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?

[Katie Wasserman]

Probably the best keyboard for DIY calculators would be something where the keycap symbols were changeable. Years ago I remember having a discussion here about something like this, although at that time Optimus Aux wasn't around then. Potentially with a keyboard like this would wouldn't even have to have a separate display. However, until the price on these comes down by an order of magnitude they're not very practical.

[BruceH]

Another possibility might be to 3D print transparent key caps, with slot in the top. A paper printed key cap can then be slid into the slot to form the top and front side label.

This company does suitably transparent 3D printing but might be a bit expensive!

As always this only addresses the problem of the key cap, not the spring mechanism.

[Chris Chung]

I was toying w/ the idea of this simple 3d printed overlay top. The ABS / PLA should give some flexibility when keys are pressed. To be mounted on top of a PCB.

Except that I haven't figure out the labeling. I also don't have a 3D-printer to try it out. Will have to go to a library for prints.

       

[mivok]

It might be worth looking to the mechanical keyboard world to see if there's anything there that would work. Quite a few companies will do completely customized computer keyboard keys with small order sizes, and I found the following company while looking at the deskauthority wiki which claims to be able to do custom doubleshot keys of different sizes too: http://keycapsdirect.com/customkeycaps.php. One thing I haven't found yet is smaller than normal (where normal is a PC keyboard) keyswitches. I suspect if you could get small keyswitches, a pcb manufactured, add an ARM microcontroller and LCD display you could have a completely custom calculator for less than you would think.

[Chris Chung]

Looks like some HP calculators uses metal dome on PCB keys. If this is the "feel" that is desired, customised metal dome overlay sheets can be ordered (minimum quantity 1000+) via alibaba. Example offer here

OTOH, most modern calculators uses these silicon membranes which again can be custom made for some minimum quantity.

I would definitely try the metal dome on PCB method to see how the keys feel like.

[Zweb]

One alternative way is repurposing cheap clone calculators in China, like what I did in calculator project ArithMax: https://hackaday.io/project/7261-arithmax

[brouhaha]

(09-29-2015 01:10 AM)Chris Chung Wrote:  Looks like some HP calculators uses metal dome on PCB keys. If this is the "feel" that is desired, customised metal dome overlay sheets can be ordered (minimum quantity 1000+) via alibaba. Example offer here

They list a price range of $0.19 to 0.21 each, but it isn't clear to me whether that is per dome, or per sheet of domes. If it's per sheet, that's a great deal, assuming that you're going to make hundreds of calculators.

The XC-42 hardware Richard Ottosen and I are developing uses Alps SKRPABE010 surface-mount tact switches which I've recently been able to source for $0.13 each in quantity 500. The XC-42 has 44 switches total, so that's $5.72 of the parts cost of the unit. Avnet lists them at $0.0784 each in quantity 4000.

[Dave Frederickson]

(09-29-2015 04:46 PM)brouhaha Wrote:  Avnet lists them at $0.0784 each in quantity 4000.

Avnet has zero in stock. Next best price is Quest Components @ $0.096 ea for a qty of 348.

[brouhaha]

(09-29-2015 05:20 PM)Dave Frederickson Wrote:  

(09-29-2015 04:46 PM)brouhaha Wrote:  Avnet lists them at $0.0784 each in quantity 4000.

Avnet has zero in stock.

True, but if I was building 90 calculators or more, I wouldn't have a problem with waiting the factory lead time, which IIRC was 12 weeks.