Union and Intersect

Use Union to keep everything either of two bodies covers, and Intersect to keep only what both cover. Select at least two bodies, then press U to union or N to intersect.

Union

The result is one body with the overlap counted once. Where the two bodies shared a face, the seam is cleaned up rather than left as a line across a flat surface. The last step of a union tidies coplanar faces into one.

Union is not the only way to join. An extrude set to Add joins as it builds, and for a shape you are drawing anyway that is fewer steps. Use Union for two bodies that already exist and were modelled separately. Each one keeps its own history until you decide to merge, and that arrangement is easier to edit later.

The seam tidy-up is no longer a union speciality: every finished body goes through it, so the coaxial ring a cut leaves and the split a full revolve leaves are collapsed the same way. It has a price worth knowing. An edge that was one of those seams is gone, so a fillet aimed at it has nothing left to round: it rounds a neighbouring edge and reports itself uncertain, or it fails outright. A reference to a construction artefact is worth less than a body whose edges are the ones you drew.

A union never takes its result apart. Two bodies that turn out not to touch still come out as one body with two lumps: joining is what you asked for, and it is how a pair of separate parts is grouped into one. Only the removals split, and Subtract covers that.

Intersect

Intersect keeps only the shared volume. Two bodies that do not overlap produce nothing, and the feature says so rather than leaving you an empty body:

Intersection is empty (selected bodies do not overlap)

The classic use is a mould or a cut-down. Build the shape you want, build a volume that describes where it is allowed to exist, and intersect the two.

Unlike a union, an intersection can leave the body in pieces, and then you get one body per piece: the biggest keeps the target's name, colour and visibility, and the rest are named after it. Subtract has the rule in full.

Keep tools

Keep tools is one switch, and it does less than its name suggests.

Keep tools spares the tools. The target was never at risk. The first body you picked is where the result lands, and it keeps its name, its colour and its visibility. Only the other bodies are consumed: with the switch on they survive alongside the result, with it off they go.

A union with Keep tools on gives you the fused body and the second block, sitting inside it. Leave the switch on when you want that block back as a tool for another cut, and turn it off when you do not.

What happens to the bodies

The result lands on the target body, which keeps everything that made it recognisable: its name, its colour, its visibility, and whatever downstream features were pointing at it. A boolean owns no body of its own, so the two names are two different things now — renaming the Union step in the timeline leaves the body alone, and renaming the body leaves the step alone. Bodies that were consumed keep their identity behind the scenes, so turning Keep tools back on brings them back rather than creating new ones.

If you need a consumed body again, edit the boolean and switch Keep tools on instead of modelling it a second time.

See also

• Subtract: removing one body from another
• Extrude and Cut: joining as you build instead
• Working with Several Bodies: when to keep bodies apart