Problem Cutting an Internal Thread in a Welded Pipe

I'm trying to cut a thread inside a length of 3/4" i/d mild steel pipe with a mini-lathe. The pipe is exactly the right size for my application but it has been made with a welded seam that's left a ridge running inside the tube.

The weld is very hard and it defeats my threading tool:

I've tried skimming the ridge off with a carbide boring tool but it also bounces over the weld without much effect.

Can anyone tell me how I might overcome the weld line please?

Many thanks,

Dave

Heat pipe to bright red and keep it there for 10 mins and then allow to cool as slowly as possible, this should help. ERW tube is nasty stuff to machine though - very grabby.

Are you sure it's mild steel ie no carbon content?

Try annealing the end where you are trying to thread? Presumably it's the result of the resistance welding process causing hardness judging by the carbide not working. Got a blowlamp? Wouldn't take long to try it and find out.

Sounds as if ERW steel is typically SAE1010 ie 0.1% carbon, so sounds plausible. No experience of it myself mind....

Edited By Muzzer on 07/01/2016 16:52:07

Are you using the biggest possible diameter boring tool? If so it shouldn't bounce over the weld.

Tony

Use a sanding bobbin in a dremel or similar and get rid of the weld first.

I have used John's method and it seems to work, tho' I doubt if NASA would approve.

Frank

Thanks guys! Good ideas AND an excuse to buy a bigger boring bar. I'll try the Dremel approach first as I'm slightly less likely to burn the house down with it.

I'm pretty sure it's mild steel because I bought it from a proper metal supplier - it's not an odd bit of scrap. I didn't know it was called "ERW" though: I'm in danger of becoming educated!

Ta,

Dave

ERW = Electric Resistance Welded.

It's made from a flat strip pulled off a roll and then rolled into a tube. They then stuff a load of volts and amps into it and it welds the seam together

A Dremel may get rid of the ridge but the hardened seam still remains.

JasonB & Muzzer's heat treatment suggestions look like the way to go.

* Danny M *

Grind the ridge off, heat treat, thickest boring bar and cross your fingers. If the job needs strength you may have to re-weld on the outside. I've never had much luck doing it, don't know why because threading the outside is usually easy.

Get a bit of seamless tubing - slightly more expensive maybe but not enough to break teh bank.

I agree seamless tubing is the way to go . It will save you a lot of problems . Heating and cooling the tube might weaken it am not from NASA lol

When I built my second Stirling Engine, I used Water pipe for the cylinder, when I bored the cylinder(no trouble with the weld), I was nearing the final size, then "ping", the weld let go, I clamped the cylinder in the vice, and ran a weld down the outside, finished the bore, the motor still runs, in fact gets better as it ages.

Ian S C

If I was a betting man I would have put money on heat-treating, but 10 minutes of red-heat didn't soften the weld all that much. (It was noticeably easier on the threading tool though. )

Grinding the weld off with the Dremel worked rather better, which suggests to me that it's only the visible weld ridge on this bit of pipe that's particularly hard.

Gordon W's suggestion would have been overkill in my case but I can well believe that his 'belts and braces' approach might be essential on other pipes. I'm learning that, when it comes to machining it, there are often huge differences between similar looking metals even when they have the same name.

Hopper and Martin both make very good points about seamless. Next time that's what I'll ask for. Another disadvantage of heat-treatment is that restoring the finish later is even more work. And weakening the seam of a pipe that's also been undermined by cutting a thread could well be problematic even if I'm not working for NASA!

Thanks again,

Dave

That looks a cracking job. I'll send you the drawings for the Shuttle steering jet nozzles and a length of scaffold pole......