Reproduction comma-indicators for the Thales pinwheel calculator

The Thales model CE mechanical calculator was missing 4 of its 5 comma-indicators. 

The originals are fairly fragile and are often missing from these pre-war Thales calculators. The bottom of the thin sheet-metal stamped construction is clamped and comes away easily - the whole thing then just drops off its rails. Although the machine 'works' also without them, to be complete a new set of these "Kommaschieber" could be made using 3D printing.

Luckily there was one remaining slider still with the machine - using this one original as a guide, a 3D design of a replacement was made that would be printable in plastic, function on the machine and still resemble the original. A set of these were then printed in PA using MJF-technology printers. This technology creates parts that are quite strong, slightly flexible and with dimensional accuracies and features good enough for this part to work. The surface as-printed is slightly grainy, but can be smoothed with sanding.

For printing (and handling) a set of the pointers were grouped (with connecting 'beads') into a single printable part-file and ordered in MJF (Multi Jet Fusion) at Shapeways. When received the parts were first washed to remove any grease or other remaining contaminants. Then the group was given a coat of primer (spray-can) before a first coat of 'chrome' lacquer. The parts are then still very grainy (and shiny), ready for separating and sanding smooth.

Using water-based modelling paints in metallic-silver, the parts can be given a proper brushed-metal appearance. Because the paint is water-based, water-based inks could be used to find a good colour match for the nickelled parts of the original. In this case, brown and yellow Ecoline was added to the silver-paint to find a colour that would not jar too much with the original. (Metallic 'silver' is a surprisingly varied colour.)

It took a few iterations of filing and sanding the surfaces smooth and re-applying a newly-mixed brushed-nickel paint to get the comma-indicators looking the part. The printed parts actually fit the comma-bar without any modifications and the PA material gives it a good, firm grip on the rails for sliding them.

Now with the complete set of 5 indicators, the Thales mechanical calculator again looks as it should and comma-positions can be again indicated with the pointers. (The comma in the lower-left UW register shows the answer to square-root of 10 being 3.162277. The indicator in the lower-right RW is between positions 11 and 12, where it started with 10 in positions 13 and 12. The RW and the upper EW registers now show the remainders of the square-root calculation. These actually make it possible to determine if the last digit of the answer should be rounded up or down - if the RW is less than half the EW, then the next digit would be less than 5 so round down, otherwise up. The EW is 632... so checking against 316.. immediately shows the RW is larger with 417..., so the answer has to be rounded up to 3.162278. Have been reading-up on the use of these calculators; fascinatingly ingenious machines!)

The 3D model of a group of 10 of these indicators is available for download at this link.