Split a CAD part into lower and upper bodies

Level: intermediate · Workspace: local CAD Editor #/model · Unit: millimetres

Split divides one native solid at a qualified plane into two separately named solid bodies. This hands-on tutorial uses the existing Split a part example. Moving the split plane changes the volumes of lower and upper without trimming a mesh and pretending it is authored geometry.

This guide uses source-backed results from a prior qualified native fixture; it does not claim a fresh browser/GPU verification, screenshot, or physical file round-trip. The example is bounded to the supported operation and geometry profile.

1. Load the Z-axis split preset

From a Node 22+ workspace that already has its dependencies:

pnpm --filter @barocss/cad-editor build:library
pnpm --filter @barocss/cad-editor-app dev

Open http://127.0.0.1:52010/#/model (or Vite's actual port followed by #/model).

1. Wait for Accepted model in Model a part.
2. Under Modeling examples → Choose an example, select Split a part.
3. Under Split axis for the new example, keep Z · lower / upper. The axis selector configures a new example; changing it does not turn the previously accepted part or its Undo history into another axis.
4. If another accepted model exists, save anything valuable, check Replace accepted model and discard its history and pending dimensions, then choose Load example.
5. Wait for Accepted model. Under Visible output body, select lower or upper for inspection.

The source-backed Z profile starts with one direct convex XY pentagon sketch, extruded to height height=8 mm, and a Z split coordinate splitHeight=2 mm. The saved outputs are two real qualified native solids, not two views of the same unsplit body.

2. Move the split plane

Under Named dimensions, locate splitHeight. This is a Z coordinate in millimetres, not a percentage, volume ratio, screen pixel, or camera offset.

1. Leave height=8 and splitHeight=2 initially. The lower result has Z bounds [0,2]; the upper has [2,8].
2. Enter 4 for splitHeight. Until Apply dimensions, the status Dimension changes pending represents a draft only.
3. Select Apply dimensions and wait for Accepted model. Switch Visible output body between lower and upper.
4. Use Undo and Redo to compare the two accepted states. Use Discard draft to abandon an unaccepted value first.
5. In a separate accepted state, set splitHeight=6. After saving/reopening the split-at-4 model, the existing provider's cold re-edit fixture demonstrates this edit as well.

The accepted native volumes are:

Accepted Z split	lower	upper	Total
splitHeight=2	1792 mm³	5376 mm³	7168 mm³
splitHeight=4	3584 mm³	3584 mm³	7168 mm³
splitHeight=6	5376 mm³	1792 mm³	7168 mm³

The interface's lower/upper labels reflect the Z coordinate relative to the plane. Both closed solids share a boundary plane, but their interiors are disjoint. The conserved 7168 mm³ is qualified for these measured fixtures; it is not inferred by counting display triangles.

3. Refuse an empty requested side

The current two-output recipe requires 0 < splitHeight < height so that both named outputs have positive volume. If you change splitHeight to 0 (the bottom face), the lower side is empty and the two-output authoring operation refuses. The provider identifies the failure as CadModelError SELECTION, operation split, including a native cause.

The current UI must keep the earlier accepted lower/upper bodies and history on a refusal. Correct the value or use Discard draft. Do not silently discard the empty output, select a different result or infer successful native geometry from a still-visible previous frame. The low-level API may separately return one unchanged side for another output contract; this example requires both.

4. Save the two authored outputs and reopen them

Under Save or open a model, choose Download model to initiate part.cad.json, and verify that the browser saved it.

Choose Choose model file and select that file. This only stages text in Saved model data; it does not update the accepted native state. After inspecting the text, check Replace accepted model and discard its history and pending dimensions and choose Open saved model. The editor re-evaluates the sketch, extrusion, split plane and named output bindings.

After acceptance, choose Visible output body → lower or upper, and make another supported splitHeight edit to confirm authored intent survived reopening. File selection and download clicks alone are not physical file-recovery evidence; the browser/device flow requires independent verification.

5. What about X, Y and beveled parts?

The same Split a part example picker can instead initialize X · negative / positive or Y · negative / positive. Those are different new-example recipes, not an axis toggle on a current accepted Z-split part.

For X/Y, negative is the material at coordinates ≤ at, and positive at ≥ at. The provider's separate X/Y fixtures measure 4096 / 3072 mm³ at splitAt=16, then 5888 / 1280 mm³ at splitAt=24; their source-pinned native authority remains in the advanced guide. An exact crossing can refuse if its native XYZ/UV rational representation is not supported, rather than approximate the plane.

Split a beveled part is another specialized preset with qualified parent/result rules. A previously chamfered parent is not permission to chamfer any resulting split side or to stack arbitrary Boolean operations. General oblique planes, repeated/nested splits, general fillets and persistent face naming remain outside this beginner workflow.

Native versus presentation

The native split creates two independently owned accepted Solids. Each bodies.lower or bodies.upper is authoritative native geometry for the supported family. The Viewer displays derived triangles and provides approximate picks; viewport visibility does not rename, remove, or certify either output.

The typed public Z-split source and frozen native evidence record the actual accepted baseline/edit/cold re-edit results. The editor CPU receiving tests reuse those genuine baseline bodies with a mocked Worker and do not constitute a fresh native campaign, a GPU readback or physical file test.

Still UNRUN for this documentation change: real browser interactions, GPU pixel verification, actual download/fresh-file Open, and screenshots. See the browser evidence protocol before claiming device acceptance.

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