DC motor spindle + tachometer

Plan of action. I ordered the motor and the controller online at the surplus center. The motor weight is very similar to the 1/8HP stock motor that came with the mill, eliminating the need to do any magic with springs / gas struts / counterweights, or whatever else people sometimes need to do to keep the weight on the z axis reasonable. The size was also similar, so it actually fit in my enclosure (a tight fit, but a fit nevertheless). And I could use the same mounting plate and belt that I used with the original motor. Of course, it wasn't quite a drop-in replacement. First, I needed to adapt the mounting plate a little because I could not drill the holes in the motor in the right places to match the plate. Also I needed to add a spacer between the motor and the plate. In addition to that, the motor had a fairly large shaft (.675", I believe). I did not want to bore out the pulley that came with the 1/8 HP motor, because I was not entirely sure that my conversion idea was not fundamentally flawed, and would not fail for one reason or another. So, I needed to make a custom pulley. On the one hand, that's a lot of extra work; on the other, making something from scratch gives you a lot more freedom. I wanted to make the pulley that is just large enough to run the spindle at 10500 RPM when the motor is spinning as fast as it can go.

DC motor testing. But first I wanted to see if the surplus treadmill motor spins at all, and whether it would work with the controller I bought. It turns out that yes, it does spin, but no, it does not spin like I'd expect it to. After powering up it revs up quickly and then coasts back down, followed by a jolt of energy to rev up again, and coast back down, rev up, slow down... Very peculiar. If I started the motor from zero and slowly increased the speed, it would keep running smoothly, but I only needed to touch the shaft to disturb the delicate balance and it started misbehaving again. Obviously, the compensation that is built into the controller to keep the motor running at a constant speed was working too well. It might have worked perfectly if the motor was loaded, but when the motor is not loaded, that "IR compensation" feature overcompensates all the time. I ended up having to turn it off before the motor started running like I'd expect it to. Oh, well... no huge loss, as far as I am concerned. If somehow the RPM sags too much on heavy cuts, I can always adjust it by hand.

Tachometer circuit. With the motor running, I was happy to start designing the new pulley. But how large should it be? Good question! In order to calculate that I needed to know the maximum RPM of the motor. The maximum RPM was printed on the side, but that's either for a 130V or 95V. I was not sure which one, and I had a 90 volt supply anyway, so I needed to perform a small experiment. I do not have a tachometer, but I was planning to eventually add a spindle RPM sensor and use mach2 software to measure RPM anyway. So this was a good place to start. I bought an OH360U Hall Effect Sensor from Digikey and a small .2" diameter, .061" thick 10K+ gauss neodymium magnet from Radioshack (the packaging says 1/8" magnet, but neither the diameter, nor the width are anywhere close to 1/8", so I am not sure what they are talking about). An old audio connector and some hot glue were used to make a rigid mounting tab that holds the sensor in place. I used a regular phone wire to connect all that back to the controller box. I needed only 3 wires, so you can see that the 4th, green, is dangling unused in the pictures. The 5V from the power supply inside my controller box is a few hundredths ov a volt higher than the 5V I measured on the parallel port. I did not want to exceed the voltage of the parallel port, so I used a couple of resistors to make sure that the voltage stays within the acceptable range.

Actual Motor RPM measurement. Now that the sensor was ready, I had to measure the true RPM of the motor. In order to do that I made a small wooden plate with two holes - one in the center, a little bit smaller than the motor shaft diameter, and the other - close to the edge, just large enough to press-fit a small magnet. After pressing the magnet into the plate and mounting the plate on the motor, I placed the assembly into my relatively bullet-proof mill enclosure. I ran the motor at maximum RPM for a few seconds, accelerating and decelerating several times to make sure the magnet does not fly away. I did not want to find out if loose items flying off of a 6K RPM motor hurt. I think they do. Everything seemed sturdy. I set the motor at low RPM and brought the hall effect sensor close to the spinning magnet. Hooray! I could see mach2 counting RPM, and the value was pretty close to what I had guessed visually. I increased the RPM to the maximum and recorded what mach2 showed. I repeated that several times. All measurements were within 100 RPM of each other. Nice! Now we know the true maximum speed of the motor! I can't remember the exact number anymore, but it was around 6K RPM.

Custom pulley. Knowing the maximum RPM allowed me to calculate the pulley ratio needed to drive the mill at 10.5K RPM. I wanted to use the longest groove possible on the spindle pulley. And yet I also wanted to be able to use the original original motor mounting plate and belt. If I chose a groove that was too large, my pulley would have needed to be large as well, and the two pulleys might not even fit inside the tiny v-belt. On the other hand, if I chose a groove that was too short, there would not have been as much surface contact between the pulley groove and the belt as I wanted, reducing maximum power transfer. It looked like I could use the third 'fastest' groove, but after doing the math I found out that the pulley would need to be larger than what I could turn on my taig lathe. Oh, well... I guess, we'll have to settle for the second groove and a smaller pulley.

Turning the pulley was straight-forward. I used a compound slide set at 30 degree angle, and ground a custom bit with smaller than 60 degree angle at the end. I can never seem to avoid chatter if I have a large surface of the tool touching the workpiece, so I cut the groove with the tip of the tool in 10 small steps instead of one large one. As an added benefit, I do not need to grind the tool to the perfect 60 degree angle - anything smaller than that will do. And I am pretty bad at grinding to start with. Doing the geometry and calculating all the moves to cut the groove was a little tedious, but after meticulously following all the steps, I had it! One side of the groove was now at the perfect 30 degree angle with shining smooth surface. The other had 10 tiny edges on the surface. I removed those edges with a piece of sandpaper while the pulley was still in the chuck. I had to grind the odd-shaped tool you can see in the photo because the pulley was larger then the largest diameter I could swing over the compound slide.

After making the groove I drilled and bored the center hole without removing the pulley from the chuck. That should ensure good cocentricity. Finally I flipped the half-finished pylley to the other side and machined the part of the pulley that was originally in the chuck. This left me with enough material on the pulley to add 2 more smaller grooves later, if needed. Finally I drilled and tapped a hole for a setscrew.

The other (spindle) pulley also needed a little modification. I drilled a shallow blind 0.200" hole in it for the magnet. When mounting the spindle pulley back on the spindle I heated the pulley with a gas torch, and I think I might have overheated it a little. The magnet appears to be a lot weaker than it was during my earlier tests. I guess, neodymium magnets are very sensitive to heat. Oh, well... lesson learned. But it still works well enough.

Connecting it all together. The last thing left was to make a small bracket that will hold the hall effect sensor close to the spinning magnet. I needed to keep the sensor close to the magnet because the magnet appeared to have lost some of its strength due to overheating. Yet I needed to make sure that the sensor does not collide with the magnet. I think I achieved that with a very crude bracket made from a piece of aluminum angle and a couple of cap screws.

Testing. First I checked the RPM range with no load. My new range is between 200 RPM and 10700 RPM (I must've made a small error when calculating pulley ratios, so I did not get exactly 10500 RPM). To test my new setup I took a 0.1" deep cut in 7075 Al with a 1/4" 2 flute endmill at 4300 RPM and 8.6 IPM. The indicated RPM almost did not drop at all. Nice! I could not do that before. How about 0.2" deep? That worked well too. I am not sure that my setup was rigid enough to take that deep a cut, so I had some excessive vibration, but the spindle showed no signs of slowing down. How about increasing RPM to 10K RPM and running it at 20 IPM? A little loud, a bit more vibration than I would have wanted, but it did cut with confidence. Nice! Now I am probably more limited by how rigid my mill is than by the power of my spindle. I like that.