For aesthetic reasons I fixated on being able to get the arms to fold up in the "parked" position as seen at the beginning and end of that video. Since almost all hobby servos only travel around 180 degrees, and ones that do travel further are more expensive, less powerful, and slower, I needed a 1::2 rotation step up; and, to maintain the sense of servo position it needs to be a timing belt or gear. Since timing belts are even harder to come-by, gears were the choice. Using a gear linkage also isolates the weight of the driven arms from the bearings in the motors themselves. It also allows the motors to be used as sorta-counter-weights, being mounted on the opposite side of the rotational bearing from the main weight of the arm. At this writing it remains to be seen how this all plays out, but here's a picture of the mechanism:
The arms are made from two different thicknesses of plexiglas, as can be somewhat discerned from the linkage photo. The lowest arm is .170" and the others are .120". In retrospect, for rigidity, I might use the thicker material for the second lowest arm as well.
The motors each have different 'vertical' spacing from the mounts to where a gear might be conveniently centered on the spline. In all cases I used the little rubber bushings that come with each motor for spacing and isolation. In addition to the bushings the HS-645MG motor needs about .250" more space, which is conveniently just about two thicknesses of the .120" plastic. The HS-755HB only needed an extra .180", so I cut a spacer from .060" material. The HS-225BB was just about right with only it's bushings mounted on .120" arm. These spacers also allow for a stronger motor and bushing mount.
All the motors are attached using #4-40 bolts, and it turns out that standard 3/8" and 1/2" lengths are nearly perfect -- for once. I found that one needs to be fairly careful with hole sizes when press fitting and bolting the various shafts, bushings, and bolts. So I used appropriate reamers and taps to make the holes work -- as cut they all seemed to be a bit undersized. Also note that the 645's will need to have some of their rubber bushings trimmed to clear the E-ring that binds, and the 755 needs to have a bit of the mounting bracket bored out as well, to clear the main bushing. These things will become apparent as you mock-up the assembly.
I've changed the design of the fixed (small, black) gear mounting slightly so I'll just describe the idea. The gear needs a spacer to the arm on which it is attached, and .120" is just about right. I cut some rings that -- should have -- fit over the gear flange while remaining clear of the teeth, with the idea that the ring would be glued to the gear (thank god for Goop!). If you try to copy this mechanism you will need to fiddle with the sizes to get it right, a close, but not tight fit is needed. Once the ring is set in place the excess gear flange needs to be cut away, so in the final assembly the gear and spacer are flush with the mounting arm, and the axle goes all the way through arm and backing plate. BTW, I used the backing plates to strengthen the axle mounting area and to locate two pins that (hopefully) will tie the whole room together. Those pins are not shown in any of the photos. They are small brass brads that run through backing, arm, and most of the gear. But I'm getting a bit ahead...
For the axle I used 1/4" DOM tube with a 3/16" hole, thinking to minimize weight and that I might run wires though the tube. The tube tends to run larger in O.D., so the bushings and gears need to be reamed out a bit. I think using tube doesn't matter, so you could easily find some nice 1/4" drill rod or something instead. The axle needs to have a notch for the E-ring clip that holds everything together. The clips are about .020" thick, but the thinnest cutting tool I could find was .040" so there's a bit of slop. The notch is around .030" deep. Careful attention needs to be paid to removing flashing from the ends and the notch edges, so the whole axle slides through the bushing cleanly. The bushing is a standard 1/4" I.D, 1/2" long, flange, that presses into a 3/8" hole.
I press fit the gear onto the axle with the bushing and E-clip in place PLUS a .015" (or so) shim between the gear and bushing to maintain some operating clearance. When the gear is positioned correctly you can remove the bushing and shim. If you haven't yet trimmed the gear flange you can put the axle in the lathe and use the clip notching tool to get it all nice.
When everything is ready the, axle can be pressed into the holding arm and backing plate, and the the bushing pressed into the other arm where it belongs -- I think a slight countersink to the receiving side of that mounting hole will help the bushing seat. Once pressed together use some of the "water thin" plexiglas adhesive (Methylene Chloride) to wick into any place where two layers of anything should not move, and clamp lightly until set.
When set, the fixed-gear arm can be drilled for the 'anti-rotation' pins and they can be Gooped in place.
Sounds simple, eh?
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