Clock Conservation Project: A.Brocot and Delletrez c.1850

By Eliott Colinge, PG/MA Conservation Studies, Clocks and Related Objects

This French clock signed A.Brocot and Delletrez presents an unusual disposition of the striking mechanism parts. The striking parts on the front plate were missing on receipt of the object in the workshop. The presence of a square section on the hammer detent indicated that there was originally a gathering mechanism working with the striking rack.

A description of the mechanism was found in Chavigny's work on the Brocot family (Les Brocot, une dynastie d'horlogers, by R. Chavigny, Editions Antoine Simonin, Neuchâtel, CH, 1991, pp. 177-181) and revealed that a patent was issued for this mechanism around 1850 by Antoine Brocot.

Following the description and the measurements available on the existing mechanism, a design was made using Autodesk® Fusion 360™, a computer assisted 2D and 3D drawing software. An animation of the mechanism shows the functionality of the missing parts (Fig.5)

This model provided a critical thought process regarding the interactions between the different components, simplifying their later making.

Figure.1 below shows the different steps of the project. On the right is the photograph of a similar mechanism found in the book by R. Chavigny, Les Brocot, une dynastie d'horlogers. In the middle is the 3D model, and on the left is a photograph of the actual mechanism with the newly made parts.

The making of the components revealed a rather organic approach involving the making of all the parts at the same time with a gradual adjustment of their respective shape to insure the proper functionality of them all.

The making of the parts required the preliminary making of tools. The rack shown in the figure below (Fig.2) illustrates one of the making processes.

Due to the length of the lever between the rotation centre of the rack and the teeth, a conventional mandrill could not be used.

On the top left is the tool made for the project, a screw maintains the plate firmly against the mandrill, therefore diminishing the vibrations and the stress applied to the material whilst being machined.

The tool at the bottom of the picture is a triangular profile cutter used to cut the teeth on the lathe.

The 3D design of the mechanism required first the drawing of the components in 2D. The figure below (Fig.3) shows the superposition of the rack on its drawing. The software also allows a quantitative evaluation of the measurements. For instance, the angle between two consecutive teeth of the rack was found on the software to be 2.43° of arch. This value was based on the centre distance of the snail and the rack and the height of the gap between the 1 hour and the 12 hour step of the snail. The snail is the part which determines the number of strikes of the mechanism (the part in the centre of the mechanism, see Fig.1. and middle/grey element in Fig.5.).

The dividing plate was chosen accordingly by calculation. The closest match found to the 2.430° was 2.429° and implied the use of 7 divisions of a 17 divisions dividing plate having one revolution every 6 degree of the workpiece (1/60th of a turn). The Figure below (Fig.4) shows the dividing plate that was used for the making.

The piece made according to the angle found and has revealed successful functionality in the mechanism. A similar methodology has been used for the other parts of the mechanism.

The project revealed the importance of a thorough preliminary planning of the steps involved prior to the making of the components. 3D drawings software products are powerful tools regarding the designing and simulation of mechanisms.