Tessera — Adaptive Keyguard for Logitech MX Mechanical (parametric, tool-free)

What a keyguard does

A keyguard is a plate that sits over the keyboard with one opening per key. Instead of holding your hand above the keys — which takes fine control and costs real effort — you rest the weight of your hand on the guard and put a finger into the opening you want. The rim does the aiming.

It is the single most useful keyboard accessory in motor-disability practice:

• essential tremor, Parkinson's, ataxia, dystonia — the guard absorbs the wobble, and neighbouring keys stop getting caught;

• spasticity and cerebral palsy — no more involuntary double presses, no dragging from one key to the next;

• typing with one finger, a headstick or a mouthstick — the pointer can no longer skate into the gaps between keycaps;

• low strength or fatigue (MND, muscular dystrophy, severe arthritis) — being able to rest an arm on something solid changes how long you can write.

Keyguards are normally laser-cut to order and cost €100–300. For the MX Mechanical there is nothing at all. So here is one.

It is a generator, not a guard

The MX Mechanical is sold in dozens of national layouts. A guard that fits only US ANSI excludes most of the people it exists for. So the model is produced by a short, readable program: you describe a layout as a table of key widths and it emits STL and STEP.

rows=[
    [1] * 16,                                # esc F1..F12 backlight mute del
    [1] * 13 + [2] + [1],                    #  1..= backspace(2) home
    [1.5] + [1] * 12 + [1.5] + [1],          # tab Q..] backslash(1.5) end
    [1.75] + [1] * 11 + [2.25] + [1],        # caps A..' enter(2.25) pgup
    [2.25] + [1] * 10 + [1.75] + [1] + [1],  # shift Z../ shift up pgdn
    [1.25] * 3 + [6.25] + [1] * 3 + [1] * 3, # ctrl..space..arrows
]

Gap(0.25) advances without placing a key; Tall(1, 2) is a key two rows high, like the numpad's +. Change the table, run one command, get your guard. ISO, JIS, whatever you have.

Why it comes in tiles

The full-size board is 433.85 mm wide. A Prusa bed is 250.

The guard splits into two tiles meeting on a seam that follows the web between key cells. Keyboard rows are staggered, so that seam is a staircase — and where the nearest web is more than ~14 mm away, as on the spacebar row, the seam simply runs through the opening. There is no material to join inside a hole, so nothing is lost: each tile carries one side of that opening and they still meet in the webbing above and below it.

Through the thickness the seam is a shallow lap, not a butt joint, so the tiles overlap, sit coplanar, and both mating faces print without support.

How it holds on — nothing clamps, nothing is drilled

The guard does not cantilever from its ends. Two continuous rails run the full length and rest on the keyboard's own keyless bezels — the strip in front of the spacebar row and the strip behind the function row. That makes the plate a continuously supported beam instead of a 434 mm span, which is the difference between a guard you can lean on and one that flexes under a palm.

It lifts off in one movement. The warranty is untouched.

The openings

19.05 mm pitch, 15.25 mm opening, 3.8 mm web, with a funnel chamfer on the top face that steers a fingertip or a stylus in. The chamfer is on the top only, not a taper through the whole plate: a full taper leaves a knife-edged rib, a top chamfer leaves a trapezoidal one that is stronger and far kinder to a hand resting on it.

Print the gauge first. Or just print paper.

Free fit check, no filament. Two A4 sheets of a 1:1 template are included. Print at 100 %, check the printed ruler really is 100 mm, tape the sheets on the register marks, and lay the result on your keyboard. If every opening sits over its keycap, the layout is right for your board.

Then the calibration gauge. It is a full-depth slice of the real guard — about an hour and 30 g — and it answers what a drawing cannot: whether the rails land on the bezels, how much air is left over the keys, whether the leg clears the side wall, and whether the tongue will go under your chassis. If it will not, use the included _notongue legs; the guard is fully located without them. If the plate sits high or low, there are _minus1mm and _plus1mm legs.

Being straight with you about the dimensions

I do not own this keyboard, and nobody has printed this yet. You should know that before you spend eight hours of filament, so here is exactly how far the design is verified.

Everything in the plane of the keys was measured from Logitech's own orthographic product renders, scaled so the pixel width equals the published body width — which turns the image into a calibrated drawing. Two independent measurements agree to better than 0.3 mm, the derived pitch comes out at 19.06–19.10 mm against a nominal 19.05, and the Mini's layout resolves to exactly 84 keys, which is what the spec sheet says. One of the images in this listing is the generated opening grid drawn straight back onto Logitech's render: every opening lands on its keycap.

What a top view cannot give is anything vertical — how high the keycaps sit, how far below them the bezel is, whether there is usable air under the chassis. Those three are assumptions. They are all named parameters, they only affect the rails and the legs, and the gauge measures them directly. That is exactly why the gauge and the paper template exist, and why this is a generator rather than a frozen STL.

If you own the board and measure something different, change the number and regenerate — or tell me, and I will.

Files

• *_keyguard_tile* — the guard, two tiles

• *_leg_left/right* — side legs: standard, ±1 mm, and _notongue

• *_calibration_gauge — print this first

• *_paper-template_sheet*.svg — 1:1 fit check, free

• src/ — the generator: layouts, geometry, build script, verification overlay

STEP files are real B-rep solids from CadQuery 2.8 / OCCT 7.9.3, not converted meshes, so they open properly in Fusion, FreeCAD or Onshape.

Not a Logitech product; no affiliation. Attaching it involves no modification to the device, but you use it at your own risk.