Strike and repeat control assemblies, quarter rack lifting and gathering pawls; rack
let down flies -
This month we continue with the fabrication of the strike
train control systems and now proceed with the rack lifting and gathering
pawls and the rack let down fly fans. In a conventional rack and snail
strike system the rack is raised by a single or dual toothed pinion, and an
accompanying pawl holds the rack steady at the newly raised level after each
engagement of the pinion. The rack continues to be raised in this manner
until the strike sequence is completed. We have chosen a more interesting
road. In our movement the role of the raising of the rack and its holding
during the strike sequence is performed by a seesaw performing both of these
functions. I had first seen this in the ecclesiastical computer contained
within the cathedral clock built by
Jean-Baptiste Schwilgue in Strasbourg, France, c. 1843. This is
another example of my borrowing an ingenious design from the masters of
horology and incorporating it into this project. We
have done this with earlier components starting with John Harrison's
visually impressive dual compound pendulums driving his equally inspiring
grasshopper escapements. The remontoire powering those escapements are
Bernard-Henri Wagner's swinging frame remontoire; a particularly stunning
example of differential gearing, along with Robert Robin's beautiful chain
drive remontoire for the accurate release of the celestial train's
functions. Also Tompion's differential system to read the equation
of time directly from the mean solar time dial is employed as an alternative
to the simpler sector dial arrangement. There are more examples to come.
Buchanan says that we are now entering a difficult phase of
fabrication. Up to this point the lion's share of the movement, excepting
the frame, was the fabrication of wheel works. Making wheels, according to
Buchanan is straight forward. Once one establishes the wheel and pinion
counts and modules, the wheel's characteristics are determined. The diameter
determines where the wheel will be planted with regards to its neighbors and
from this the locations of the wheel pivots. Here we are creating lever,
rack and interconnecting assemblies that will need to be made to conform to
specifications in all three dimensions. Any change in one component affects
all of the others. Lengths, angles as well as visual esthetics must all be
considered in unison. Concepts must first be drawn out, then initially
fabricated in wood or plastic to test their viability and then either tested
again in brass or cut directly to the final steel material. This
material will later be polished and blued.
We now begin the quarter strike lift and gathering pawl
rocker assembly. The first row of photos show the raw stock with the outline
of the rocker frame drawn on the surface. Holes are drilled to accept the
pivot jewelling. Next the jewels are assembled. The flat jewelling is made
in-house. The middle photo shows one of the parts that serve both as a
spacer between the rocker plates as well as a pivot for one of the pawls. The last photo shows
the two pair of pivot jewels which will later support our allegorical bird
pawls. The center chaton is the pivot upon which the entire rocking pawl
assembly is mounted. It is double-flanged and is the structure that holds
the two parallel rocker plates between which the two pawls are pivoted.
Next the vertical base stock for the bird pawls is fitted. The
second photo shows the outer rocker plate set on top of the two vertical
pieces. The last photo shows the plastic mockup which was made to test the
working concept set upon the raw steel stock as a final check that its
incorporated within the entire material available.
assembly is then fitted to the arbor upon which it will be
supported. Now the exact positions for the jeweled beaks of our birds can be
determined in relation to the teeth of the rack. The flat jewelling
representing our bird beaks are next fitted to the steel stock and tested
for fit and functionality.
of the pawls as represented by the plastic mockup were never meant to be the
final shape. That mockup was used to test the design's functionality. To create
the final design Buchanan first began by drawing his concept on a photograph
of the actual parts mounted to the movement. These drawings flow over
several layers of parts as shown in the picture above.
That picture was then carefully cut up and these were used as
templates to cut the outlines of the metal pieces. The size of the photocopy
had to be carefully calibrated to match the actual dimensions of the real
components. One can begin to see the labor intensive nature of this
part of the project.
The steel components are cut by hand. The electrically
powered jeweler's fret saw used for most of the brass flat stock in this
project is not suited to cutting steel stock of this thickness and a larger
machine would not afford the delicate pattern that we want to achieve. This
sort of hand work will give the project an 18th century feel. What I call a
'warmth' to the project that simply cannot be had from a computer aided
The final product is shown above. In a conventional system
one would have between the rack pinion and gathering pawl maybe a half-dozen
components. Here we have twenty five.
The final assembly is mounted. The biasing spring is not yet
shown. Look at the very far left of the assembly to see the wheel which by
rolling against the heart-shaped cam, (later to be skeletonized), will induce
the back and forth movement to the rocking pawl assembly, see videos below.
Under conventional design standards that cam would have been the single or
double bladed pinion that would have been positioned next to the rim of the
rack teeth with a simple pawl to hold that rack during striking.
We now begin the fabrication of the quarter and hour rack
let down flies. These will mediate the fall of the quarter and hour strike racks. While not
completely necessary, the racks are fairly large and their tails will fall
onto the thin rims of the skeletonized snail cams, and besides it gives us
another opportunity to introduce another eye-catching feature to the
mechanism! These four photos show Buchanan trimming material from the inner
opening of the let down fly ratchet wheels. These parts are small enough to
employ a watch lathe. The scale of the parts we will be making,
in most cases, from now on will be on this order as we move on to the various
The first photo shows the exploded view of the fly fan pair;
approximately 32 parts. Next the rough-assembled fan. The third photo shows
Buchanan polishing the pinion teeth with a wood blank charged with polishing
compound that has the exact
tooth profile milled into its surface. All pinion teeth are polished in this
way. The last photo shows the
beginnings of the fly fan pillar mounts.
Next are some of the various machining steps necessary to
create the pillars.
The fly fans are now mounted their pillars. These are still
far from finished. There are a lot more refinements to be made to the
pillars as well as the fly blades. The screws holding the fly blades will be trimmed to size and the
heads made smaller to be better proportioned.
These photos show the fly fans mounted within the movement.
We now have the ability to set the quarter strike train in motion. The
videos below show the initial tests before the completion of the fly let
down fans. The quarter strike rack simply falls in the conventional manner
via gravity without the let down fly fan's assist.
For those who cannot use the built-in viewer above, I have
added the YouTube viewers below. You can also click on the individual video
links above and a clip will open on your computer's built in video viewer.