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...