Saturday, August 12, 2023

Fitting new key-tops on Comptometer key-stems

Having printed a new set of key-tops for the Comptometer, these have to be fitted to the stems. The key-tops of a Comptometer cannot be pulled off, but the entire key-stem has to be removed from the machine. (This is actually easier than it may seem - at the bottom of the mechanism unhook a spring + tilt a bracket and a stem comes right out.)

Because there may be small differences between the columns - either from adjusting when new, or from wear over time - the key-stems were kept as sets per column. To start with replacing; a column of old (and dirty) key-stems and a set of new key-tops.

To remove a key-top from its stem, the stem is clamped tight in a vice (using card to prevent scratching of the surface). With the tips of pliers held against the key-top, a few light taps of a hammer against the pliers near the stem will make the key-top come off cleanly. It's bad practice to use pliers; it's bad for the pliers - a custom grooved bit of steel or hard wood should have been made, but the taps are not hard and it was a cheap-n-rusty thrift-store find.

Every stem was thoroughly cleaned. Rust and dirt was removed with steel-wool, where needed some filing of the sides. A scrubbing in soapy water then to further clean and remove any steel-wool particles clinging to the part (not something to get inside the Comptometer mechanism!).

As a finishing step, the stems were rubbed with aluminum - this gets smeared into any rough, rusty areas. This helps to protect and also makes any defects less noticeable. Working per column-set and one key at a time to keep track and not mix-up any numbers and stems - all the stems are different lengths and a '3' key-top should go on a '3' stem.

The new key-tops are a press-fit on the stem - in normal operation a key only gets pushed on, the only 'pull' it gets being the momentum from spring-back when a key is released. They're on pretty tight and should be fine. Some glue could be added later, if it turns out that key-tops do jump off or get lost. Working through a column, the end-result is a re-keyed set of key-stems. (Probably the cleanest Comptometer keys in the Northern hemisphere :-)

Cleaning and replacing the key-tops on all the 8 columns of this Comptometer, gives 8 sets of clean key-stems ready for re-assembly into the machine - likely much the same as what the earlier 1940s refurbishment would have had lying ready :)

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