Octagonal complement-key (for the Burroughs Class 5 machine)

A fancy shaped octagonal keytop without any text, a blank key.

That is the CAD-model for the top-left key of a Burroughs Class 5 Calculator. As is well-described elsewhere on the internet, this machine was a direct competitor to the Comptometer. It was cheaper and lacked the carry-suppression tabs. This means that when subtracting with the complement-method, zeroes (i.e. the 9-key) need to be added all the way to the left of the register. That is why the Class 5 has a lone 9-key over the 10th register position; as seen here in the image from 1927 advertising material (see the complete page at rechnerlexikon).

This key is only for chasing the overflowing, fugitive '1' out of the register. So it does not have any marking. It is a blank version of the same octagonal pattern as the rest of the keyboard 9-keys. This is more clearly visible in e.g. this snippet from a 1921 advertisement (see complete page at rechnerlexikon, a site btw well worth exploring!)

This key being a bit exposed at the edge of the machine probably contributed to many of these Burroughs Calculators missing this key. Or, as in the case of my specimen, the original replaced with any key that was available at the time - in this case a round, small key with the 9 legend sideways.

The original-pattern blank key is today well within capability of a regular FDM 3D-printer. So from the CAD model a mesh, then 3D printed in black PLA material. In the below image the 'raw' print straight out of the printer placed on the stem. The dimple is clearly showing the layers - printed at 0.12 mm.

With a little sanding to smoothen the dimple's layering and roughen the surface, it was painted black. With oil-based hobby-paint, the net-effect is a smooth surface that is also quite robust.

The STL is available in the Thingiverse here and the CAD-file with several mesh file-formats for printing is available here.

The finished, new key on the calculator - complete keyboard with a decently matching top-left key that is perhaps still looking a bit new and shiny: