Homebrew Drum Computer

[Allison]

>
>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:
>
>> 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...

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

>Gordon