Zigzag turntables
Brian Loomes’ articles are always a pleasure to read and his latest on the zigzag dial decoration is no exception. A very interesting account of a branch of clever dial and movement making as done by a group of penny pinching, oppressed but very skilled artisans.
However I have to take issue with him on one point. He says that ‘… the dial was pinned down to a turntable … the engraving tool was rested in a guiding brace … then the dial was slowly rotated on the turntable as the chisel was worked from left to right and back, thereby scoring a continuous zigzag line’.
I am sorry, but as written I can’t see that this would work no matter how skilled the artisan. These ancient machinists had to be a lot cleverer than that. For the dial shown in figure 2, I suggest that the following is needed.
1. The dial is pinned down to a turntable that is also separately capable of controlled lateral (or vertical) movement.
2. The turntable has some sort of dividing ability in rotation. I do not suggest that it is a full-blown dividing head but it is capable of controlled equal division.
3. The turntable is capable of continuous smooth rotation as well as rotation in defined steps.
4. The engraving tool, in its brace, is capable of controlled downward movement probably by simple hand pressure.
Item 1 above is not difficult to imagine—a calibrated turntable whose spindle is mounted on a calibrated dovetail (or similar) slide so that the whole can both rotate and can move horizontally (or vertically). To keep the device simple there were probably rubbing supports under the engraving point.
In use the proceedure would be something like as follows.
1. The dial is mounted by its dial pins on the turntable.
2. The under-dial support is brought up to the underside of the dial in order to take the thrust of the engraving against the probable slop in the assembly.
3. The circular equally-spaced double rings are engraved first probably using the engraver’s eye measurement. Now one of the zigzag engravings can be started.
4. The tool is brought down against the outside of the inner ring and the first zigzag started as the turntable is moved from left to right on its horizontal slide in a steady smooth manner (probably by a screw) to the completion at the inside of the next ring.
5. The tool is lifted and the turntable is rotated by a fixed step and the zigzag repeated.
The calibrated rotation and the smooth horizontal movement thus gives the neat regular layout of the zigzags within the circles.
All or part of the different steps may have been made by the engraver’s eye and hand alone. These old craftsmen were wonderful operators—that was a major part of their skill. Nevertheless it must have been something like this, mechanised or hand or both, in order to produce such a neat and properly divided engraving in both directions. Any slight deviation from the plan would have been so obvious as to make the dial almost unsaleable.
Ted Wale, Canada
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