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geekhack Community => Keyboards => Topic started by: tebla on Sat, 13 February 2021, 08:17:51
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I'm making an ergodox and planning on using the Lister case design but made from wood. I'm considering getting layer 3 (the plate layer) cut from aluminium by a mate. He's asked me what the machining tolerances would need to be, anyone got any idea?
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Avoid wood
Wood also swells and shrinks with humidity. It changes much more than metal.
The wood inlays you see in cars and stuff is a very thin veneer with a thick acrylic coating.
If you are set on a wood top, then make it slightly smaller than the metal so that it doesn't protrude over the center plate edge if it bloats in high humidity. Use a chamfer so it looks more 1 piece. with the rest of the board.
The same applies if you're using wood bottom.
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The plan is to get the most fiddly layer (3) CNCed from metal and cut the other layers by hand from plywood. My understanding is that layer 3, the plate, needs to be the most accurate for a snug fit of the switches. My mate who is going to machine layer 3 for me is asking what tolerance I need it to be machined to. I guessing it doesn't need ridiculously tight tolerances but I don't really know.
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Standard +/-0.005" was good enough in my case. 13.8mm generally can still fit the switches whereas it gets a bit too tight at 13.6mm (also keep in mind my caliper isn't the most accurate).
Also I'd be cautious about using CNC for the plate due to internal corners. Not sure what internal radius is needed though you can add a bunch of dogbones.
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Thanks for the input! I don't have CAD software, so would have to have a look at downloading some if I'm going to need to edit the .dxf to add dogbones (had to look up what they were lol) Could I just file the corners by hand after its machined?
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I mean surely you can but it's 76 keys, 304 corners. I'm not sure I'd want to do that.
Depending on where you are in the world, maybe easier to just get a laser cut plate (and not have to deal with internal corner issues).
You should ask your friend (that's machining the piece) whether his CAM software has an easy way of adding dogbones. If so, then you may not need to make changes.
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The tolerances are in the Cherry MX datasheet:
The holes should be 14±0.05 mm on a side.
Max corner radius is 0.3 mm.
If 0.3 mm corner radius is too small then you could modify the cutout for each switch to be a leaning H shape (an H rotated 90°). But that shape would require you to fit the tolerances for the gap across each hole properly or the left and right edges of the hole will not provide support. (discussion here (https://geekhack.org/index.php?topic=59837.msg1375621#msg1375621). See also this thread (https://geekhack.org/index.php?topic=65747.0) about dogbones)
Note also that Cherry MX had been designed for plates of metal with a layer thickness of 1.5mm±0.14 mm.
Litsters case design is instead made for laser-cut acrylic, and because acrylic is more fragile than metal, the plate layer was increased to 4 mm thickness — but that means that the switches get press-fitted instead of snapped-in. (In other words, the latches on the side of each switch are always pressed in)
So, if you're making a plate in metal then by all means cut it from 1.5 mm thick stock and use thicker layers below it.
BTW. It is also possible to build an ErgoDox without a plate altogether. Each PCB has eight holes for screw-posts, and by using wide washers you could use most of those to bolt the PCB to the bottom plate.
Later revisions of the PCB also support PCB-mounted stabilisers for the long thumb keys (but stabilisers are actually not necessary for those keys weirdly enough).
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Thanks so much for all the info! super-useful :)
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The tolerances are in the Cherry MX datasheet:
The holes should be 14±0.05 mm on a side.
Max corner radius is 0.3 mm.
If 0.3 mm corner radius is too small then you could modify the cutout for each switch to be a leaning H shape (an H rotated 90°). But that shape would require you to fit the tolerances for the gap across each hole properly or the left and right edges of the hole will not provide support. (discussion here (https://geekhack.org/index.php?topic=59837.msg1375621#msg1375621). See also this thread (https://geekhack.org/index.php?topic=65747.0) about dogbones)
Note also that Cherry MX had been designed for plates of metal with a layer thickness of 1.5mm±0.14 mm.
Litsters case design is instead made for laser-cut acrylic, and because acrylic is more fragile than metal, the plate layer was increased to 4 mm thickness — but that means that the switches get press-fitted instead of snapped-in. (In other words, the latches on the side of each switch are always pressed in)
So, if you're making a plate in metal then by all means cut it from 1.5 mm thick stock and use thicker layers below it.
BTW. It is also possible to build an ErgoDox without a plate altogether. Each PCB has eight holes for screw-posts, and by using wide washers you could use most of those to bolt the PCB to the bottom plate.
Later revisions of the PCB also support PCB-mounted stabilisers for the long thumb keys (but stabilisers are actually not necessary for those keys weirdly enough).
Got my plates back from the laser cutters today, (laser could not guarantee the corner radius so I did go for the design with dogbones for the cutouts)
the switches have absolutely no play in the horizontal plane left and right and the tiniest amount up and down. But this is before soldering the switches.
Thanks again for your advice on the design!