ODDS AND ENDS
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.
When it carne to fitting dock drives to my telescopes something better
was required. Here are my calculations.
|Earths Period of Rotation.
||23 hrs. 56 min. 4.091 sec.
|Closest approximation for gearing
||23 hrs. 56 min. 30 sec. ( 86,190 sec. )
||17 x 15 x 13 x 13 x 2
||169: 1 - 34 : 1
||4 rpm. ( 15 sec. )
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
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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