Motor Maintenance

Cutting the Comm.

Cutting the comm. is the single most important part of motor maintenance you can perform. You can use a comm. stick, eraser, steel wool, or many other various techniques to clean the surface of the comm. but eventually you will need to have it cut. Having the comm. cut at proper intervals, such as every 10-12 runs, can help maximize the life of your motor. That's a very good reason alone, especially if you're using a more expensive motor hand-wound motor. Not having the comm. cut at proper intervals will result in the motor having a dramatically shortened lifespan. In addition to maximizing the life of the motor, cutting the comm. will allow the motor to perform to the best of it's ability.

I use an Integy X-MOD Super Lathe. Instead of the standard ball bearing v-blocks I opted for the optional carbide v-blocks. The carbide v-blocks are now standard instead of an option. Normally bearings are a good thing in the RC world, but over time the ball bearing blocks introduce some play, and therefore harm accuracy when cutting. So most machinists will agree that the carbide blocks are the way to go on a lathe such as this.

If you are just starting out, you'll definitely want to avoid the diamond bit. While the diamond bit may give you a slightly better finish on the comm., most people will never even notice. The other drawbacks are the cost and durability. A diamond bit often cost 3 times what a carbide bit does, and while the diamond bit will last longer than a carbide bit, it can break very easily if you make a mistake while cutting.

You might want to shop around for a lathe, even give your local hobby store a chance. I found out mine could get me the lathe much cheaper than buying it through Integy. I purchased the lathe and the optional carbide blocks for only $99.00. On top of that I avoided shipping charges. It may also benefit you to know that the box this lathe comes in, makes a great carrying case. So if having something to store the lathe in was a concern, you may want to consider saving a few bucks and skip ordering the optional case.

In addition to the lathe, you'll need a few other items. One of those is oil. I prefer 3-in-1 oil, but there are many other items you can use as well such as bearing oil.

You'll also need a motor to run the lathe. The instructions for the Integy lathe suggest a 55-turn motor. Good luck finding one of those. Instead I've found a use for one of my stock Stinger motors I have lying around. You'll find you don't necessarily want high RPM's, and using a 6-cell pack the Stinger turns a little fast. So I broke apart one of my old 6-cell packs and removed two of the cells, and just wrapped the 4 remaining cells together with some electrical tape. This setup works perfectly for me, and also gives me a 4-cell pack that works well for breaking in new motor brushes.

Here is the comm. we'll be cutting in this example. This armature is from my P-94 13x2. Notice the gray area on the comm. where the brushes have been riding. This is what we'll be getting rid of.

Before placing the armature across the v-blocks, you'll want to put a drop or two of oil on each of them.

How the armature is driven by the lathe will vary depending on the brand and model of lathe you purchase. With the this Integy lathe, you use a rubber o-ring that loops around the motor driven shaft and the armature. The easiest way I've found to install this is to first hook the o-ring in the notch around the lower shaft that's driven by the motor first. Then as you're lifting the o-ring up, slide the comm. side of the armature into the o-ring and seat the armature on the v-blocks. Then simply roll the o-ring up onto the armature stack (the part that covers the motor windings) and center it over the notch on the lower shaft.

Now take and put a few drops of oil on the comm. itself. This will help lubricate the comm. and carbide bit while you are cutting. You can drop the oil on the comm. and spread it around it with your fingers, or you can apply it with a small paintbrush. Either method works fine. It also won't hurt anything to turn the armature by hand to make sure you thoroughly coat it with oil if you find that is necessary.

Another method of lubricating the comm. is to use a permanent marker to "blacken" it. This method is good for those who are new to cutting a comm. as the permanent marker also makes it really easy to see what you are doing when you are cutting the comm.

Either way the lubrication step is important. Failing to use a lubricant will result in premature wear on the carbide cutting bit.

Now attach your battery and lets get ready to start cutting!!!

The first thing you'll need to do is to find the correct cutting depth. Check you bit to make sure it's far enough back that it won't contact the comm. Then move it over the comm. Now turn the lathe on, and very slowly start turning the knob that controls the cutting depth and bring the bit closer to the comm. Keep a close eye on the comm. at the area where the bit is positioned, as pinpointed in the picture below. You're looking for the first faint line of copper to appear. This will tell you when you first make contact with the comm.

Remember keep it slow. If you cut too deep, you'll waste comm. material because you'll have to even that cut all the way across the comm. It's also possible to dull your bit by cutting too deep.

As soon as you see that first trace of a copper line, shut off the lathe leaving everything where it is. You may find, after turning off the lathe, that the cut does not extend all the way around the comm. That's not important. What's important is to catch it when the cut first starts to appear.

Now check the depth knob to see where it is set. This is you initial cutting depth, remember this number. All lathes will be slightly different as to how their depth markings are. On the Integy, you have markings on both the end of the knob and on the side of the knob which is visible at the top of the lathe. These markings correspond to each other. Use both of these locations to your advantage. They'll make setting the lathe much easier.

Next you'll need to set yourself up to make your first horizontal cut. However, before you move the bit, you'll need to back the bit away from the comm. some so it doesn't drag across the comm and dull the bit. Using the cutting depth knob, decrease the cutting depth to give you some room between the bit and the comm. Then move the bit to the right so it's beside the comm. This way you'll start your cut before the beginning of the comm., and you'll cut all of it.

Now reset your cutting depth to what is was in the step above, and you'll be ready to cut.

Turn your lathe back on, and slowly move the bit across the comm. When you get to the side of the comm. where the windings are, make sure to leave a small gap between where you stop and where the windings are secured to the comm. Otherwise you'll snag the bit and damage both the armature and your bit. After making your first pass towards the windings, turn off the lathe and move the bit back past the end of the comm.

Unless you have only run your motor a couple of times you'll probably find you need to cut again, as the areas near the edges of the comm. segments often will not get cut enough on the first pass. If that's the case, increase the cutting depth slightly and make another pass. Remember, easy does it. Make several small cuts, rather than one big one. Big cuts will prematurely wear your cutting bit and waste material on the comm.

This particular armature only had 12 runs on it, and I used 4 very light cuts to restore it.

Once you are done, you will have a nice fresh copper finish all the way around the comm. However, there are a couple of other things you must do before you're ready to reinstall the comm. in the motor can.

Take a hobby knife and using the non-cutting side, clean the grooves between the comm. segments. You want to remove all cutting material out of the grooves. Then take a ballpoint pen and run it along each of the grooves to help taper their edges and to remove burrs. Make sure you use a ballpoint pen with a wide enough ball that it rolls on the edges of the grooves. You want the ball to do the work, not the tip of the pen.

Following that clean the armature good with some motor spray or compressed air to remove any dirt, and also any metal shavings that may have become lodged in the windings.

Our finished armature, awaiting reinstallation.