Well, it's been a little over a year since Hexabot was completed, and I've finally gotten around to making this Instructable which features work done only a few months after Hexabot's completion.
The robot got a lot of use by enthusiastic riders in a few months (among them was my young cousin who drove it back and forth for hours when I brought it home over Christmas break), and this led to discovery of a few design flaws.
This project was funded directly out of my pocket, so in the interest of saving money I skimped on a few things. One of those things was not adding bushings to any of the rotating joints. It soon became apparent that the main drive linkages weren't going to last long without them. This Instructable is going to show you what you need to do repair them.
Step 1: The Damage Done
The drive linkages are machined are made from aluminum, and the drive axles are steel bolts. Steel on aluminum contact is pretty high friction, which combined with the bolt thread's small contact area lead to some heavy wear on the aluminum.
Also, the steel legs rubbing on the drive bolt did some hefty damage to the threads. The steel on steel contact isn't that much of a concern to me at this time, so I'll only be focusing on the aluminum-steel interaction.
Step 2: What You'll Need
To make this repair you'll need:
2 flanged bronze bushings for 3/4" shaft diameter, 1inch length under flange, 1 inch outer diamater (I used McMaster-Carr part # 6338K434)
2 grade 5 3/4-10 x 5 inch long hex bolts (McMaster-Carr part # 91247A855)
Drill press, or mill
1" drill bit
Step 3: Hole Enlargement
To make sure the bushing stays permanently seated, we want to drill the hole to achieve an interference (or press) fit with the bushing. The bushing's diameter is 1 inch with diameter tolerance of +0.003" to +0.004". According to this webpage +0.002" is the maximum interference for a press fit. So, it'll be a bit of a tight squeeze with the 1 inch drilled hole, but should still be manageable (use a large arbor press if you can!).
This whole repair is pretty simple. Disconnect the legs on Hexabot so you can remove the drive linkages. Clamp the linkage securely on your drilling device of choice and align the worn drive hole with the drill spindle. Chuck up the bit , and enlarge the hole.
Step 4: Press Fitting the Bushing
Once the hole is drilled, you can press the bushing in with an arbor press. Chamfering the hole a little can make it a little easier to guide the bushing into position.
Please note that the side of the linkage the flange is on is important. When the linkage is reattached to the robot, you want the bushing flange facing outwards. See the other pictures on this step for further clarification.
Step 5: Reinstallation
Once you've added a bushing to each linkage, they can be reinstalled. With the flange on the bushing facing outwards, you'll only need to add 2 washers as opposed to the 3 that were there previously.
Reattach all the legs and linkages, tighten locknut onto the drive bolt, and enjoy the smoother ride of your hexabot!