Why are MX stems not symmetrical?

[br4n_d0n]

I sadly just noticed today that the MX style keycaps can only be put on two ways; normal or upside down and are unable to be mounted sideways. Why don't they make the keycap stems symmetrical so one can put the keycap on in any orientation they want?

[Rob27shred]

This is just a guess, but I would imagine it was a proprietary thing. When Cherry first came out with MX switches they did patent the design & also manufactured keycaps & stabilizers to fit with them.

[br4n_d0n]

Whatever the reason, it's a real bummer for me because I was hoping to mount some of my keycaps sideways.

[Findecanor]

Huh? What kind of switches and keycaps are you using?

The stem of most keycaps for Cherry MX is four-way symmetric so it can be attached north, south, east or west on the switch. The switches under 2u vertical keys (ISO Return/Enter, Numpad + and Numpad Enter) do have different orientations on different keyboards.
While the stem on the switch is not four-way symmetric, it is symmetric enough to fit inside the symmetric hole in the keycap's stem.

However, the shape of a keycap's walls might require a particular orientation to be able to mate with the shape of the switch's housing. This is especially a problem with the sloping front wall on thick-walled Cherry profile (GMK, EnjoyPBT etc) on the home row — when the key is only 1u on the vertical.
Also, the orientation does matter when using Costar-style stabilisers: the stabilising wire has to be on the same side as the diode window.

[br4n_d0n]

On the keycaps I have, they look symmetrical until you look super close and you'll notice that the horizontal line is a little fatter than the vertical line.

[rinkaan]

On the Stems they are not symmetrical, but the keycap base seems to be symmetrically.. I tried using my 1u arrow left key, I can put it in all 4 orientations.

If cos the R is different as you rotate it, so it depends on R ur keycaps are. Would still look odd though

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[bansheetango]

Look up keycap profiles. You're looking for a uniform keycap profile.

[Darthbaggins]

The stem is fine, it's the cap's profile that is not.

[br4n_d0n]

I know that a uniform profile would work, but I'm trying to use keycaps that I currently own.

Here are some diagrams of the MX stems/keycap dimensions to prove my point for those non-believers.

[tweety]

I know that a uniform profile would work, but I'm trying to use keycaps that I currently own.

Here are some diagrams of the MX stems/keycap dimensions to prove my point for those non-believers.

Show Image

Show Image

And what is that image supposed to prove really?

The MX standard specifies that the keycap stem is symmetrical. You image shows it as symmetrical.

That literally means you can turn a spec compliant keycap any of the four ways, regardless of the width and spacing on the switch stem, which you correctly state is non symmetrical. Its is, but it doesn't matter if the keycap is following spec. A non-standard keycap not following spec though, then it matters.

[Findecanor]

The switch stem is narrower in one direction because when you push it in, there is air inside the keycaps's shaft — and that air needs to be come out, which it does along the sides of the narrower sides.
If it does not come out, that air would push back - and possibly not allow the switch stem to go in fully. Or it might, but get pushed out again.

Because of how keycaps are injection-moulded in two-piece moulds, it is not possible to have an air-hole at the top of the keycaps' stem for letting air out.

Some older switch designs with cross-shaped holes in the keycaps, have the switch's stem be T-shaped even, for the same purpose.
Other types of switches with inverse keycap mounts (where the keycap has a peg that goes into a hole in the switch), have instead one or more air-holes right through the slider, allowing air to escape through the switch.
You can see this inside every Alps slider.

[Volny]

The switch stem is narrower in one direction because when you push it in, there is air inside the keycaps's shaft — and that air needs to be come out, which it does along the sides of the narrower sides.
If it does not come out, that air would push back - and possibly not allow the switch stem to go in fully. Or it might, but get pushed out again.

Because of how keycaps are injection-moulded in two-piece moulds, it is not possible to have an air-hole at the top of the keycaps' stem for letting air out.

Some older switch designs with cross-shaped holes in the keycaps, have the switch's stem be T-shaped even, for the same purpose.
Other types of switches with inverse keycap mounts (where the keycap has a peg that goes into a hole in the switch), have instead one or more air-holes right through the slider, allowing air to escape through the switch.
You can see this inside every Alps slider.

Interesting! I'd wondered about why this is myself, as I've noticed the asymmetrical stem myself.

In my experience, most keycaps are fine to rotate 90°, and fit snugly despite the slight assymetry, as their stem socket is symmetrical anyway, so it makes no difference. But I've seen a few keycaps that actually mimic this assymetry in their stem socket and are therefore too loose unless at 0° or 180°.