Published 2026-08-27 · 2 min read
Union, difference, intersection: the three operations behind any part
Almost any technical object can be described as simple shapes added to and subtracted from each other. Three operations are enough. Once they click, you stop drawing complicated shapes and start composing them.
Difference: the one you use most
Subtract one shape from another. A cylinder subtracted from a plate makes a hole. A smaller cube subtracted from a bigger one makes a recess. A torus subtracted makes a circular groove.
The practical trick: make the cutting tool slightly longer than the part. If the cylinder's face coincides exactly with the plate's face, the result is ambiguous and the program may leave an infinitely thin film of material.
Union: real fusing, not grouping
Union melts two solids into one and deletes the geometry left inside. Grouping merely holds them together, hidden interior faces and all.
For visualisation the difference is invisible. For printing it matters a great deal: leftover interior faces confuse the slicer into printing invisible walls.
Intersection: rare, but it saves you
Keeps only the volume two shapes share. It seems exotic until you need a double-profile shape: intersect two perpendicular extrusions and you get in one move a part that would otherwise be awkward to draw.
It is also the simplest way to round a complicated corner: intersect the part with a sphere or a large cylinder.
Order matters
Booleans are not commutative: A minus B is not B minus A. And when you chain several, order changes the result — especially if you cut a hole and then fillet an edge running through it.
Practical rule: build the large volume first, cut holes last. Add fillets once the shape is stable, because they are the most fragile to later changes.
Why it sometimes breaks
- Exactly coincident faces — move the tool by 0.1 mm and the problem disappears.
- Shapes touching at a single point or along an edge, with no shared volume.
- Imported models that were not watertight to begin with: a boolean cannot repair what was already broken.
- Too many stacked operations on one object: if the result turns strange, rebuild in a different order.
In GrafitGo
All three work on real geometry through a CSG engine, not through display tricks. The result stays a solid you can cut further, section or export straight to STL.
They also apply to whole groups, so you can subtract ten cylinders from a plate in one move instead of repeating the operation ten times.
Can I undo a boolean afterwards?
Yes, with undo while you are working. Once saved, the result is a new solid — which is why keeping a copy of the parts before big operations is worth it.
Why is my hole not perfectly round?
Because the cutting cylinder had too few segments. Raise its segment count before subtracting.
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