git:20261005.db0b505 to git:20261008.13091b2

---

name: kernelcad-sheet-metal

description: Folded sheet-metal parts — sheetMetal(profile, opts), .bend(edge, angle, radius), .flattenPattern(). Use when building L-brackets, U-channels, service panels, or any part fabricated from a flat blank + folds.

---

# kernelCAD — sheet metal

Build folded sheet-metal parts (L-brackets, U-channels, service panels) from a closed Sketch + thickness + K-factor. Bend along a linear axis line; recover the flat blank as a `Region` for laser / CNC consumption.

```typescript

// L-bracket: 100 x 60 x 2 mm, one 90 degree fold along x=50.

const s = path().moveTo(0, 0).lineTo(100, 0).lineTo(100, 60).lineTo(0, 60).close();

const blank = sheetMetal(s, { thickness: 2, kFactor: 0.38 });

const bracket = blank.bend({ atX: 50 }, 90, 3);

return bracket;

// Recover the flat blank as a Region.

const flat = bracket.flattenPattern();

console.log(flat.outer); // [[0,0], [100,0], [100,60], [0,60]]

console.log(flat.bendLines); // [{ start: [50,0], end: [50,60], angle: 90, radius: 3, ordinal: 0 }]

```

## API

```typescript

// Top-level constructor — captures kind:sheetMetal record.

sheetMetal(profile: Sketch, opts: {

thickness: Editable<number>; // mm, > 0

kFactor: Editable<number>; // unitless, in [0, 1]

faceLabels?: FaceLabelsMap;

}): Shape;

// Shape methods — chain on a sheetMetal-rooted Shape.

shape.bend(

edgeRef: EdgeSelector | string, // slice-1: { atX: n } | { atY: n } | { face: 'top' }

angle: Editable<number>, // degrees, signed (positive = fold +normal)

radius: Editable<number>, // mm, inner bend radius

): Shape;

shape.flattenPattern(): Region; // derived view — no FeatureRecord

```

## Bend-allowance math (K-factor approximation)

```

BA = (pi * |angle_deg| / 180) * (kFactor * thickness + radius)

```

- `BA` = developed length of the neutral axis through the bend arc.

- `kFactor` = neutral-axis offset ratio. Typical values: 0.33-0.45 (mild steel / aluminum).

- Slice 1 supports the K-factor formula only.

## Slice-1 limits

- Bend axis derived from `{ atX }` / `{ atY }` EdgeQuery or `{ face: 'top' }`. Other selectors emit `feature.bend.edge-not-linear`.

- `radius >= 0.5 * thickness` recommended; tighter bends emit `feature.kernel-failed`.

- `flattenPattern()` supports at most 2 bends in the chain; 3+ emits `feature.flattenPattern.multi-bend-unsupported`.

- Sketch profiles must be polylines (`path().moveTo / lineTo / close`). Arcs in the profile are not supported by `flattenPattern()` in slice 1.

- The lowering pipeline (slice 1) omits the curved bend section in favor of a sharp-corner fuse — agents see the rotation but not the radiused inside corner. The K-factor math still holds for flat-pattern recovery.

## MCP introspection

- `flatten_pattern` — returns the unfolded `Region` as JSON (outer ring + bend lines + holes).

- `inspect({ of: 'bend-table' })` — lists every bend with its K-factor bend allowance, axis line, and parent `sheetMetal` opts.

## Sheet-metal diagnostic codes

- `feature.sheetMetal.kfactor-invalid` (error) — `kFactor` not in `[0, 1]` or non-finite. Hint: pass a value in `[0, 1]`; typical 0.33-0.45.

- `feature.bend.edge-not-linear` (error) — resolved bend axis cannot be derived from the selector. Hint: pick a straight perimeter edge; slice-1 supports `{ atX: <n> }`, `{ atY: <n> }`, or `{ face: 'top' }`.

- `feature.flattenPattern.multi-bend-unsupported` (error) — chain has 3+ bends. Hint: flatten an upstream Shape with `<= 2` bends, or wait for slice 2.

## Verification gates

After authoring a sheet-metal part, run before reporting done:

| Gate | Pass criterion |

|------|----------------|

| G-eval | `kernelcad evaluate` exits 0 — no `feature.sheetMetal.*` / `feature.bend.*` / `feature.flattenPattern.*` diagnostics |

| G-kfactor-valid | `kFactor` is a finite number in `[0, 1]`; pick 0.33-0.45 for mild steel / aluminum unless you have a measured value |

| G-bend-edge-linear | Every `.bend(...)` selector resolves to a straight edge (slice 1: `{ atX }`, `{ atY }`, or `{ face: 'top' }`) |

| G-bend-table-exists | `inspect({ of: 'bend-table' })` returns one entry per `.bend(...)` call with non-negative bend allowance and a finite axis line |

| G-flatten-roundtrip | `shape.flattenPattern()` produces a `Region` whose `outer` polyline area equals the source profile area within `1e-6` (chain has <= 2 bends in slice 1) |

| G-flatten-bend-count | `flattenPattern()` chains have at most 2 bends — 3+ emits `feature.flattenPattern.multi-bend-unsupported` |

| G-profile-polyline | Sketch profile uses only `moveTo` / `lineTo` / `close` — no arcs (slice 1) |

## Mounting sheet on a frame

A sheet (flat panel or bent part) that sits on tubes or a frame touches them: put its underside at the frame top (`z = tubeHeight` for tubes on `z = 0`), not a few mm above. If sheet and frame are fused with `union()`, `evaluate_script` fails with `union.disconnected` when the sheet floats (the message gives the gap) and `union.member-overlap` when members finished with the same material share more than 1 mm³. Prefer `assembly().part('sheet', ...)` + one part per frame member when they are separate BOM lines; recipe: `welded-tube-frame-tray`.

## Related skills

- `kernelcad-authoring` — `path()` + `Sketch` shape this skill operates on; rotate / translate the bracket after folding.

- `kernelcad-features` — fillet / chamfer / hole apply to the folded result; remember the slice-1 sharp-corner approximation when placing features on the inside corner.

- `kernelcad-params` — `thickness`, `kFactor`, `angle`, `radius` all accept `Editable<number>`; wire to params for fab-iteration.

- `kernelcad-mcp` — `flatten_pattern` and `inspect({ of: 'bend-table' })` are the inspection surface for downstream CAM / nesting tools.