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Calculation and fabrication of equation cam contours, springs for sunrise/sunset dials 

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I have asked Buchanan to provide what I call a 'forensic' report. That is to record his observations as he goes along. I will provide the .MP3 audio file for each segment but just in case your security settings will not allow you to open this file I have also transcribed each session. My additional comments will be inserted into the text from time to time and this will be in red text. Buchanan refers to each photo by the number of that photo which can be followed by each photo above the captioned text. The .mp3 audio file will appear at the beginning of the photo sequence in blue text. Click on this text and you can then follow along with the audio file by scrolling downward through the photos as they are narrated one by one in the voice of the restorer.

Photo 29 009. Here we have the equation of time differential and compared to the previous photograph we will notice the rocking lever is in the ‘down’ position virtually at 9 o’clock it’s rotated the small pinion on the differential arbor in the clockwise direction and lifted the differential pinion to the 11 o’clock position. And one can then follow the equation of time hand has accelerated somewhat in comparison with the mean time dial, on the main dial. Photo 29 010. Is a back view of the equation of time mystery dial. It gives quite a good impression of the mystery aspect of this dial. Photo 29 011, is the initial marking out of the seventy three steps on the equation of time cam. The cam blank has been mounted on the seventy three toothed gear wheel or drive wheel and the teeth are used as indexing points to get our seventy three equally spaced lines at the correct arc to match the arc of the leading lever for the equation of time mechanism. The target diameters for the equation kidney cam are 73mm outside and 30mm inside. We have assumed that Pouvillon would have used the latitude for Paris for this mechanism which is 480 51’N.

Photo 29 012. Here we can see equation of time blank, again on the marking object, the dividers have been set at the distance of the pivot point of the reading lever to the reading point and the point of the divider on the big brass sheet is at the same distance from the equation cam arbor as on the clock itself. It’s necessary to mark this line in a curve to match the stroke of the lever Photo 30  002 is the equation of time blank with the various points marked out on it (using center punch marks) which reflect the measurement from the clock and gives the correct equation of time dial reading for the correct date. Photo 30 003. We have the initial or the cutout according to the making out on the one year arbor. The blue marks on the teeth of the year wheel are to facilitate checking our cam as we step it around through all 73 positions to compare it with the equation of time chart.

Photo 31 001. Here we have my error chart where we have in the first column numbered 1 to 73 we have 73 stations. The next column what the equation of time cam, the amount of minutes the equation of time cam should lead or lag the mean time hand. And then in the third column we have the actual error or the difference between what the equation of time dial should read and what it actual reads. In other words I now have my errors. On the cam blank you can see on each station I have marked a dot for each half-minute error. So we have errors varying from approximately minus 1 minutes to plus 1 , or about a three minute error. One has to, of course, only remove metal so we then convert these errors to all metal removal amounts and this is what’s marked out on the cam. Photo 31 002. Here we have a close up of the cam with the various error marked out I think you can see our worst error was two minutes at this stage. And we now start filing more or less away on the portions according to the errors. Photo 31 003. I think this is about the third time around where we are still removing errors. As you can see the errors are becoming more equally spread, and I always end up with a metal removal. So on the right hand side you can see one portion where we have zero error and everywhere else metal has to be removed.

Photo 31 004. Here we have yet a further stage and you will see that our errors are slowly reducing. Photo 31 005. As you can see our error chart is full and we are getting toward the end of our whole exercise. Photo 31 006 is the total numbers of tries or adjustments required to produce the cam it took me at least eleven comparison and filing cycles to get a correct cam. One can see the laborious process this actually is. These charts show the methodical process and patience needed to compose these comparison tables needed to create the correct cam outline.

Photo 31 007. Here we are approaching the final stages and we’re just removing metal according the number of dots on each station. It’s a slow process and one has to be careful not to remove metal where one needs it to remain. Photo 31 008 is the final cam replaced in the blank and shows the final amount of adjustment required, possibly due to the lack of my skill but we have the correct adjustments in. The third photo shows stock watch balance wheel packets.

Next the spring is attached to the indicator hand's arbor used in the length of day/length of night dials. The next photos show the retaining taper pin used just like it would be in a watch to secure the other end of the spring. We are careful to use the posts and holes that were already present to reverse engineer what we believe were present when the clock was originally finished. These springs are used to eliminate lash in the gear meshing so the hands will read off the dial with greater accuracy.

Pouvillon-33-000.MP3. Photo 33 001. At the bottom of the picture, out of focus, we have the hand we’re hoping to replace. The coil on the right hand side is the spring we have selected to use as material of suitable thickness and on the left hand side the actual piece we have cut off. We’ve drilled a hole in the foreground we’ve started to round off to make the boss of the hand. So here we have gone from the delicacy of a watch balance spring, to using the material from a full fledged clock spring material. In fact this material is very well suited for the making of clock hands. Photo 33 002 is the embryo hand and the hole drilled in the end fitted onto a pair of filing buttons used to control the width of the rim around the boss. Photo 33 003. Another view of the filing buttons. The tubular section pushes up the spigot and prevents one from filing any further than is required.

Photo 33 005 is the vise we use for filing and in the foreground the binocular magnifier used to see filing right up to the line in other words to ensure suitable accuracy. Using diamond files here as first of all the hand is small and secondly I have not softened this material as I want to preserve its springiness. It helps prevent bending during working working (filing). Just next to the file on the bench itself you can see the actual copy hand or the hand we’re trying to copy and the brush on the right hand side used to get rid of the filings so we can see the scribe line clearly. Photo 33 006. A closer view of the embryo hand held in the vise you can see the filing buttons just to the right hand side of the jaws and you can see we haven’t started to do too much of shaping as yet. Pouvillon-33-006a.MP3. Photo 33 007, (not shown). A close up of the blank for the hand. We’ve filed the boss around until we’re approaching the root of the hand.

Photo 33 010. Here we have the two hands, (for the length of day/length of night dials). The hand on the left hand side is the new hand and the one on the right hand side is the original. You can see there are subtle differences it’s also difficult to match the differences perfectly. Photo 33 011. Another view of the two hands. The hand in the foreground (lower) is the new hand and the one in background is the original to be supercritical you can see I haven’t thinned down the pointer section or the front section of the hand quite as thin as the original. This should be attended to finally. Photo 33 012, (not shown). Another view of the two hands side by side. These are about 1.5cm in length or just over inch.


Pouvillon-34-012.MP3. Photo 34 012. We have the lever operating mechanism for the sunrise/sunset dials (shutters) and also the length of day/length of night dials. The arbor projecting rightwards from the drop down pillar is the one year arbor that will finally revolve the five operating cams. Working from the right hand side the brass lever with the two little pins just above the arbor the equation of time mechanism. Then the two straight black levers operating the sunrise shutter and the sunset shutter and then we have the round wire with the counter weight which operates both the length of day and length of night mechanism. Photo 34 013. We have another view of the sunrise/sunset-length of day/length of night dial and its operating mechanism. In the background we can see the one year arbor.

Photo 34 014 is another view of the operating levers for the sunrise/sunset-length of day/length of night dial. What we are showing in these three photos are the pair of 'C' shaped springs Buchanan used to give bias to the sunrise/sunset shutter levers needed to keep these levers seated on their respective cam perimeters. As was done in the case of the cam pack assembly as well as the small hair springs installed earlier we were careful to design our parts to comply with what was already present in the clock. If you look closely where the spring ends are anchored they fit neatly onto the grooved area on the drop down post as well as their counterparts on each lever. Without actual photographic evidence we had no idea what Pouvillon had originally done. It is also possible that he simply used a set of conventional coiled springs; these too would equally fit within the original clock's configuration. We think this design is more elegant than coils; notice the beautiful blued finish. Photo 34 016. We have a front view of the length of day/length of night dial. Showing some fairly extensive tarnish or corrosion as well as the shutters lifted far too high.

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