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Because of its complexity I consider the waist plate/bearing/donut/motor as an assembly. But really it is just a
connection between the structure below and the torso above...
For the torso to rotate there needs to be a bearing between the torso (that does rotate) and the donut (that don't rotate).
Fortunately this is one of the easier parts to get. It is a lazy susan bearing: a Rockler 12451, about 17.5"
in dia. The B9Creations donut is designed to fit and work with it.
Since I am not following the standard design a few minor modifications to the bearing are required to securely connect
the torso to the bearing and then to the donut. The white plastic "feet" are removed, holes in the outer race are drilled
out to the clearance dia of a 10-32 screw (screw pointing down), holes in the inner race are drilled out and tapped for 1/4-20
threads (screw pointing up).
And for the same reason a few spacers are required. A 1/4" high spacer goes under the bearing and a 1/4" spacer
above the bearing. More on this latter. This will give a gap between the torso and donut of about 1/8".
There are a number of different ways to make the torso rotate. On TV they put a person inside. Worked good but not exactly
how I intend to spend my time. Some people use car windshield wiper motors, some have used car starter ring gears (heavy!)
and some have used fabricated plastic gears. All work well.
I decided to do some things slightly different: Unlimited rotation, a motor at the center of the torso/donut and a "Quick
release" ability. And instead of creating parts, like a ring gear, I will use an existing device: a simple DC gear motor.
Searching e-bay found this motor assembly for about $40:
Its a 12 vdc gear motor with a 600:1 reduction with a rotating platform with another gear reduction. With 12 v the platform
takes about 12 sec for a full turn. Just about perfect.
Make a hole in the donut to fit, a hole in the waist plate, its in.
The center of the platform (the part with 4 holes around its edge) is the part that rotates. This will be "connected"
to the torso or lazy susan bearing so that when the motor platform moves the torso rotates. And at some latter stage
of development it will be simple to connect an encoder so the electronics will know which way the torso is pointing.
But there is a problem. It may be that I am being too picky but my machining is off a little and the central motor is
off center of the lazy susan bearing by around 1/32". If this is not corrected there will be either less then smooth torso
rotation or some slop. So to make the connection between the motor and bearing compliant I came up with a simple linkage system
using some rod ends and plexyglass:
To attach the Torso to the bearing a simple design using rubber bushings was
used. The idea is to allow some minor slop between the Torso and the bearing as well as provide some vibration/sound
reduction. First the screws sticking up from the bearing were modified by removing the threads from the middle. This allows
the rubber mounts to be both captive and still move around a little:
The rubber bushings were found at a local Mega home improvement store (Ok, Lowes)
in the special parts bins. They
have 1/2" shank dia, a brass threaded insert (1/4-20 thd) and most important to me, a 1/4" thick lip. Perfect for a spacer
between the bearing and Torso. Having the threads from the screws removed holds the bushings captive and allows the bushings
to float a little. This prevents a nasty tolerance build up developing between the holes in the Torso and the screws.
When its all together (Waist plate, donut, bearing, motor and mounts) it looks like this:
As you can see in the last picture there are 4 toilet flanges bolted to the underside of the Waist plate. These allow
the leg structure to connect to the donut assembly. Very cheap, very strong.
One last detail. To hold the Torso to the rubber mounts a few threaded knobs are used but they don't actually touch
the Torso. When the knobs are tightened down the rubber expands in the holes in the lower Torso flange, Torso locked in place.
The idea is that to seperate the torso just slide open some of the vents, reach in and unscrew the knobs (I will probably
only use 3) and lift off the Torso: Quick release! I may epoxy some suitable washers in place at each hole to make this more
durable.
This was more work then expected but it came out very nice. Its strong, it allows smooth continous rotation and it allows
quick release from the Torso. And supprisingly it does not weigh that much, only about 21.8 lbs. Keeping the weight of any
parts from here on up down is going to be critical to keep the CG low.
So far, even with the extra weight of 2 cookies the torso turns very well. As you can see it takes around 10 seconds
for a complete rotation:
Rotation test
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