Re-keying a Comptometer model J

Another, second Comptometer got a full new keyboard. 

This one is a common model J that was bought as a parts machine - it was severely rusted (all keystems rusted red) and half the columns didn't work. 

But it had good white keys, so these were all harvested to refurbish the Supertotalizer. Having taken its keys, couldn't quite bring myself to 'bin it' - so made an attempt to get it working again.

As often with these machines, deep cleaning of some rods and levers internally got it all working again. All keystems taken out and refreshed (steelwool!) and remaining green keys removed made it look much better - then it needed a full set of 90 new keytops.

The very labour-intensive method for making keys as used before on a model H was too much for this J. Besides, the keys on a model J should not be fully opaque as on a model H. Especially the white cellulose-acetate keys of the J are a bit semi-translucent. So, no expensive resin-printing and lacquering+polishing, but a simple FDM print for this machine.

New 3D models were made, this time suitable for less-hassle FDM printing. Keytop legends were copied from some remaining good specimens. A modern ' hobby printer' can then make pretty good parts, even with a regular 0.4 mm nozzle. The STL files can be downloaded from Printables, with extra hints on print settings.

To do some simple visual smoothing, the keys were given 3 or 4 layers of clear varnish. That also stops the lettering-paint creeping between layers when filling the lettering.

Latex wall-paint again was used, filling the lettering (well, numbering...) - letting it dry, then any spillover on the keytop can be wiped off with a damp cloth. Hold the cloth taut, flat - then it will only soften and take off the paint on the key surface and not draw it out of the recessed lettering.

The white and the green plastic is a bit 'bright' still, but the documents of the time do state that the Comptometer had green-white keys, not green-cream! 

Several columns were by the way printed using a raft, which isn't really needed - no rafts gives a cleaner base. Nevertheless; result was a full set of 10 columns of new, reproduction keys in PLA plastic.

Fitted on the machine and comparing side-by-side with aged originals on a 1930s white-green Comptometer, the keys do look very bright - yet this might be what these machines looked like when new.

The new 3D models and method of manufacture really lower the cost for making new keys for a Comptometer. In case one or two keys need replacing, a printed part can of course be painted to be a good colour-match with the rest of the original keys. In case a full keyboard is replaced, this is less critical and ease of manufacture becomes key. (pun!)

Result is that we now have yet another Comptometer; a very common model J - with uncommonly bright keys :-)