I’ve finally gotten back to working on the Hardinge. This time I’m addressing the feed motor on the carriage. When I hooked it up previously, I was disturbed to find that the motor only ran in one direction despite the direction switch being changed. I checked it out with my multimeters and found that the switch worked fine and was reversing the voltage. I started checking out the motor a discovered that one of the field motor wires had continuity with the motor’s case. Not good.
The motor is a right angle 240V DC motor that moves the carriage and cross slide. This isn’t a motor that would be easy to buy a replacement for. So, I decided to dig into it and see what was wrong.
Since I had continuity between the field coils and case I knew that I was looking for some kind of contact between the two. A field coil is just a continuous piece of coated wire folded into a bunch of loops. When electricity is connected it generates a magnetic field. Once I opened the motor I quickly found a charred area. Here’s a better view of the charred area. As the coil is a continuous wire, any break in the wire means the coil won’t work any more. Electrical contact between the coil and the motor frame will also cause problems as it shorts to ground.
Other than the bad coil the rest of the motor looked fine. So, I started trying to figure out how to fix it. I took the motor around to a couple of motor shops but most didn’t work on motors this small. One did but said they wanted $250 + parts to fix it. Me, being cheap, though about possibly winding the coil myself. Sure, I’ve never wound a coil before but it can’t be that hard right?
First, I took some measurements of the bad coil. The wire was 34 gauge magnet wire and I was able to get a spool off of eBay. I also made some drawings of the coil and thought about how to wind it. I measured the resistance of the other coil and found it to be 944 ohms. I looked up the resistance per length of the wire to figure out how much wire would be in the coil. The result? 3,557 ft of wire. About 0.67 mi or a little over 1 km. Looking at the bad coil I could see that there were an uncountable number of turns in the coil. I did some math on the coil and decided that I needed about 4500 turns in my coil. I planned on doing more turns than that because my winding would be more random than my mathematical model.
To count the turns I picked up a magnetic switch and wrote a Arduino program that would count up every time the switch was closed. I stuck a small magnet to the lathe’s chuck and mounted the magnetic switch near it. I tested it by hand to make sure it worked as planned. Once the bugs were worked out, I turned on the lathe and watched the output on my netbook’s screen. The program steadily output the number of revolutions and current RPM.
Given that many turns I’d definitely want to use the lathe. The bad coil is rigid and lumpy which made it hard to measure with much precision. If the coil were flat it wouldn’t be that hard to wind but this one is curved. I thought about the possibility of winding a coil flat and bending it. I eventually talked myself out of that and tried to figure out how to wind it in place. Luckily, some PVC pipe has about the right size inside and outside radii to match the bend of the coil. With the PVC and some MDF I built a curved form. I quickly found out that I’d have to hand guide the wire or design some kind of mechanism to guide the wire. I started trying to hand guide it and found that the max speed I could achieve was 17 rpm. At that rate I’d be there about 5 hours.
So, I went back to the idea of winding the coil flat and bending it. I made a much simpler form, worked the bugs out, and started winding. Once I got to around 4700 turns I stopped winding, crossed my fingers, and cut the wire to measure the resistance. It was right at the resistance I needed. (In retrospect this wasn’t a good sign as I did 200 extra windings and should have been greatly above the desired resistance.) I pulled the coil off, taped it together in spots, and found it would bend without issues. Next, I tried to fit it in the motor frame and found it wasn’t wide enough. Ok…I just call this one the prototype.
Comparing my prototype to the bad coil I found that I needed to make my new coil about 3/4″ wider. I used the same form design for the prototype and this coil. To aid in bending I made the width of the form increase over the outer 1/2 of the coil thickness. I don’t really know if this made a difference but it seems like it would in my head. The form is in two pieces to enable me to remove the coil. The center hole is for mounting on a bolt and the smaller holes were for string that I placed before winding. After winding, I would carefully remove the top of the former and use the strings to keep the coil together.
I put the form in the lathe and set the lathe’s speed to 90 rpm. I positioned the spool of wire on the carriage and the netbook in a good spot to easily view it. If you look carefully, you can see the magnetic switch (attached via blue tape) and magnet on the chuck. I turned the lathe on and started winding. I found it better to guide the wire with my fingers to even out the bumps in tension and get the wire where I wanted it. I shut the lathe off periodically to check how it was winding. After about an hour, I arrived at 4700 turns and checked the resistance again. It was a lot higher than I needed but that’s ok. I unwound turns off and checked the resistance frequently until I got the resistance I wanted.
Tada! I marked the inside of the coil so I wouldn’t forget which side was supposed to be inside.
I wrapped it completely in tape leaving the ends hanging out. Playing with the prototype coil, I found that it would bend but wouldn’t hold the shape which isn’t surprising. It would stay bent some though.
To see if I could permanently bend it I put into the pieces of PVC from my curved form and left it sitting for a few days. Unfortunately, but not expected, it didn’t hold the shape.
After that I decided to see of I could hold it in place with some spray varnish I bought for insulating coils. From what I’ve read, normally coils are dipped in a varnish to protect them but didn’t want to buy an expensive can for a one time use. I tried the spray instead, which isn’t really a varnish.
I soldered some thicker wire onto the coils ends and tapped them in place.
You may recall that a magnet has two poles: North and South. The magnetic field of a coil also has two poles. The orientation of the poles are determined by the direction that the electricity flows through the coil. Looking from the inside of the coil, my coil is wound counter clockwise though this isn’t critical. More on that in a few. To determine the orientation of my magnetic field, I used a compass and ran 12V DC through the coil. As you can see, the North pole is to the left and the South pole to the right in the pic. To change the direction of the poles, you swap the positive and negative wires. To hook up easily to the existing coil is the reason I wound mine CCW.
The new coil has to have its magnetic field in the same direction as the other coil. To check this, the wires are hooked up to the other coil and the compass direction is noted. I actually, checked the direction of the magnetic field on this coil before winding but I’m showing it here.
I spray varnished my new coil in PVC bender showed above but once again it didn’t hold its shape.
Finally, I decided on epoxying the outside of the coil. Some coils are dipped in epoxy which penetrates the entire coil but the bad coil didn’t appear to be dipped. So, I settled for brushing on some epoxy in my modified form. Plastic wrap was used to keep the coil from sticking to the form.
I ended up with a horrible finish but the coil held its shape! I tied the coil in place like the other coil but didn’t get a picture of it. After that, I replaced the bearings on the armature, trimmed, and soldered the wire before reassembling the motor. Next, I checked the motor over to make sure I had continuity where expected and that there were no shorts to the case.
Finally, I hooked the motor back up to the lathe and gave it a try. It actually works!! I expected it to of course but I still half way expected the magic smoke to come out. I tested it apart from the carriage to make sure it moved and went in both direction. After the successful test, I reattached it to the lathe, filled the oil reservoir in the carriage, and tested it again. The speed and direction worked correctly.
This is a short video of winding the prototype coil:
This is a video of testing the feed motor on the lathe:
Here is a drawing of the form I made. It is constructed from 0.25″ thick MDF and all dimensions are approximate. All edges where wire crosses should be filleted and smoothed. Drill holes as needed to assemble, mount on an arbor, and tie the coil temporarily together to remove from the form.