enclosures

name	enclosures
description	Two-part boxes, housings and cases - wall thickness per process, shelling, lid lips and grooves, screw bosses, PCB standoffs, vents, draft.
triggers	["enclosure","box","housing","case","lid","cover","lip","tongue","groove","boss","standoff","vent","slot","shell","cavity","pcb","wall thickness","parting line"]
version	1.0.0
license	Apache-2.0
author	AgentCAD core
requires	[]

An enclosure is the part class where a plausible-looking model fails at assembly: the wall is unprintable, the shell refuses, the lid binds, or the boss splits on the first screw. This skill is the body/lid pair end to end - wall thickness per process, shelling in the order OCCT tolerates, lid lips and grooves with a real clearance number, screw bosses, PCB standoffs, vents, and draft when the part is moulded. snippets/two_part_enclosure.py is the whole pattern as a working script (body + lid as one two-solid Compound). Use snap-fits instead for a screwless closure, fits-and-clearances for the numbers behind a press fit or a heat-set insert, sheet-metal for a folded chassis, and fdm-design-rules for printability in general.

Wall thickness by process

Process	Working wall	Absolute min	The constraint
FDM, 0.4 mm nozzle	1.2-2.4 mm	0.8 (2 perimeters)	a wall is an integer number of extrusion widths
SLA / DLP	1.0-2.0 mm	0.5 supported, 1.0 unsupported	thin walls cup and warp in post-cure
SLS (PA12)	1.0-1.5 mm	0.7 mm	powder removal and part handling
CNC, aluminium	>= 1.5 mm	0.8 mm	tool deflection and chatter, not strength
CNC, plastic	>= 1.5 mm	1.0 mm	clamping and springback
Injection moulding	1.5-3.0 mm (ABS/PC/PA)	0.9 mm (PP)	fill length and sink

Rules and their limits:

• Keep the wall uniform. Moulded: variation beyond about +-10 % of nominal shows up as sink and warp. FDM: round the wall to n x extrusion width (0.8 / 1.2 / 1.6 / 2.0 / 2.4 on a 0.4 nozzle) - an off-multiple wall gets a gap-fill bead that is weaker than either neighbour.
• Ribs on a moulded part are 0.5-0.6x the nominal wall, height <= 3x wall, 0.5-1.5 deg draft per side, base radius 0.25-0.5x wall. A full-wall rib is a sink mark on the show face. FDM has no sink, so a rib may be the full wall - the floor there is 2 extrusion widths.
• A tall thin box wants the upper end of the range: panel stiffness goes as t^3, so 1.2 -> 2.4 mm is 8x stiffer, not 2x.

Shell the body (order of operations)

Box -> outer fillet -> shell -> bosses/standoffs/ribs -> holes -> cosmetic fillets. Not taste: each step breaks the next if moved.

from agentcad.toolkit import safe_fillet, safe_shell

body = Box(length, width, height, align=(Align.CENTER, Align.CENTER, Align.MIN))
r_out = max(r_out, wall + 0.5)                       # inner radius stays > 0
body, r_out, _w = safe_fillet(body, body.edges().filter_by(Axis.Z), r_out)
body, warn = safe_shell(body, wall, [body.faces().sort_by(Axis.Z)[-1]])

• offset(amount=-wall) fails when an outer radius is <= the wall: the inner corner radius r_out - wall would be negative. Clamp it, as above.
• Shell before you add bosses. Shelling a body that already carries bosses hollows the bosses too.
• After the shell, the topmost face is the rim ring, not the cavity floor. plane="top" in agentcad.toolkit.holes / features resolves to the largest outer face along the axis, so for anything inside the cavity pass an explicit Plane.XY.offset(wall).
• safe_shell's last fallback is an approximate boolean shell that is not uniform on curved or slanted walls (measured ~20 % thin on dome mid-sections). It says so in the warning - surface it, do not swallow it. The rest of the playbook: selectors-and-occt-failures.
• Prove the wall instead of asserting it:

from agentcad.toolkit.specs import check_wall

SPECS = [check_wall(min_mm=1.2, grid=10, requirement="ENG-wall")]

check_wall is a sampled ray cast, so it finds fillet runouts: the measured minimum sits below nominal by design. Pick the limit from a measurement.

Lid interface: lip, tongue and groove

Three closures, cheapest first:

1. Flat lid on a rim - no location, no light seal. Only acceptable when four screws locate it.
2. Tongue (lip) - a ring on the lid underside dropping into the cavity. Depth 2-4 mm and at least 1.5x the wall; thickness 0.6-1.0x the wall. The default, and what the snippet builds.
3. Tongue and groove - a groove in the rim taking the lid's tongue. Needs a rim wide enough for 2 x groove_wall + tongue, so the wall grows to ~3x nominal there; use it when a gasket or a light seal is needed.

Clearance per side - it is per side, so the box-to-box gap is twice it:

Process	Sliding/seating clearance per side
FDM	0.20-0.30 mm
SLA / SLS	0.10-0.20 mm
CNC	0.05-0.10 mm
Injection moulding	0.05-0.10 mm, plus the draft loss

The tongue must follow the cavity's rounded corners: the cavity inner radius is r_out - wall, so the tongue's is r_out - wall - clearance. A square tongue in a filleted cavity binds at the corners as soon as r_out - wall exceeds ~3.4x the clearance.

lo_x, lo_y = length / 2 - wall - clear, width / 2 - wall - clear
tongue = Box(2 * lo_x, 2 * lo_y, lip_h,
             align=(Align.CENTER, Align.CENTER, Align.MAX))
tongue, _r, _w = safe_fillet(tongue, tongue.edges().filter_by(Axis.Z),
                             max(0.4, r_out - wall - clear))
tongue -= Box(2 * (lo_x - lip_t), 2 * (lo_y - lip_t), lip_h + 2.0,
              align=(Align.CENTER, Align.CENTER, Align.MAX))

Gasket: for an O-ring face seal, groove width ~1.3x the cord and depth ~0.75x the cord gives about 25 % squeeze; keep the groove corner radius >= 3x the cord so the ring does not roll.

Screw bosses

• OD = 2x the screw nominal (M3 -> 6 mm). Below ~1.8x the boss splits on the first drive; above ~2.5x a moulded boss sinks.
• Self-tapping into plastic: pilot = 0.8x nominal (M3 -> 2.4 mm), engagement >= 2x nominal, boss height = engagement + 1-2 mm so the screw never bottoms.
• Heat-set insert: boss bore = the insert's specified hole (usually its OD minus 0.1-0.3 mm), boss OD >= 2x the insert OD, 0.5-1 mm of boss below it. Numbers in fits-and-clearances.
• Tapped or clearance holes: use the hole wizard so the hole carries a record and reaches the drawing callout - holes.tapped bores the tap drill, holes.clearance the ISO 273 diameter (see holes).

from agentcad.toolkit import holes

body, _recs, warn = holes.tapped(body, boss_pts, "M3",
                                 plane=Plane.XY.offset(height), depth=6.0)

• A boss merged into a wall is a thick section. Moulded: stand it off by

  = 1 wall thickness and tie it with a rib 0.5-0.6x wall. FDM: merging into a corner is fine and stronger - embed it ~0.5 mm into the wall so the fuse is an overlap, never a tangency (a zero-area contact from which OCCT may hand back a non-manifold solid).

• For a free-standing boss, use the feature helpers - and note the plane:

from agentcad.toolkit import features

floor = Plane.XY.offset(wall)                 # "top" can only be an OUTER face
body, w1 = features.boss(body, (bx, by), 6.0, boss_h, hole="M3", plane=floor)
body, w2 = features.rib(body, [(bx, by), (bx, width / 2 - wall)], 0.6 * wall,
                        to=boss_h - 1.0, plane=floor)   # gusset to the wall

features.boss warns that less volume arrived than the seed contains whenever the boss overlaps existing material - the signature of a correctly merged boss, not a fault. The warnings that are faults read "floating solid", "added no material" or "N disjoint solids". More in ribs-bosses-draft.

PCB standoffs and internal clearances

• Standoff OD 5-6 mm for a 3.2 mm (M3) board hole; pilot 2.4 mm for a self-tapping M3, or 0.2-0.4 mm radial clearance so the board drops in.
• Height >= 3 mm above the floor: through-hole solder fillets and clipped leads stand 1.5-2.5 mm proud of the board.
• Keep a standoff centre >= 1x its OD from a wall, or merge it in deliberately - a 0.3 mm slot between the two neither prints nor fills.
• Leave >= 2 mm over the tallest component and >= 1.5 mm around the board edge; a connector breaching a wall needs its cutout plus 0.3-0.5 mm all round.

Vents and slots

• Slot width sets the ingress rating. IEC 60529's first digit is a probe diameter: IP3X excludes a 2.5 mm tool, IP4X a 1.0 mm wire. A 2 mm slot is IP3X and not IP4X; there is no arguing around it.
• FDM: slot width >= 2 extrusion widths (0.8 mm on a 0.4 nozzle; 1.5-2.0 mm prints cleanly). Ligament between slots >= the slot width and >= 2 walls.
• Cut with a solid thicker than the wall (wall + 2 mm), so the cutter passes clean through instead of landing tangent to the outer face; a tangential cut leaves slivers that fail the next boolean.
• Keep slots >= 3 mm clear of the floor, the rim and the corner bosses, so the wall keeps a continuous band.
• Vertical slots in a side wall need no support; slots in a top face are bridges - keep them under ~10 mm (fdm-design-rules).

Draft and the parting line (moulded or cast only)

• 1 deg per side minimum on any face parallel to the draw; 1.5-2 deg is the safe default. A textured face adds ~1 deg per 0.025 mm of texture depth.
• Deep bosses and ribs still need >= 0.5 deg per side; check the top does not close - a 6 mm boss 20 mm tall at 2 deg loses 1.4 mm of diameter.
• Draft eats clearance. A 4 mm tongue drafted 1 deg per side narrows 0.07 mm root to tip. Specify the fit at the tightest section and add the draft loss at the other end.
• In build123d, draft before you shell or fillet. A shelled enclosure refuses features.draft above ~0.25 deg (measured through the worker), while a tapered extrusion (features.rib(..., draft_deg=...), extrude(taper=...)) cannot fail that way.
• Put the parting line at the lid joint and pull every outer face from it. Anything undercut relative to that pull - a side port, a snap hook - needs a side action or a redesign; printed, the same feature only needs the right build orientation.

Checklist

1. Wall from the table, uniform, rounded to n x extrusion width for FDM.
2. r_out >= wall + 0.5 before shelling; clamp every dependent dimension inside build() (see robust-parametrics).
3. Shell, then features, then holes, then cosmetic fillets.
4. Lid clearance per side from the process table, applied to the tongue and to its corner radius.
5. Bosses at 2x nominal OD, pilot 0.8x nominal, engagement >= 2x nominal.
6. Tongue notched around every boss: notch radius = boss radius + clearance
   • ~0.3 mm.
7. Slot width matched to the ingress rating; ligaments >= slot width.
8. check_wall in SPECS; read every toolkit warning instead of discarding the second return value.
9. Ship body and lid as one Compound with SOLID_LABELS = ["body", "lid"], so the fit is measurable in one part and each solid takes its own material.

Sources

• IEC 60529:1989+A2:2013, Degrees of protection provided by enclosures (IP Code) - first-digit probe diameters (2.5 mm, 1.0 mm).
• DuPont, General Design Principles for DuPont Engineering Polymers, module I - nominal walls, rib proportions (0.5-0.6 t, h <= 3 t), boss and self-tapping-screw boss proportions.
• Covestro (Bayer MaterialScience), Snap-Fit Joints for Plastics: A Design Manual - boss/rib/draft proportions, undercut handling.
• Parker Hannifin, Parker O-Ring Handbook ORD 5700 - face-seal groove width and depth for ~25 % squeeze.
• Protolabs, Injection Molding Design Guidelines and CNC Machining Design Guide - resin nominal walls, draft minima, minimum machined wall.
• Formlabs, Design Guide for SLA 3D Printing - supported and unsupported minimum wall.
• UltiMaker, Design for FFF, and the Prusa Knowledge Base "wall thickness" article - extrusion-width multiples, minimum printable wall.
• SPIROL, Inserts for Plastics - Design Guide - boss OD vs insert OD and hole preparation for heat-set inserts.
• ISO 273 (clearance holes), ISO 261/262 (metric threads), vendored in agentcad.toolkit.hole_standards.