Prompted by someone else’s success at improving his RA axis backlash, this evening I returned to mine and ended up disassembling the entire axis once again just to make sure I’d put everything back together as well as I could.
In the process I discovered that whilst the instructions suggest that the entire process can be done with the mount on the tripod, actually to insert the RA axis back into the lower section of the mount I found it easier to take a different approach. The “middle” bearing on the RA axis can be a bit of a pig, it seems. In use it sits right next to the upper bearing (which is inside the brass RA gear) and fits into place in the lower section of the mount. If it goes in anything other than perfectly true however, eventually something ends up binding and it isn’t possible to get the mount head into place properly. So what I ended up doing was sitting the entire RA and DEC assembly on a flat surface so the polar scope cap is down and the RA axis pointing straight up, like this:
The bearing can then be fed onto the RA axis all the way down. I found it still “grabbed” a bit in places, but rotating the bearing seemed to make that easier. I assume the RA axis isn’t perfectly round. I then took the lower section off the pier and fed that over the top as well. This is much easier because it’s lighter and it’s now possible to see the end of the RA axis as it passes through the mount, so it’s possible to keep everything pretty well lined up. The tapered thrust bearing can then be dropped into place easily followed by the retainer. With everything done up finger tight it can be returned to the pier/tripod for final reassembly.
I suspect the backlash I was seeing was due to poor engagement of the worm and RA gear ring. I couldn’t see any particularly obvious end float in the worm, certainly. Now it is significantly reduced, to the point where I’m not sure I could get it any better.