Sketch first

Many printed parts are a 2D shape made solid: a bracket or a plate is an outline extruded, a knob or a pulley is a profile turned about an axis, a hook is a section swept along a path. For those, drawing and agreeing on the 2D sketch before any 3D gives a cleaner part than stacking boxes and cylinders, and it is quicker to check: one flat picture with its dimensions.

Sketch first is optional, per part. The AI offers it when the part fits: outlines extruded flat (brackets, plates, clips, spacers, gaskets), profiles revolved (knobs, pulleys, spool parts, bushings) and sections swept (hooks, handles). It does not offer it for boxy enclosures (a box, a shell, a lid) or for organic shapes, which have their own recipes. Say no and it models the part as usual.

The sketch_bracket example's outline: overall size, holes grouped by size, the slot, the inside radius, hole and slot centres measured from the lower-left corner.

How it goes

1. Dimensions first, as always: the AI lists what it read and waits for you to confirm.

2. The sketch. new_part(name, sketch_first=True) (CLI b123d-print new <name> --sketch) starts a script with a sketches(d) hook next to build(d):

   from b123d_print.recipes import bed_chamfer, rounded_outline, slot
   
   def outline(d):
       pts = [(0, 0), (d.ARM_X, 0), (d.ARM_X, d.ARM_W), (d.ARM_W, d.ARM_W), (d.ARM_W, d.ARM_Y), (0, d.ARM_Y)]
       s = rounded_outline(pts, [d.CORNER_R, d.CORNER_R, d.CORNER_R, d.INSIDE_R, d.CORNER_R, d.CORNER_R])
       s -= Pos(d.ARM_X - d.HOLE_INSET, d.ARM_W / 2) * Circle((d.HOLE_D + d.HOLE_COMP) / 2)
       s -= slot((d.SLOT_FROM, d.ARM_W / 2), (d.SLOT_TO, d.ARM_W / 2), d.HOLE_D + d.HOLE_COMP)
       return s
   
   def sketches(d):
       return {"outline": lambda: outline(d)}  # lazy, like pieces: made only when asked for
   

   A sketch's name becomes part of its picture's file name: 1-40 letters, digits, _ or -, starting with a letter or digit.

3. The picture and the checks. render_sketch(name) (CLI b123d-print sketch <name> [<sketch>]) draws the sketch on a mm grid, writes it to out/images/<name>_sketch_<sketch>.png, lists the sizes, holes, slots and corner radii, and runs the sketch checks below. The AI fixes any failure, shows you the picture and waits for your OK.

4. The solid. build(d) extrudes, revolves or sweeps the same outline(d), so the part is the sketch you approved, then adds what only exists in 3D: countersinks, bed_chamfer, a lip. From here the usual flow: build, preview, sections, check, design review, export.

def build(d):
    p = extrude(outline(d), d.THICK)
    return bed_chamfer(p, d.BED_CHAMFER)  # chamfers the inside radius and the holes too

A sketch is drawn on Plane.XY, in the script's own coordinates. A profile to revolve can be drawn on Plane.XZ (Plane.XZ * sketch); the picture then reads it in that plane's own axes. An outline traced from a scan gives the points for rounded_outline.

The dimensions on the picture

• Overall size: width along the bottom, height along the left.
• Holes by diameter, identical ones on one label (3× ⌀4.4); slots by width and centre-to-centre length; inside corner radii grouped the same way (2× R4). Outside radii are listed under the picture.
• Centres of holes and slots as ordinate dimensions from the datum, the lower-left corner of the sketch: one per distinct X along the top, one per distinct Y down the right.
• Findings are circled in red: a sharp or tight inside corner, the narrowest neck, a hole too small to print.

On a busy sketch, labels are placed so they do not overlap, values close together move out a row, and with more than 14 distinct centres along an edge the centres are listed in the text result instead of drawn. dims=False (CLI --no-dims) draws the shape and the grid only. The dimensions are only on the picture: they change nothing in the part or the checks, and the same numbers are in the text result.

The sketch checks

Check	What	Level
region	one closed region: an open outline (its two ends and the gap), a closed wire that is not yet a face, or several separate regions	FAIL
neck	the narrowest material in the sketch, between two boundaries or across a wall, against min_wall (WARN) and one line width (FAIL); a point that tapers to nothing warns	WARN / FAIL
corners	inside corners with no radius or one under 0.5 mm (BP-R1); outside corners print rounded anyway	WARN
holes	holes under 1 mm do not print round	WARN
bed	with a printer profile, the outline laid flat against the bed	WARN

They are 2D only and quick: the part's own checks still run on the solid.

Recipes

• rounded_outline(points, radii): a closed outline through the points in order, one radius per point (0 = sharp) or one for all. It refuses radii that do not fit on an edge, naming the two corners, where FilletPolyline alone would make a bad face.
• slot(a, b, width): a slot whose round ends are centred on a and b, at any angle; a hole when a == b.

b123d-print recipes rounded_outline shows the code, an example and the notes.