This month we address the missing demonstration crank
assembly for the orrery. The design for this was discussed in the November
2011 chapter along with the other two missing complications on the movement.
I will begin by first first showing the design drawings prepared by Buchanan
that were depicted in that section. Pouvillon had built this clock as an
advertisement of his skills and it was shown at various venues where he
received official recognition for this clock. The ability to demonstrate his
orrery would have been essential to this end. Again I stress as with all
other areas of this project, all restored parts are fully reversible
with no alterations occurring to the original artifact.
The first diagram shows a three-quarter view of the
crank itself. This is positioned using the existing witness marks for the
now missing cocks to hold the winding crank arbor. Notice that it is set
diagonally to the edge of the orrery base plate and is in the same fashion
as the arbor with the worm drive to the earth-moon system is set in at the
opposite angle. The use of odd angles for arbors and connecting rods is used
in other parts of the movement and adds to the visual interest Pouvillon
built into his design.
Here a three-quarter side view shows more clearly the
position of the winding arbor cocks (1 and 2). There is an open space on the
vertical arbor below the existing pinion adjacent the orrery ratchet wheel
(this pinion is hidden by the rim of the ratchet wheel in this photo and
drives the orrery). That open space is positioned perfectly for the location
of the restored bevel pinion, (3) to mesh with the crank bevel drive wheel.
The orrery is driven off the strike train. However it
is not geared directly to that train. That train drives a single arm cam (7)
which rotates once every 12 hours and this engages a ratchet wheel advancing
the orrery one click every twelve hours. When the train is at rest and the
drive arm is not in contact with the ratchet wheel, the ratchet wheel is
held stationary by an existing ratchet pawl (4), not shown but is drawn in
for illustrative purposes. We will restore a missing lever (5) which will be
attached to the same arbor as is the existing ratchet pawl. The entire
winding crank arbor can slide slightly forward and back on its elongated
pivots. Under normal operating conditions the bias spring on the existing
ratchet pawl which is also connected to the same arbor as the restored lever (5) will push
that lever against the collar (6) mounted to the winding crank arbor keeping
it pushed forward. This results in the winding crank bevel wheel remaining
disengaged from its mating pinion, (3) shown in the prior diagram. When the
crank key is inserted onto the winding square the winding crank arbor is
pushed backward; engaging the bevel wheel to its mating pinion and causing
the collar (6) to push the lever (5)
rearward. This lever is also attached to the same arbor as the ratchet pawl (4)
and will cause it to swing away from the ratchet wheel thus allowing the
smooth rotation of the entire orrery forwards or backwards. When the key is
withdrawn, the ratchet pawl's spring returns it to the normal position,
disengaging the bevel wheel and locking the
orrery into a stationary position.
The following photos show the fabrication process in the making of the
orrery demonstration crank. First Buchanan begins by making the steel drive
pinion on the Bridgeport mill.
Next the mating brass bevel wheel is cut.
The two raw-cut blanks are shown in the first photo. Next the pinion begins
to reach its final shape. Next the bevel is positioned onto its arbor. This
will later be spoked and the collet beveled in the same style as Pouvillon
used elsewhere on the clock.
The center photo shows the the restored pinion below the existing ratchet
wheel and its drive pinion, note the pawl engaged on the rim of the ratchet
wheel. That pinion below the ratchet wheel is connected to the orrery. The
third photo shows initial positioning of the brass blank blocks (see arrows)
which will be used to make the winding crank cocks.
Next the blocks are drilled and shaped.
The first photo shows the initial testing of the winding crank arbor for
fit. Next the cocks are milled out to their initial form, with further
refinements to be done before completion.
The first two photos show the bevel gear and pinion arrangement as well as
the rear cock. Next an overall view of the winding crank arrangement.
Here Buchanan is using a jewelers saw to cut a slot into a screw head.
Notice the clever jig used to ensure the cut is perfectly straight and
centered on the head even though a hand saw is used. This will be the grub
screw used to fasten the restored ratchet release shown in the third photo;
in the same manner as the existing ratchet pawl just above it, to the arbor.
The upper lever is the existing spring-biased pawl which holds the orrery
stationary when it is not being advanced by the strike train. The lower
lever has a small pin mounted to its end. This pin is engaged by the winding
crank collar when its key is inserted and since the lever is connected to the same
arbor as the ratchet pawl, it will move the pawl out of mesh with the
ratchet wheel, thus allowing the smooth movement of the orrery during
forward or reverse winding. See film clip below.
The first photo shows from the rear the existing and restored levers in
place. The grub screw has yet to be attached to the restored lever and the
blue winding arbor push collar will later be trimmed down. Next an overall
view of the winding crank assembly.
The first photo shows the two winding cranks for the clock. The smaller is
for the orrery demonstration and the larger to wind the clock itself. These
are based on the winding key shown in one of the photographs taken in 1983
by Jean-Pierre Rochefort. The handles are made from the same ivory material
as the orrery planets.
This video shows the restored orrery demonstration crank which allows that
system to be displayed in fast forward or reverse. The crank arbor is spring
loaded so when the key is inserted, it automatically disengages the orrery
from its normal drive train allowing it to 'freewheel' under the control of
the crank. The orrery is being cranked backwards in this video and so the
accompanying dials are also moving counterclockwise. The Earth-Moon system
does not function because it is normally driven from separate drive to that
which the crank is attached. In other words, the orrery is driven from the
strike train while the Earth-Moon is driven off the time train. The crank
substitutes for the strike train drive only.