend and teacher. Parallel.

According to the Drill & Tap charts I have, you just use the same drill size as BSP.

What's interesting is the sizes on one chart differ a bit from another.
One chart says 15.25 mm while the other says 14.25 or 14.75 mm.

it doesn't really matter as its the tap that makes the taper and one drill size will allow the tap to penetrate more or less than the other.

Certainly not stupids questions, i've often wondered the same thing. I have only ever started with a parallel hole but not sure it that's the correct technique.

Be interested to see what others say. I recently wanted to cut a male 3/8 NPT thread on the lathe but I assumed I would need a taper turning attachment to do this.

Simon

For tapping bspt or npt holes you can start with a parallel hole, it just makes it a little bit harder to thread. I have found that some of the tapping size charts can be a bit on the small side for your hole. I'll check my black book on Monday when i am back at work and check what size works. For making the male thread I would pre turn the taper on the bar, then I would roughly screw cut the thread and finish it with a die. roughing the thread will make finishing with a die a lot easier especially on a tapered thread. I hope this helps.

Just for interests sake, the taper on a BSP thread is 1/32" per inch.

Phil

If you are cutting the thread with the typical round die, then you would just cut as per usual..

I have to get a set of small BSPT dies, but at the moment since I do not have any but I do have a lot of BSPT chasers I cut the threads in the lathe like this...

Prepare the part to the threaded, then I put one of the thread chasers in the tool holder, with the side square to the work. Then I set the lathe for threading..

I thread witht he leadscrew like I was single pointing... It works OK inthe smaller sizes in brass material.... You have to be quick in that you have to retract preferably before or at the same time you disengage the half nuts (if you thread like that) otherwise bad things happen.... reversing the lathe is a bit less risky, but who wants to be dull and not take risks..

As the thread chaser is tapered, it cuts the taper automatically..