BSP threading

Hi,

I am trying to cut an external 1/4 19 BSP thread. The data book I have states the basic major diameter as 0.518 inches. Is this the diameter I have to turn the part to before starting treading? Or is it more complicated than this?

All comments gratefully received.

Hi George,

Yes it's that simple, as long as you note that this is measured across the rounded crests of the thread.

I think I'm correct in saying it's meant for 17/32" pipe which is 0.531" diameter (the size the threads would be if NOT rounded).

Neil

George, you are correct, O.D. is 0.518 for a parallel thread ,

Depth of cut is 0.6403274 divided by 19 = 0.0337.

Top & bottom thread form radius is 0.1373292 divided by 19 = 0.0072

I guess you are cutting this with a die rather than screw cutting on the lathe.

Dinosaur,

I AM screw cutting on the lathe so is the answer different?

George,

answer is same for screw cutting . To accurately form the top & bottom radi with a hand ground single point tool is a bit problematic. Using a full form carbide tool makes life a lot easier. Most turners would just stone off the hand ground sharp tool tip & round off the top of the thread form tips with a chaser or small file. It is possible to use a tap as a chaser. If you have the 1/4 BSP tap then this is ideal to use as the chaser.

It is permissible to substitute a flat instead of a radius at the top of the threads as this doesn't affect the thread strength to any significant degree.

It would make sense if you haven't had much screwcutting experience to read Martin Cleeve's screwcutting book before starting.

Many thanks for posting this question, I am about to start plumbing up my boiler using 1/4 bsp fittings.

Ta

Mike

It is possible to use a tap as a chaser. If you have the 1/4 BSP tap then this is ideal to use as the chaser.

It is not possible to use a R/H tap as a chaser for an external thread but it is OK for an internal thread. Think about it !.

It is possibe, the helix angles match and you just run the lathe backwards to make sure the cutting edge on the tap is doing the work

Hi there, George,

I'd screw-cut it as a roughing process to get rid of most of the un-required material, and then finish with a die.

Best regards,

Swarf, Mostly!

Many thanks for the comments one and all.