Strike and repeat control assemblies, continue main fly detents - May 2011
This month we continue with the fabrication of the strike
train systems. We now proceed with the main fly fan detents which were begun
in February. Two are
necessary, one for the quarter and one for the hour strike functions. These
detents are directly attached to their respective racks and in conjunction
with them allow the strike train to operate for the appropriate amount of
time to count out the correct number of strikes on both the quarter and hour
bells. Buchanan will continue with our convention of using a bird-like
allegory for most functions requiring a pallet or stopping action.
The first photo shows the detent mockup completed last
February in the upper left hand corner. One can see the bird caricature. We
must include the same dimensional characteristics present in this mockup in
the final part. The entire bird is mounted to an arbor that pivots within a
blocky rectangular bracket. One can see more of the initial setup of this
mockup on the movement in the
chapter. The final design must eliminate the bracket and have the bird pivot
inconspicuously within the slot cut into either end of the horizontal plinth
upon which they will be mounted. This is the large piece in the middle of
the photo. First the raw stock is cut on the reciprocating hack saw, second
photo, from stainless steel since the final finish will be a polished silver
finish. All silver parts are made from this material. The two angle brackets
are cut in the third photo and the arbor upon which the bracket will pivot
is fitted in the last photo.
Last month I described a bit on how this arbor was attached,
The two photos show this in more detail. This part must withstand the force
of the main strike fly whip as it is stopped by the bird's beak to terminate
the strike sequence. While the force is relatively low, this is a critical
safety issue since failure of the detent would result in a strike train
runaway. The end is therefore squared so as to be able to be tightened with
a conventional clock key. The entire part is fully recessed within the
plinth and so it would be difficult to get a good purchase on this part for
the purpose of tightly securing it with conventional methods. A jeweled
chaton will later cover this area shown in the second photo.
This photo shows one angle bracket in place. The associated
strike train fly whip is visible just below the bracket corner. Those folks
with wheel cutting experience can appreciate the several different wheel
cutting techniques shown within the main fly fan assembly in the lower right
hand corner. From front to rear: Conventional external rim cycloid teeth.
Next, teeth cut on the interior of the wheel rim and finally an angled bevel
These four photos show the various stages in the fabrication
of the detents. In the second photo Buchanan uses the first completed bird
beak as a template to hand file the second for a close match. The extensive
hand working of parts throughout this machine will give it a quality of
craftsmanship, what I call 'warmth' that is absent in a computer controlled
The two bird detents are shown in their respective positions
on the horizontal plinth. We were able to accomplish the attachment of the
detents with minimal disruption to the decorative lines of the plinth. The
detent arbor pivots are in a pair a jewel bearings. One is located with a
blind hole, just beyond the outside front surface of the plinth, with the
other located on the rear; out of sight.
now turns to making two pillars that will hold the rack let-down flies.
These flies mediate the speed at which the racks will drop. In conventional
clock movements the racks simply fall under the influence of gravity or if
positioned in a way that this is not possible, are pushed via a spring. In
our movement the racks could fall simply by gravity, but they are rather
large and the rack tail will contact a snail cam whose rim will be
delicately skeletonized. So the prevent any damage, we will employ these
flies to let the rack down at a controlled speed. Besides, two more fly fans
will add to the visual fascination of the entire strike sequence, and in
truth this is the driving reason. The first photo shows fabrication of the
interior mounts to which the brass pillar uprights will be attached. Two of
these mounts, one secured in place on the plinth surface, are in the next
The first photo shows the mount next to the pillar with the
next photo showing it tucked flush into the pillar's lower end. A rod, the
length of the pillar with threads on each end will screw into the center
mount hole and also be secured to the top end of the pillar. In this way we
are able to attach the pillar to the plinth with no visible fastener points.
We want the frame to look as if it were made from one milled piece and
without the distraction of obvious fasteners. Theoretically it would be
possible to create such a frame out of one piece, however that would make it
impossible to assemble the myriad parts needed within the frame. We realize
that such clever assembly techniques could pose a problem in the future for
someone trying to take the movement apart for service. I will have these
points printed out and hopefully this will be kept with the clock for any
future repair person to consult.
The pillars are now shown in place. These will later be
shaped into the same decorative format as the rest of the movement frame.
An overall view of the rack let down fly pillars and bird
detents within the movement.