I have spent 2 days fixing my milling machine.
The PC that I used to control it stopped working; the floppy drive had failed and none of my 6 spare floppy drives worked, so I guss it was a motherboard problem.
I found another old PC to run the software on. Fortunately, I found the source code for my PC control ( "Heeksatrol" ) which I wrote in about 2001. So I was able to modify it.
I took the opportunity to automatically turn off the spindle when the tool needed changing, or the program had finished running.
I looked at the relays and tranformers in the machine, but decided to simply add a RC servo to physically turn the spindle switch. I made my program output on one of the unused parallel port pins and connected an Arduino with the servo attached ( source code here ).
While calibrating the speeds, I blew some of the machine's power transistors, so I had to replace them.
Anyway, now my machine cuts metal and turns the spindle off, so I know when to jump up and change the tool. :)