Retrofitting a 1978 CNC mill with a modern PC based control


After looking the mill over, the only thing I could find that was broken was a piece of the Aluminum housing on the Y axis stepper motor. This looks like a weak point in the design and the heavy duty liquid-tight conduit that goes to the motor is probably the culprit for the break. Maybe one rainy day I'll pull it all apart and weld the piece back into place.


I also noticed that the Y axis motor coupler retaining ring was missing so I took everything apart to investigate. It looks like the rubber spider is OK, but it is a strange design that I hadn't seen before. It actually wraps around the outside of both couplers as opposed to being held captive in the center of the couplers like most I have seen. A hose clamp was temporarily used to keep the spider in place by the previous owner, so I'll keep that in place until I can find the right retaining ring.


While I was in there, I decided to pull the stepper motor and see if my Arcsin controller could drive it. I was amazed to find that the motor worked quite well while under control of the Arcsin. Yes, that is a big stepper motor! I showed this in comparison to a Nema 23 200 oz/in motor that I usually work with on my CNC machines. These are 8 amp motors when run in Unipolar mode and I'm driving them with 3 amps in Bipolar mode. Motors driven in unipolar mode deliver roughly 40% less torque compared to bipolar mode but I am also using less than half the current on the Arcsin. So, I would expect to see 50% to 60% of the motor torque that these things were designed to run at.


Yep, that is my little ol' Arcsin 3 amp driver moving the ways around on a 3000 lb mill!


After playing with all the axes for a while with the Arcsin. I decided that I had better get this mill up and running if I was going to do something useful with it. So, I opened up the electrical box and started yanking wires out. The box was amazingly clean and I was surprised at the craftsmanship that went into this mill. It is even more amazing to think of all the man-hours that went into building this thing and to think that I paid less than the price of an Ipod for it!


I don't have a schematic, so I'm doing a lot of guessing here, but it looks like the original motor control switch was replaced with a relay-controlled setup. This is so that the spindle motor could be started from the CNC program or the Bandit control rather than by reaching over to turn the motor control knob.


I'm also seeing what looks like a magnetic motor starter switch and reversing contactor in the box above the electrical box. I'm not sure how this thing is wired just yet, but I will figure it out. I can't think of a reason I'd use reverse on the spindle motor, but I'll probably wire it up just in case I need it.


I've been carrying around this aluminum control panel box for a couple of years and now I have the perfect use for it. The Arcsin will be mounted in here until I save up the cash to get a set of Geckos. To install the new driver box, I removed the spindle motor and coolant pump switch panel and associated wiring, then routed all of the stepper motor wiring, relay controls, and even another 6 pair of shielded wires from the electrical box to the driver box. To do this, I had to fish the wires inside the base of the machine, and while in there I got a good look at the coolant pump and the interesting sludge inside the column. Probably should clean that out oneday as well.


Under the driver box, you can see the old Bijur mist-coolant reservoir. Eventually, I'll have to yank it off as well and see if I can salvage it. When I pulled off the cap, it looked pretty gunked up, but it might be worth trying to salvage.


Here is a shot of the phase converter schematic that I came up with. This design is a mix of a few designs and is based on a couple of parts that I got off Ebay and a few parts that I had on hand.