[Help] How to Measure Spacing for Switches on a PCB

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12 Aug 2019, 07:54

Hello everyone, I've been trying to make a PCB for Futaba MA switches for a while now, but I don't entirely understand the spacing between switches on the plate. Is there any way to measure this accurately? I've seen online that most plates are 19.05mm from center to center, but I want to make sure before I go ahead and work on the PCB. I'd also like some help figuring out how to measure the spacing on larger keys like the spacebar and modifiers. I already have the plate, which I salvaged from an old board.

Apologies if this is hard to understand at the moment, I don't entirely know the terms for it.

Thanks in advance, any help would mean a lot.


12 Aug 2019, 12:42

One standard key "Unit" (u) is 19.05 mm = 3/4 inch. Only some small form-factor keyboards have smaller spacing in one or both dimensions (width,height).
Larger keys are expressed in number of whole units, and fractions of units: 1/2=0.5 and 1/4=0.25 units. Switches are typically in the centre of each key.
Blank spacing between groups of keys is often also expressed the same way.

I suppose that you'd want to support Futaba switches with Cherry MX-compatible mounts with support for "standard" Cherry MX keycaps.
For Cherry MX (compatible) keycaps, stabilisers are the the same for 2u, 2.25u and 2.75u keys: 1.25u between mounts.
For space bars (3u and wider), the stabiliser mounts are (typically) located 0.5 unit from each end: (i.e. you could put a 1×1 key on each stabiliser mount ... )
See Space bar dimensions in the Wiki for some examples.

If you go outside the de-facto "standard" for Cherry MX keycaps (followed by Poker and Quickfire Rapid, listed on Keycap size by keyboard in the Wiki) you would have to decide which keycaps you'd want to support and measure those.
Many keyboards have a stepped key for Caps lock, where the key is 1.75u but the switch is in the centre of the leftmost 1.25u (Cherry) or the leftmost 1.5u (many Alps).

Every keycap is of course a little bit smaller all around than its key size, or they would rub against each other. If you design a case with a border around the keys, you should keep in mind to also widen the opening a similar amount.

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