ODDS AND ENDS

Worm Drives.
When I first made telescopes and needed worm drives on the axes. I did what other amateur telescope makers did. I cut them with a tap in the lathe. In fact I still do. Refer Fig.3 ,Page 369. ATM. Book 2. A lot of force is required to cut the worm teeth in bronze. The tap has to be fed in full deapth from the start. It is a jerky operation with the interupted cutting of the four rows of teeth engaging one after the other. I later used a spiral fluted tap which gave a continuous cutting action. The tap pulls itself around the blank. When the blank makes one turn the tap may or may not drop back into the first tooth. The only thing to do is feed the tap in a Ii the deeper an let the blank do several revolutions when it will sort itself out. There will be a little irregularity in the drive at this point but if a hand cable is used to guide the telescope it does not matter. However with a clock drive for photographic purposes it might. Also there is no guarantee of the number of teeth that will be cut. I got variations of four teeth.

Clock Drives.
When it carne to fitting dock drives to my telescopes something better was required. Here are my calculations.

I was able to purchase a 4 rpm. synchronous motor and gearbox from Phillips. Recently I found they do notmake a 4 rpm. box. The next thing was a 169 worm wheel. This was a problem. I did not have a dividing head and in any case it would require compound indexing. A pretty tricky operation. But I did have a circular milling table that could be set to a tenth of a degree. Each division was 2.13 °. I wrote out on a piece of paper the whole 169 settings. If in those days electronic calculators were available I could have had it on the machine beside me with 2.13 entered and just kept hitting the °+" button. ( Hewlett- Packard Calculator with RPN.). You cannot do this with calculators using Algebraic Notation.

I was going to make up a milling device using a master worm wheel. I machined up a bronze wheel. I set the circular milling table up on the lathe saddleand using a single tooth cutter in the chuck made the 169 gashes around the circumference of the wheel. Then I replaced the cutter with a tap, engaged it with the worm and let it run for approximately fifty revolutions of the wheel. That would do as far as I was concerned. I now built an attachment to sit on the saddle of the lathe. This had the master worm and wheel on a spindle at the top of which was fixed the worm wheel blank that had to be cut. The master worm was geared to a spiral fluted tap held in the lathe spindle and tailstock. In operation the master worm and wheel control the number of teeth cut in the blank. The tap is fed in full deapth and the blank allowed to rotate several revolutions to ensure, I hoped, a smooth running drive. Fig. 1 illustrates the set-up for cutting the wheels.

I originally made my worm out of stainless steel and the wheel out of bronze. Recently I changed to a brass worm wheel and an aluminium wheel. As the turning of worm and wheel is so slow, wear can be forgotten about. The wheel could be anodised if a harder working surface was preferred. Also these materials are easier to machine.

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