modify-structures

Modify Structures

Overview

modify-structures is Pass 2 of the CAD pipeline. It takes the base STEP files produced by generate-structures and applies detail features specified in each part's modifications list: radial holes for fasteners, through-holes for screws, rectangular pockets for electronics bays, cylindrical pockets for camera mounts, vent holes, weight-relief cuts, and any other subtractive or additive detail operations that aren't part of the structural shell.

The split exists because:

• Iteration speed — changing hole positions or adding a camera mount doesn't require regenerating the structural shell
• Review boundaries — you verified the base geometry in Pass 1; Pass 2 is purely "apply these details"
• Future-proofing — rail buttons, altimeter bay cutouts, payload mounts, and custom user requests all plug into the same modification framework

This skill is rocketry-agnostic — it knows about build123d boolean operations and modification recipes, not about rockets. The manifest's modifications list drives what gets applied.

Core principle: Modifications are applied to base STEPs produced by Pass 1. Do not re-generate base geometry here. If a modification requires changing the structural shell, update the manifest's features block and re-run Pass 1.

Never edit component_tree.json directly to add or change modifications. The modifications list is populated by manufacturing_annotate_tree based on DFAM rules and user-specified overrides. If the modifications list is missing entries the user wants (e.g., rail button holes, vent holes), re-run manufacturing_annotate_tree with the appropriate parameters — do not hand-edit the JSON.

When to Use

• generate-structures has completed Pass 1 successfully
• At least one entry in manifest["parts"] has a non-empty modifications list
• The user asks to add retention hardware, accessory mounts, vent holes, or other detail features

If every part's modifications list is empty (which is the default for a freshly generated DFAM manifest — retention defaults to "none"), skip this skill entirely.

Inputs

• <project_root>/gui/component_tree.json — the parts[].modifications lists are the authoritative spec (populated by manufacturing_annotate_tree, never hand-edited)
• <project_root>/cadsmith/step/<name>.step — base STEPs from Pass 1

Output

• <project_root>/cadsmith/source/<name>.py — the canonical script for this part, overwritten with the modification version (imports the base STEP and applies all modifications)
• <project_root>/cadsmith/step/<name>.step — overwritten with the modified version
• gui/assembly.json — regenerated via cadsmith_assembly(action="generate") if any part was modified
• <project_root>/gui/assets/png/<name>.png — re-rendered after modification

Steps

1. Load the Manifest

manifest = read_json("<project_root>/gui/component_tree.json")

2. Identify Parts Needing Modification

parts_to_modify = [p for p in manifest["parts"] if p["modifications"]]

If the list is empty, skip the rest of this skill.

3. For Each Part With Modifications

1. Verify the base STEP exists at <project_root>/<step_path>. If missing, run generate-structures first — do not fabricate a base here.
2. Write the modification script to <project_root>/cadsmith/source/<name>.py, overwriting the Pass 1 script. The script imports the base STEP, applies each modification in order, and exports back to the same step_path (overwriting).
3. Execute via cadsmith_run_script. The tool runs the script in isolated mode and returns the path of the overwritten STEP.
4. Re-render via cadsmith_generate_assets(step_file_path=<step_path>, out_path=<images_dir>/<name>.png) and Read to visually verify the modifications are in the right place.
5. Re-extract via cadsmith_extract_part to confirm the bounding box hasn't changed unexpectedly (modifications typically only remove material, so bounding box should match).
6. Pause for user feedback. Modifications are detail features that interact with the base geometry in ways that are hard to verify autonomously — hole placement relative to shoulders, pocket depth vs. wall thickness, angular alignment of through-holes with mating parts. Show the user the re-rendered PNG, describe what was modified (e.g., "Added 4× M4 heat-set holes at Z=25mm on the upper airframe shoulder"), and ask whether the placement looks correct. See User Feedback on Modifications below.

4. Regenerate the Full Assembly

If any part was modified, the assembly layout is stale — regenerate it via cadsmith_assembly(action="generate", project_dir=<project_root>). The GUI's 3D viewer will update automatically.

Modification Script Structure

Unlike Pass 1 scripts which build geometry from scratch, Pass 2 scripts import an existing STEP and apply operations to it. Like Pass 1, the paths are resolved relative to the script's own location so the project stays portable:

"""
<Part name> — detail feature modifications.
Pass 2 (modify-structures): applies holes / pockets / mounts to the
base STEP produced by generate-structures.
"""
from build123d import *
from pathlib import Path
from math import cos, sin, radians

# --- Resolve paths relative to this script's location ---
# This script lives at <project_root>/cadsmith/source/<name>.py
# Base STEP and output STEP are at <project_root>/cadsmith/step/<name>.step
SCRIPT_DIR = Path(__file__).resolve().parent
PROJECT_ROOT = SCRIPT_DIR.parent
BASE_STEP = PROJECT_ROOT / "step" / "<name>.step"
OUTPUT = BASE_STEP  # overwrite in place

# Import the base
base = import_step(str(BASE_STEP))

# Apply each modification in order
with BuildPart() as modified:
    add(base)
    # ... modification operations below, in manifest order ...

OUTPUT.parent.mkdir(parents=True, exist_ok=True)
export_step(modified.part, str(OUTPUT))

Key rules:

• Paths are relative to __file__. Resolve PROJECT_ROOT from the script's own location. This keeps the script portable across machines and checkouts — never hardcode an absolute path like /Users/someone/rockets/... in a modification script.
• Import the base STEP, don't rebuild the shell. If you find yourself re-declaring LENGTH_MM and OD_MM to rebuild a cylinder, you're in the wrong skill.
• Apply modifications in manifest order. Some modifications interact (e.g. a pocket inside a heat-set boss region); order matters.
• Overwrite the base STEP in place. The manifest's step_path is the canonical location; don't emit a separate _modified.step file.
• Imports limited to build123d, bd_warehouse, pathlib, math, typing. Same isolated-mode constraint as Pass 1. Use bd_warehouse.fastener for spec-correct fastener geometry in boolean cuts (heat-set pockets, counterbores, nut traps).

Modification Recipe Reference

Each modification entry in the manifest has a kind field that identifies the recipe. Below are the supported kinds and their build123d implementations.

radial_holes — cylindrical holes drilled radially inward from a tube surface

Fields:

• count — number of holes in the polar array
• angular_positions_deg — explicit list of angles in degrees, or omit to evenly space
• z_mm — Z position of the hole centres (single value for all holes in this modification)
• radius_mm — radial distance from the Z axis (where the hole axis passes through; typically the shoulder OD / 2)
• hole_diameter_mm — diameter of each hole
• hole_depth_mm — blind depth (for heat-set inserts) or pass 0 for through-wall

Recipe:

positions = modification.get("angular_positions_deg") or [
    i * (360 / modification["count"]) for i in range(modification["count"])
]
for angle_deg in positions:
    ang = radians(angle_deg)
    cx = modification["radius_mm"] * cos(ang)
    cy = modification["radius_mm"] * sin(ang)
    hole_plane = Plane(
        origin=(cx, cy, modification["z_mm"]),
        x_dir=(-sin(ang), cos(ang), 0),
        z_dir=(-cos(ang), -sin(ang), 0),
    )
    with BuildSketch(hole_plane):
        Circle(modification["hole_diameter_mm"] / 2)
    extrude(amount=modification["hole_depth_mm"], mode=Mode.SUBTRACT)

Use this for heat-set insert receivers and shear pin holes. For heat-set inserts, prefer using bd_warehouse for spec-correct pocket dimensions — call cadsmith_bd_warehouse_info(generator_class="HeatSetNut", generator_params={"size": "M4-0.7-8", "fastener_type": "Hilitchi"}) to get exact nut_diameter and nut_thickness, then subtract the geometry directly:

from bd_warehouse.fastener import HeatSetNut
insert = HeatSetNut("M4-0.7-8", "Hilitchi", simple=True, mode=Mode.PRIVATE)
# Use insert.nut_diameter and insert.nut_thickness for the pocket,
# or subtract the insert geometry directly with mode=Mode.SUBTRACT

radial_through_holes — through-wall clearance holes

Same fields as radial_holes except hole_depth_mm is ignored (the hole passes all the way through the wall). Use this for screw clearance holes on the mating side of a heat-set joint.

Recipe: identical to radial_holes but use a large extrude depth (e.g. 2× tube OD) to ensure the hole passes through both walls if the tube is hollow. Alternatively, use Hole(...) which automatically extends through material.

positions = modification.get("angular_positions_deg") or [
    i * (360 / modification["count"]) for i in range(modification["count"])
]
for angle_deg in positions:
    ang = radians(angle_deg)
    cx = modification["radius_mm"] * cos(ang)
    cy = modification["radius_mm"] * sin(ang)
    hole_plane = Plane(
        origin=(cx, cy, modification["z_mm"]),
        x_dir=(-sin(ang), cos(ang), 0),
        z_dir=(-cos(ang), -sin(ang), 0),
    )
    with BuildSketch(hole_plane):
        Circle(modification["hole_diameter_mm"] / 2)
    # Extrude far enough to punch through both walls of a hollow tube
    extrude(amount=200.0, mode=Mode.SUBTRACT)

Future: rectangular_pocket — surface-mounted cutout for electronics, cameras, access panels

Not yet implemented. Reserved fields: target (Z and angular position), width_mm, height_mm, depth_mm, fillet_mm. When this lands, query the cad_examples reference collection for a known-good implementation before improvising.

Future: cylindrical_pocket — circular recess for camera lenses, altimeter ports

Not yet implemented. Reserved fields: target, diameter_mm, depth_mm.

Future: rail_button_mount — protruding boss for an adhesive rail button

Not yet implemented. When this lands, it will likely be an additive modification (the only one in this skill that adds material rather than removing it).

For any kind not in the recipe reference, query rag_reference(action="search", collection="cad_examples", query=f"build123d {modification['kind']}", n_results=3). If no results, ask the user rather than improvising.

Verification After Modification

After each part is modified:

1. Re-render: cadsmith_generate_assets(step_file_path=<step_path>). The tool auto-routes to png/<name>.png, overwriting the Pass 1 render — the modified version is the current truth.
2. Visual check: does the render show the modifications in the expected positions? Heat-set holes should appear as small dark circles around the shoulder mid-length. Through-holes should appear at matching angles on the mating tube.
3. Dimensional check: cadsmith_extract_part — the bounding box should be unchanged (all current modifications are subtractive). If the volume dropped by more than ~5% of the base, flag it — you may have subtracted too much.
4. User feedback: pause and ask the user to confirm the modifications. See below.

User Feedback on Modifications

All modifications require user feedback before proceeding to the next part. Unlike Pass 1 where only complex features need a pause, Pass 2 modifications are inherently detail-oriented and their correctness depends on assembly context that only the user can fully judge.

What to show the user

1. The re-rendered PNG — show the path or display inline
2. A summary of what was applied, e.g.:
   • "Applied 4× radial heat-set holes (M4, 5.7mm × 7mm) at Z=25mm, evenly spaced at 0°/90°/180°/270° on the upper airframe shoulder"
   • "Applied 4× through-holes (4.5mm clearance) at matching positions on the lower airframe mating end"
3. A targeted question:
   • For hole patterns: "Do the hole positions align with where you want the retention hardware? Are the angular positions correct for your assembly?"
   • For pockets: "Does the pocket depth look right relative to the wall thickness? Is the placement where you expected?"
   • For paired modifications (heat-set + through-hole): "These two parts mate at this joint. Do the hole patterns on both sides look aligned?"

Handling feedback

• Approval — proceed to the next part's modifications.
• "Move the holes" / "Change the angle" — update the manifest's modification entry, rewrite the script, re-run, re-render, and ask again.
• "Skip this modification" — remove it from the script (but leave it in the manifest for traceability), re-run, and proceed.

Red Flags — Stop and Fix

• A modification script rebuilds the base shell instead of importing it via import_step
• A modifications list is empty but the Pass 2 script still produces output — the part should be untouched
• A hole pattern appears at the wrong Z position — cross-reference the manifest's z_mm against the part's features["length_mm"] to confirm the shoulder position was computed correctly in DFAM
• Clearance through-holes and heat-set insert holes don't align at the joint — the angular positions and Z positions must match between the shoulder-emitting part and the mating part
• The assembly render shows holes in the wrong place or missing — check each part's modification script independently before blaming the composition
• A modification adds material where the base was hollow (e.g. a pocket that punches through a fin root) — check depth_mm and make sure you're subtracting, not adding

Handoff to the cadsmith Subagent

After every modified part is verified and the full assembly is regenerated, hand control back to the cadsmith subagent. The subagent reports to the orchestrator which then hands off to the prusaslicer subagent for slicing.

Do not re-run generate-structures after a successful modify pass. The modified STEP is the current truth; regenerating base geometry would overwrite your modifications.

Quick Reference

# the Pass 2 loop
for part in manifest["parts"]:
    if not part["modifications"]:
        continue
    verify_base_step_exists(part["step_path"])
    write_modification_script(part)
    cadsmith_run_script(script_path, out_dir)
    cadsmith_generate_assets(step_file_path, out_path=images_dir/<name>.png)
    Read(png_path)
    cadsmith_extract_part(step_file_path)
    if check_failed: fix_and_retry()
    # always pause for user feedback on modifications
    ask_user("Applied <modifications> to <part>. Does the placement look correct?")
    wait_for_response()
    if user_requested_change: update_and_retry()

# regenerate the assembly if anything changed
if any_part_was_modified:
    cadsmith_assembly(action="generate", project_dir=<project_root>)
    ask_user("Regenerated assembly with modifications. Do the joints look right?")
    wait_for_response()

# then handoff
report_to_cadsmith_subagent()