German Equatorial Mount (GEM)

The Drives

For some time I have wanted an equatorial mount so I could venture into
astrophotography, at least the wide field class for starters. I wanted a
mechanically sturdy mount with a simple RA drive with slow motion control to
both axies to get me going. I also wanted to design the mount in such a way that
I could upgrade the drive system later, possibly going all the way to a Mel
Bartel's goto system with microstepping stepper motors, digital setting circles
and peecee control. That was the dream.

It all fell into place one day when my observing mate Wayne offered me a pair
of motors and hand controller from his old Vixen mount. He purchased the Vixen
GP mount about 2 years previously and immediatly upgraded the drives to Sky
Sensor 2000 which is a goto system that came with new motors. The old motors
were put in the sock drawer and forgotton until I mentioned my wish. They were
perfect.

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.

It was time to sit down and design the rest of
the drive, incorporating a clutch to protect the teeth. The rest of the drive
was pretty normal turning, boring and screwcutting. Everything was machined on
my lathe, the only exception being the machining of the step on the worm wheel
housing for the worm block. This was done by a friendly toolmaker.

It was a good experience to cut the keyways
for the clutch thrust plate in the lathe. Doing this type of work only shows
what a versatile machine the lathe is.

Here are the pair of GEM drives, one assembled and one in
pieces.
Onto the rest of the mount now.

Last updated - 27th May 2001

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