alt-azimuth mount

I've been studying the alt-azimuth mount on an 80mm Meade model 312
telescope. Unlike my earlier studies of mechanisms in recent times,
this is not based on reading a book on how the thing is made, but on
direct examination of a telescope and tripod someone loaned me.
I tend to be impressed by the ingenuity of mechanical devices, but maybe
that is because I haven't seen as many of them and am much less sophisticated
about them than the readership of rec.crafts.metalworking. Anyway, after
one has fixed the approximate altitude and azimuth and turned some
knobs to clamp it in position, one can make small adjustments in the
altitude and azimuth by turning some plastic knobs attached to flexible
shafts. In each case, the flexible shaft terminates in a metal tube with
an allen screw in it that lets it be attached to a screw thread. The
screw thread has been machined on one side to present a flat face to
the allen screw. The other end of the screw thread passes through a threaded
hole and emerges on the other side to push against vertical cylinder V which
is fixed in place by virtue of the original clamping. This causes the
threaded hole to move away from the vertical cylinder V and causes
a horizontal cylinder H on the other side of V, the end of which horizontal
cylinder has been cut in a special kind of curve to fit snugly against V, to
press against V. The horizontal cylinder sits on one side of a vertical
piece of metal M, on the other side of which is a larger horizontal cylinder L.
As H presses against V, it tends to pass through M and retract into L.
(I've described the adjustment control for the azimuth. A quite similar
arrangement is used for adjustment of the altitude.
My guess is that L and H form a kind of unit and that L has a spring inside.
I think the spring must be pretty strong, since I don't seem to be able to
cause H to retract just by pushing on it with a screwdriver.
I would also guess that this is a pretty standard part and a standard
trick for fine adjustment of position.
What I'd like to know is what this unit of H and L is called and whether
this trick for fine adjustment has a name. I'm also interested in knowing
who sells this particular H-L unit as a standard part, even though I don't
have any immediate plans to build something using it. It just seems like
a good thing to know about.
I hope my description of the mechanism wasn't too confusing. I couldn't
think of an easy way to supplement it with pictures.
Reply to
Allan Adler
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