14 Dec
2007
14 Dec
'07
4:07 a.m.
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:
Most of the old Drum machines spun at 1200, 1800, 3125 or maybe a screamigly
fast 3600. Though 1800 is the more common.
Also most used "fast tracks" that had more than one head per track
radially so that a partial rotataion was needed.
A good machine to study is the Minuteman Missle guidence computer. It
was bit serial and used a disk as a "drum" rotating memory.
Allison
Allison
Certainly I could trash a few old cassette decks,
or even get some
floppy R/W heads to experiment with audio recording tape super-glued
to a soda can, but I really want to get at least the performance the
old machines could produce, so that means a reasonably fast drum RPM,
somewhere around 6000 RPM, say.
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