Threading increasing OD?

[PerryRT]

OK, so I've been trying to thread a piece of pipe 3/4-10 as part of a hand crank for my 9x30 lathe.

(Problem 1, of course, is that I don't really know what the pipe is made from. It's a scrapyard piece, but by dimensions etc, it appears to be 1/2" nominal schedule 80 steel seamless pipe)

I ran into something interesting that I don't understand, though.

I did what I usually do - I turned the diameter a couple of thou's under (0.747, I think in this case), and set up for threading. 30 degrees, fish tail, blued the OD, all that. I took passes of 0.003 until I was down to a razor's thin line on the bluing then started checking for fit with a nut.

Couldn't get it to fit no matter what I tried. The pitch was right (first thing I checked), the initial entry/champher was ok. I finally tried running a die down it, and couldn't get that to pass either (even with signifigant work.) I checked the pitch diameter with the three wire method and it was ok. Curious, I checked the OD with a mike. It had grown from 0.747/0.748 to 0.756.

Is this steel just really really ductile and being "grown" as I take passes? 8 thou seems a lot for that. Or is there something else going on?

More importantly, how do I fix it?

What I did this time was, knowing that (with the three-wire test) I was at the right pitch diameter, I decided to just shave a few thou off the OD. It didn't really work well - but I could get the nut to fit and eventually made it work. It was a one-time screwon anyway - I locktited the pipe into the handle and figured the extra friction would help it to hold, but I'm curious about the "right" way to fix this.

 

[Tony Wells]

I've often seen the major diameter grow a little on an OD thread, as well as the minor diameter shrink on an ID thread when tapping. Sometimes it's just a bit of a burr, sometimes the material is relieved and does actually swell. If you had an optical comparator you could look at the profile in detail and magnified, and I'd imagine you'd see what would look like a small step on the crest of the thread. I usually hit the tops of the thread with a smooth lathe file to get rid of any burring, and some fine paper or Scotchbrite to smooth it up. As you know you have the PD right, the only thing you can do is reduce the major. If you choose to skim turn it, that's fine, but it will push a small burr over into the thread, and you'll have to deal with that anyway. Back to the file/paper/ScotchBrite.

And yes, you'll see this more often on softer metals, and regular iron pipe is not all that great to thread.

 

[Metalmann]

Like Tony says...pipe is also famous for being squishy steel, hehe! Too soft to get perfect threads.

Just flat file the OD, and you'll be OK.

 

[PerryRT]

Like Tony says...pipe is also famous for being squishy steel, hehe! Too soft to get perfect threads.

Just flat file the OD, and you'll be OK.

Well, it wasn't too bad in the end. After all, it was one of those "only has to fit once" sort of jobs. The interference fit probably helped, honestly. Next time, I buy some decent steel (not that I'll ever do this job again, I suspect.)