ribs-bosses-draft

name	ribs-bosses-draft
description	features.rib, features.boss and features.draft - stiffening ribs, screw bosses and mould draft, the plane predicate, draft's low ceilings, and the silent fuse when a feature misses.
triggers	["rib","ribs","gusset","boss","bosses","screw boss","standoff","draft","draft angle","taper","mould","molding","stiffener","features","neutral plane","plane"]
version	1.0.0
license	Apache-2.0
author	AgentCAD core
requires	[]

Ribs stiffen, bosses receive screws, and draft lets a part leave a mould. All three are agentcad.toolkit.features one-liners, and all three share the same failure: they fuse happily onto nothing. A rib that misses the part and a boss standing in mid-air both raise the volume by exactly the right amount, report is_valid True and leave you a second solid nobody looked at. Use this skill when adding stiffening to a thin wall, a screw boss to a housing, or draft to anything that will be injection moulded or cast. The enclosures skill covers where bosses belong in a housing; this one covers how to make them and how to know they landed. Draft in particular has measured ceilings far lower than intuition suggests — read that section before choosing an angle.

The three helpers

from agentcad.toolkit import features

part, warn = features.rib(part, profile, thickness, to=8.0, plane="top",
                          draft_deg=None)
    # `profile` is a polyline of (u, v) points in the plane's coordinates,
    # traced to `thickness` and extruded away from the seat. to=<mm> is
    # exact. to="part" intersects the part's BOUNDING SOLID -- an envelope,
    # not the part: on a convex part it adds 0 mm3, on a shelled one it
    # runs to the bbox top. It always warns. draft_deg TAPERS the
    # extrusion; it never calls the draft operation (see below).

part, warn = features.boss(part, at, d, h, hole="M3", draft_deg=None)
    # a cylinder standing h above the seat; hole="M3" bores the tap drill
    # through holes.tapped so the screw boss carries a RECORD.

part, achieved_deg, warn = features.draft(part, faces, angle_deg,
                                          neutral_plane, min_angle=0.25)
    # `faces` is a list of Face objects or a selector callable f(part) ->
    # faces. NEVER indices. On failure it binary-searches DOWN.

The plane predicate, and which way things grow

plane is holes' predicate and the normal points OUT of the material: holes drill along -z_dir, ribs and bosses grow along +z_dir. After you add a rib, "top" resolves to the RIB's top face — pass an explicit Plane for anything inside a cavity.

That last sentence is the ordering trap in one line. Adding three ribs with plane="top" in a loop puts the second rib on top of the first. Resolve the plane once, before the loop, and pass the Plane object:

from agentcad.toolkit import features

def build(p):
    part = _shell(p)
    seat = Plane(part.faces().filter_by(Plane.XY).sort_by(Axis.Z)[0])
    for u in (-20.0, 0.0, 20.0):
        part, warn = features.rib(part, [(u, -30.0), (u, 30.0)],
                                  p.rib_t, to=p.rib_h, plane=seat)
    return part

to="part" deserves its warning: the bounding solid is an envelope, so on a convex part the rib is entirely inside the envelope and adds 0 mm³, and on a shelled one it runs all the way to the bbox top rather than to the wall you had in mind. Use an explicit to=<mm> whenever you know the height, which is nearly always.

Draft's ceilings are low and its failure is quiet

DRAFT'S CEILINGS ARE LOW AND ITS FAILURE IS QUIET. Measured through the worker: box 35 deg, box+boss 15, box+fillets 10, shelled box 2.5, gusset_plate 17.5, prototyping/enclosure_base 0.25, and rocketry/nozzle and construction/angle_bracket refuse EVERY angle. Failure is monotone in the angle (no islands). Only the extreme angles raise — most failing angles RETURN a shape with is_valid False and a plausible volume, so a hand-written draft() must check is_valid itself. Draft before you shell or fillet; when nothing works features.draft returns the part unchanged and says so.

Read that table as a design constraint, not a bug report. The moulding rule of thumb — 1° per side, 3° on a textured face — is below every ceiling above except the shelled box and the enclosure base, and those two are exactly the parts a real mould tool would be drafted on the sketch rather than by a draft operation. The reliable pattern for a moulded part is to build the taper into the profile (extrude(taper=…), a lofted section, or draft_deg= on a rib) and reserve features.draft for touching up a face you cannot reach that way.

features.draft returns achieved_deg for exactly this reason: it is what was actually applied, and it can be less than you asked for or zero. Read it, and if the part is user-facing, say so.

Both a rib that misses and a boss in the air fuse happily

Both a rib that misses the part and a boss placed in the air FUSE HAPPILY: volume rises by exactly the right amount, is_valid True, nothing raised. The only tells are the solid count and an intersection probe, which is what the helpers' warnings report.

So: read the warning, and back it with a spec.

from agentcad.toolkit.specs import check_that, check_valid

SPECS = [
    check_valid(),
    check_that(lambda part, metrics: len(part.solids()) == 1,
               name="one_solid"),
]

Sizing, in short

• Rib thickness: 0.5–0.6 × the wall it stiffens for injection moulding (thicker sinks the outer surface), and up to 1.0 × the wall for FDM, where there is no sink mark to worry about and the rib is only as strong as its perimeters.
• Rib height: up to ~3 × the wall. Beyond that the rib buckles before the wall yields, and two shorter ribs beat one tall one.
• Rib spacing: no closer than 2 × the wall, or the two heat-affected zones merge into one thick section.
• Boss outer diameter: ~2 × the screw's major diameter (enclosures has the table), wall 0.6 × the nominal wall, and never a solid post — a cored boss cools evenly and prints without a blob.
• Boss to wall: connect with a rib or a gusset rather than merging into the wall, so the thick junction does not sink.
• Root fillet on every rib and boss, r ≈ 0.25–0.5 × the rib thickness. Add it last, after all the ribs exist, and use safe_fillet.

Checklist

• The seat Plane is resolved once, before any loop that adds features.
• to= is an explicit millimetre value, or the to="part" warning was read and accepted.
• A boss that receives a screw uses hole="M…" so it carries a record.
• Draft angle is at or below the measured ceiling for a part of this shape, and achieved_deg was checked, not assumed.
• Draft runs before shell and fillet.
• The build reports one solid and is_valid, proving nothing fused into the air.

Sources

• AgentCAD toolkit source: agentcad/toolkit/features.py — rib, boss, draft, the plane predicate, the binary search down on draft angle and the per-feature engagement warnings.
• AgentCAD toolkit source: agentcad/toolkit/holes.py — the record a bossed screw hole carries.
• AgentCAD toolkit source: agentcad/toolkit/specs.py — check_valid and check_that, the assertions that catch a silent fuse.
• build123d documentation, Objects and operations (extrude(taper=…), loft, fillet): https://build123d.readthedocs.io/
• Open CASCADE Technology documentation, Modeling Algorithms — BRepOffsetAPI_DraftAngle and its failure behaviour.
• Injection-moulding rib and boss proportions: DuPont, General Design Principles for DuPont Engineering Polymers, Module I; G. Erhard, Designing with Plastics (Hanser, 2006).