Boolean operations — cut, combine, keep
Union, subtract, intersect. The three commands that turn LEGO bricks into actual designs.
Union — combine two shapes
Drop two cubes that overlap and hit Union. The result is a single shape with their footprint merged. Used for: building larger forms from primitives, fusing a handle onto a body, adding embossed text to a plate.
Tip — Union is the default behaviour in ForgeSlicer. Two positive shapes that touch are automatically unioned when you slice or export.
Subtract — cut a hole
Place a cylinder inside a cube and mark the cylinder as Negative. When ForgeSlicer composes the scene, it subtracts the cylinder, leaving a hole through the cube.
This is the single most useful operation in printable-part design. Screw holes, ventilation slots, cable channels, engraved text — all subtracts. Every "how do I make a hole" tutorial ends here.
Intersect — keep only the overlap
Less common but useful: keep only the region where two shapes overlap. Used for: clipping a tall object to a curved surface, isolating the "diamond" pattern between two crossed cylinders, building lens shapes.
Why positives + negatives instead of "apply now"
Most CAD tools make you commit each boolean immediately. ForgeSlicer keeps the parts as positives and negatives in a scene tree so you can edit dimensions after the fact — make the cylinder hole 0.5 mm bigger and the whole part rebuilds. No undo gymnastics.
Union = combine. Subtract = cut (mark as Negative). Intersect = keep overlap. Edit a positive or negative anytime; the result rebuilds automatically.