I considered 3 options. A large servo motor, a brushless DC motor, VFD with 3 phase motor. I really wanted to find a large servo motor or brushless DC motor that was thin and tall so it would have a similar footprint to the stock motor. I couldn't find one cheaply enough or the right size. There was also very little documentation for controlling some of these motors and the servo drives were expensive too. The reason I didn't want to do the VFD motor is because there are no small 3 phase motors. Most are 56c sized which is frankly pretty massive on the mill. In the end, due to some great ebay deals I bought a 1.5HP 56c motor and a 2HP Emerson Commander SK Vectorless VFD.
There are some large benefits to a Vectorless VFD. They are all designed to take 0-10v speed inputs, have inputs for forward/reverse and most have modbus control so you can fully control them from software. They can deliver all their torque at very low rpms and are almost as cheap as a standard VFD. You don't need any encoder feedback to the VFD. I chose the Commander SK because Tormach picked it for their mills and I got a great deal on one on ebay. I did have to run a 240v circuit to my mill but I have a subpanel in my basement and it was quite easy. I put a heavy duty 240v switch in front of the VFD so I can turn it off when I'm not using it rather than having to flip the breaker.
The motor is a Marathon Blackmax Model Y538. Again, I found a screaming deal on ebay for it. I also would have bought a Marathon MicroMax motor. The Blackmax is an 1800rpm 1.5HP totally enclosed non vented motor that delivers 1000:1 constant torque and is class F rated which means it can be used with VFD's. It has no fan and can technically deliver all it's torque at 2 rpm. It also has a max mechanical speed of 5400rpm. I wanted to go with a 2HP motor but above 1.5HP they only have cast iron frame motors and those weigh 75 lbs or more, I set a limit of 50 lbs of weight because the z axis motor needs to move all this weight up and down.
I hooked up the motor to the spindle using a belt drive based heavily on a fellow named CodeRage. In order to fit a 56c motor and still have a power drawbar you must do the y axis mods and have a head spacer. I made the biggest head spacer that Hoss had plans for (it looks like a tombstone). My belt drive conversion is like all the others. You have a set of mounting plates that sit on the mill head and have a raised section for the motor to mount and a lower section that you'll put the power drawbar assembly on. To fit a 56c motor your mounts will hang off the side of the millhead and extend back past the millhead onto the head spacer.
 |
| Belt Drive mill head mounts with motor and mount in back |
I used GT2 5mm profile belts in a 15mm height. The GT2 profile is a timing belt profile designed for less noise and higher speeds than the standard trapezoidal (L) profile. I bought aluminum pulleys and a belt from sdp-si.com and made heavy use of their pulley center distance calculator. Due to my desire for a 10k rpm spindle speed and the 1000:1 constant torque VFD and motor, I decided to run a 1:2 pulley ratio.
I used 24 and 48 tooth pulleys (6A55-024DF1512 and 6A55-048NF1516) with a 92 tooth belt which was perfect for my 5.44 inch center distance between the motor and the spindle. The smaller pulley goes on the spindle and the larger pulley on the motor.
The smaller 24 tooth pulley is the perfect size for the G0704. Mill off the hub so you have just a pulley. Bore it out to .851 so it will fit over the splined spindle shaft on the G0704 and then bore out a 1" wide hole recessed .10-15 deep in the top so you can have more threads to secure the drawbar tophat to. This also clamps the pulley between the rpm sensor disc on the bottom and the tophat on the top. To secure the pulley to the splined shaft I drilled 3 holes near the bottom of the pulley, 120 degrees apart in the teeth of the pulley. I threaded them 10-32 and used 1/4" set screws. This leaves the head of the setscrew below the tooth of the pulley which is important since the belt is going to cover the holes.
To use the original rpm speed sensor disc you will need to take a dremel (since your mill is probably in pieces for this conversion) and notch the flange where the two bolts that hold the rpm sensor disc are.
I did not have a lathe at the time. I tried to bore the pulley out on the mill but you just can't center it perfectly even with a co-axial indicator. It was a great excuse to move up the purchase of a lathe. I bought a G0602 and the pulley was bored out perfectly on the lathe. If you don't have a lathe I would recommend buying/borrowing one for boring out the pulley.
Another tip for boring out the pulley is to use the V grooves in the 3 jaw chuck on the lathe to clamp the rim/flange of the pulley. This keeps the pulley perpendicular while you're boring it out. To make the tophat I used the lathe and then used thread milling with a single point cutter to make the M20x1 threads that are needed.
 |
| Belt Drive mounts, pulley bored out, tophat on |
 |
| There are no bolts in the front belt drive mounts because I measured wrong and the holes are off a smidge |
 |
| The fancy engraving on the side is because I bought a diamond drag engraver and of course you have to use it once you have it. |
The belt drive conversion works great. I'm very happy with the performance of the VFD and motor. My z axis moves up and down just as fast as before and machining aluminum at 6k rpm is much better.
