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Retrofitting an old Orac lathe to new CNC control
3:00 PM 4/4/2006











The ORAC lathe was a fantastic piece of British engineering from circa 1983. It's main use was as an education tool used in Universities and Colleges.

When I first saw the Orac, it was in a friend's garage and looked like a grubby little ancient machine. We pulled out all of the boards and original electronics from it and you could definately tell they were from 1983! Thick bakelite connectors, ribbon cables and resistors as thick as pencils! Despite it being a CNC machine I had a pretty strong impression that a lot of the functionality of the device was done in digital electronics rather than on the processor. Hence, tons and tons of logic boards all intrically connected together.

You'll notice in the top picture how the spindle has a disk with several dozen evenly-spaced slots in it. These break a beam of light, thus giving several pulses as the spindle rotates. This is used when creating screw threads as it allows the main axis to be advanced in sync with the spindle. A lot of modern CNC apps simply need 1 pulse per revolution and are able to extrapolate from that, but I highly suspect the reason they had it like this was because they were using a basic counter circuit that would count a (user) set number of pulses before advancing the stepper motor one position. Thus the original Orac probably only had a limited number of "divisional" threading values that could be set. A lot like gearing really - and also imposing a limitation: if the Orac was originally metric, it wouldn't be able to cut english/imperial threads and vice versa, somethich which is not a problem on modern CNC machines...

So the old electronics were a bit dated and sucky: but I soon began to realize that the mechanical and electromagnetic specs on the machine were nothing to be sniffed at. A beefy 3 phase motor (multiple speed, including reversing directions) and ballscrews on both axis, and a very, very nice, well-thought out design. This really surprised me.. I can only begin to guess at what this machine used to cost in it's day. Sadly (or in our case fortunately) these machines were used for little more than making chess pieces out of plastic by disenthused snotty students. :)



Variable speed/direction from a 3-phase motor
10:09 PM 3/30/2007

A simple challenge then - get the spindle motor running. It's a nice 350W 3-phase motor. Now this is not going to run off a domestic power-supply, as that's only single-phase. The ORAC came with some weird built-in phase convertor, but it looks like a field-generator from a TIE-fighter. Some of the capacitors were bloated as well. A TIE-fighter that was in the Empire Strikes Back, then. And there are strange connectors coming off it - maybe they control the shield polarity/diffusion? Maybe they provide some strange feedback to the antiquated controller? These insecurities made me decide right out to just use an off-the-shelf phase-converter. The one I got is a bit old, but still has all the functionality I need. You can perfectly control both the speed, and the direction of the motor. And it doesn't require any TIE-interface circuitry.

I did need to build a little circuit board to mount the potentiometer dial and start/stop switches on though. In time I'll tidy this up, and eventually move it to PC control as well (a speed controller board from Homann Designs allows the PC to control the potentiometer and direction switch).

For now manual works. Nicely.


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