Re: ATM Gary Gears

[Andy Saulietis <iss@pvtnetworks.net>]

Re split worm A-B gears,

>> 3.Now if ya want kinky, pin 2 blanks together and do the Gary-Walk as
>> before. Then split the pair, add a spacer and horseshoe spring. You have
>> just made a VERY expensive anti-backlash gear.
>
>I understand that the method to reduce cutting error in large gears is to
>step the two gears against each other while lapping against the worm, so
>that they are at 0, 120, 240, 60, 120, 180, 240, 300, 360, 30, 60, 90, 120,
>etc degrees offset. In this way small errors on either gear are pressured
>towards conformity.  This is kinda like the process of making flats by
>grinding A against B against C. When done the surfaces don't depart more
>than an arcsecond or so.


A long time ago, I came across an old radar antenna azimuth mount, this
appeared to
have a worm gear in it ~7" diameter. Bought it ($5 at a surplus place in
Cambridge MA,
the good old days..;) & got it apart..it was bronze, split and spring loaded.
Before long, it was the RA drive for a large GEM I built in the MIT student
shop
back in the early '60's. Once there was a scope on the mount, the sad news 
showed that the gear had a large PE of several arc min. As an experiment I
pinned the 2 halves of the gear together, using the worm to aline the halves.
The PE was considerably less but still not acceptable for astrophotography.
I then lapped the gear/worm together..this got a marginal improvement but
still not good enough. Moved to Houstn, the monunt got a new Byers gear, and
is currently supporting a C-14 at Houston AS observatory site near
CVolumbus TX.
I have the mount here now to install a stepper motor upgrade on it, with a
Bartels
controller.

I'm not sure that split AB worm gears are the answer based on this experience.
BTW I'm still producing large HDPE worm gears using a home-built gearhobbing
machine.

Chuck Shaw ended up with the split gear & tried to use it in 10" GEM, 
eventually we gave up on it & used an HDPE gear instead.


Andy Saulietis
ISS Enterprises
39 Silver Fox Trail
Mayhill NM 88339

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