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Strike and repeat control assemblies,
initiators, March 2010
This month we continue the fabrication of the strike/repeat
initiators. What these wheels do is begin the sequence of
events that lead to the quarter strike train’s release. For
example, in the instance of the pull-repeat function, the
operator initiates the repeat function by either pulling on
a cord, thus the ‘pull’ part of the pull-repeat moniker or
will be able to pull down on a lever to initiate the repeat
function. We employ both methods. The cord works with the
case closed; with the lever being used when one has the case
open and is operating the machine in full demonstration
mode. Either of these actions cause the wheel to rotate
nearly 180 0 and by doing so compresses a leaf
spring. When the wheels reach the point where the spring is
fully depressed the wheels are allowed to rotate back in the
opposite direction, driven by the compressed spring.
However, now in the opposite rotation, a ratchet gear which
is attached to the wheel is engaged and this will cause a
cam to lift the rack pawls away from the quarter strike
rack, thus allowing it to fall to the correct position as
determined by the rack’s tail as it meets the snail cam. In
the case of the clock striking the quarters under normal
operating conditions, a cam attached to the quarter strike
train performs the same initiation function as would the
pull cord or lever being activated. This system incorporates
what Buchanan calls an “all or nothing” concept. For either
initiator wheel, the wheel must be fully rotated to engage
the ratchet pawl and thus release the rack. If one merely
pulls the cord or pushes the lever half-way, the procedure
is aborted and the wheel simply spins back to its beginning
position without activating the cam that releases the rack
pawls.


Above are the two wheels spoked out. One is used for the the
pull/repeat function and the other is connected to the
strike train for normal automatic striking operations. The
last photo shows a small box of matches for scale. The
highly curved springs are attached to pivoted detent levers
which are either pushed by the four lobed quarter strike cam
for the regular quarter strike sequence initiation or the
pull-repeat lever. In normal operation each detent is
positively seated against a hard stop and kept in this
position by the curved spring. As the cam lobe pushes on the
detent that energy is transferred to the spring.
However, if one were
to turn the clock backwards, the cam lobe would approach the
lever from the wrong direction and cause a jam. But in our
design, the lobe will simply pivot the detent away,
depressing the spring and allowing the lobe to pass by
safely. When the lobe has cleared the detent from the wrong
direction, the detent is snapped back to its original
hard-stop position by the spring.



The first photo is the machine
setup for the counter-sunk holes to be drilled that will
contain the ball bearing races which the fly fan arbors will
rotate in. The third and fourth photos show the checking
procedures for proper fit of the bearing into the hole.
These are all press fit applications requiring a tolerance
of one thousandth of an inch for snug fit.


These photos show a series of
steps in the making of the fly fan armatures. They are
designed to match those of the main strike train fly
armatures completed last December.


One can now see the emergence of the armature in the same
design as those on the main fly fans

The first two photos show the two custom tool cutters
Buchanan uses to obtain the shape of the base that will
connect to the fly blade holders. The third photo shows the
initial rough blanks that will become the fly blade holders



The first eight photos above show the several steps
taken to form the fly blade holders. First the blanks are
machined to size. Compare how these look inserted into the
fly fan armature with the eighth photo. This gives an idea
of how much material is eventually removed in the process of
machining throughout this project. Next the sides of the
parts are milled flat. The taper ends are fabricated and
then a slot in which the blades will be held is cut using
the slitting saw. The last photo shows the beginnings of the
pinions that will drive the fly fans.

The fly blade holders are now complete.
The screws to be made that will attach the blades to the
holders are 72 thousands of an inch or 1.2mm.


The fly assemblies are now mated to the initiator
wheels. Once again, we see how complex a mechanism can
become when one might expect a simpler solution. While we
may not use the easiest way, we always choose the most
interesting one! The second photo shows the assembly where it
will reside within the movement and the builder's hand gives
the viewer a sense of scale.

Shown above is the procedure Buchanan uses to depth
the wheelworks. The wheel is mounted to a fixed pivot and
its mating pinion is secured in a pivot that is mounted onto
an old lathe slide. He can then adjust the depth of the
teeth until they run smoothly. Then with a microscope,
second photo, he carefully measures the distances between
the arbors and this information is then transferred to the
holes to be planted within the movement frames. This
illustrates another example of the exceptional quality
control parameters used throughout this project and that are
absolutely necessary for a design of this complexity.

Here the additional chatons in connection with the
strike initiator assembly are fabricated and inserted into
the movement frames.
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