Subject: Re: Homebrew Drum Computer From: Gordon JC Pearce <gordonjcp at gjcp.net> Date: Fri, 14 Dec 2007 10:01:09 +0000 To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org> On Thursday 13 December 2007 07:48:00 Robert Nansel wrote: That's actually pretty fast, and anything you stick to the drum will need to be stuck very firmly! By comparison, a washing machine in fast spin goes at roughly a quarter of this speed, and a typical car engine is reaching its "red line" at around 6000rpm... You would need an extremely good workshop to machine up a drum that would stay in balance at this sort of speed. It's actually the sort of thing that you might want to farm out to a specialist machine shop. I would be inclined to make the drum out of a bit of thick-walled aluminium tubing, carefully bored to remove any imperfections from the inside that might affect the balance. A couple of aluminium end caps would take a thickish steel axle and the bearings (which would be tricky in themselves - they'd need to be sealed to prevent grease being thrown out, or something capable of running fairly dry at high speed). Then you'd skim the drum to ensure it was perfectly concentric - any runout would very quickly destroy the bearings at 6000rpm. Some motors use little sealed ball races. I'd consider looking at teeny-tiny taper roller bearings like car wheel bearings, so that you could use a shim to set the preload very accurately to remove any play. I can probably provide a scan of a car gearbox manual that shows *exactly* how to do this bit. You'd probably need to balance it after painting or otherwise coating it with some sort of magnetic material. I don't know what you'd use for that. You could probably research what they used in early drives. I am now entering the realms of speculation, but my gut feeling is that you'd need to somehow spray it on and rub back the layers to get a perfectly smooth finish - when it's done it shouldn't quite be hot-rod shiny but it should be very flat and polished. Over to Tony, I think...

Gordon