I rely on the various machines in my workshop and when one has issues I try to resolve it quickly. So when my Schaublin 70 lathe began tripping the house RCD it was time to investigate the problem, as the household management was not impressed.
I’ve had it a while as it came with a business I acquired, manufactured in 1967 it’s well built and very heavy and has done some work but is extremely accurate and fairly fast at 4000 RPM. The motor is 3 phase and large at 3HP, and being an industrial machine it’s built to run for long periods. After doing some investigating it would appear that the motor is at fault, as I fitted a small 3 phase converter rated for the motor, but unfortunately modern converters have issues with older type motors over a period of time.
Original motor in place
So the starting point was to try and replace with a new Schaublin motor, that was quickly dispelled as the cost was eye watering and difficult to obtain.
Old motor removed, not easy as quite heavy and restrictive access in the lathe
I had another motor/controller combination in mind as I’d used one before upgrading a friends lathe, it’s marketed for Myfords and Boxfords. But the benefit is the controller is pre configured to the motor, so a simple fit. The down side is its only 1HP and a slower motor with a smaller drive shaft diameter. As the clutch and belt arrangement is very free running on the lathe I’m not concerned with the power output on the new motor, it’s not going to run continuously so wouldn’t overload it. But achieving a similar top end speed is important, and as a new drive pulley is needed I could make a new one larger to get closer to the old top speed.
Like most of us I’ve collected bits and bobs over the years, and found a un-machined large chuck back plate. Some quick calculations showed that with this machined to the largest diameter this would give speed of around 3600 RPM, which I’m happy with.
Machining the new pulley did take a bit of time as the casting had a thick hard skin, but using a selection of different tools the basic shape came out OK. Boring the shaft to size was straightforward. The pulley is fixed to the shaft with a key and a couple of locking screws, as a don’t have a key way cutting broach a different approach was required. I’ve used the Myford lathe in the past, so I ground up a tool to cut the key way and cutting small amounts using the carriage winding in and out produced it.
Boring to shaft size.
Testing hole with shaft gauge.
Machining the V for the belt.
Cutting Keyway.
Test fit on motor shaft.
The new motor has rubber bushes on a mounting plate, and as the holes on the plate didn’t match those on the machine. I used the machine ones to fix a steel plate to, this in turn had the new motor fixed to it. The new motor is fairly heavy and with limited access inside the machine and I couldn't get it inside to get the fixing position correct, so the motor was removed from the mount plate and the new pulley was used with a jig to establish the position. This was easily achieved and to tension the belt the weight of the motor pulling it self down on the mounting slots fix this.
Mounting plate in using original fixing points.
Jig being made.
Jig fitted to to set motor fixing points.
Motor fitted, just need to fix the wiring into position.
The control unit was fixed to the front of the lathe, and testing found the top end speed to be around what I’d estimated. This has been a good little project that’s got the lathe up and running. And it’s actually quite nice to machine some big lumps, as the majority of my bits are tiny in comparison.
Simon