As seen in my draft plastic assembly, the required alignment between the rotor and stator in the gearbox is relatively tight. The difference in diameter between the inner race of the rotor and the outer surface of the stator is only about 0.2mm, which gives 0.1mm of clearance in normal operating conditions. A plastic drive train was never terribly likely to succeed. My next steps have been to machine the pieces of the gearbox critical to alignment out of aluminum, so as to ensure that the rotor and stator, (and also the gears) are held within some approximation of appropriate tolerances. The path of joints between the rotor and stator looks roughly like this:
The rotor attaches to the sun gear holder. The sun gear fits inside the holder, the shaft fits inside it. The shaft then fits into a bearing (unpictured above) that is inside the planet output. The planet output is then fit inside the output bearing, which fits into the front housing. The stator rests on the front housing. The rest of the components are necessary to transmit torque to the planet gears, or mount the overall motor, but I currently believe they shouldn’t be critical to maintaining rotor and stator alignment.
To verify that this would indeed work, I’ve sent off to have the sun gear holder, the planet output, and the front housing to be machined in 6061 Aluminum. I ended up using Xometry, which just farms the CNC jobs out to shops across the country. This was my first time using them, or having any parts CNC’d for that matter, so I was a little paranoid about getting appropriate drawings in place and trying to get all the tolerances into a suitable state.
In this drive train, there are several joints between parts that matter from a tolerance perspective. The sun gear fitting into the sun gear holder, the shaft bearing that sits in the planet output, the output bearing surface on the planet output, the bearing surface on the front housing, and the stator surface on the front housing. Ideally, each of these would form a relatively rigid connection under load. Thus I wanted to have something close to an interference fit.
The first part I made was the sun gear holder, and its primary interface is with the sun gear. The sun gear has an 8mm diameter, and my micrometer measured one sample at 7.998mm. Looking at Machinery’s handbook 30th edition, there is this chart showing standard tolerances for metric fits.
Then, for an 8mm shaft, the tables list:
|Close Running||Sliding||Locational Clearance||Locational Transition||Locational Interference||Medium Drive|
However, using the quick turn Xometry feature only allows one to specify tolerances in the tightest as +- 0.001″, or +-0.025mm. +- 0.025mm is more than the entire range between the lowest size medium drive hole and the largest locational clearance hole. I also wasn’t exactly sure how rigid things needed to be, and didn’t want to mess with shrink fits or further post-machining operations, so did my initial drawings erring on the side of having too much clearance. I figured I could always use some retaining compound if need be. Thus I specified +0.04 -0.01 for that particular interface.
1 week after submitting CAD and drawings the sun gear holders arrived in the mail!
I also was surprised to receive an inspection report! Xometry claimed that I needed to pay extra to get one, so receiving it was a pleasant surprise. Reading the report, the measurement on the bore was claimed to be 7.99 or 8.00 for all 20 parts. So, I broke out my telescoping gauge and micrometer to measure that critical shaft interface myself to double check.
Lo and behold, smaller than I had specified, and right at the bottom end of a medium drive fit. Sure enough, the sun gear felt like it was going to take a significant amount of force to install. I went through and measured the dimensions on the first 10 parts using the same technique, for each part measuring at 4 different radial angles.
Overall, that is a relatively narrow spread, just +-0.015mm, however it is centered out of the tolerance band I had specified. Granted, I may not have yet mastered my telescoping gauge measuring technique, however, the impossible to manually press in gear seems to validate my results.
Next up, trying to assemble the sun gear into the holder anyways.