# Concept 28 R1 - Chalk Ember / Fusion handoff

Status: supplier-review prototype. Not released tooling or a production sign-off.

## File authority
Open `cad/Walkie_One_28_R1_Chalk_Ember.f3d` in Autodesk Fusion. This file was edited in Fusion from the completed final Concept 27 R2 STEP assembly, with the UC1 local casting-release approach adapted to its revised boss geometry. Imported baseline bodies do not acquire a reconstructed historical sketch timeline. New native fillet/engraving features and cut-outline sketches are included. No Adam MCP generation was used for 28.

`REFERENCE_ASSEMBLY.step` includes the unchanged OEM electronics. `CASE.step` contains manufactured/custom stock parts plus 18 fasteners. Individual STEP/STL exports are normalized to assembly coordinates in Fusion. The 92 rods in part 61 are one purchased mesh component, not 92 fabrication jobs. The DXF cut outlines and stock specification govern that purchase. F3D and assembly STEP retain the source occurrence organization.

The rigid body remains 78 x 150 x 24 mm. Overall nominal bounds remain 83.35 x 152.7 x 24.7393 mm including rim and controls. Front z=0; X across, Y up. Play center (0,-47), visible light (0,2), original collector/LED optical datum (0,5), microphone (0,-14), speaker (0,42), all in mm.

## Changes from the final 27 R2 functional baseline
A separate graphite mesh, stainless bezel, black scrim and adhesive stack replace the molded hole pattern. The acoustic opening is 35 mm diameter, inside the final two-ring gasket stack's 38.0 mm inner diameter. The speaker and gasket do not move. The visible grille is larger than the underlying speaker. Bezel is 48 x 50 mm with R11.5 corners; visible opening 44 x 46 mm R9.5. The skin rebate is 48.4 x 50.4 mm, 0.30 mm deep. Original speaker pilot bores are retained.

The 27 mm solid brushed-metal play cap sits over the revised round upper carrier. Original lower retention wings, actuation contact and clamp remain. The shell bore is 27.9 mm; the upper carrier is 27.3 mm. The cap is 1.10 mm thick plus a 0.10 mm bond film. Its 0.04 mm laser outline is cosmetic.

The light has a 4.7 mm circular visible optic and 4.9 mm shell aperture, moved 3 mm down to (0,2). The original rear collector and internal LED/coupler stay at their original positions. The single-piece optic has an offset output; qualify brightness and uniformity physically. A 0.592 mm3 nominal interference at the collector flange is inherited from 27 and remains subject to fit qualification. The PTT uses a smooth capsule skin and 0.6 mm exposed edge radius; concealed carrier engagement is unchanged.

## Casting route
Cast front and back shells in tough rigid urethane; cast the rim and PTT separately in elastomeric urethane. Shore A 80-85 is an evaluation target. RPS selects grades, compensation and release construction. The four files in `cad/casting_masters` are nominal master-pattern candidates, not mold halves. No shrink scale or finish stock is applied. The front now has a single 35 mm acoustic opening. The old 121-hole blank-master/drilling schedule is obsolete for 28 and is intentionally not included.

The UC1 local 1-degree release approach is applied to 16 actual final R2 bosses. Speaker mounts use Y=18 and 66 mm; LTE boss tips use Z=-20.33 mm. A 0.10 mm root land is retained. This is not a globally drafted injection-molding design. Review thin features, pilot durability and all undercuts in the selected silicone-tool strategy. Metal caps/bezel, mesh, optics, PA12 internal carriers, stock pads and adhesives use separate processes; do not cast every STL.

The 0.30 mm grille rebate leaves nominally 1.90 mm of the original 2.20 mm face skin outside the acoustic opening, and 0.80 mm above the lower speaker pilot ends. Supplier review is required before committing to tooling. Do not add unselected threaded inserts to the inherited bosses.

## Assembly
1. Inspect and finish the cast shells. Check pilot bores, closure interfaces, openings and rim fit before bonding parts.
2. Bond black scrim at the perimeter, with its separate 0.05 mm film. Keep the 35 mm acoustic opening clear of adhesive.
3. Bond the cut woven mesh into the bezel rebate using the 0.09 mm edge bond allowance. Fully contain sharp cut edges. Bond the bezel into the shell using the 0.20 mm perimeter film.
4. Install the circular lens with its visible center at (0,2), 3 mm below the former center, using the inherited collector seat. Retain the original internal right-angle optical coupler and LED mount. Verify brightness and uniformity across the offset output before permanent bonding.
5. Bond the circular metal cap to its carrier. Install the inherited clamp, then check free return and travel. Install smooth PTT grip and unchanged volume carriers/caps.
6. Use the final 27 R2 electronics, pads, keepers and countersunk fasteners in their reviewed locations. The two LTE service frames lift separately. Fit four hidden rim pads to the front shell only and attach the flexible rim after rigid closure; no pad bridges the service seam. Check harness, USB, RF and acoustic performance with actual hardware.

## Verification and limitations
Fusion measured zero material difference in the front shell below z=-2.21 mm relative to the final R2 casting-adapted baseline. All 436 unchanged assembly-occurrence bodies retain their native face counts, volumes, world bounds and transforms versus final R2. Independent STEP validity, full static fit, 60 directional retention checks, four sampled control sweeps, cover insertion and fastener/service access evidence are supplied. These are nominal geometry checks, not force or overtravel certification. See Trace_Reconciliation.md for the fresh results and remaining holds.

Cross-kernel coincident-shell subtraction in OpenCascade was numerically unstable on the inherited toroidal shell surfaces, including the unmodified back shell. Native Fusion differences establish preservation; independent non-coincident overlap checks establish the changed interfaces. The only detected changed-part overlap is the unchanged lens-flange interference. Acoustics, mesh resonance, coating adhesion, drop strength, sealing, resin behavior, optics and RF remain unqualified.

The drawings are nominal prototype dimensions, not approved production tolerances. Suggested acceptance values require supplier agreement. No water-ingress rating is claimed.

RPS describes silicone-tool urethane casting, rigid/rubber-like materials, pigmentation and secondary finishes: https://www.rapid-products.com/service/urethane-castings/ (reviewed 2026-09-10). This is process context, not supplier acceptance of this design.