Butter Churn

A plunge butter churn: a tall body of sixteen oak staves that narrows toward the top, three forged iron hoops, a bottom of three boards set in a croze, a lid of two boards with a hole for the handle, and a maple cross dasher whose handle runs up through the lid. A showcase piece, not an example — it witnesses no API contract. It asserts that generated geometry meets declared asset budgets, recomputed from the finished mesh.

What it composes#

The budget that matters#

A churn is for plunging. The body narrows toward the top, so a plunger that clears the staves at the bottom can jam partway up the stroke, and nothing else notices:

• the plunger still sits inside the body without touching it;
• the handle still clears the lid's hole;
• the bounding box does not move.

The piece reads the inner wall off the staves ring by ring and the plunger's reach off the dasher's slats, about the handle's axis. It finds the height at which the wall comes within WALL_CLEAR (10 mm) of the plunger, stops at the lid if that comes first, and asserts the travel from the plunger's rest height as a stroke floor of 400 mm. Measured: 513.8 mm, for a 100.6 mm reach.

--wide-dasher is the falsifier built for exactly this. It widens the slats to 125 mm. The plunger still clears the staves at rest, but it can travel only 140.4 mm before it would meet the wall, and the run exits

1. Every other budget passes and the AABB is unchanged.

Budgets#

Declared in the script as named constants, recomputed from the generated mesh. Measured values are from Blender 5.2.1; every one is byte-identical on 4.5.11 and 5.1.2 (only the glTF file size differs, by 8 bytes, which is exporter metadata and not a budget).

Real-world size: 0.34 m across the foot, 0.75 m to the rim and 1.05 m to the top of the handle. It is a farmhouse plunge churn.

Construction#

• Staves. Sixteen staves are lofted on a straight taper (R_BOT 170 mm to R_TOP 120 mm), with a gap of STAVE_GAP between neighbours.
• Each stave's section has four vertices across its width at fractions 0, ¼, ¾ and 1, outer face over inner face, and quads close both ends.
• Ring heights are a uniform run plus both edges of every hoop and of the croze, so a hoop's edge and the bottom's faces meet stave vertices.
• Each stave's foot and top are cut sloping inward (END_CANT, doubled on alternate staves), so the churn stands on its outer edges.
• Every real edge is chamfered at 1.5 mm; the few-degree edges across a stave's width are left alone.
• Hoops. Each hoop is a banded iron section: flat inner face, chamfered outer edges. It is sampled at every stave's two interior vertex angles, so every hoop vertex lies radially over a stave vertex at the same height. The inner face is HOOP_BITE inside the stave's outer face, read from the same taper; the outer face is HOOP_PROUD outside it.
• Bottom. A 48-gon disk sized to the inner wall plus CROZE, cut across X into three boards with a named seam (Sutherland–Hodgman clip). Caps are chamfered as n-gons and then triangulated.
• Lid. Two half-annulus boards with the seam across the hole. The hole is HOLE_CLEAR wider than the handle. The lid rests LID_SEAT on the stave tops and overhangs by LID_OVER.
• Dasher. Two crossed maple slats, the second a step higher. The handle's foot is driven HANDLE_BITE into them. The handle is a 12-segment lathe with rings at both faces of the lid, so its clearance is measured where it passes, and it ends in a swelled grip.

Findings the budgets forced#

• Neighbouring ends in one plane. The first build had 240 coplanar cross-shell pairs:
• adjacent stave feet and tops;
• the two lid boards;
• the bottom boards' faces and a rim chord the seams split.

The fixes:

• stave ends are canted inward, doubled on alternate staves (at one cant, neighbours 5.6° apart in bearing still matched as one plane);
• one lid board stands 0.6 mm proud;
• the middle bottom board steps up and in by 0.6 mm.

The count is 0.

• A falsifier stolen by the collider. --float-hoops exited 11 on the collider ceiling (242 against 240) instead of 18. The hoops stand 5 mm proud and add nothing a character could hit, so they were taken out of the body hull rather than the ceiling raised. The collider is now 178 triangles, and the falsifier exits 18.

Conventions walked#

Every convention in showcase/README.md, and whether it applies here.

Falsifiers#

Each breaks one pipeline stage so a named budget fails. All ten were run on 4.5.11, 5.1.2 and 5.2.1 and exited the same declared code on all three.

Exit codes#

File-local and sequential. 9 is a valid check code. 1 is the FATAL wrapper — a crash, never a named check. 19 (plumb and real-world size) is reserved across pieces and unused here.

Run it#

# Budget check, no render. ~1.8 s on 4.5 and 5.2.
blender --background --python butter_churn.py --

# Falsifier: the plunger jams partway up the narrowing body. Must exit 20.
blender --background --python butter_churn.py -- --wide-dasher

# Falsifier: the lid's hole is narrower than the handle. Must exit 18.
blender --background --python butter_churn.py -- --tight-hole

# Render the gallery still (EEVEE; --engine cycles on a GPU-less host).
blender --background --python butter_churn.py -- --output butter_churn.webp

Smoke runs the check-only path. It does not pass --output or any falsifier.

Cross-version measurements#