On Wed, 5 Sep 2001, Barry Young wrote:
> In searching for cheap alternatives to expensive
> racks,
Probably the classic source for rack and pinion gearing is Boston Gear
http://www.bostgear.com/
in particular:
http://www.bostgear.com/products/open/spur.html
Might also be interested in gear theory
http://www.bostgear.com/products/open/theory.html
They don't have any prices listed that I could see, but you can put
together a shopping list, and submit it for a quote.
The last time I checked, you could buy a couple of pinion gears and
something like six foot of matching rack for roughly $100, maybe
less. I ended up spending around $80 for the sprocket and chain assembly
that was recommended in the View Camera plans by John Layton. So ... this
is at the root of my suggestion that if you plan on buying this stuff, it
is probably worth pricing a set of rack and pinion gears.
> I have tried a toothed belt fastened to a flat
> surface with the cog rolling along it, They have
> massive backlash and slop. I never got it to work well
> enough for a camera despite trying the finest toothed
> belts I could find.
>
> I tried miniature bicycle type link chain with MUCH
> better results, but the chain is tough to get fastened
> in an acceptable way and cost nearly as much as the
> dang racks I was trying to avoid.
Yes, the sprockets have very sharp teeth so that they can catch the belt
or chain link and pull it around the curve of the sprocket wheel. If you
lay it out flat the parts will still mesh, but the end of the sprocket
teeth is too small causing the slop, and the pressure angle is just too
shallow. Basically the sprocket teeth are like little wedges and any
pressure on the mechanism makes the belt or chain slide up to the tip of
the wedge. You need a more square set of teeth for the linear forces.
> Small Parts Inc. sells reasonably priced nylon racks
> which I will experiment with next. A pair of racks is
> about $20.
Yes, or you can get brass or stainless steel for a bit more. In any case,
the price isn't really that much considering the hassles you end up with
when you try using these other items for something they are not designed
to do.
I liked the suggestions of attaching the focusing rail to the chain or
belt (at a small point) and then running the loop over the sprocket as it
was intended. Other options are:
1. Use a threaded rod with a nut attached to the focus rail. Your focus
knob could then be at the back of the camera under the ground glass.
2. If you don't have a notched belt or chain and matching sprocket you
are still in luck! Attach a piece of string to each end of the focus
rail and then wind the opposite ends of the string in opposite
directions around a focusing shaft. Turning the shaft then lets out
one end of the string while pulling the other. (If you've ever taken
apart an old radio this is how the tuning knob would run both the
dial and the rotary vane capacitor.)
3. If you have the notched belt or chain, use them in a similar fashion
to suggestion 2.
4. Buy (or scrounge) an actual rack and pinion combination
5. An idea for a really big camera would be to use servo or stepper
motors to control the camera motions. Then just stand behind the
glass and push buttons to focus
6. etc.
- Wayde
([EMAIL PROTECTED])
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