Last time I covered getting to the point of having the rotor installed into the gearbox. Here we’ll look at making it actually work in that configuration.
When I first got the rotor in place, it was clearly not centered properly. Although much closer than in the plastic gearbox, it did interfere with the stator during a portion of a revolution. The first obvious problem was that the primary shaft wasn’t making it all the way through the front shaft bearing. That should have been an easy fix, but for two different very annoying reasons.
When I made the sun gear holder, one of the tweaks I made during my 3d printed iterations was to leave a hole where you could press on the primary shaft. That was intended to be used to install it all the way into the output bearing. Unfortunately, that hole is behind the position sensing magnet. The one that I superglued in place last time overly eagerly.
That magnet has a recess in the sun gear holder which nearly exactly encloses it, with maybe only a few thousandths around it in all directions. There was no way to grip the magnet at all. I tried soaking the junction in acetone for some time, I tried using the heat gun, and in the process ruined the 3d printed outer housing, but none of those loosened it up enough to allow another magnet to retrieve it.
Eventually I pressed on the primary shaft from the front of the motor, and used that to pop out the position sensing magnet.
Now that I had the magnet off, I could press the shaft back into place. Or at least I should have been able to. I made it into the output bearing, but then the pressing became inordinately difficult. I think even more so than in any of my test fits. This may have had two causes… one is that I was using a dowel for the main shaft which is slightly oversized, and second the retaining compound had seeped around into the interior of the bearing, and fractionally cured. Those combined made it both really hard to push the shaft in, and as I later discovered, impossible to press out.
Aligning the rotor and stator
At this point, I had the rotor installed seemingly properly, and it still was interfering with the stator. After much experimentation, including partially exploding the rotor bearing on the sun gear holder, I arrived at a technique that allowed me to align them properly and simultaneously discovered that my intended mechanism for registering the rotor to the sun gear holder was, at the least, sub-optimal.
The sun gear holder has 4 M3 flat head bolts which fasten it to the rotor. I had oversized those holes to 3.1mm, and put tapered countersinks on each. I had planned on the countersinks forcing the rotor to be centered. However, it didn’t look like that was working as I had expected. Each time I would loosen up the bolts and re-tighten them, the rotor would interfere with the stator in a different way. Eventually, I was able to center them by sliding slips of paper all around the stator between it and the rotor, then tightening the bolts down.
Then I spent a few hours, painstakingly under the microscope, picking out grains of steel that had managed to find their way to the rotor, most likely from when I partially exploded the rotor bearing.
At this point, the rotor did seem to be aligned properly to the stator, everything moved freely, and the gearbox worked as expected. I just need to disassemble everything in order to install a new outer housing to replace the one I destroyed while attempting to remove the position sensing magnet. That is its own story, for next time.