Wednesday, July 19, 2023

Comptometer keys - making new

The project Comptometer model H got 'new' keys at its refurbishment in (probably) the 1940-ies, however these cream/green keys of a model J don't age well. Also, the keys of a model H Comptometer should be black/white instead of green/cream. These are one-piece moulded 'plastic' keys, similar to the keys of e.g. an early Oliver typewriter. As part of this model H's current 2023 refurbishment, decided to go for a full set of new keys in black and white. Replacing all keys makes it actually a lot easier, because blending in with any original keys won't be a problem.

In a 3D CAD-program (mesh-editor) new keys were designed, dimensions taken from the least-degraded remaining keys.

The dimpling was of course varied for the even and odd keys, as well as the practical depth of the lettering was worked out with some test-prints.

Also from some (failed) test-prints, worked out a way to make the pockets for the key-stem a bit flexible to get a workable press-fit on the stems. The open areas around the stem-pocket could later be filled after fitting on a stem (e.g. with glue), in case the extra strength should be needed.

Some more experimenting was done to find the best way for finishing the keys. Printing vertical gave the crispest lettering, the visible layering steps in the dimpled key-top could be sanded smooth without problems. Fine-grit sandpaper held over e.g. the ball-end of a file- or screwdriver-handle to press and smoothen properly into the dimple.

Then two layers of enamel hobby-paint to further smoothen the surface and give them an even black (or white) surface finish. How this wears will remain to be seen, but the solid material underneath is at least also black (or white-ish).

For picking-out the lettering, best results were had from filling the numbers with latex paint after lacquering the key with enamel - latex from testers; cream and black latex wall-paint (Black is surprisingly hard to find in latex wall-paint, by the way.)

A whole batch of keys was then printed in black and in white resin. All subsequently sanded, lacquered and lettered to get a full of keys set for refurbishing an 8-column Comptometer model H.

The STL models for a full column of keys for a Comptometer can be downloaded here.


  1. Excellent work! Really points out the usefulness of 3d printing

  2. Was it hard to remove the old caps? I have understood that you need to break them to get them off.

    And do the new caps feel good and sturdy?

    I bought myself a Comptometer model L, and while the green key caps are in good condition, the white ones are flaking apart. I might need to replace them at some point.

    1. Removing the old keys was actually surprisingly easy - see my Aug 12th post. The leystems do need to be taken off the machine though.

      The base of the new keys should've been made a bit thicker - they are a wee bit fragile (but are holding up). Resin is more brittle than the MJF-nylon they were originally designed for. So, probably good-enough, but could be sturdier.

      The white keys are the problem indeed - they are of cellulose-acetate and don't have enough white pigment to act as antacid - so degrade and leech vinegar.

      Good luck with your Comptometer! (a J?)