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How can I replace/repair broken Atari 1040ST key caps using 3D Printer?

Switches are in good condition. Only the key caps are to be replaced. Did Atari followed any industry standards like Cherry MX, Alps or such to allow me to copy the dimensions? Are there any reliable 3D model for key caps? And which filament I should use?


Edit: Photo of the missing key and another one for reference is added

Missing Key

Reference

  • Are you able to snap another key off and copy that? And of course, it'll help to know which key you want to replace. IIRC the function keys are different from the rest. – Wilson May 22 '18 at 8:41
  • The question is coming from a friend. He was asking for a replacement to the DEL key in numeric pad. I didn't mention this in my question above to keep it more generic. – wizofwor May 22 '18 at 11:58
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    You may be able to cast replacements for cheap if you don't mind some bubbles. Make a 2-part silicone mold and cast in polyurethane. – snips-n-snails May 22 '18 at 16:12
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    Agree with the silicone mold proposal. Should result in a 200%better surface than what you can expect from 3D-printing – tofro May 23 '18 at 16:05
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    @wizofwor Yea, that's an original keyboard (and an early-model ST, nice). The original key is '.' (no 'Del' label on that key). I'm pretty sure that key has a unique profile so if you're going the 3D printing route I'd copy the upper profile from the adjacent '0' key. IIRC, the lower profile is the same for all the square keys. The dome switches look like this: picclickimg.com/d/l400/pict/162073354557_/… (from ebay.co.uk/itm/162073354557 - no affiiation). – Alex Hajnal May 28 '18 at 18:05
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Rather than try to 3-D print replacement keycaps, you might consider casting them in polyurethane resin. The advantages are:

  1. Lower initial cost (about $75 not including dye if you don't mind some minor imperfections, see below).
  2. No need to do any CAD modeling.
  3. The cast keycaps take the same texture as the original. No 3D print layer lines. A nearly perfect reproduction.
  4. Easy to repair broken keycaps by recasting the missing areas.

I successfully cast a missing Amiga key by copying another key this way, but the result had a bubble on the bottom lip of the keycap. To prevent bubbles, I would need a vacuum chamber to eliminate bubbles from the silicone when making the mold, and a pressure pot to eliminate bubbles from the resin when casting the part. Also an oven to bake the part after casting will add strength, but do not use the same oven to cook food!

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I don't know if Atari followed any industry standard for their key designs. Also, I've never encountered a buckling spring ST keyboard; original ST keyboards used collapsing rubber dome switch contacts (domes from ebay, no affiliation). For the upper portion, your best bet is to copy one of the other keys in the same row. You should be able to pop the caps off pretty easily with a flat-head screwdriver. FWIW, all of the Atari keycaps I've seen (520 STFM, 1040STF, 1040STE) use the same design; not sure about the detached TT keyboard but going by feel, the non-function keys also appear to be the same design (the TT function keys feel totally different though).

As far a creating a 3-D model, I'm not aware of any existing ones. However, the external Atari key designs are much blockier than what you'd typically find on e.g. an IBM keyboard so should be much easier to model (no complex curves). The Delete key on ST and TT keyboards is in the main block of keys (to the right of Return, below Backspace) and not on the numeric keypad. The key labeled '. Del' on PC keyboards is just labeled '.' on the ST/TT.

AFAIR, the specfic key you're missing has a unique shape so you wouldn't be able to directly copy one of the other keys on that keyboard. Honestly, your best bet might be to purchase a replacement key.

1040STF Image source: Wikipedia (Bill Bertram, CC-BY-2.5)

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    Here’s a good photo of a 1040ST with its keyboard in evidence, illustrating the various points made in this answer. – Stephen Kitt May 24 '18 at 8:48
  • I add photos of the keyboard in question. Apparently, I was wrong about the springs. So I removed the spring part from question. – wizofwor May 28 '18 at 9:07
  • Updated answer. Kept the bit about the switch/spring design though. – Alex Hajnal May 28 '18 at 18:27

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