Here it is! My First Thing! I dunno what it is, but it’s lovely! After fiddling around working out gear ratios for thread cutting, I was desperate to give it a go, and it worked out reasonably well. Conclusion: cutting threads on a lathe might take some practice… who’d have thought?
I got a spare bit of aluminium bar, pre-loved and with holes in awkward places, and lightly skimmed a 27tpi spiral onto it, just to see how it looked. It looked temptingly promising.
Then I turned the bar down a bit, experimenting with feeds and speeds ’til I got a reasonable finish apart from where the holes were, and despite a cutting tool which is, as I understand it, far too pointed to get a very clean finish. I made a 5/8″ diameter collar followed by a 3/8″ diameter bit at the tailstock end. I was supporting the end using a dead centre into the (M6 threaded) hole already in this particualar bit of scrap. I put grease in there to stop it getting too hot, and I noticed that the only chatter I got, despite general ineptitude in the feeds-and-speeds dept. to start with, was when the centre wasn’t quite tight enough.
I started with the 3/8″ section, and set up to put a 16tpi thread on that. I figured that as I could engage the half-nuts anywhere and still pick up the thread, using the same pitch as the leadscrew was a good idea for a first attempt.
All went reasonably well until I allowed myself to be distracted during the rather short run and crashed the tool into the 5/8″ collar behind the thread. I was impressed at the depth of cut this little lathe attempted to take before stalling and blowing its fuse. I was further impressed that, after a scramble around the house for fuses of the same silly size turned up only a 500mA one (the blown one was 3.5A) the lathe nonetheless ran fine without blowing the much smaller fuse. Until I crashed it a second time. Light-duty fuses might be the way to go here, ’til I get the hang of stopping in time.
I tidied up the hideous graunches in the 5/8 bit, making it a bit shorter and more sloping of face than it started out, and whipped the piece off the lathe to try my new thread out in a suitable nut. I didn’t have the foresight to have a 3/8 UNC nut to hand, but I did have the foresight to mark the piece where chuck jaw number 1 was gripping it, just in case. I tried it in the only 3/8 x16 female thread I had handy, that on the underneath of a tripod head. It didn’t fit. Noticably too big. I’d cautiously made the outside diameter pretty much exactly 3/8, possibly a hair more, and the thread was both too large on the outside and too shallow. Back to the lathe, I lined up the chuck mark and, remarkably, it seemed to run true. Fiddled around getting the tool back into the existing thread groove at the right starting point, having to re-seat the tool in the toolholder as the thread was really too close to the end of the bar and clearance of the toolpost against the tailstock was an issue, but managed to resume cutting the same thread, and deepened it to a proper full depth, having lightly skimmed the peaks off to reduce the o/d. Second time around, it fitted the tripod head nicely.
I then had to pack up and drive off to a conference, so I didn’t have time to add further elaboration to my Thing, but, well, there it is, My First Thing. I’ll link to pics big enough to show off the slightly cruddy finish, especially where the tool’s caught the pre-drilled holes.