The Dome Motivator

I've been working on the next construction step for my R2 for far too long. For a while I thought I'd try to make my own "Rockler" bearing and then include the gear drive with it. I worked up a design, but decided that given the material costs that just buying a bearing from Rockler when they were on sale would be cheaper. I purchased my bearing from Rockler [Lazy Susan, Heavy-Duty Swivel - Item # 12451].

I'd seen the COM-8 Instructions on "preparing" a Rockler in several places:
http://astromech.net/forums/showthread.php?21320-anybdy-know-size-of-the-long-screws-that-poke-through-the-rockler-and-up-to-dome  - post #3
http://astromech.net/forums/showthread.php?21272-possible-answer-to-rockler  -  post #3

As well as instructions for removing the Steel balls and replacing them with Acetal balls to make the bearing quieter.
I've been unable to find a dimensioned drawing of the bearings currently being shipped, so I took dimensional measurements from my bearing and used those when I designed the motivator. I made one assumption that turned out to be false. There are two sets of holes in the inner ring. The set that is countersunk (incorrectly) for 10-32 bolts and the set that originally had plastic pads in the holes. My error was in assUme'ing that the holes were concentric with the bearing. Most folks use the 10-32 bolt set to mount their dome and I used that set of holes for the outer gear on the dome motivator. Imagine my surprise when I mounted the bearing and discovered it was not on center!

Here's what I found.


The upper side of the Rockler, with the countersink.

I turned the bearing over and rigged up a marker to draw a line on the bearing. The 10-32 holes are numbered, the larger holes lettered.

note the spacing change in #3, #4 and #6 in comparison to the other holes!

Fortunately, the larger hole set seems to be consistent. 
 
This Rockler has already had it's steel balls swapped out for Acetal ones, but I'm going to remove them again so that I can countersink the holes without fear of contaminating the bearing. I also want to see if the countersink breaks thru into the bearing race (it should not, but who knows?)


Here's a look at the current state of the Dome Motivator. As a result of the problems described above, I improvised and used the 1/4 holes to mount the internal gear on.

Rockler bearing mounted to Ring4, Internal drive gear slid onto 1/4-20 bolts.

Normally the nuts would be holding things together. Here they are providing clearance for the bolt heads since my Rockler has not had it's mounting holes countersunk.

The spur drive gear was temporarily spaced up on the motor shaft using washers as a way to get the spacing correct for it to mate with the internal gear mounted to the Rockler.

The core insert in incomplete, so for this test, we propped it up by running a couple of pieces of plastic between the shoulder rings.

Above: Here's the hole pattern I cut for the motor mount.

Left: The motor that supposed to mount in that position!

And here's why you do a prototype!
I don't know where I got that motor mounting bolt pattern.
I've looked back over the information I downloaded from Pololu and that's not it as you can tell from the Motor



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