MK-52 Calculator

The MK-52 is a Russian calculator that is interesting for a number of reasons. Being Russian, it has Russian manuals and Russian keys. This is a bit of a problem for me as I don't speak Russian. I find it difficult to remember the key markings and what they relate to on the keyboard. So, I thought it would be worth having a go at creating some English keys for the MK-52, just to make using it more comfortable.

My first idea was to machine the keys out of plastic and engrave the legends into the top of the key. This could then be filled with a coloured material and there you go. After some research I found that nylon has a high enough melting point that I could maybe machine a set of keys from it and then fill the engraving with polymer clay and then cure that at 105C. Hopefully the nylon will stand up to that treatment and the polymer clay will be robust enough if it is cured and recessed into the key.

Setting all this up is a bit tricky as the parts have features on all sides and work holding and work order is complicated. So, as a trial I had a go at 3D printing the keys. This actually turned out fairly well. I have printed in PLA, so polymer clay may not work, but I have a couple of keys that I've printed and engraved with legends. I then used a pencil to colour the engraving, rather than use polymer clay.

The result is a bilingual MK-52:

The two white keys have English legends. Here's a close up:

The engraving is a bit wonky, that's just the setup I used. this was just a trial and I was more concerned about the engraving. Engraving PLA is not easy, but after a few passes at different depths and RPM I managed to get pretty good 2mm high text. This is about 0.5mm deep which is OK for the pencil colouring, but for polymer clay more depth might be needed so more material is in the keytop.

I tried using the keys after I put them back in the calculator and they seem to work fine. The pin on the back that presses into the membrane mechanism is a bit shorter on the 3D printed part, but that doesn't seem to affect operation at all.

I also had a look at getting some keys 3D printed by Shapeways. I hoped that they'd do full colour printing but they don't, a single colour is all that they do, and that comes out at $6 per key. This makes it too expensive to consider, I think, which is a bit of a shame.

Holding the keys is a problem on the engraver, then I had a thought. The keys were held perfectly well on the 3D printer, why not use the print bed on the engraver?

 On the Fisher the print bed is sort of circular and removable. So, I tried that and it worked:

I managed to engrave a set of 13 keys, albeit a bit off centre.

I've used an amalgam of English wording that I found on the internet. I've had to change the letters so that there's a maximum of three per key, as four doesn't fit with the text size I've chosen.

There's a problem using polymer clay with PLA: it requires too high a temperature to cure. Instead, I tried air-dry clay. I got some white and used it to fill the engraving, so there's a contrast between the letters and the button. I also tried a mould-able glue product (Sugru).

These are some of my attempts, together with an original button on the right.

The air dry clay has worked best I think, as it doesn't colour as much from dust when I sand and polish the top. Maybe if I left the mould-able glue  to cure longer than 24 hours then it wouldn't pick up so much dust. The dust gives the letters an off-white colour that isn't as nice as the whiter finish of the clay.

I polish the tops of the keys which gives them a nice shiny finish. The sides are still the normal 3D printer layer finish. The bottom with the 2mm cylinder that pushes the key is a bit of an untidy mess.
I've decided to print the keys with a hole in the base and fit a length of 2.5mm styrene rod there instead of trying to print the feature. You can see the lower side of one of the keys here:

Printing this part has the advantage that it's flat on the bottom and sticks well, and the keytop is at the top of the part which means it has a nice finish which polished well and also doesn't suffer from the gaps that the first layer of a print seems to have. These gaps filled with white clay and created a pattern of thin lines on the top. You can also see a finished key in the above photo and also several keys that have had clay applied and are drying before I sand and polish the top surface.

I then made a full set of keys (well, the Russian ones, I left the original number and function keys) and fitted them in the MK-52:

These are air dry clay filled except for the R/W and ADR keys, they are filled with paint. I've also used some 2.5mm diameter styrene rod for the part of the keys that pushes on the membrane. I glued the rods in a hole that is printed on the bottom of the keys.

This is very much a trial, so we'll see how well they work.

You can see the keys working in this video:

Here's a short timelapse of the keys being engraved. You can see that one of them popped off as the tool came down to touch it. The fixing method isn't perfect! I could always engrave more keys and get some redundant ones for this sort of mishap. I engraved a block of keys like this several times and that key always fell off. At least it's consistent.