Strike and repeat control assemblies, initiators, March 2010
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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 1800 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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