Roko.ca

(Just a disclaimer, I wrote this late at night, so if may not make perfect sense. I’ll edit it for clarity when I get a chance)

Rewinding a few months back to the design phase of my robot, I had the issue that I wanted to use both radial ball bearings, as well as Teflon/bronze washers thrust bearings in the robot’s leg joints in order to provide as smooth motion as possible. Sounds easy right? I found some 8mm diameter bearings that I got for a really good price, and still but needed to find a large outside diameter Teflon washer with a small inside diameter to fit over the shaft. I couldn’t find such a washer at low volume and low cost. After some thought, I decided that I could get some standard 1/4″ ID washers, with a .750″ OD, which is slightly less than the diameter of the robot’s frame at the joints. The theory being that I could then bore out the inside diameter to 8mm, and have that slip over the bearing which I would then only seat part way into the frame. Sounds easy, right? To do it well, I needed to make a jig.

With the arrival of my new boring head for my Sherline mill today, I figured what better way to learn how to actually use it than by making a jig for boring my washers out. This was a better option than trying to make the bearing seats in the frame without having ever done any real boring before, and risking ruining some parts I’ve already put a lot of time into. I did a few quick tests on some scrap metal, and then got down to business. I cut two pieces of some thin bar stock aluminum, drilled, tapped and screwed them together, and then began to make the center hole, which was to be 8mm. I drilled a rough hole approximately 6mm in diameter into the aluminum, and then put it into the mill’s vice. Using the center finder, I (obviously) found the center of the hole to line it up, and proceeded to bore out the 8mm center hole. After that was done, simply unscrew the two pieces of metal that make up the jig, and in the bottom piece I made a flat bottomed hole 0.750″ diameter, and a bit under 1mm deep to seat the washer in place. The key, I discovered, is to do it slowly and patiently. I ended up cutting the recommended 20mil at a time, then smaller and smaller increments as I got closer to my desired final diameter for the hole.

The end result turned out perfect. The teflon washers are simply seated into the jig, and held in place with the top plate. Since the material is soft, I can use just an exact-o knife to cut away the excess material on the inside, to a perfect 8mm diameter using the jig. For the bronze washers, I chose to re-align the jig in the mill and use the boring head to to bore out the center, a few washers at a time. The reason I’m using both Teflon and bronze is due to the fact that the Teflon washers are approximately 0.63″ thick, and the bronze washers are thin. I use the Teflon as the main thrust bearing washer, and then have the bronze washers to act as shims to make sure the joint is nice and snug, despite any tolerance issues in machining the frame. (I am far from an experienced machinist, so several parts of the robot are designed such that I can make mistakes and can recover from them instead of having to rebuild complicated parts)

[caption id="" align="alignright" width="150" caption="Perfect Fit of Bearing assembly"][/caption]After seeing how well the boring head worked out on the jig, I decided to try it out to bearing seat for the leg joints. After carefully measuring with the center finder, I began to make the 8mm diameter x 3mm deep cut to seat the bearing into the frame. It came out surprisingly well, and aligned concentrically with the 3mm reamed shaft/pilot hole I had to start with. I can run a shaft through both bearing and 3mm hole without binding. Now I only have another 23 bearing seats to go… On that note, the rest of the parts have been water-jet cut, and should arrive later this week. A lot of work goes from turning the water-jet cut roughs into final parts, but progress is definitely being made. I have also gotten some work done on the motor controllers in the past little while, but still need to finish up the design revision before sending out to get more PCBs made. When those get back, I have to build them and finish programming them. If all goes well, I’m hoping to have the robot workign with a very basic walking gate in a few months. Then the fun begins with experimenting with more complicated control algorithms, although I have a feeling progress will slow down as the weather turns nicer and the diving season kicks back into high gear.