Re: ATM [Fwd: Re: Equatorial mount musings]

[kevin@aquilagroup.com (Kevin Ferguson)]

Mel forwarded from David :

> I stumbled across something called a
>"dividing head" in an old Enco catalog. This appears to have a 40:1 worm gear
>with clutch and heavy bearings and can be tilted from 0-90 degrees for $335.

MSC had one on sale at ~ $250 a couple of months back, and I bit. (I think MSC
puts these on sale once or twice/year) I intend to use it for it's intended
purpose,
but will give you a rundown now, and can try to field any questions you might
have. Note that MSC now owns Enco. (last Enco order showed as MSC on my 
credit card statement) Rec.crafts.metalworking discussion indicates that 
most of the imported dividing heads seem to be nearly identical.

There are no oil seals on the case, and when tipped more than a few degrees
(from spindle horizontal) it will not hold (for long) sufficient oil to keep 
the worm dipping in lube. In a mounting, I think grease would work better 
than gear oil, and the case would do a fairly good job of keeping the grease 
from drawing dirt...Of course you then run into issues of viscosity v.
temperature.

>The thing weighs 38 lbs. Adding some declination bearings and a counterweight
>should complete the job. 

The catalog weight includes a fair number of heavy accessories  which would be 
useless for making a scope mounting (tailstock for example) ...without them
I think
the weight is closer to 25#...Easy to handle with two hands, but a real handful
for one. I used to carry 30# (4 gal) cases of paint daily at work so am 
pretty confident of my estimate.

There was at least one report in "Home Shop Machinist" magazine of some of these
imported heads having "metricated" 1-1/2"-8 thread which will not mate with
a "real" imperial thread.  The spindle on mine does thread into the chucks
for my Logan lathe, but the register (unthreaded part of spindle near seating
shoulder) is ~.018" oversize and will require grinding to mate with my chucks.

One of the accessories included was a 5" diameter CI chuck backplate
threaded to fit
the spindle. That might be handy for making a slip clutch, or mounting dec.
bearings.

>My question..Am I way off base here? Any guesses wether the worm gear will
>have a bunch of slop? Would the worm gear bearings be too stressed? All
>comments appreciated.

I think it is probably plenty strong for any scope I can imagine. 
As for rigid...Thats a lot tougher.  The
Body and base castings are pretty solid (CI), and fit togethor with a mildly
angled
(~75 degrees?) dovetail.  The locking between the two is solid, (two bronze
wedges 
opeeated by metric version of ~3/8" bolt) but painful to make fine
adjustments, as the large non-precision bearing area is pretty grabby. 
(for latitude changes..shouldn't be an issue if on a fixed pier).

The "latitudee" markings are probably only good to +/- 1 degree.  

The spindel has a timkin (in style anyway) roller bearing at the north end,
and a 
bronze plain bushing the south end. The south end of the spindle is ground
and polished to a mirror like finish, it fits with only maybe
.001 - .0015" clearance....Just right in my book. There is a needle thrust 
bearing outboard on the south end, with a lockable adjustment nut to take 
up axial play, and so all radial slop in the north bearing. (because it is
tapered) 

The length of the spindle may be a little on the short side (maybe 4.5"
between the bearings) and the plain rear bearing  (south ) has no 
provision for taking up slop, though it has minimal clearance new, and would 
see less loading than the north bearing.

The worm shaft is in a bronze bushing, but has only polished steel plain
bearings
to take thrust. I don't think it would be hard to add some thrust bearings.

You are wrong about there being a clutch on the spindle: The spindle can't
be rotated without turning the wheel. The worm's shaft is
eccentric in it's mounting bushing, and when that bushing is rotated the
worm can be brought out of engagement with the wheel, or conversely
be adjusted for zero backlash. The wheel is keyed and set-screwed to the
spindle, and mine is a VERY tight fit.  Gave up on trying to remove it
when cleaning out case (I found none, but foundry sand has been found inside
some asian manufactured machine tools)

The gearing is 40:1, and much heavier duty than I've seen on any telescope
mounting,
though the wheel diameter is only 3 or 3.5". Offhand I can't think of a way
I could easily
measure the geartrain for errors. I have been able to set the clearance for zero
backlash, and the friction feels constant over a full revolution, indicating
that the elements run fairly concentric. Dividing heads are typically not up to
"astronomical" standards WRT accuracy, which has a lot to do with why really
good
gearsets are so rare and expensive...ordinary gears are only as good as the
ordinary dividing head which was used to space the teeth. 

There is a lock which grips a clamp around the spindle, preventing rotation
or loading
of the gear...not sure what use that might serve in a mount.  Ditto for the
"direct
indexing" provision, which puts a tapered locking pin through one of 36 holes in
a plate attached to the spindle nose...that plate is marked in degrees, not
hours <g>
but is only held on by 3 screws.

The internal spindle bore is a B&S #7 which is a self-locking taper...locks even
tighter than MT....not suitable for use as a cone clutch.  Replacing the
spindle 
might solve a number of problems, and
it would be a fairly stright forward turning with 3 diameters and a keyway.
Be warned
that it may be very difficult to remove the bearing race from the spindle...
Advice I recieved on how to correct the spindle register diameter indicated that
others had been unable or unwilling to remove this race.  The other race for
this
bearing is in a removable round cover for the front of the case, so it also
might
not be too hard to modify/replace if a different bearing were needed/desired.

If you have no experience with asian machine tools, be warned to expect
"crude but
effective" level of workmanship/finish. In this case, I still don't have the
spring
loaded crank handle working quite right, and I will need to rework the nut
which goes around the rear of the spindle, as the locking screw stripped....
I put up with such due to the low (you should see what a real B & S head
goes for) 
purchase price. It is useable as recieved, but needs some work before I will
enjoy using it.

David, I couldn't find your email adress in Mel's forwarded message.  If you
have
more specific questions, please email direct, as I feel I've used enough
of the group's bandwidth...unless you feel your questions are of very general 
interest.

-Kevin