Printing without supports, and reading slicer warnings

These lessons come from one Claude Code session on 2026-11-01 (session 47707e1e-b3c1-4114-b10e-9fc722a426bd): a 1/8 Cat 323 excavator, 46 printed parts (178 pieces) for a Prusa XL 5T, designed in CadQuery with cad-agent and taken to PrusaSlicer 2.9.4 slicing every part with no supports and no stability warning. Jeff Patrick contributed them (#260); the project is in his private repository. Its file paths below are relative to that project. Nothing was printed or test-fitted in the session, so these are rules for files that slice cleanly, not proof that a print succeeds.

flowchart TD
    G["geometry<br/>teardrops, 45° cutter ends,<br/>no faces meeting tangentially"] --> O["print orientation<br/>splice face down,<br/>shells upside down"]
    O --> S["overhang screen:<br/>regions, not one total"]
    S --> P["slice, warnings kept"]
    P --> B{"long bridging?<br/>read the G-code"}
    B -->|"internal bridge<br/>over sparse infill"| C["slicer setting"]
    B -->|"real overhang"| G
    E["one part per class<br/>sliced early"] -.->|"mass and time"| G
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef near fill:#eef1e4,stroke:#6f8a80,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class G,O,S,P,B here
    class E near
    class C out

Timestamps are UTC from the session transcript. The Check lines name the cad-agent work these lessons asked for: #246 (keep PrusaSlicer's warnings), #247 (G-code bridge report and overhang regions), #249 (robust booleans and the 1 µm weld) and #251 (range-of-motion clearance sweep). In the session the project's own tools/slice_all.py, tools/bridges.py, tools/overhangs.py and tools/clearance.py did those jobs. In cad-agent, #246 is cad-agent slice with cad.slicer.engine: prusa, and #247 is cad-agent overhangs and cadagent.manufacturing.gcode.bridge_report.

Geometry

Cut horizontal holes as teardrops

• Rule: a hole whose axis lies flat in the print orientation gets a 45° roof (a teardrop) above its circle. If the part prints upside down, point the roof the other way. Make a slot the hull of two teardrops, not a box over them.
• Check: in the overhang screen, no region past 45° sits on the top of a horizontal hole. Overhang regions with position and angle (#247; tools/overhangs.py in the project) point at the hole directly; a single downward-area total does not.

End clearance cutters in 45° cones, not flat discs

• Rule: a cutter that clears room for a moving part (a swept cylinder, a bore, a channel) must not change radius in a flat step facing along its axis. Limit every radius change to 45° and end the cutter in a cone or a point. A flat end prints as a ceiling, and a step prints as a ring.
• Check: an overhang region list (#247) after every new cutter, not only after new solids. The cutters' own slopes can be checked in the build: no profile step steeper than 45° along the axis.

Print a section with a clevis slot standing on its splice face

• Rule: when a beam is printed in sections, look at each section's slots and forks in the print orientation. If a slot's upper cheek would be held at one end only, stand the section on its splice face, so the cheeks become walls that grow straight up.
• Check: a print-orientation table with a reason for each part (PRINT in cad/build.py), and the slicer's bed-adhesion warning kept (#246), since standing parts are tall and narrow.

flowchart TD
    F["a face pointing down<br/>in the print orientation"] --> S{"flatter than 45°?"}
    S -->|"yes"| R["reshape it: teardrop roof,<br/>45° cone, column boss"]
    S -->|"no"| P["no overhang flagged"]
    R --> T["overhang regions:<br/>none left there"]
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class F,S,R,T here
    class P out

Fasten shells printed upside down with 45°-capped bosses, not inward flanges

• Rule: a shell open at the bottom and printed roof down has its bottom edge at the top of the print. An inward flange there is a ceiling. Put the screws in bosses that are columns tied into the wall with a 45° cone on their free end, which prints either way up.
• Check: for each part printed upside down, list the features at its bottom edge and check each is a column, cone or slope, never a shelf. The overhang screen in the print orientation confirms it.

Open a shell's bottom without cutting away the walls' first millimetres

• Rule: when a cut opens a shell's bottom, keep the cut inside the walls on every side. A cut that spans the full length takes the bottom of the end walls with it, and those walls then start in mid-air.
• Check: after any cut that opens a part, every wall reaches the bed: no overhang region at a constant small height above it (#247 reports each region's height range).

Make long arch roofs steeper than 45°

• Rule: a face at exactly 45° is at the slicer's limit. On a long roof it can still print as overhang perimeters and raise a "long bridging" warning. Use about 40° from vertical for long arches and roofs, and lower anything above the arch below its apex, so no floor spans across it.
• Check: after slicing, list the longest Overhang perimeter moves (#247; tools/bridges.py). Long runs on a 45° face mean the face should be steeper.

Keep cutter tips off the faces they meet

• Rule: do not let a cutter meet another face tangentially, or end exactly on it. Offset it by a few tenths of a millimetre into the void, and give overlapping sampled solids distinct pointed tips.
• Check: every boolean result is checked for validity, with a fallback that never returns a bad shape silently (#249; union and cut in cad/cq_util.py). The print mesh is welded at 1 µm before the watertight check (_mesh in cad/build.py), and a hole that survives the weld is a real fault. A range-of-motion clearance sweep (#251) catches a cutter that cut nothing.

Slicing

flowchart TD
    W["PrusaSlicer:<br/>long bridging extrusions"] --> G["bridge_report:<br/>the longest bridge moves, z, xy"]
    G --> I{"inside a solid region?"}
    I -->|"internal bridge"| S["a slicer setting:<br/>solid_infill_below_area"]
    I -->|"at an outer face"| C["change the geometry"]
    classDef here fill:#fffdf5,stroke:#173d3c,color:#173d3c
    classDef out fill:#ffe1cc,stroke:#bd3a12,color:#173d3c
    class W,G,I here
    class S,C out

Find a "long bridging" warning in the G-code before changing the geometry

• Rule: keep the slicer's warnings, and then find what each one is about before redesigning. PrusaSlicer's "long bridging extrusions" and "floating bridge anchors" are often internal bridges: the first solid layer laid over sparse infill. Those are fixed with a slicer setting, not geometry.
• Check: the slice report counts warnings and stability issues per part and fails on any (#246). For each long bridging warning, the longest bridge and overhang-perimeter moves with height and position (#247): a bridge inside a solid region is internal, and one at an outer face is real.

Slice one part from each class before finishing the design

• Rule: slice one representative part of each kind (a shell, a structural beam, a link, a small part) as soon as it exists, with the real profile. Use the slicer's grams and hours to set the design's budget. A volume-based estimate is not a budget.
• Check: a slice of each part class is in the project before the last parts are designed, and the README states the sliced total, not an estimate.