Saturday, September 2, 2023

Improvising new feet for the Comptometer and a window-sheet

Comptometers have feet - although today many are sat flat on the table like a shoebox, they'd originally have had rubber feet. These feet were press-fits into holes in the bottom of the case, found in grey or black and usually completely decayed. They're either completely lost or pressed flush with the base of the machine, the rubber being pushed with a bulge into the interior. (Remnants of a foot can also sometimes be found floating about the inside of a case. Or stuck to the cork lining.)

To make a quick-fix set of feet, these were built-up from rubber. Of itself, the feet could be a good candidate for 3D printing replacements (FDM, PU rubber), but disk-stacking works gooed-enough. Using small drops of cyanoacrylate glue, disks of 10mm (one), 16mm (one) and 18mm (several) were glued together in a stack.

Using hole-punches, disks are cut from a sheet of 2mm rubber. Place dot of glue, place disk on top, centre properly and then press together firmly. As final element, self-adhesive felt-pads to get 'dampening' feet. 

These improvised feet are almost a press-fit into the recessed holes with the 10mm disk (10.5 would've been good, but no such hole-punch :-). To keep them in place, some extra hobby-glue is good enough (not cyanoacrylate!). This stacked method is a quick, improvised alternative to 3D printed feet. They simply work and Comptometer feet are not too visible anyways.

The windows over the numeral-wheels are covered with a single sheet of celluloid. The original sheet of celluloid on this machine was very yellow. Now that the keys were bright again, also wanted to have clear windows over the numeral wheels. A strip of 0.2mm thick clear-plastic was cut from a sheet - it was sold as 'mica-sheet' for crafting. No idea what plastic it really is, but it's fairly clear and can be cut easily. It is also flat, the window must be curved to fit under the front window-panel.

To create the curvature needed, it was placed over a ~33mm diameter tube held in place with elastic-bands. A hair-dryer then was used to heat the new window-strip to have it relax and settle in a curve. This sort-of worked; it is correctly curved, but it got some extra 'wrinkles' as well from the elastic bands. 

With better clamping of the sheet and better temperature control (oven?) this method could probably be improved on, but the quickly improvised new window looks fine. More importantly, it works: gives a clear view of the numerals and will stop dust and dirt getting into the mechanism.

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