MX KeyRail – Snap to Key Typing Guide for Prosthesis & Mouthstick Users (Logitech MX Mechanical)

WHO IT HELPS

People who cannot press individual keys with their fingers: upper-limb amputees (body-powered hook, myoelectric hand, residual limb), mouthstick and head-pointer users, people with tremor or reduced dexterity, and one-handed typists.

THE PROBLEM

Hitting a 14 mm keycap with a prosthetic hook, a stick or a shaking hand is slow and error-prone: you press the neighbour key, slip between keys, or press two keys at once.

HOW IT WORKS

A carriage rides on two 8 mm rods across the keyboard. The rods sit on two shuttles that slide front-to-back on rails clipped to the ends of the keyboard. You only interact with ONE head:

- Push it sideways → the carriage clicks every 1/4 key (4.76 mm). Every row of the MX Mechanical is offset by multiples of 1/4 key, so the plunger ALWAYS stops exactly on a key centre, never between two keys.

- Push it forward/back → it clicks into each of the 6 rows.

- Press it down → a guided plunger presses only that key.

ENGINEERING DETAILS

- Snap-to-key indexing: 86 X detents on a 3-piece rack + 6 row detents on each rail, engaged by compliant printed pawls (no springs, no screws).

- Printed folded leaf springs lift the plunger back after each press.

- Hard stop at 4.6 mm: the switch actuates (~3.7 mm of plunger travel) but never bottoms out, which protects the keyboard.

- Soft TPU tip, 2.4 mm above the keycap at rest.

- Side brackets are generated directly from the official Logitech MX Mechanical CAD file, with 0.25 mm clearance. Non-destructive: they slide onto the keyboard ends; no drilling, no glue on the keyboard.

- USB-C port and power switch stay accessible.

- Fully parametric Fusion 360 file: change kr_xc / kr_zc to preview any key position, adjust clearances, pawl stiffness, detent depth.

3 INTERCHANGEABLE HEADS

- A – Ø46 mm soft pad (TPU) for a residual limb, forearm or fist.

- B – Ring (Ø19 mm opening) for a prosthetic hook: hook in, aim, press.

- C – Cone cup for a mouthstick or stylus tip.

PRINTED PARTS (14 files)

Bracket L, Bracket R, Shuttle L, Shuttle R, Rack segment 1/2/3, Carriage, Plunger, TPU tip, Head A, Head B, Head C, Rod caps (x4).

NON-PRINTED HARDWARE

- 2 × 8 mm smooth rods, cut to 457 mm (standard 3D-printer linear rods).

- 2 × Ø3 × 20 mm dowel pins (or 3 mm brass rod) to join the rack segments.

- Optional: a drop of dry PTFE lubricant on the rods.

ASSEMBLY

1. Join the 3 rack segments with the Ø3 pins (a drop of CA glue is optional).

2. Insert the plunger into the carriage bore from the top, then push the TPU tip onto its lower end.

3. Press both rods into the LEFT shuttle (tight fit) and insert the rack end into its pocket.

4. Slide the carriage onto the rods: its pawl must sit on the rack teeth.

5. Press the RIGHT shuttle onto the rods and the rack, then fit the 4 rod caps.

6. Slide the two brackets onto the left and right ends of the keyboard.

7. Slide the whole gantry onto both rails from the front until the shuttles click into a row.

8. Push the head you need onto the top of the plunger.

TIPS

- Turn on Sticky Keys (Windows / macOS) so shortcuts like Ctrl+C work one key at a time.

- For tremor: the detents act as a guide; pair with Filter Keys to ignore double presses.

- Place the keyboard on a non-slip mat.

STATUS

Designed and fully verified in CAD around the official Logitech reference: no interference, end-of-travel positions checked from Esc to the right end of the Space row. Not yet physically test-printed: makes and feedback are very welcome!

IMAGES

All pictures are Fusion 360 renders of the CAD model. I did not have time or access to a physical Logitech MX Mechanical keyboard to photograph prints, so the hand, hook and mouthstick shown are illustrative stand-ins, not real photos.