Booleans and Fusion

FluidCAD automatically merges (fuses) solids that touch. This guide explains how auto-fusion works and how to use explicit boolean operations when you need more control.

Auto-fusion

By default, when you extrude a sketch and the result touches an existing solid, they merge into one:

sketch("xy", () => {
    rect(60, 60).center()
})

extrude(30)                    // creates a box

sketch("xy", () => {
    circle([0, 0], 40)
})

extrude(50)                    // this cylinder auto-fuses with the box

The result is a single solid — the box with a cylinder sticking out of the top.

Creating separate solids

Use .new() to prevent auto-fusion:

sketch("xy", () => {
    rect(60, 60).center()
})

const box = extrude(30)

sketch("xy", () => {
    circle([80, 0], 40)
})

const cyl = extrude(30).new()  // separate solid, no fusion

Explicit boolean operations

Fuse (union)

Merge two or more solids into one:

import { fuse } from 'fluidcad/core';

fuse(solid1, solid2)

Subtract (difference)

Remove one solid from another:

import { subtract } from 'fluidcad/core';

subtract(solidToKeep, solidToRemove)

Common (intersection)

Keep only the volume where two solids overlap:

import { common } from 'fluidcad/core';

common(solid1, solid2)

When to use what

Scenario	Approach
Adding a boss to an existing solid	Just extrude — auto-fusion handles it
Creating a hole	sketch on a face + cut()
Merging two separate objects	fuse(a, b)
Subtracting a shape from another	subtract(keep, remove)
Keeping only the overlap	common(a, b)
Building multiple independent parts	Use .new() or part()