Replacement platen knob for Corona 3, late model, typewriter

To replace a missing platen knob on a late 1920s Corona folding typewriter, a simple 3D model to print a replacement. The early folding Corona's have a more 'chubby' platen knob, this is a model for the thinner, 30-rib pattern used from the late 1920's on, as e.g. on the Special models.

Printing it simply with the stem-down and using supports will get a one-piece print with the scarring from the supports on the rear-face of the knob. This face is not really visible when mounted on the typewriter.

Thread tapped in a plastic part is not really strong enough to take the force needed for a set-screw. As solution a slot is included in the stem to take a steel hex-nut. Press a regular M2 nut in the slot in the stem. A short M2 screw then as set-screw.

The forces now needed are minimal anyways - this screw only needs to prevent the knob from sliding off the platen rod. The flat of the hole already prevents any slipping of the knob on the rod.

With matte black hobby paint (water-based acrylic), several layers and some very light sanding and rubbing, the layer-stepping of the print is hidden. In below picture, a printed reproduction is next to a genuine original knob.

In the side-view it is of course easier to tell the reproduction from the original.

Fitted onto a ~1930 Corona typewriter, it is functional and looks the part.

The 3D model is available for download on Printables.