Split a beveled part and keep its cut centered

Level: intermediate → advanced · Workspace: CAD Editor #/model · Unit: mm

You can combine two supported operations—Chamfer followed by Split—without turning display triangles into authoritative geometry. You can also make the split plane follow a part's width using a saved parametric dimension. This exercise compares two different built-in models, not a switch that converts an existing model between free and formula-driven splits.

For the simpler prerequisites, use the CAD modeling reference for two chamfers and the plain split guide.

Verification status: Source-backed local experimental workflow. All volume and refusal figures below come from previously executed, source-pinned CAD provider fixtures. This documentation change has not executed a fresh browser/GPU/file-roundtrip campaign, and it does not certify manufacturing quality.

1. Start the actual Modeling workspace

From a repository checkout with existing dependencies, using Node 22+:

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

Open http://127.0.0.1:52010/#/model or the local Vite port with #/model. Wait until Model a part shows Accepted model.

To load a different preset, expand Modeling examples, use Choose an example, and select the desired entry. An existing accepted model requires the explicit Replace accepted model and discard its history and pending dimensions checkbox before Load example is enabled. Merely changing the picker or axis selector does not change your accepted geometry.

2. Split the finished, twice-beveled part

Choose Split a beveled part, keep X · negative / positive under Split axis for the new example, acknowledge replacement, and press Load example.

This model uses a 32 × 24 × 8 mm direct rectangular extrusion. Its first top-edge chamfer has frontDistance=1 mm; the second top-edge chamfer has backDistance=2 mm. The second chamfer's selector belongs to the actual first chamfer result. Only after both bevels are accepted does the model split the finished result.

There are three exposed drivers under Named dimensions:

Field	Initial value	Meaning
frontDistance · front bevel distance	1 mm	Physical setback on the first supported top edge
backDistance · back bevel distance	2 mm	Physical setback on a distinct top edge of the first result
splitAt · selected-axis split coordinate	16 mm	Actual X coordinate of the split plane, not a bevel distance

The outputs are independently named negative and positive, meaning the lower-X and higher-X occupied sides of the plane. Select each result with Visible output body → Choose one output to inspect; hiding one does not remove it from authored data.

Execute the measured edit sequence

1. At splitAt=16, inspect both accepted outputs.
2. Change splitAt to 8, then press Apply dimensions. While the status is Dimension changes pending, only draft text has changed. Wait for Accepted model before interpreting a new result.
3. From that accepted X8 state, change backDistance from 2 to 3 and apply again.
4. Use Undo and Redo to review accepted states. If text is pending, choose Discard draft or apply it first.

Accepted state in the source-pinned native sequence	negative volume	positive volume
X16, front1/back2	3032 mm³	3032 mm³
X8, front1/back2	1516 mm³	4548 mm³
X8, front1/back3	1496 mm³	4488 mm³
Cold reopen of saved X8/front1/back3	1496 mm³	4488 mm³
Reopened model, X16/front1/back3	2992 mm³	2992 mm³

For quick comparison with the frozen provider's negative / positive notation: X16 = 3032 / 3032, X8 = 1516 / 4548, X8/back3 = 1496 / 4488, and reopened X16/back3 = 2992 / 2992 (all mm³).

These figures are qualified only within the bounded convex-planar family of the measured fixture. They are not reconstructed from browser pixels or guaranteed for unrelated imported models.

Moving only splitAt can reuse the unchanged model features leading to the finished chamfered part. Editing backDistance changes the second chamfer and the subsequent split. Feature reuse counts are not timing or total native-call benchmarks; the app still regenerates display geometry from accepted outputs.

Another axis is a new model, not an edit

This preset also offers Z · negative / positive when you load a new example. The separate source-backed Z4 fixture yields 3072 / 2992 mm³. The two-chamfer preset does not offer Y; the plain Split a part preset has different X/Y/Z choices and output names.

Split axis for the new example resets example replacement acknowledgment. It does not convert a currently accepted X model into Z or preserve the old example's history. Save any existing work before acknowledging a new load.

3. Load a different, automatically centered split

Now select Keep a beveled split centered from Modeling examples, acknowledge replacement, then choose Load example. This is a new authored recipe with the same kind of two serial edge bevels but with no editable splitAt field.

The model owns a derived X coordinate:

split X = width × 0.5 + offset

Its only user-editable Named dimensions are width, offset, frontDistance, and backDistance. The UI describes width · rail X size and offset · X displacement from center. offset is a millimetre displacement, not a normalized fraction or the width of the bevel.

The expression is authored by the preset using the public model.dimension.scale and model.dimension.add operations. It is saved as a model-owned formula tree; the editor does not offer a formula text editor, general JavaScript evaluator, or a switch that retrofits the independent Split a beveled part model with this expression.

Change the offset, then change the width

1. Starting from width=32 and offset=0, the X plane is at 16 mm.
2. Edit offset from 0 to 4 and choose Apply dimensions. The cut moves to X20.
3. From that accepted state, edit width from 32 to 40. The plane becomes X24 automatically, even though the offset stays 4.
4. On the accepted width40 model, set offset back to 0 to recenter at X20. Use Undo/Redo to compare prior accepted states.

Accepted source-pinned model state	Computed split X	negative / positive native volume
width32 / offset0	16 mm	3032 / 3032 mm³
width32 / offset4	20 mm	3790 / 2274 mm³
width40 / offset4	24 mm	4548 / 3032 mm³
width40 / offset0	20 mm	3790 / 3790 mm³
Cold reopened width40 / offset0	20 mm	3790 / 3790 mm³
Reopened width40, offset4	24 mm	4548 / 3032 mm³

Each table row describes its own qualified native state, not a new live browser observation. Notice that changing width also changes total material volume, so keeping the split plane centered does not mean the positive/negative volumes remain equal after adding a nonzero offset.

4. Safely test an unsupported split

In the direct-coordinate beveled model, after reaching the accepted X8 state, a splitAt=40 edit requests an empty positive side. The existing provider fixture refuses with CadModelError SELECTION / split, reporting the outside positive side. The previous X8 accepted snapshot remains usable.

In the derived centered model, from the accepted width40/offset4 state, offset=40 would place the plane at X60: again one requested output is empty and the measured provider refuses SELECTION / split. In the fixture, retrying offset=0 from the last accepted state succeeds. Do not continue editing a failed candidate as if it were accepted.

The editor must keep previously accepted native bodies, authored text and history on refusal; correct the field or use Discard draft. Preserve actual failure diagnostics. Do not silently snap a plane into the part, drop one named output, weld disconnected material or treat a stale viewport image as proof of success.

5. Save, reopen, and re-edit the actual recipe

Open Save or open a model and choose Download model to initiate part.cad.json. Confirm the browser really saved the file; clicking Download is not durable-storage evidence.

Choose Choose model file to stage the saved UTF-8 text in Saved model data. File selection does not accept geometry, update the viewport or grant replacement permission. Review the staged text, check Replace accepted model and discard its history and pending dimensions, then choose Open saved model.

After acceptance, select negative or positive in Visible output body as needed. Generic Open chooses the last validated authored output, not your previously displayed output. Re-edit the split coordinate for the direct model, or offset for the centered model, to establish that authored intent survived a cold rebuild. The saved file contains the expression tree and numeric drivers, not cached meshes, private feature reuse frames, camera pose or GPU pixels.

The staging reader refuses invalid UTF-8, empty text and inputs over 262,144 bytes. A readable text file still undergoes independent native validation during explicit Open; no parser acceptance is implied by the file chooser.

6. Hard boundaries and evidence tiers

The source has a bounded profile: one supported sketch/extrusion, up to two serial owned chamfers, then one supported split. The split parent must descend from the original extrusion through that admitted chamfer chain; arbitrary sibling jumps, cut-rooted chamfer trees, repeated/nested splits, arbitrary oblique planes, general fillets and persistent topology naming are unsupported.

The centered preset's formula grammar admits only model-owned add and scale, with no string expression parser or general evaluation language. The existing formula limit is 32 expression nodes, depth 6; all intermediate/resolved dimensions must be finite and within ±10,000 mm. Native exactness and displayed-mesh admission remain distinct. Being under the formula budget does not guarantee successful geometric representation.

Provider evidence: the beveled split fixture executed its admitted X/Z native states and cold rebuild/re-edit; the owned derived dimension fixture separately executed its width/offset and refusal cases. Their source pins and precise counters belong to those references.

Consumer limitation: the editor's tests replay source-qualified baseline transport through a mocked Worker. Edited-state display geometry, full browser React interactions, real GPU pixel capture, and physical Download/Open were not freshly demonstrated for this guide. UNRUN here means unverified, not passed. Follow the browser evidence protocol before upgrading a manual claim.

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