Allan Adler wrote in message ...
I'm pretty much convinced now that the upright rail, although moveable,
can only move away from the motor, not towards it, from the upright position.
The argument is as follows:
(1) Let the rivet for the upright rail be A, the rivet for the release lever
be B, let the rivet joining the release link pivot to the release link be
C and let the rivet joining the release crank to the release link be D.
Assume the motor is on the left, so that A is the left of B. Then if one
tries to push the upright rail towards the motor, the motion of the
upright rail and the release lever will both have to be counterclockwise.
(2) Let E be a point on the base to the right of B. Then the angle CAE is
less than 90 degrees and angle DBE is more than 90 degrees. That means
that when the rails are rotated to the left, C moves up and D moves down,
and therefore, since D is lower than C to begin with, the vertical
distance between C and D increases.
(3) On the other hand, the right side of the release link pivot blocks the
left side of the release crank, which makes it impossible for the
horizontal distance between C and D to decrease. However, the actual
distance between C and D is always the same, since C and D are rivets
located on the release link, which is one piece of metal.
(4) The square of the distance from C to D equals the sum of the squares of
the horizontal and vertical distances between C and D (Pythagorean
theorem). If the motion towards the motor were possible, the horizontal
distance would not decrease and the vertical distance would definitely
increase, so the distance between C and D would have to increase, which
it can't.

Therefore, motion towards the motor is impossible.

I think that aspect of the design of the frame assembly is pretty clever
and is not explicitly mentioned by Gingery. There is nothing actually
blocking the path of the upright rail from moving toward the motor, but
it still can't move that way. And that feature would have to be preserved
by anyone proposing to modify the design for use with another motor. The
moral is that one has to make sure that the release link pivot and the
release crank have a significant overlap of their vertical edges.

There must be a lot of tricks like that which apparently everyone takes for
granted, but this is the first time I've seen anything like that. There is
an expensive (one could buy a Clisby instead) four volume work called
"Ingenious mechanisms" which might be the place to look for things like
this and might not. Does anyone know of a book that definitely has this
trick in it (besides the Gingery lathe book)?

Ignorantly,
Allan Adler








I had to get my copy of Gingery's book out of storage. His terminology
for the linkage parts is possibly confusing but the mechanisim is
clever and simple. The countershaft frame, when locked, is prevented
from moving from the extreme of its locked position by the support
piece (he calls it a "crank") for the release lever end of the
pivoting link striking the similar support on the c. shaft frame, and
also by the pull of the belt. In any case the motor, in his design, is
bolted to the c.shaft frame and will move along with it. I departed
from his plans here and mounted my motor under the lathe bench on a
swiveling sub-base which kept the motor belt tension constant using
the motor's own weight. Also, since I condidered this to be an
experimental assembly and subject to change, I used 5/16" fine thread
bolts with double locknuts for the pivots instead of rivets in the
locking linkage and 1/2" bolts for the c.shaft and release lever frame
pivots. His comment about belt slippage is accurate, and the lathe
would be greatly improved by a backgear drive. A DC variable-speed
motor will help also and I considered it, but 15 years ago when I
built mine small DC motors were not as easy to find and industrial
motors were around $300.00 from Grainger. A later book shows how to
make change gears for threading using a milling machine from another
book in the series, though I never got that far with mine. The lathe
is really best suited for turning plastic, wood and non-ferrous
metals. While I managed to coax mine into doing fairly heavy work on
steel, the machine's lightweight design made it prone to chatter and
using it at near its maximum capacity could be a slow and frustrating
experience.
Mike.