Action Rail Hole Spacing

[Sarah Fox]

Hi Ric, Terry,

> It would strike me as a good idea to use the old rail as a guide, unless
> you know it was full of problems.  Otherwise its jig making time.
> Shouldnt be too hard to contrive some kind of a hole spacing jig.
> Perhaps a retractable dowel swing mounted so that for each new hole to
> be drilled the rail is held in place by the dowel inserted into the
> previoiusly drilled hole ??

The problem with such a jig would be cumulative error.  If you're off by 0.2
mm per hole, you'll be off by 4 mm by the time you've done a series of 20
holes.

A better way that would create no cumulative error would be to use a long
threaded rod, preferably with an even number of threads per distance between
rail holes.  For instance, if your holes needed to be 1/2" apart, that would
be 10 turns (i.e. 10 threads) of a 1/4-20 threaded rod.  Mount a couple of
nuts perhaps 6" apart at the end of a track (which you clamp to the platform
of a drill press), and use the threaded rod as a stop for the rail (which
slides along the track).  Advance the rod x number of turns, drill, advance
x more turns, drill, etc.  If you aren't lucky enough to find a rod with an
even number of threads per hole, just do a quickie spreadsheet to calculate
the number of turns required for each hole in the entire series.  To count
turns, you will of course need to fashion a crank handle for the end of the
threaded rod, and the drill bit should be shortened as much as possible to
minimize runout.

If you want to get fancy, use a third, unmounted nut, with a spring between
it and one of the mounted nuts (pushing the threaded rod away from the rail,
not towards it).  You'll need something to keep the nut from turning.
Perhaps you could mount it in a free-floating block of wood?  The spring
will preload the mechanism and minimize backlash.  Alternatively, you can
have an adjustment mechanism that moves the two main nuts together or apart.
It should be adjusted tightly enough to remove play, but loosely enough not
to bind.

Don't forget to grind the end of the threaded rod smooth.  ;-)

Peace,
Sarah


Peace,
Sarah