Cut a pocket from the top of a plate

Lesson 5 · Level: beginner → intermediate · Workspace: CAD Editor #/model · Units: mm

Make a real top-entry recess in a 32 × 24 × 8 mm plate, deepen it into a through-window, then save and reopen the authored part. This differs from Lesson 1's underside-entry pocket: the cutting tool starts at the top, not Z=0.

This is a source-backed guide for the local experimental editor. The native provider measured the values below; this lesson has not independently captured browser screenshots, GPU output or a physical file round-trip.

1. Load the correct example

Use a repository workspace with Node 22+ and its existing dependencies:

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

Open http://127.0.0.1:52010/#/model (or the port Vite printed, plus #/model). Under Model a part, wait for Accepted model.

1. Expand Modeling examples → Choose an example, then select Cut a top-entry pocket.
2. Save any earlier part you need. If a model is already accepted, check Replace accepted model and discard its history and pending dimensions.
3. Click Load example and wait for Accepted model. Choosing an example in the list alone does not replace the accepted model.
4. Under Visible output body, select part if needed.

The preset retains three named outputs: plate (original stock), tool (the rectangular cutter) and part (material left after the cut). Hidden outputs are still authored and validated.

2. Learn which direction the tool moves

The 8 mm-thick plate occupies Z=0..8. The tool's X range is fixed at 8..24 mm. Its Y range comes from Tool Y origin and Tool Y span; their initial values are 8 mm each.

Inspector label	Parameter	Starts at	Role
Pocket depth downward from top Z=8	depth	4 mm	How far the cutter enters from the top
Tool Y origin	toolY	8 mm	Lower Y location of the cutter
Tool Y span	toolSpanY	8 mm	Extent in Y, not depth

The model owns the derived dimension from = 8 - depth. The cutter extrudes upward by positive depth starting at from, so its top stays at Z=8 while its bottom moves down. There is no separate free-standing from field in this preset; entering an offset in a generic imported graph is a different authoring contract. Unlike the underside pocket, this one leaves the lower plate face intact while the recess is shallow.

3. Deepen the recess

Change only the labeled Pocket depth downward from top Z=8 field, one accepted step at a time:

1. With depth 4, the pocket has a 4 mm-thick floor at Z=4. Inspect part, not the cutter.
2. Type 6. Dimension changes pending means this is only a draft. Select Apply dimensions, wait for Accepted model, then inspect the deeper recess. Its floor is at Z=2.
3. Type 8 and Apply dimensions. The cutter now spans Z=0..8; the aperture becomes a through-window with no floor.
4. Use Undo/Redo to compare accepted steps. If a draft is pending, use Discard draft or apply it before asking for history changes.

These are previously measured qualified native volumes for the part output, not values inferred from new screen pixels:

Accepted depth	Tool lower Z (from)	Result	part volume
4 mm	4 mm	Recess with floor Z=4	5632 mm³
6 mm	2 mm	Recess with floor Z=2	5376 mm³
8 mm	0 mm	Through-window	5120 mm³

The existing native API recipe and accepted provider evidence establish these particular dimensions. The viewport mesh is derived from accepted native data; it does not independently certify volume or exact material.

4. Understand an intentional refusal

You can recreate the provider-qualified refusal inputs with the visible controls, after reaching an accepted depth=8 through-window: change Tool Y origin to 0 and Tool Y span to 24 in one pending batch, leaving depth at 8. The cutter then crosses the full Y extent of the plate, leaving two disconnected remaining components (1536 mm³ each in the native fixture). The single-result part authoring contract refuses instead of silently choosing one disconnected piece. The previous accepted through-window remains usable; correct or Discard draft. This is a Y-extent change, not a separate X-width slider: tool X stays fixed at 8..24. The exact provider refusal is known, but this documentation change has not independently tested the live browser UI.

The current UI can also refuse invalid or nonrepresentable tool dimensions, positive-height violations, geometry qualification, or the 64-panel display limit. A refused Apply dimensions preserves the previous accepted snapshot, history and authored text. Check the actual diagnostic, then correct or Discard draft. Do not infer that a native acceptance guarantees a GPU frame.

5. Save and reopen the authored model

Open Save or open a model. Choose Download model to initiate part.cad.json and verify that a file actually appears on disk. Then choose Choose model file and select it. File selection only places UTF-8 text into Saved model data; it neither evaluates geometry nor replaces an accepted document.

Review the text, check Replace accepted model and discard its history and pending dimensions, and select Open saved model. The model is rebuilt from the authored sketch, extrusion, derived lower-origin expression, cut, and named parameters—not a saved mesh or camera.

Important: the preset initially selects part, but generic Open chooses the last saved output in canonical order part,plate,tool, which is tool. Use Visible output body to choose part again before deciding the pocket disappeared. Editing depth after Open establishes an editable recipe; simply seeing an old preview does not.

The file reader refuses empty/malformed UTF-8 and files larger than 262,144 bytes; explicit model Open still performs its own native validation and may refuse a syntactically readable file. Failed staging/Open retains the old accepted model.

Qualification and evidence

This is a bounded integer-cell rectangular difference, not a general curved Boolean or unrestricted cavity tool. A successful native result has one positive-volume connected material component and needs a supported exact representation. Source-receiver checks preserve original cut surfaces; no shortcut from displayed triangles to B-Rep identity or persistent face naming is implied.

The existing baseline provider retained 46 genuine part display packets, with 184 vertices and 92 triangles for that baseline. It did not retain genuine meshes for every hidden operand or the depth-6 and through-window edited states. Their separate CPU controller/display exercises use explicitly mocked or topology-derived data; those are not fresh native captures.

External validation remains open: live React controls, changed-state browser/GPU pixels, physical download/reopen, and real screenshots are UNRUN in this documentation change. Track the dedicated top-pocket device gate #1867 without marking it complete from the Manual. See the browser evidence protocol.

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