Key Findings

[kusmi]

Recently I'm thinking about calculator keys – and how to make them DIY-style. Looking at the forum, there are many very interesting DIY calculator projects – and from the electronics standpoint they are already on the finishing line. Most of those are only intended to be produced in small batches (say 50 or max. 200 calculators?) – so large scale manufacturing of keys/housing would probably be too expensive.

One remaining hurdle seems to manufacture old-style HP quality keys: To produce them in double-shot injection molding technique is very expensive (as I'm told – I never actually saw a figure how much it would cost, e.g. on a Chinese factory?).

So I'm looking to find a method to make decent quality keys – and I'm also interested what a prospective buyer would pay for good quality keys? (Obviously, if you are willing to spend $100 on a complete key set and produce 100 calculators, you could probably already produce an injection mold. But if you only want to spend – say $10 – for a set, it's not possible...).

I don't have the required tools available to produce the keys, but I'm looking to find some "creative" ways, on how we could solve our key problem ;-)

So, to start the brainstorming, here are my first ideas:

1.) DIY-calculator projects could share keys between them. Basically we could produce an extended set (say 60 keys) with different key markings. Let them produce a batch of 1000 key sets and then use them in all of our DIY calculator projects – we then share the cost of the injection molds between those projects. This technique does only work for HP 42S style keys (only one single marking on key) – alternatively we could produce slanted keys, but leave the slanted part empty – we could then use ink-printing or laser markings on the slanted part.

2.) 3D print the keys and use some surface finishing techniques. Now we are approaching DIY style thinking :-) I don't know what kind of plastic is used in old HP calculators (ABS?). I do have a 3D printer at home, so this looks somewhat interesting. I'm not happy with the surface finish yet - the result is too rough. But we could find some smoothing techniques to make the keys look&feel nicer. Of course we would first need a printer capable of printing two materials at the same time (e.g. white for markings, black for actual keys).
2.a) Traditional smoothing involves putting the 3D print in an Acetone vapor bath.
2.b) Another approach could be to CNC-mill an aluminum plate with key-molds in it. Then stick the 3D printed keys in those molds and re-heat it in an oven to the melting temperature of ABS plastic. The plastic should then get a bit "thick-honey-like" and reflow into the mold, producing a shiny surface. Hopefully not destroying the key marking too much.

3.) There are some DIY injection molding techniques – but I think double-shot injection molding might be too difficult – especially because you would need two very tiny molds (one for the key markings - one for the overall key). But I might be wrong...

Then we have the question on how to "mount" those keys to the actual calculator. Here I'm thinking of either doing it the old way (domes - but then we would need to produce the dome sheet as well...) or just use micro tact buttons (something like this) but with elongated buttons, so we could drill a small hole in our DIY-buttons and glue the tact button on it?

At the moment method 2.b) and 1.) look the most promising to me – but I just have started thinking, I'm sure some of you have even better ideas?