{
 "id": "83-boundary-loop-thicken",
 "name": "Boundary surface through a loop, thickened",
 "description": "Four sketch curves on XY (a 10x10 square's bottom, right and left sides, and a half circle of radius 5 bulging up for its top) make one boundary surface; thickened 2 mm symmetrically, the surface then removed.",
 "exercises": [
  "boundary_surface",
  "thicken"
 ],
 "units": {
  "length": "mm",
  "angle": "deg",
  "area": "mm2",
  "volume": "mm3"
 },
 "conventions": {
  "world": "right-handed; Z up. XY/XZ/YZ are the origin planes.",
  "sketch_frame": "sketch-local 2D coords; world = plane.origin + x*plane.x_dir + y*plane.y_dir (origin in mm). Profile 'loops' list sketch entity ids.",
  "point_refs": "'<id>.start' / '<id>.end' / '<id>.center' reuse (share) that existing sketch point; 'at' is its position when referenced.",
  "extrude_direction": "positive = along sketch plane normal unless stated",
  "selections": "selected faces/edges carry 'point': a world-space point on the face / the edge's parametric midpoint; pick the nearest face/edge to it."
 },
 "parameters": [],
 "features": [
  {
   "op": "sketch",
   "name": "Loop",
   "plane": {
    "ref": "XY",
    "origin": [
     0.0,
     0.0,
     0.0
    ],
    "x_dir": [
     1.0,
     0.0,
     0.0
    ],
    "y_dir": [
     0.0,
     1.0,
     0.0
    ],
    "normal": [
     0.0,
     0.0,
     1.0
    ]
   },
   "entities": [
    {
     "type": "line",
     "start": {
      "at": [
       0,
       0
      ]
     },
     "end": {
      "at": [
       10,
       0
      ]
     },
     "id": "s1"
    },
    {
     "type": "line",
     "start": {
      "ref": "s1.end",
      "at": [
       10,
       0
      ]
     },
     "end": {
      "at": [
       10,
       10
      ]
     },
     "id": "s2"
    },
    {
     "type": "arc_three_point",
     "start": {
      "ref": "s2.end",
      "at": [
       10,
       10
      ]
     },
     "end": {
      "at": [
       0,
       10
      ]
     },
     "mid": [
      5,
      15
     ],
     "id": "a3"
    },
    {
     "type": "line",
     "start": {
      "ref": "a3.end",
      "at": [
       0,
       10
      ]
     },
     "end": {
      "ref": "s1.start",
      "at": [
       0,
       0
      ]
     },
     "id": "s4"
    }
   ]
  },
  {
   "op": "boundary_surface",
   "name": "Patch",
   "sketch": "Loop",
   "curves": [
    "s1",
    "s2",
    "a3",
    "s4"
   ],
   "result_bodies": [
    "Sheet"
   ]
  },
  {
   "op": "thicken",
   "name": "Slab",
   "bodies": [
    "Sheet"
   ],
   "thickness": 2,
   "symmetric": true,
   "operation": "new"
  },
  {
   "op": "remove",
   "name": "Clear",
   "bodies": [
    "Sheet"
   ]
  }
 ]
}