In article ,
Allan Adler wrote:
The telescope I've referred to in recent postings has a pair of knobs
joined by an axis whose center is a pinion that engages a rack on a tube
that holds the eyepiece. This enables one to focus the telescope. The
rack seems to be well-oiled but still moves somewhat unevenly in some
places as one turns the knobs.
When I look at it so that I can see all the teeth below me: |||||||||||
it seems that they have not all worn evenly, that some are apparently
[ ... ]
I'd like to be sure of what kinds of conditions contribute to this kind
of uneven motion in a rack and pinion. It mostly turns evenly but in some
places it seems more prone to offer some resistance.
[ ... ]
Continuing with the hypothetical, suppose I want to be really fanatical
about figuring out what exactly is wrong with the rack. Since eyeballing
it suggests some irregularities, suppose I want to measure all of the
heights of the teeth and all of the widths of the peaks and of the valleys.
Is there any convenient way to do this? One way that occurrs to me
is to take the rack, clean it, ink it and press it against a piece
of paper to make a print of the rack. Then I can photocopy the inked
page with enlargement and measure it. There would still be a lot of
measurements to make, but they wouldn't be so small and easy to mess up.
Also, if the print were faint in some places, that would confirm my
impression that it was badly worn in some places.
What I would do to examine the rack is to check it on an optical
comparator (after a through cleaning, of course). Mount it so you see
the rack teeth in profile. Put some clear plastic film under the clips
on the display, select the largest magnification you can manage with
the optical comparator, and trace the outline of the profile on the
plastic film. Now, move to one of the worst teeth, adjust so the
bottom of the tooth profile is the same, and trace this one, The area
between the two profiles should give you a good idea how badly worn
it is.
To *make* one, the ideal way (with HSM tooling) would be a
horizontal milling machine, a proper gear tooth profile mill (for the
rack gear -- which is either #8 or #1 -- I forget which end is correct,
but when you buy it you can look that up in the catalog. One end of
the set is for 135 tooth through rack, and the other end is 12-13 teeth.
Obviously, you first have to know what the size of the rack
teeth are -- both diametrical pitch (or "module" for metric gears,
IIRC), and the pressure angle. And if the teeth are cut at an angle
instead of straight across the gear (more like "/////////" than
"||||||||") you will need to angle the blank, and to calculate a
correction factor to your feed using a bit of trig to get the tooth
spacing to come out right.
Note that the angled teeth are a lot smoother in the feed, but
you will need better bearings on the pinion, as it will have a side
thrust which the straight tooth will not.
Now -- if you want to avoid the side thrust, but benefit from
the smoothness, then a herringbone gear would be ideal -- but more
difficult to make. It would look somewhat like this:
/ / / / / / / / / / / / /
\ \ \ \ \ \ \ \ \ \ \ \ \
There is one other issue related to the rack and pinion. There seems to
be no barrier between the rack and pinion and the interior of the
telescope tube. It occurs to me that this might let vapors from the
oil diffuse into the tube and possibly also coat the inner surface of
the telescope lens at the other end. That sounds undesirable and makes me
wonder whether someone might have made a mistake by oiling the rack and
pinion in the first place. If so, the source of the mistake might have
been that someone noticed the uneven motion and resistance to turning,
didn't realize the role that wear might have played in it, and instead
tried to solve the problem by oiling it without realizing that this
might be bad for the optics.
The typical lube used in the focusing rings of cameras
(multi-start threads) is typically a quite thick grease, and has fairly
minimal vapor -- unless it is kept in the direct midsummer sun. :-)
On the other hand, I don't know anything about scopes or racks and pinions,
so my 'pinions aren't worth anything. If someone is better informed, please
inform me. Thanks.
Well -- you have most of what I know about the subject above.
Good Luck,
DoN.
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