Allan, what if you ask for a digital camera for Christmas?

GWE

Allan Adler wrote:

Anthony writes:


You might want to investigate the pinion. There may be nothing wrong with
the rack at all. If the center hole on the pinion is offset, or at an
angle to the teeth, this would also explain the uneven wear. Even a mis-
alignment of the two bores the pinion rides in could cause this.


I ruled out a problem with the pinion on the grounds that the resistance
wasn't repetitive as one turned the knobs completely a few times. I don't
know if that is a valid argument.

The pinion and the knobs form a single piece. So, unless it was made
incorrectly, I don't have any reason to suspect the pinion itself. I'm
not sure what you mean by "at an angle to the teeth". One meaning might
be that it crosses the teeth. With that meaning, it might not work at all,
so I'm guessing you mean that the planes containing the teeth of the pinion
and of the rack are parallel but the peaks of the pinion are not parallel
to those of the rack. To put it another way, I think you're saying that
the axis of the knobs might not be parallel to the bottom line of the
rack valley it passes through, so that it dips to one side. That is
conceivable but again it would seem that it would result in consistent
problems all along the length of the rack, not just in one place.

On the other hand, there still might be a problem such as you describe.
I unscrewed the "box" that covers the pinion. It is not perfectly rectangular,
since it has to fit against the cylindrical tube and also it has cuts
to allow the axis of the knobs to pass. I turned the box over, so that
these cuts were all on top and the box was open, and noticed that there
was a flat rectangular piece of metal inside of nearly the same length
and width of the box. The rectangular piece had a rectangular hole in it.
When I opened it, it was jammed in so as to run diagonally in the box from
one edge on the bottom to the opposite edge on the top. There were two
more identical pieces of metal sitting flat on the cylinder and the
axis of the knobs, enclosing the little towers that the screws screwed
into and the pinion. The bottom of the box has four "pedestals", one in
each corner. I don't know whether the flat piece of metal was correctly
placed. Possibly someone else opened it up to oil the pinion and put the
flat piece back in incorrectly and possibly it is really supposed to be
in the diagonal position in which I found it. If the three flat pieces
are removed entirely, then the axis of the knobs wobbles badly. I think
the three flat pieces are intended to sit flat on the four pedestals.
When they are stacked that way, they fit in the box but rise to block
off most of the hole the knob axis passes through. So, I think the flat
pieces are used to clamp the knob axis in a fixed position. The flat
pieces also rise above the bottom of the circular arcs that are intended
to fit the cylinder body. I think that when the box is screwed back onto
the cylinder, the flat pieces are forced down to the bottom of the circular
arcs, forcing them to bend to conform more to the cylindrical shape. This
has the effect of pushing the other two, still straight, edges of the flat
pieces to clamp the knob axis even more tightly. This arrangement makes
so much sense to me in terms of the operation of the rack and pinion that
I find it hard to believe that the diagonal arrangement I found could
possibly be correct, but maybe there is something I don't know, such as
the possibility that the diagonal arrangement eliminates some lateral
movement.

I didn't find that correcting it, even if it was wrong, made any difference
in the resistance, but this is all kind of subjective. Unfortunately, I can't
see the pinion engaging the rack, so it is hard to know what the problem
is exactly, except by trying something that has the effect of curing it
and concluding that what I thought I was correcting was actually what
was wrong. But I do have one idea: I can count how many teeth there are
on the pinion and how many are on the rack and how much of a turn I have
to make of the knobs to move a given number of teeth along the rack and how
many teeth there are between the visible part of the rack and the part that
actually engages the teeth. That means I may be able to wait until I find
some resistance and then turn the knobs the appropriate amount to move the
bad part of the rack to where I can see it.

After I've gotten that crackpot idea out of my system, I'll probably just
clean and re-oil it, as was suggested by others. But I'm still concerned
about the possibility that oil fumes might be contaminating the telescope
tube and coating the inside surface of the telescope lens.