I bought a pair of KBWT-210 controllers online. The best price at the time was through
www.walkeremd.comNote that the KBWT-210 is rated for up to 10 amps continuous output (at 220vac supply) with a limit of 17 amps. The maximum current output and DC voltage output must be set to avoid over-driving the motor and damaging it. This is just about the heaviest duty controller I could find. PWM controllers for big DC motors just aren't that common, I guess. They make a smaller model (KBWT-26 for 220v supply or KBWT-16 for 110v supply) that is rated 6amps continuous, 10 amps max, but my motors are rated 6.7 amps.
Setting the voltage is easy -- simply set the speed control to max, measure the motor's shaft speed with a tach and adjust the max voltage trim pot from a minimal setting upwards until the shaft speed meets the rated speed. Setting the max current requires a DC ammeter rated as much as the motor. Fortunately, I have an active clamp-on that measures DC current, but most meters will require an in-line connection. Because the overload shut-down function will kill the power if you exceed the setpoint, you can't simply lock the rotor and wind up the current trim pot like with an SCR controller. You have to load down the spinning motor while measuring the current. An assistant would be a big asset here!
Other features include IR compensation (monitors armature current to maintain regulated speed under load), field outputs, adjustable accel and decel, auxilliary stop contacts (I'm eventually going to wire limit switches to those), computer- or potentiometer speed control and selectable inhibit function (prevents accidental start-up on power-up until pot is rotated to 0). There's an overload shut-down if current exceeds the max setpoint and finally the output is fused.
These things are
big. Too big to fit in place of the original controller. About 7" square and 3" tall, IIRC. Each. Most of that is heat-sink. (The smaller versions have smaller heat-sinks, so they are not as wide) I built a new enclosure for them. The re-wiring was planned anyway so I can now use the lathe motor as a power-feed when milling.
Control is fairly precise. I can set the lathe to turn at 1 rpm (with the belt in low range). My mill spindle maxes out at >5,000 rpm (with the belt in high range). I have the accel set to ramp up the speed somewhat gradually, so there is a delay when adjusting the speed. I have the decel set to 0 so it shuts down as quickly as possible.
I think I mentioned the motors run cooler, smoother and quieter. By quiet, I mean you can converse in a normal voice on the telephone while the lathe is spinning at full speed. Scary quiet. They also have vastly improved low-speed torque -- now I can actually use that huge swing!
The downside is they aren't cheap. And as I said in the earlier post, I think the torque the 1.5hp motor can develop is probably more than the mill can really handle. I now have the mill motor current limit trimmed down to 6 amps to limit the torque.
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) and find I can get very good results provided I take care to ensure a rigid set-up and appropriate cuts. For example, cutting a 1/2" slot in 316 with an end-mill, I would limit each roughing pass to ~0.030" - 0.040", undersizing the slot by .005" on each side before taking the final .005" or .010" of depth at full width. I also try to minimize the quill extension by using a tilt-table or riser block to support short pieces. This adds considerably to the surface finish quality. 
