Combining and splitting solids

Combining solids is always a command you issue. Core concepts explains why Hew never does it for you. Every input to these commands is a watertight solid, and so is every result.

Union, Subtract, Intersect

1. With the Select tool, click the first object or group, then Shift-click the second.
2. Choose Object ▸ Union, Object ▸ Subtract, or Object ▸ Intersect.

Union produces one object containing all material from both, and cleans up after itself: where two coplanar faces meet, the seam dissolves and they become a single face. Subtract cuts the second selected object away from the first. Intersect keeps only the overlapping material.

When the result is one connected solid, it’s a single new Object. When it comes apart — a Subtract that severs the target, or operands that never touched — each piece is its own watertight solid. The pieces arrive together in a group named after the operands (“Bar − Cutter”), so nothing scatters. Painted materials survive the operation on the faces that survive. Like everything, booleans are undoable in one step.

If an input isn’t solid, the operation is refused with an error naming the problem. Fix the leaky object first (check Object Info to find it).

Combining groups

Either side of a boolean can be a whole group instead of a single object. You can mix, too — subtract a group of cutters from one solid in a single command. Because you asked for volume algebra, Hew first fuses the solids inside each group operand into one composite (grouping alone never merges anything), then applies the command between the two composites. Every solid under a group operand has to be watertight — a leaky one anywhere refuses the whole operation, and the message says which side to fix.

One thing a boolean won’t do is quietly consume a component instance — instances share their geometry with every other copy, and a boolean destroys its inputs. If an operand is (or contains) an instance, the command refuses and tells you so. Explode the instance first (or Make Unique, then Explode) and combine the result.

Booleans also work between two individual members while a group is open for editing — Union, Subtract, or Intersect them just like top-level objects — and the result folds back in as the group’s member when you close it.

Slice

The Slice tool (Tools ▸ Slice) is the opposite move: cut one solid into two independent, watertight solids along a plane. It’s the tool for splitting a model into printable halves.

1. Activate Slice and move the cursor over your solid. The cutting plane previews live:
   • Hovering a face lays the plane flat on that face’s plane.
   • Otherwise the plane is perpendicular to a world axis: press → for X, ← for Y, ↑/↓ for Z (Z, a horizontal slice, is the default).
2. Hold Shift to pin the current plane so it stops re-orienting as you move; press Shift again (or Esc) to release.
3. Position the plane — it follows the cursor’s snap point, and you can type an exact offset along the plane’s normal.
4. Click on the solid to commit.

Both halves are complete, closed solids — Hew caps the cut faces. One of them is selected afterward so you can immediately Move it apart. Slicing empty space, or a plane that misses the solid, is refused with a message. Slice works on a group’s members too, while the group is open for editing — both halves fold back in as members when you step out.

Which tool when?

For a hole or a pocket you usually don’t need booleans at all: draw on the face and Push/Pull inward or through (Push/Pull). Union is for merging parts into one printable body. To cut a complex shape out of another, model the cutter as its own solid, position it, and Subtract. To split a model for the print bed, Slice and move the halves apart. And if you only want parts to stay logically together, don’t combine them at all — use a group.