solid.shell in SolidCraft: how it works, how it fails, how to get around it

Syntax

solid.shell {faces:[[x,y,z]], thickness, direction:"inside"|"outside"|"both", tangent_chain?, body?}

• faces are points on the faces to REMOVE (open). For an underside cavity pick a point on the bottom face z=0 that lies inside the outline and outside bosses and bores, for example [-38,0,0].
• direction:"inside" keeps the outer shape. body names the target body.

Failure 1: "offset: a corner whose faces don't meet the same way after the move"

• Cause: arcs that are meant to be tangent (for example a convex R12 around a hole boss and concave R75 side arcs) were entered with 5-decimal coordinates, so the faces are only nearly tangent. The offset treats the joints as real corners and fails for any thickness, even on the bare extruded base.
• Fix: compute the tangent points analytically (the point on the line between the two arc centres at distance r1 from the first centre) and enter the coordinates with 10 decimals in sketch.arc.three_point (start, end, through). The shell then works on the base and on base plus cylinder.
• solid.fillet on such an edge answers edge is smooth when the joint is truly tangent, so you can use it as a tangency test.
• tangent_chain:true did not help for this failure.

Failure 2: shell hangs SolidCraft (minutes, needs a restart)

• Seen when the body already has a bore whose wall equals the shell thickness: a bore R25 inside a cylinder R30 with thickness 5 (4.9 as well). The offset of the outer face and of the bore face meet and collapse into degenerate geometry and an endless computation. A 60 s timeout does not mean it stopped: check running and do not resend. Nothing can be done until it ends or the app is restarted.
• Prevention: shell BEFORE cutting bores or holes whose wall to another face is <= thickness (plus a small margin). A working order is base, cylinder, ear or joins, shell, bores, small cuts. Save a checkpoint before the shell.
• A batch that contains a failing shell rolls back everything, so run the shell as a separate call.

Safe order that worked

base extrude, cylinder join, shell (thickness 5, bottom face open). Adding the bore afterwards is the open point to test: the cavity wall at R25 coincides with an R25 bore, so expect a degenerate boolean. Consider a slightly different shell thickness or build the bore as part of a pocket.

Fallback: emulate an underside shell with a pocket cut (validated)

1. Sketch on XY at the open face with the outline offset INWARD by the wall thickness t: convex arcs R become R-t and concave arcs R become R+t (same centres), keeping tangency exact.
2. Add circles for the material that must stay: a boss ring (for example R30). Rings around holes are already in the offset outline (hole radius plus t).
3. solid.extrude {operation:"cut", start_offset:-1, distance: depth+1, profiles:[{point:[x,y]}], exact_regions:true}. Pick a point INSIDE the wanted region only. A point outside the offset outline gives unknown profile.
4. Verify with measure (volume, shells=1) and tell the user it is an emulation, not solid.shell.

Checks after a shell

measure: 1 shell, closed, and a plausible volume drop (area x t). Then take a screenshot from below (camera yaw 225, pitch -40) to confirm the cavity.