CIM Reduction?

I’ve already purchased an AM Planetary Gearbox, but the FP motor doesn’t seem to get the job done. What does it require to get this to work with a CIM motor. AndyMarks instructions aren’t very detailed. Specifically, I don’t understand this statement in the instuctions "The bore of the Sun Gear of the AM Planetary needs to be reamed to 0.3105-0.3110 for a press fit onto the CIM motor shaft (we do not offer this part) ". Can you help me with this? Thanks alot for the help!

Planetary gears have a ring gear (outside), planet gears, and a sun gear (center). The sun gear must be reamed–that means that you need a reamer and at least a very straight drill press, if not a mill. A drill bit might work, but I’d go with a reamer.

What you’re doing is expanding the hole slightly and smoothing it out. Press fit means that you have to push really hard on the gear to get it onto the shaft of the motor.

AndyMark also sells a planetary gear box for the CIM. From my experience, it’s much easier just to buy the one from AndyMark then it is to make the fittings for the CIM to the FP gearbox. it is, however, kind of expensive(120$).

We actually just did this the other day, so here are some tips. First of all, I would suggest buying a few extra pinion gears, especially if you don’t have a lathe, just in case one gets messed up. To get the press fit to work, your hole needs to be very precise. We used this precision reamer, and the diameter worked well:

McMaster part number: 3002A27

Also, in order to bolt the CIM motor onto the gearbox you need to use the slots in the side of the gearbox. However, these have very little clearance. We bought an L allen key (I think 5/32" but I could be wrong), and cut it down to about .5" to get it in the slots.

Also, once you’ve mounted everything, check the amperage running through the CIM motor mounted to the gearbox. It should be pulling around 3.5 amps, but if it’s pulling more, you should check that you cut down the CIM shaft far enough and that the hole you reamed down on the pinion gear is centered.

This piece needs to have be reamed to 0.3105-0.3110. Then it will fit with the cim.

These are the directions take directly from the website

Modification for CIM Motor input

  1. The bore of the Sun Gear of the AM Planetary needs to be reamed to 0.3105-0.3110 for a press fit onto the CIM motor shaft (we do not offer this part)
  2. The end of the CIM Motor shaft, sticking out past the Sun Gear needs to be cut off
  3. The cap plate on the AM Planetary needs to be flipped over so that the 3 counterbore holes for the #10-32 mounting screws are facing outward.
  4. The cap plate needs to be mounted to the AM Planetary before the CIM Motor is mounted.


If I read this correctly, the OP has already bought the gearbox and wasn’t satisfied with the fischer price, so he’d like to convert it for it to be usable with a cim. Otherwise your right, definately save yourself the time and buy the AM Cim Planetary.

The AM Planetary combined with th FP motor has an output at approximately the same speed, but with reduced torque as a CIM motor. If you just need more power, but same rpm, you should be able to omit FP+Planetary and directly replace it with a CIM.

Perhaps a better description of the mechanism you are trying to power would help.

Thank you everybody for the help. you were all very informative.

I have a mill and I am quite familiar with it. I’ve never used a reamer before, but if it is anything like a drill, then we should be good to go. thanks for the help.

You could also ream it out on the lathe, depending on which one is more accurate. With the lathe your hole should be concentric. Just another option.

Just a note, traditionally you drill to as close as you can get while remaining a few thousandths under size, and then ream. Also, reaming is traditionally done and a much lower speed and feed than drilling.