Ideal keycap mount?

squizzler

12 Jan 2020, 13:54

I am interested in whether there is an optimal design of interface by which the keycap is mounted on the switch.

The problem with MX and Alps is that the plug and socket by which the keycap mounts is directly over the switch mechanism. This creates what I would call additional 'stack height'. Thats's a bicycle terminology to describe the amount of height that the headset adds to the head tube and affects how low the handlebar stem can be placed.

Some low profile switches like Cherry ML and Kailh Choc use pairs of horizontally separated tabs which I presume straddle the central spring and enable the unit to be shorter.

I personally like the design Omron have produced for their RGB switch marketed as Alpha Zulu or Romer-G depending on vendor. This has four tabs arranged round the edges, with its wide stance it ought to be the most mechanically stable of all designs. The central void used as a light well as in present applications offers potential for a switch designer to house parts of the switch mechanism within the cap itself allowing for lower board profile, or permit more complex switch mechanisms like beamspring within a manageable stack height, albeit at expense of thru-switch lighting. Omron are one of the world's biggest microswitch manufacturers, doubtlessly know what they are doing, and presumably designed their interface with future potential in mind.

Whilst I am sure the first few responses will be along the lines of 'MX = industry standard, duh!', I am interested in what would be the optimal from an engineering point of view, and whether there are designs forum member particularly like.

anmq91

12 Jan 2020, 14:18

It depends on what you are going for. Is max stability more important than shine through lighting for example? Different very stable and interesting designs always have their pitfalls. The Buckling Spring (BS) design is super stable and I would assume the 'stacked height' would be negligible due to the lack of having an actual slider. It does however mean that you cannot have in/under key back lighting. In terms of stability I think the Fujitsu Leaf springs sliders are also super stable, but again have the lack of back lighting in keys. Romer G's have good stability and back lighting, but do not have the best reliability track record.

Would be interesting to hear the opinion of other more experienced members of the keyboard community here. I personally feel there is no such thing as a perfect interface - only the one that resonates best with the user for what they expect from the switch :)

Findecanor

12 Jan 2020, 15:12

I like these properties in switch sliders and mounts:
* Backlighting of Omron Romer-R, like you mentioned. I think that it would also be able to distribute light to legends that are in the corners instead of the middle. One thing I would like to see is the slider divided into two or more compartments, so that different legends on the same keycap could be illuminated by different LEDs depending on the keyboard's modifiers or other mode.
Also, I think the sliders should be opaque and black, with the inside painted white for containing and spreading the light only to the legend.

* Topre switches are not only very smooth and stable. Keys with short stabilisers don't have stabilisers directly under the key but instead as part of the slider underneath the mounting plate. This makes the stem even more stable so that no stabiliser mounts under a keycap are necessary. Also, the mechanism is protected.
On Topre Realforce, even the 1.75u Caps Lock is stabilised this way: and there wouldn't even be room for stabiliser mounts in the keycap. (BTW. On the HHKB, the corresponding key is not stabilised...)

* Some keyboards, such as IBM Buckling Spring keyboards implement stabilisation of longer keys by using two stems: one for a functioning switch and one for a dummy switch with no spring. This allows keys to be split into two by replacing a dummy with a real switch and changing keycaps. Thus, one single design of the underlying switch matrix/barrel plate/mounting plate is often able to support both ANSI, ISO and JIS (and HHKB etc.).
Many of these designs that depend only on a dummy switch are not very smooth though, and do suffer from binding on longer keys if they are not lubricated enough.
The Ericsson switch has what I think is the best solution to this problem: there is a stabiliser bar, but it is connected between the two sliders: not connected to the keycap directly or via any special mounts.

* Some switches, such as Ericsson switches, Space Invaders and Fujitsu Peerless have sliders with a wider flat top than e.g. Cherry MX and Alps.
This allows the switch to both have a longer spring (going up to the top) while not being too tall or needing to penetrate the PCB or backing plate.

(Edited for grammar)
Last edited by Findecanor on 16 Jan 2020, 11:54, edited 4 times in total.

squizzler

13 Jan 2020, 12:29

It depends on what you are going for. Is max stability more important than shine through lighting for example?
In my opinion, fastening to the inside-edge of the keycap improves on designs that use a central stem and socket in both these aspects.
In terms of stability I think the Fujitsu Leaf springs sliders are also super stable, but again have the lack of back lighting in keys. Romer G's have good stability and back lighting, but do not have the best reliability track record.
* Some switches, such as Ericsson switches, Space Invaders and Fujitsu Peerless have sliders with a wider flat top than e.g. Cherry MX and Alps.
This allows the switch to both have a longer spring (going up to the top) while not being too tall or needing to penetrate the PCB or backing plate.
It should be feasible to design switches with most of the above mentioned interfaces using a 'Romer-G' style mechanism that incorporates light wells. The ultimate would probably be for the slider to be an interference fit to the inside of the keycap without tabs and grooves but I suspect this would require tight tolerances to stay put and presumably also limits the opportunity to offer differently shaped keycaps.

Of the examples given I like the 'Space invader' since the ridge and groove in each corner would support the cap over the broadest area (like wheels on a car) and leave the greatest amount of internal space for the switch mechanism. The linked Wiki suggests the space invaders were successful in reducing stack height allowing the board as a whole to be less than 3/4 inch off the desk. That of the Fujitsu (Peerless, leaf spring Gen 3) also looks extremely elegant.
* Topre switches are not just very smooth and stable. Keys with short stabilisers don't have stabilisers directly under the key, but instead instead as part of the slider underneath the mounting plate. This makes the stem even more stable so that no stabiliser mounts for the keycap are not necessary. Also, the mechanism is protected.
On Topre Realforce, even the 1.75u Caps Lock is stabilised this way: and there wouldn't even be room for stabiliser mounts. (BTW. On the HHKB, the corresponding key is not stabilised...)

* Some keyboards, such as IBM Buckling Spring keyboards implement stabilisation of longer keys by using two stems: one for a functioning switch and one for a dummy switch with no spring. This allows keys to be split into two by replacing a dummy with a real switch and changing keycaps. Thus, one single design of the underlying switch matrix/barrel plate/mounting plate is often able to support both ANSI, ISO and JIS (and HHKB etc.).
Many of these designs that depend only on a dummy switch are not very smooth though, and do suffer from binding on longer keys if they are not lubricated enough.
The Ericsson switch has what I think is the best solution to this problem: there is a stabiliser bar, but it is connected between the two sliders: not connected to the keycap directly or via any special mounts.
I haven't yet considered stabilising the long keys as I run an ergonomic board whose long keys are only 1.5x that of letters.

In short, I feel those designs with central stem and socket such as MX and Alps mount are legacies from a time before backlighting or keyboard depth were even considered, and are sub-optimal for today's designs. I notice the Fujitsu leaf spring already mentioned here moved over the course of thee generations from a central stem with cruciform plug and socket to an inside-edge mount, suggesting that firm felt there were benefits to the latter.
Whilst I am sure the first few responses will be along the lines of 'MX = industry standard, duh!', I am interested in what would be the optimal from an engineering point of view, and whether there are designs forum member particularly like.
Thanks for proving me wrong there!

Gadroon

16 Jan 2020, 11:44

Buckling Spring (BS) design is a good choice.

Findecanor

19 Jan 2020, 13:28

Another issue that should be taken into consideration is how easily it is to break a stem, either from external forces on a keyboard or from pulling a key.
Alps mount is relatively fragile. Longer stems also tend to be fragile.

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Man_In_Blue

Yesterday, 05:05

As far as stability, HiTek space invaders have just about the most stable mount I have ever felt. I also quite like the Micro-Switch hall effect mount, as it allowed for a cap to be nearly solid.

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