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Calendar readout control levers, begin digital year indicator  - April 2015


These arbors are for the various release functions to set the dial work. The three brass components are the sequencing cams that control the release of each calendar function; day, date (uses the same cam) and month and year. They all mount on the calendar input arbor which revolves one-half of a revolution daily for each operation. Similar to the four legged cam on the strike chime release created four years ago in April 2011.

These are the rocking levers which turn the forward or reverse motion of the sequence cams shown above, into a smooth forward or reverse direction for the connecting levers that will control each of the bird analogue detents. There are a total of six, for the forward and reverse functions for the day and date (one pair), month and leap year. These rocking levers working together with the cams, surprise pieces within the perpetual module, bird analogue detents along with the matching crenulated output wheels will act as computational ‘logic gates’, often referred to in computer design as AND, OR and NOT gates. These mean that when a certain input is registered, the output will be the result of the programmed logic gate and will follow a certain pattern. This computational logic is the result of the surprise pieces controlled by the various cams within the perpetual module.


Logic gate truth table. Now let's see how these gates apply to our mechanical perpetual calendar calculator.

In the Gregorian calendar three criteria must be taken into account to identify leap years: First, the year is evenly divisible by 4; second, if the year can be evenly divided by 100, it is NOT a leap year, OR the year is also evenly divisible by 400; the third order. This means that 2000 AND 2400 are leap years, while 1800, 1900, 2100, 2200, and 2500 are NOT leap years. When these criteria are accounted for the calculator is permanently perpetual; it is a third-order perpetual calendar calculator.

A basic first-order perpetual calendar accounts for the quadrennial leap years. That is, a four year perpetual calendar. The next complication is the 100 year calendar where at the end of that 100 year period the leap year is skipped. To keep the calendar in synch that skipped leap year must be continued for the next three 100 year intervals. That is for the next 300 years, every 100 years a leap year is skipped. The 400 year perpetual calendar then allows the leap year to be re-inserted once every 400 years; making it a perpetual calculator. Of course it goes without saying that the calendar properly accounts for the regular sequence of the months ending in 30 or 31 days.

Illustrated above is the method used by Mr. Martin Burgess for the calendar dial's crenulated wheels. In essence they are freewheeling, but are so light that they will run along with the underlying toothed drive wheel since they are mounted upon those drive wheel arbors through the use of a cannon arbor not unlike the arrangement used in a conventional motion works for the hour and minute hands, but instead of these being isolated from each other they are allowed to contact. So when the drive wheel turns the crenulated wheel upon which the dial hands are mounted will turn with it. This is because there is contact between the inner arbor of the drive wheel and the cannon tube that surrounds that inner arbor. Buchanan describes the process as follows: "The first time I examined Martin Burgess clock, the one installed at Schröder Wagg Bank in London, which used the same principle, it took some time to realise just how simple the principle was, and there were two of us and we had it in our hands! I think you will also understand the need for poising and that I cannot use a castellated wheel to perform any work although we can obtain information off it. It also makes hand setting easy as all we do is lift the latch and move the hand." That castellated wheel is beautifully illustrated in the February 2015 installment. We now can see why it was so important that the entire perpetual module be perfectly poised. Here we are employing another fairly unique design feature to this machine.


These videos demonstrate the Martin Burgess clutch used to drive the crenulated output wheels which are connected to the readout hands.


Buchanan has made a temporary front plate for the calendar multi dial cluster from clear plastic so as to give the viewer a better understanding of the fabrication process.

This photo shows the rough blank for the lever that will read the crenulated daily index wheel of the perpetual module.

This photo shows that blank refined into its final, curvaceous shape which follows the rid line just to the left. The red arrows highlight the decorative organic plant spurs that are used throughout this project.


The sinuous trip linkages which will control the bird analogue detents begin to take shape. The circled area shows a flat on one of the levers that connects with a pin on the date crenulated wheel. This provides for the once monthly trip. See how this lever becomes more refined from left to right, red arrow. Next the collet used to attach that lever to the one below it and rotate both around a common pivot point.


The lever before and after decorative shaping. The videos below show demonstrations of the calendar drive.

The first video demonstrates a 28 day February, the second a 31 day month.

This video demonstrates a 30 day month.


These photos show the toggle mechanism which replaced the solid, immovable wire as illustrated towards the end of the February instalment, which was used to read the various cams and crenulated ring on the perpetual calendar module. It solves the lag time for the change of the date as it relates to the rest of the calendar dials when the calendar is run in reverse.

Here we see a demonstration of the calendar being run in reverse for a normal 28 day February.

Can we possibly cram in any more parts? Yes we can, and we will!

Quadruple frame calendar module 

The complex calendar mechanism is housed between the quadruple frame assembly yet it is completely removable as a unit from the rest of the movement. From left to right the frames contain: forward remontoire, reverse remontoire, calendar works. Below the horizontal pillar is the overdrive safety clutch built last month.

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 Buchanan now begins the fabrication of the digital year indicator


The first step in trying to create a mechanical digital counter is to see how these function. It is not the sort of mechanism one normally encounters in a clock. The first photo shows a couple of modern inexpensive digital counters which were taken to pieces. In the next photo one can see a digital counter in the background on a clock by the custom clockmaker La Vallée of Como, Italy. Their digital readouts have ten flat surfaces, in other words the ring has ten facets one for each digit. I found this to be a nice innovation over the conventional cylinder readout and when done properly is far more legible.


One of the key things learned from the forensic observation of the commercial digital counters is the type of tooth profiles used in those devices. Conventional clockwork uses cycloid tooth profiles for the wheels and pinions. That design, however, has too much lash to be useful in this mechanism. Here one needs to have each digit disc align as accurately as possible to its neighbor so that all of the four digits for the year are perfectly in alignment as possible. This required Buchanan to make a custom tooth design to achieve this goal of minimal lash yet having a smooth drive between the digital discs. The type of tooth profile for the discs was very close to a perfect circle. So a drive pinion had to be made to mesh properly with this type of wheel tooth profile.


Here we see the making of the ‘carry’ pins. These are located on the opposite side of each digit disc and after a complete revolution of the disc will advance the adjacent decimal wheel to the left by one digit, thereby carrying the calculation forward. This allows the additive calculation to continue through to the next decimal counter and so on. In our example we have four decimal wheels so the first, rightmost wheel would have to rotate ten times to advance its neighbour by one digit, and turn 10,000 time to completely turn over the entire set of four decimal wheels.


Here is how how a new custom tooth profile fly cutter is made. The first photo is a 50x paper pattern of the pinion tooth gap. The second photo shows the optical profile grinding machine that will convert this to the actual size needed to grind the fly cutter needed to make the pinions. Buchanan describes the process: "The gear to be copied is placed on the work table. Above it is a microscope attached to a 50:1 pantograph arm (red arrow), at the end of the arm is a pencil. By manipulating the arm one guides the cross hairs of the microscope along the profile of the gap in the gear tooth and a 50 times over size copy, yellow arrow, is drawn on the drawing table. One then replaces the sample gear with the cutter blank and then by plotting along the line, or cut out, on the paper, one manually manipulates the vertically oscillating grinding wheel to grind away the tool until the cutting edge is up to the cross hairs of the microscope. Then the microscope is moved along the profile a little further and the next portion of the tool is ground back. In theory one produces many small steps along the profile but the edge of the stone has a radius and the steps blend somewhat. Some practise is needed and good hand eye coordination helps a lot. The line of oscillation of the grinding wheel can be adjusted to give the correct cutting angles. Once cut the 8 tooth wheels need half a tooth removed the work correctly."


The custom fly cutter is shown next to one of the pinions produced from it. The cutter was made to fabricate only three drive pinions. The last line Buchanan writes above about a half tooth being removed is shown in these two photos. Half of the width is removed from every other tooth. This is necessary since the eight teeth of the pinion on the unaltered side are in constant mesh with the ten pins on the decimal wheel to the pinion’s left. However on the opposite side the pinion must mesh with the paired ‘carry’ pins that are on the neighbouring decimal wheel to the right. The alternate milled areas are required because the paired carry pins require twice the area than do the single ten decimal pins. If one tried to drive the pinion without its neighbouring tooth milled down, it would immediately jam.

Now the ten facets are milled onto each decimal wheel.



Buchanan has now fretted out each of the decimal wheels. Once more I must point out that he has taken the ‘high road’ in his quest for the maximum quality and the pursuit of what I believe sets his work apart from all others, his skillful fretting out of every conceivable part. These wheels are close-mounted to each other and it would be difficult to see if the outer, let alone the two interior wheels were spoked; not to mention that these are only about one half inch, (1.3 cm) in diameter.

Year digital counter demo showing a minimal amount of lash between the decimal wheels; allowing for good alignment, yet a smooth meshing between them.

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