While updating the mjmech code-base to interoperate with the moteus servos, I simultaneously was updating it to use C++17.  C++ rarely removes or deprecates features, but one of those which actually was removed in C++17, after being deprecated in C++11, was auto_ptr.  unique_ptr is strictly superior in all regards, and so there is no real reason not to switch.

However, mjmech depends upon a large amount of third party software.  Amazingly, only one package actually was broken by this removal of auto_ptr, log4cpp.  It actually saw some updates for C++17 compliance back in 2017, but otherwise hasn’t been updated since then.  I went ahead and forked it, and got it compiling with clang in C++17 mode at least:

After completing one gearbox, I needed to build at least 4 more of them to replace the lateral servos on Super Mega Microbot (2).  So, I got to work.  First, I disassembled 5 more BE8108 motors.

Then, I drilled out the rotors, this time using the mill at AA.

Next I removed the stators from their backing.  This was painful enough last time, that I tried a new technique using the mill to do most of the work.  Unfortunately, one of the stators was critically damaged during my initial experimentation.  So, now down to 4 survivors.

dThe original chassis I had built for the brushless servo quadruped was designed around the aluminum bracket that was used in the Titan XOAR 6008 leg.  For a quadruped that uses the BE8108 gearbox for the lateral mechanism, a new attachment mechanism was necessary.  While I was at it, I made some other improvements as well.

1. The battery is switched to use an off the shelf cordless power tool battery.
2. The chassis is a shell, where most wiring can be run inside, including the IMU junction board.
3. Dedicated inserts are in place on the top for a suspension fixture to be attached.

This was once again a record for the longest print I’ve made on my Prusa mk3s, 31 hours and change.

After finally getting the darned thing apart, and printing a new outer housing, I went about re-assembling the whole mechanism.  This time, I tried to take care to make the future disassembly less painful.

To start with, I filed down the problematic outer bearing interfaces of the sun gear holder so that the bearings were a slip fit over them.  These two interfaces don’t need to be particularly snug, so that was easy enough, if monotonous, to accomplish.  I also machined out a some pockets around the magnet hole, to make it possible to just hot-glue the position magnet in place and more easily extract it.

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.

When I initially created rpi_bazel, I set it up to use a host provided clang.  I decided to update that so that the rpi_bazel rules themselves download a binary version of an arbitrary clang.  This lets you decouple the version of clang from what is available in any given Linux distribution and improves the reproducibility of builds, since you are no longer dependent on whatever PPA you used to grab clang from.

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: