ATM 5 hour straight bolt guide curve

[doug kniffen <dk@usmo.com>]

It has occured to me that making the guide curve for the 5 hour straight
bolt tracking platform may seem to many people as the most difficult part
of the project. It doesn't need to be. If making the curve out of plywood
there is an option that permits a smooth surface for the sliding contact to
slide against. What kind of curve is it??? Hyperbolic is a fairly good
guess, but it also seems to be a section of one of those elliptic curves.
It isn't important for construction.

As mentioned in the sketches, once the actual bolt drive travel rate has
been determined, you can easily plot out a full size curve on fine ruled
graph paper with an accuracy of half a millimeter. Smooth is more important
than perfect since we can adjust the position of the curve during platform
assembly. Once the full size curve has been graphed, it is simple matter to
find the maximum radius of a compass arc which will fit inside the curve.
This radius can be used to make a smoothing disk, basically a plywood
circle with sand paper attached around the edge.

Glue the paper plot onto the wood piece from which you will cut the guide
curve. With a coping saw, cut the wood close to the plotted curve. Attach
the smoothing disk to a table and slide the curve against the disk as
needed to finish to the final dimensions. As always, average all potential
errors at each step.

Take the basic tangent drive, move the hinge to the same side of the board
as the bolt, and add a small curved guide. Maximum tracking time (50mm
lens) then increases from 26 minutes to more than 300!     That's all there
is to building a better barn door. 

Doug Kniffen