Boolean operations treat solids like sets of space and combine them: union merges, subtract removes one from another, intersect keeps only the overlap. Nearly every real part is a stack of booleans — a block, minus holes, plus bosses.

The three operations

  • Union (add): two overlapping bodies become one watertight solid. Use to merge sub-shapes you modeled separately.
  • Subtract (cut): the "tool" body is removed from the target — drilling holes, carving pockets, splitting keyways. The tool is consumed.
  • Intersect (common): only the shared volume survives. Underrated: model a part's side profile and top profile as two extrusions, intersect them, and you get the 3D shape that matches both views.

Automatic vs. explicit booleans

Modern direct modelers hide most booleans behind gestures: pull a face to union, push to subtract (see push/pull modeling). You still want explicit control sometimes — keeping an overlapping extrusion as a separate body, or forcing an intersect. In OpenShape3D, every extrude/revolve/sweep carries a result badge (Auto | New Body | Union | Subtract | Intersect), and the Combine tool runs explicit booleans between existing bodies, computed off the main thread so the UI never freezes.

Practical patterns

  • Drill with cylinders: place a cylinder through your part and subtract — the universal hole.
  • Master-tool workflow: keep a "cutter" body as New Body, position copies with Transform, then subtract them all.
  • Two-view intersection: extrude the front silhouette and the top silhouette; intersect.
  • Split via subtract: subtract a thin box to slice a part for printing (or use a dedicated Split tool with a plane).

Why the kernel matters

Mesh booleans (in polygon tools) intersect thousands of triangles and often leave slivers, non-manifold edges, or shells that aren't watertight — problems you discover in the slicer. B-rep booleans intersect the underlying surfaces exactly and re-stitch clean topology. OpenShape3D's booleans run on the OpenCASCADE kernel, so results stay exact through any number of operations.

When booleans fail

Coincident faces are the classic trap: two bodies sharing an exact face can confuse the intersection math. Nudge one body a hair to overlap properly, or make surfaces clearly intersect rather than kiss.