Monday, September 1, 2014

Keep them spinning

I feel like I'm backsliding on machining. Does not do much for my confidence in finishing that submachine gun...or doing a limited run of Jubal Early's for sale at the RPF.

I've got several orders of solid-body grenades to finish. Those do go faster, but I'm still at almost two hours (with set-up and clean up) each. So I tried a couple of tricks to speed things up, with mixed results.

The lathe has a quick-change tool post and the ability to remember tool offsets. But that doesn't help if you move the post or rotate the cross-slide. So the first trick was to clamp my cutter at a slight angle, meaning I can leave the tool post aligned for parting off and not have to change that.

The next trick was to leave the cross-slide at 15 degrees instead of 45. That meant I didn't have to rotate between cutting the chamfer (and doing the knurl) and the rest of the cuts. It does make it slightly harder to align some of the other cuts, though, and it means I'm bringing the knurling tool in at right angles to the work instead of along the normal of the resulting knurl.

Unfortunately, that put the cutter to where the work was rubbing along the relief. I figured out what was going on soon enough to rescue the piece, though. But I didn't get so lucky with the next one. Cut the first groove at .174 instead of .274, realized the mistake, parted off. Then realized I could chop the front off and re-use it for the other style of grenade. Except I'd parted already. I cut down the rim, and was just able (with a lot of gauge work) chuck it in over the rim and make the cuts. But I was cutting backwards now to make the final cuts. I cleaned up a battered left-handed cutter from the bin, but it still made a poor finish. And I'd slipped up in a couple other places trying to take the measurements backwards from my usual references.

So two hours of clever improvisations and I had a ruined piece of metal anyhow. Well, at least I learned a little. Didn't help me, by the way, that all the way through this another TechShop member was plowing through some really pretty lightsabre hilts. With the cleanest knurling I've seen done in that shop, too.

Well, it helped in one place. Seeing him hog on 4" billets made me rethink my first cuts. The book value for Aluminium with HSS bits is 250-350 surface feet per minute. I'm starting with a 1" billet and coming down to .790, and that means the machine RPM should be in the 9600 range. However; carbide tools can be run at twice the speed of high-speed steel.

Well, I'd been running at 9100. I bumped up to 1250 RPM and it felt pretty good. And I reduced it down to two roughing passes instead of four, taking over a tenth of an inch of metal off with each pass. This does stress the metal a little, but I'm leaving .004" consistently (and adding it to the other cuts), for a single finishing pass. Since these don't have to be fitted into other machinery, only the nominal finished dimension, and having a smooth finish, actually matter.

Unfortunately I've still got several tool changes, and because I can't chuck around the rim, I can't remove the main body until I've made that finishing pass. And the solid-body ones require a flawless cap, so I can't drill those out and hold between centers. But, still, there's probably some way I can make this a little more efficient. Oh, yes, and the tool holders seem to be going astray; there's not even enough to leave all my tools in holders as I work (which kind of negates the usefulness of the quick-change turret).

I got into a conversation with a volunteer at one of my theaters (the one where I'm trying to fix up the sound system), and realized I don't have any of the music I've created for plays on my current laptop. So I spent an afternoon or two fixing that. And listening to a lot of old work. And might post some of that here some day. Yeesh. I've got that new keyboard, and the new library, and I really should hook that up some day...

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