Cutting a taper thread...

[AdeV]

...is it as simple as setting the angle on the topslide, taking a skim (just so the tool doesn't bury itself ass-deep in the metal), then single-pointing as normal?

I am about to cut a thread in some 60mm pipe, and I'm pretty sure I need it to be tapered to fit the elbow fittings I've got...

[Reckless_Engineer]

If you set the angle on the top slide when you engage the lead screw it will still cut straight just with the tool at the wrong angle.

Ive only ever cut taper threads on external stuff using cnc which is easy but i wouldnt know how to do it on a manual machine :scratch:and i always thought the matching internal threads were straight to allow the threads to lock into it but i may be just day dreaming here lol.

[AdeV]

If you set the angle on the top slide when you engage the lead screw it will still cut straight just with the tool at the wrong angle.

Good point, that hadn't occurred to me 

Bugg'rit, I'll just cut 'em straight, I'll make them fit together somehow...

[Lew_Merrick_PE]

Ade -- Taper Turning Attachment.

[Rob.Wilson]

What thread size is it Ade ?

What's it for 

Rob

[AdeV]

TTA (gah, of course!). I still haven't made the hangy-ony-bit for mine yet (oops)...

Rob: 11TPI on a 60.3mm OD pipe. It's going to be an exhaust heat exchanger; once I've wrapped it in (or wrapped into it) a bunch of copper piping...

I've cut the threads straight, and I can screw the pipe about 1/4" into the elbow; going to skim a tiny bit off the top of the outer threads (at a taper) until I can get at least 1/2" screwed in. 

[Jasonb]

Sounds like 2" BSP in which case the OD of the thread should be 59.614 not 60.3mm that may give a better fit.


Pipe (male thread ) is usually cut parallel BSPP and the female fittings tapered BSPT

Jason

[AdeV]

Sounds like 2" BSP in which case the OD of the thread should be 59.614 not 60.3mm that may give a better fit.

Pipe (male thread ) is usually cut parallel BSPP and the female fittings tapered BSPT

Interesting.... The original pipe is clearly tapered (male thread), it's not obvious which the female fitting is, I'd have to get the yardstick out for that.

Anyway... I cut the thread straight, then skimmed a few thou off it to make it "tapered" (sort of). In the pic, I'm about to start skimming:



For this test version, it works a treat. I screwed it into the engine & fired her up (NOISY), the ali got toasty warm in no time at all.... There was no sign of any blow-by around the threads, so - in best West Country voice - praaahper jaahb!.

[Pete.]

Using your fixed steady (or set-over the tailstock) you could have set the tube over a bit to produce a tapered thread. Might have to take measures to stop it walking out of the chuck though.

[Ned Ludd]

Another way to cut tapered threads would be with a hand held thread chaser, an under used bit of kit but useful all the same.  They give a full form thread, that is to say they cut peaks and troughs, and they are good for making fine adjustments to machine cut threads as well.
Thread chasers, for those who don't know them, are the opposite of "single point threading" in that they cut several threads at once. They are available in machine and hand held varieties.
A certain amount of practise is need to cut a completely new thread but if wood turners can do it, so can we.
Ned

[spuddevans]

but if wood turners can do it, so can we.

Speaking as a former wood turner ( and a pretty below average one at that  ) it takes a lot of practice. And it gets exponentially harder as you attempt coarser and coarser threads, try 22tpi and finer to get your eye-hand co-ordination into gear.

Tim

[Ned Ludd]

Hi Tim,
Thread chasing in wood is made much easier with variable speed, the coarser the thread the slower you want the lathe to turn. Coarse threads need very slow speed and they also rely on crumble free wood.
If you just want to clean up threads that you have screw cut then they are much easier to use as they follow a true path.
Ned

[AdeV]

Thread chasers are a sore point with me.... ever since I turned down about a cwt of them - brand spanking new in boxes - for little more than the price of scrap, basically because a) I wasn't sure about spending the money, and b) I didn't think I'd ever find a use for them 

The bloke selling them weighed them in. Probably got £20 for them. Bloody shame. I won't let that happen again, I can tell you.

[bigmini]

Using your fixed steady (or set-over the tailstock) you could have set the tube over a bit to produce a tapered thread. Might have to take measures to stop it walking out of the chuck though.

I did this to make a new manifold from a chunk of aluminium for my compressor. It fits on to the tank using a 1/2"BSP thread. It worked really well, but I drilled it out after cutting the thread. I'm not sure how easy it would be to do this with a pipe.