Having successfully upgraded the crane function to a ballscrew, my attention has turned to the rotation. I was always a bit unhappy about this, as the gears are quite small and the arm is long, with lots of inertia. It worked for my tests, but seems a little jerky in action – stiction taking control!
I have decided to upgrade. The dolly itself could benefit from being rebuilt, as I used standard WBP plywood for the original, which is not great stuff. So I will rebuild using 15mm birch ply. At the same time the bearing and drive will be changed. These will now have a 450mm diameter ball race, routed directly into the ply and using 20 14mm dia bearing balls and a cage made from hardboard. I found a youTube video on making a wooden lazy susan bearing, and this seems to work well. The big diameter should make for a very stable platform – another case of my converging on standard design after trying the alternatives!
To prevent the dolly from tipping up I shall reuse the original thrust bearing and post to locate the dolly. Around the perimeter of the dolly top plate a flexible gear rack will be fitted, meshing with the gear from the motor, mounted on the dolly base. This will give a much better ratio, less effort required to turn, and goes a long way to overcome the inertia issues. Pics as soon as I have constructed the modifications.
Additionally I have changed the geared stepper motors on the pan and tilt axes to ungeared motors. This will eliminate the backlash at these points.
And the Big Easy Drivers have finally exhausted my patience! For no apparent reason another stopped working. Probably nothing wrong with the units themselves. It is the connections that are proving so tricky. The green screw-in connector blocks that fit onto the BEDs have holes that are almost too small for the motor wires, so they are ridiculously fiddly. I have retired them all and gone for the black box CNC type (nice easy connections!), 3 of which are working perfectly at the moment, 3 more on order.
With luck these modifications will finally give me the accuracy and repeatability I need.