# Printable finger attachments and eight-motor tray

All STL units are **millimetres**. Generic bench prototypes, not patient-fitted or clinically validated parts. The user's chosen interface is fingertip caps with adjustable strap retention and flexible clip-on tendon guides; no thumb actuation.

## Print files

| File | Material | Quantity for one hand | Purpose |
|---|---|---|---|
| `motor_base_cover.stl` | PETG | 1 | Lower guard under the motor cases; add soft padding. |
| `tray_standoff_32mm.stl` | PETG | 4 | Separate upper tray and lower cover with M3 through-bolts. |
| `eight_channel_sheath_anchor.stl` | PETG | 2 | Anchor the outer sheaths to padded supports at transmission ends; see below. |
| `fit_coupon_16_19_22.stl` | TPU 95A | 1 first | Choose an opening without compressing a fingertip. |
| `cap_small/medium/large.stl` | TPU 95A | 4 total, select size per finger | Split fingertip sleeve, two tendon anchors, proximal strap slots. |
| `guide_small/medium/large.stl` | TPU 95A | Up to 8 total | Two guides per finger, one on each phalanx shaft. Do not cross joint creases. |
| `eight_motor_tray.stl` | PETG | 1 | Eight MG90S-class openings, two rows of four. |
| `servo_ear_clamp.stl` | PETG | 16 | Two clamps per motor, clear of the central case; four M3 fasteners total per motor. |
| `horn_mounted_spool_r8.stl` | PETG | 8 | One tendon spool per motor, attached to the original servo horn. |

Openings, before print tolerances: caps S=16×13, M=19×16, L=22×19 mm. Guides have 2 mm more internal width/depth than the corresponding cap, because they sit farther down the finger. The lateral split and a soft strap allow limited adjustment; this does not make one size fit every finger. Default cap length is 16 mm; its cuff must clear the distal joint crease. Shorten it for a smaller distal segment. Print the coupon and one cap first. Change `config.json` and regenerate when measurements are available.

## Printing and assembly

- Start at 0.20 mm layers, at least 3 perimeters and high infill for the small parts; check the slicer's thin-wall preview. Export geometry is stepped at 0.5 mm in attachments (1 mm in the large tray). Smooth/deburr all tendon and skin-contact surfaces; these STL files are a conservative kernel-free draft, not smooth production CAD.
- Orient cap with its **open cuff on the build plate**, closed rounded end up. In the STL, the finger axis is Y; rotate by ±90° about X in the slicer. Flexible cap tendon lugs may need supports. Guides similarly print with the finger axis vertical. Verify overhang/support settings in your slicer.
- Print the tray flat, ear clamps flat on the bed, and spools with an end flange flat. Do not print servo splines: retain the supplied horn and its original centre screw.
- Cap anchors/guide holes are nominal 3 mm. Use smooth low-stretch textile tendon and optional PTFE sleeves; do not run bare metal wire against skin. Hole dimensions must be checked after printing.
- The spool has an 8 mm core radius and two mounting holes nominally 12 mm apart. Check your supplied horn before printing; drill/adjust spacing in the generator rather than forcing misaligned screws. Inspect screw length so it cannot damage the servo body.
- Tray body openings are nominal 24×14 mm, suitable only after checking your actual servo case. The tray does not assume clone-specific mounting-ear screw spacing. Fit ear clamps over the mounting ears with washers/locknuts, adjusting spacers for ear thickness as needed.
- Attach soft hook-and-loop bands through cap slots and around guide bodies as required. Fit straps without compressing the finger; worn fit and comfort still need verification. No exposed fastener, sharp tendon or rigid tray edge should contact skin.
- Secure the tray to a **padded forearm support** with two straps through the long slots. The tray is a carrier, not an anatomical brace. Motors underneath must have a spacer/cover and clearance from the forearm. Route flexion tendons along the palmar side and extension tendons dorsally, with anchored sheaths between forearm and hand.
- Provide a mechanical tendon disconnect reachable with the other hand. Cutting power or removing PWM does not guarantee release from geared motors.

## Travel check

At an 8 mm core radius, 180° rotation gives about 25.1 mm ideal take-up. A 150 µs pulse change is only a small fraction of a typical servo's full rotation, and clone pulse-angle mappings differ. Measure actual usable travel. Do not expand pulse endpoints just to compensate for a routing error or to overcome a stiff hand.

## Reproduce / verify

`python3 generate.py` requires NumPy and writes thirteen binary STL files plus `manifest.json`. Run `python3 export_assembly.py` after generation to rebuild the GLB; `python3 preview.py` (Matplotlib) rebuilds the inspection sheet. The generator checks every mesh for two incident faces per edge, consistent winding, one connected surface and positive enclosed volume. Those checks establish mesh integrity, not printer compatibility, fatigue life, mechanical strength, fit, comfort or safety. None of these parts has been physically printed or load-tested yet.

## Sheath reaction anchors

The added eight-channel anchor has 4.5 mm sockets for 4 mm OD PTFE/Bowden outer sheaths and 2.5 mm tendon exit holes, separated by a shoulder. The shoulder reacts the outer sheath compression; the inner tendon slides through. Four channels sit in each of two rows. Confirm actual tubing dimensions and retention before loading.

Use two anchors, securely strapped/fastened to the padded forearm support at the transmission endpoints. Arrange the sheath socket face toward the sheath run. A loose outer sheath cannot transmit predictable motion. The strap slots secure the block; they do not clamp the tubing or replace a sheath fitting. Retain tubing with appropriate fittings/strain relief, and check that it cannot lift out when unloaded. The block is generic, rigid and untested; it must not bear directly on skin.

## Motor enclosure stack

The upper tray and lower guard share four corner holes at (6,16), (160,16), (6,68), (160,68) mm. Four 32 mm posts separate the 4 mm upper tray from the 3 mm lower cover. With 24 mm tall servo cases below the upper tray, this leaves about 8 mm nominal internal clearance above the cover. Verify your actual servo case, mounting ears and wire bend before tightening the stack. Use suitable M3 through-bolts, washers and locknuts (roughly 45–50 mm total fastener length depending on hardware). Add padding under the lower cover and retain the whole assembly with forearm straps.

`assembly.glb` provides an animated assembled layout with the actual printed meshes, four finger proxies and eight simplified motor boxes. The 32-second motion is illustrative; it is a visual assembly study, not an anatomically fitted brace or a single printable solid. Print the individual STL files.