The first thing I needed to do was to time the shaft rotation to work out the gear ratio required.
I made a pair of 120mm pointers and fitted these to each shaft, connect up a 12volt battery, switch on the hand paddle and away the RA motor went.... tick,tick,tick...
A vice to hold the motor steady, a mark on the wall and a stopwatch let me see that the output shaft took 10 minutes to revolve. A quick calculation meant that a ratio of 144 to 1 was required between the motor and the sky. This was confirmed by a quick visit to the Vixen website.
Next I had to decide on the worm pitch - worm wheel diameter combination, remembering the worm wheel had to have 144 teeth. I remember reading in Advanced Telescope Making (Macintosh) that a wormwheel could be cut using a tap if setup correctly. A thread of M12 with a pitch of 1.75mm would yeild a wormwheel diameter of about 80mm.
Aluminium Alloy 2011T6 was selected due to it's high tensile strength and hardness and 3 blanks were turned and grooved to a diameter that would equal a circumference of 144teeth x 1.75mm.
A mandrel was designed that would hold the worm wheel blank at the lathe spindle height and was mounted on the tool post shaft. The mandrel could also be mounted in the four jaw chuck to hold the blank if adjustment of the groove diameter was necessary.
A tap tolder was made from 25mm mild steel and it was made long enough so all components would clear the chuck safely.
The tap was a H.S.S. ground thread straight flute intermediate hand tap, nothing special, a good quality tap.
It was then a simple matter to plunge the blank into the revolving tap and the tap drew the blank around to suit itself. Cutting oil was applied liberally and swarf was brushed away to stop clogging. I allowed plenty of time for the cut to wash out before inspection.
The first worm wheel I made had 148 teeth so a recalculation of the groove diameter was in order. I mounted the blank in the 4jaw chuck and turned the diameter to the new size then recut the teeth as before, 144 this time. I then turned the other 2 blanks to the new diameter and cut the other wheel to match. To my good fortune I had no problem with teeth overlap that I feared all along.
