# Turning & Sections

*Faceplate turning, profile sections, and turned-part cards*

**Names:** `stock` `section` `turn` `profile`

## When to use

- A part with a **round** cross-section made on a lathe: a spindle, baluster, leg, bowl,
  platter, pedestal-table top or foot.
- A **rectangular member that changes size along its length**: a tapered spoke or a leg
  narrowing to a foot.
- You want the cut list to say how the grain runs on a disc (`across axis (faceplate)`).

Not for a flat round outline with the faces left flat (use `cut_path`), nor for a curved
(barrel or ogee) rectangular profile (not built: RFC-0101 stays partial for that alone).

## Syntax

`section` defines the part's **base solid**, the role `stock`'s cube plays for every other
part. `circle`/`profile` turned sections are `axis=z` only (one spindle axis). A `rect`
section may sweep `axis=z` (default), `axis=x` or `axis=y` (#459, ADR-0034's "not done" bullet
— now built): the physical bound names `rect` and `at` take follow the sweep axis.

| `axis=` | `at` key | `rect` bounds | sweep runs |
| --- | --- | --- | --- |
| `z` (default) | `z=` | `x_min= x_max= y_min= y_max=` | `z = 0` to `z = -size.z` (top face to foot) |
| `x` | `x=` | `y_min= y_max= z_min= z_max=` | `x = 0` to `x = size.x` |
| `y` | `y=` | `x_min= x_max= z_min= z_max=` | `y = 0` to `y = size.y` |

Stations and bounds are otherwise in the part's own frame (D4): for `axis=z`, `z = 0` is the
top face and down is negative.

```kdl grammar

stock <Material> shape=round diameter=<d> z=<length> [grain=across]   // round blank
stock <Material> <x> <y> <z>                                          // rect blank

section axis=z [interp=linear|spline] [samples=<n>] {
    at z=<expr> { circle r=<expr> }                         // spindle: radius at a station
}
section axis=z { profile { line … arc … polyline { point x= y= } } } // faceplate: drawn outline
section axis=z { at z=<expr> { rect x_min= x_max= y_min= y_max= } }  // tapered rect member
```

- **Spindle (`circle` stations).** Grain is derived lengthwise. `interp=linear` joins the
  stations; `interp=spline` (`samples=`, default 64) draws a centripetal curve through them.
- **Faceplate (`grain=across`).** The blank is a disc cut from a board; the grain crosses the
  axis. Pair it with a `profile`: a closed outline in **(radius, z)** (`x` is the distance
  from the axis), written with the same `line` / `arc` / `polyline` children as `cut_path`,
  chained end to start. The **closing edge back to the first point is implied**, so write the
  outline from the axis at one face, out around the profile, and back to the axis. Winding is
  normalised; `interp=` and `samples=` do not apply; a `section` is stations or a `profile`,
  never both.
- **Curved rect member (`interp=spline` on `rect` stations, ADR-0034).** Each of the four bounds
  is a smooth curve through its keyframes; `samples=<n>` (chords per interval) or
  `tolerance=<len>` (default 0.01) sets the fineness, one or the other. See the worked case in
  `examples/trestle_table.kdl`.
- **Tapered rect member (`rect` stations).** Straight between keyframes. Stations must run the
  whole part (the sweep axis's own `0` to its far face; repeat a rectangle to hold a run
  constant), and no rectangle may leave the blank. `rect` bounds are corner-anchored: for a
  taper that keeps one face flush, hold the lower bound at `0.0` on every station; for a
  centred taper, inset both sides equally.
- **`axis=x` / `axis=y` member (a curved apron or shaped rail, #459).** The apron's own length
  runs along its *local* sweep axis, whichever way it is placed in the assembly (a side apron
  turned 90 degrees still sweeps its own `axis=x`, never a second `axis=y` case). Worked case:
  `examples/curved_apron_side_table.kdl`'s `Apron` (`axis=x interp=spline`, full height where
  it tenons into a leg, waisted thinner at mid-span).
- A square pommel over a turning (square `stock` plus `circle` stations that stop short of the
  top) is the RFC-0127 form in `docs/KDL_AUTHORING.md` §6.

## Example

```kdl
project name="Turned And Tapered" units=mm schema_version=1 {
    materials {
        material Maple type=hardwood
        material Oak type=hardwood
    }

    // A faceplate bowl: a disc cut from a board, grain across the lathe axis. The
    // profile is a closed outline in (radius, z); the edge down the axis is implied.
    component Bowl role=bowl {
        stock Maple shape=round diameter=200.0 z=70.0 grain=across
        section axis=z {
            profile {
                polyline {
                    point x=0.0   y="0.0 - 70.0"
                    point x=60.0  y="0.0 - 70.0"
                    point x=100.0 y="0.0 - 20.0"
                    point x=100.0 y=0.0
                    point x=92.0  y=0.0
                    point x=0.0   y="0.0 - 60.0"
                }
            }
        }
    }

    // A spindle: stations of diameter, grain derived lengthwise.
    component Baluster role=baluster {
        stock Maple shape=round diameter=40.0 z=300.0
        section axis=z interp=spline {
            at z=0.0            { circle r=14.0 }
            at z="0.0 - 60.0"   { circle r=20.0 }
            at z="0.0 - 150.0"  { circle r=10.0 }
            at z="0.0 - 300.0"  { circle r=16.0 }
        }
    }

    // A tapered rectangular member: straight between keyframes, whole length.
    component Spoke role=spoke {
        stock Oak 44.0 44.0 420.0
        section axis=z {
            at z=0.0           { rect x_min=0.0 x_max=44.0 y_min=0.0 y_max=44.0 }
            at z="0.0 - 40.0"  { rect x_min=0.0 x_max=44.0 y_min=0.0 y_max=44.0 }
            at z="0.0 - 420.0" { rect x_min=7.0 x_max=37.0 y_min=7.0 y_max=37.0 }
        }
    }

    assembly Set {
        part Bowl1 instance=Bowl {
            by top_center
            pos x=0.0 y=0.0 z=0.0
        }
        part Baluster1 instance=Baluster {
            by top_center
            pos x=300.0 y=0.0 z=0.0
        }
        part Spoke1 instance=Spoke {
            by top_sw
            pos x=500.0 y=0.0 z=0.0
        }
    }
}
```

The compiled cut list prints `across axis (faceplate)` for the bowl, `along axis` for the
baluster and `lengthwise` for the spoke. Each turned part's drawing card carries a `TURNED`
line with one `depth below top → ⌀` pair per authored station (a tapered member prints
`→ width × thickness`); on a `rect` section, "depth below top" is `axis=z`'s own wording — an
`axis=x`/`axis=y` member's card instead reads "depth from `{entry face}`" (e.g. `depth from
x=0`), since it has no top or foot. Full worked examples: `examples/pedestal_table.kdl`
(turned), `examples/curved_apron_side_table.kdl` (`axis=x` `rect` section).

## Pitfalls and diagnostics

- **`grain=across` on rect stock** is refused: rect stock states grain as its fifth positional
  argument. On round stock `across` is the only value.
- **"declared blank is …mm across, but the `profile` reaches …"**: the `diameter` must cover
  the widest point. Likewise a profile taller than `z=`.
- **"reaches x=-…"**: `x` is a distance from the axis and cannot be negative; the kernel would
  silently clip it, so it is refused.
- **"does not close into a loop that encloses an area"**: segments must chain end to start,
  and the outline must not cross itself. Do not repeat the first point at the end.
- **A `spline` child inside `profile`** is not a segment; use `line`, `arc` or `polyline`.
- **"a `rect` section must run the whole part"**: add a station at `z=0.0` and one at
  `z="0.0 - <length>"`.
- **"leaves the blank"**: a section only removes material; widen the `stock`.
- **`circle` beside `rect`**: one `section` is one shape family.
- **"overshoots the blank"** (`interp=spline` on `rect` stations): the smooth curve bulged
  past the stock between keyframes. Add a keyframe where the curve turns, or declare the blank
  wider. A curve that "doubles back in `z`", or "pinches the member", is refused too.
- **`samples=` / `tolerance=` without `interp=spline`**, or both together, are refused.
- A `kdl-check` warning on `by top_sw` (or any corner/edge anchor) of a round part: use
  `top_center`, `bottom_center` or `center`, which are the axis.
- Turning is a bench operation in the fabrication pack: no CNC setup is generated for it.
- A mark on the curved surface of a turning is not reachable; only the flat ends
  (`normal=pos_z` / `neg_z`) are.

## See also

- `docs/KDL_AUTHORING.md` §6 *Turned stock*, and §7 for `taper` (still supported; a `rect`
  section builds the same solid, and `examples/wagon_wheel.kdl`'s spoke now uses one)
- `docs/adr/0018-turned-stock.md`, `docs/adr/0031-faceplate-turning-and-profile-sections.md`,
  `docs/adr/0034-curved-rect-sections.md`
- `docs/rfc/0129-faceplate-and-profile-path-turning.md`, `docs/rfc/0101-functional-sections.md`,
  `docs/rfc/0127-round-tenon.md`
- `docs/superpowers/specs/2026-10-11-curved-rect-section-axis-x-design.md` (#459, `axis=x`/
  `axis=y` on a `rect` section)
- `examples/pedestal_table.kdl`, `examples/chess_pawn.kdl`, `examples/spindle.kdl`,
  `examples/wagon_wheel.kdl`, `examples/curved_apron_side_table.kdl`

## Names

Every name this page declares, how the example library uses it, and the shortest real use.

### `stock`

feature; node in 114 of 116 examples (910 uses).

Shortest real use, from `glazed_vitrine_whiplash_crest.kdl`:

```kdl
stock Glass dw 6.0 dh
```

### `section`

feature, word-valued property; node in 47 of 116 examples (93 uses); property in 18 of 116 examples (36 uses).

Shortest real use, from `pop_laminate_stepped_sideboard.kdl`:

```kdl
section axis=z interp=linear {
    at z=0.0 { circle r="drum_d / 2.0" }
    at z="0.0 - drum_h" { circle r="drum_d / 2.0" }
}
```

### `turn`

not used by any example yet.

### `profile`

word-valued property; node in 11 of 116 examples (12 uses).

Shortest real use, from `arts_crafts_cabinet_on_stand.kdl`:

```kdl
profile "square"
```

## Related terms

- [Baluster turning](/almanac/techniques#baluster-turning)
- [Bamboo turning](/almanac/techniques#bamboo-turning)
- [Round tenon](/almanac/techniques#round-tenon)
- [Spindle](/almanac/techniques#spindle)
- [Turned leg](/almanac/techniques#turned-leg)
- [Turning](/almanac/techniques#turning)