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Grinding down / reducing RC Motor shaft diameter - The low-tech Approach

Grinding down / reducing RC Motor shaft diameter - The low-tech Approach

Old 09-17-2014, 05:10 AM
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Default Grinding down / reducing RC Motor shaft diameter - The low-tech Approach

I thought I'd start a little thread for those looking for some practical info and experiences on reducing RC motor shafts with minimal special tools. The info below is not new and has been posted already somewhere in build threads; my aim with this thread is to make it a bit more findable / accessible and I invite others to share their experiences (failures, problems?) here too.

The reason for wanting to reduce motor shaft diameter is to be able to match some less common motors to more commonly available pinion sizes, which basically increases the range of motor choices or prevents one from having to find exotic / expensive pinions. Examples are the Leopard 2845-2860 motor series for minis (4 -> 3.175 mm), Castle's old 1717 Vertigo helo motor (6 -> 5 mm) and the TP4070 or any 56 mm can motors (8 -> 5 mm).

I know some might oppose to using motors that are not necessarily intended for a certain vehicle size or class (too big, powerful, unnecessary, crazy, etc.). This however is not the thread to talk about that. If you don't see the point of reducing shaft diameter to run certain motors, this thread is not intended for you.

As a little word of caution, before anyone starts rushing into ordering a big motor and getting out the good old dremel, it is important you are aware of this:
  • Reducing the shaft diameter of a motor obviously weakens the shaft, making it more likely to break than in its original form - there might have been a valid reason the manufacturer opted for the bigger size shaft...
  • If not done correctly and precisely, grinding down the motor shaft can ruin a perfectly fine motor or cause problems with the gear mesh, leading to other failures / issues.
My Approch
To start off, I would never have tried this myself without the encouragement of a fine Portuguese gentleman called Targetingxmod.

I used no special tools and have no machining skills. All I used was a cheap dremel-tool copy, a cheap (13 bucks) little suction-cup desk vise and a caliper. In advance, I thought of all possible ways and constructions to help grinding straight, as that was my biggest concern, but in the end that got all too complicated or required buying more and expensive tools, so I just did it by hand.
  1. First step is taping-off all the holes to make sure no metal dust is pulled into the motor or bearings by the rotor magnets.
  2. Hook up a battery, ESC and Rx / servo tester so the motor can run, keeping the shaft nice and round.
  3. Then with the motor spinning on low RPM (I just used max throttle trim on my radio, but you can also strap your throttle into the desired position), moving a dremel grinding stone disc (I used those green ones in the pic, took about 5-6 of them) spinning the opposite way, along the shaft back and forth as smooth and straight as possible. I've even seen others do it on smaller Leopard 4 -> 3.175 mm motor shaft reductions with just a grinding stone, no dremel, but wouldn't do that myself for 8 -> 5 mm as that is quite a bit more to chew off.
  4. Stop and measure often to monitor progress and check whether you're grinding straight, keeping shaft diameter uniform; better measure 10 times too many than one too little.
  5. At the end I used one of those sanding drums (also in the pic) and some sand paper to get a smoother effect. The sanding drum I just held with my hands, so not on the dremel. Start this step when the pinion is just about to fit - if you do this when it already fits, it will become too loose.
  6. Use a dremel cutting disc to make a little flat spot for the pinion grub screw.
That's it! Most important thing is to have patience and take your time. I think it took me about an hour or so, with lots of stops to take measurements and change worn out grinding stones. So, was it worth the trouble? Absolutely!

Last edited by Dr_T; 11-29-2014 at 01:04 PM.
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