So, last time I had a functioning gear-train, I just needed to disassemble everything in order to replace the outer housing.  As I was putting things together, I realized that several custom fixtures would likely be needed in order to disassemble various parts cleanly.  Here, I made a giant 3D printed cylinder that the front housing and planet output would bolt to, and then the central shaft could be pressed out.

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.

Shortly after receiving the sun gear holders, I received the first iterations of the planet output and front housing.

Both of these seem to have actually adhered to the tolerances I requested, so thankfully it won’t be too hard to fit everything together.  However, getting everything together for the first time did involve a comedy of errors – a lack of planning for assembly order, a lack of foresight into how things would be *dis-assembled*, a stubbornly stuck shaft, and plenty of broken parts.

As discussed last time, the sun gear holders I had CNC machined unintentionally had a slightly undersized bore that the sun gear was going to fit into.  The allowance was large enough, that there was no way I was going to press it into place as is.  So, I decided to try a shrink fit, but before I did I wanted to do some math to verify that it was possible with the temperatures I could easily achieve and that I wasn’t going to explode (or even just fracture) the aluminum part from over-stressing it.

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:

Now with the post machining operations complete (disassembly, stator, rotor, internal gear), I used all the plastic printed pieces to put together a fit test article.  I wanted to use this to verify that all the pieces would not interfere and would function correctly and had an outside hope that the result would be functional.

One of the post-machining operations required for the BE8108 gearbox was reducing the outer diameter of the internal gear.  Stock internal gears seem to come with a large amount of outer material, likely because they are intended for stationary process control applications.  For a mobile environment, a 100 tooth gear with a 50 mm pitch diameter is somewhat useless when it has 70 mm outer diameter.

I ended up turning this down on a lathe using a custom 3D printed mandrel.

After getting the stator out of its housing, the next step is to do the post-machining on the rotor.  Here, I didn’t want any of the original bearing housing, and just needed to drill out a hole in the middle big enough to put my sun gear holder through.  I figured I would experiment with some custom fixturing, so broke out Fusion 360 and drew up a set of “soft jaws”… i.e. *really* soft jaws, since they would be 3D printed.

After separating the rotor from the stator, next I needed to remove the stator from the aluminum backplate that holds it into place on the rotor assembly.  It looked like the assembly was at least pressed in, but given that there were no fasteners, it wasn’t clear if it had adhesive, or if perhaps it was a shrink fit.

First, I tried heating up the assembly with a hot air gun, hoping that any adhesive would loosen up and possibly the stator itself would expand or break free.  No dice.

To build the 8108 class gearbox with the geartrain inside, first I need to get the motor apart and separate out the individual pieces that I needed.  Here’s the first in a few posts about the modifications.

The original unmodified 8108 motor looks like:

To begin with, I needed to separate the rotor from the stator.  First, there is a small screwed on plate which needs to be removed – it just has normal Phillips head screws.