Clearance Cut
Use Clearance Cut to make the pocket a part slides into. Select the body to cut first, then the parts that go into it. There is no keyboard shortcut, so start it from the toolbar. It is the tool for "this goes in here": a nut, a magnet, a bearing, a PCB, a printed part that has to mate with another.
Why a subtraction is not enough
Subtract a part from a block and you get a cavity the exact shape of the part. Two things are wrong with it. It has no clearance, so nothing fits, because printed plastic is never exactly the size it was modelled. And it has no way in: a cavity in the middle of a block is a void, not a pocket.
Clearance Cut fixes both in one feature. It grows the part by a clearance, then sweeps the grown shape along an insertion direction so the cavity opens out into a channel.
| Field | What it does |
|---|---|
| Target + parts | The body to cut first, then the parts that go into it |
| Insertion | The axis of the channel. The part is understood to slide in against it. A straight model edge, a reference axis, or a world axis |
| Clearance | The gap around the part, in millimetres. Zero gives a tight silhouette fit |
| Flip direction | Reverses the axis. An edge carries no sense of which way is "in" |
| Keep parts | On by default, unlike every other boolean. The parts survive the cut |
Reading the clearance number
The gap is grown along the three world axes rather than perpendicular to each face, and that has a consequence: a face square to an axis gets exactly the clearance you asked for, and a sloped face gets more. A 45° face is loosened by about 1.4 times the number, and a corner by about 1.7.
So the number means "at least this much, measured squarely". On a part that mates on flat, axis-aligned faces, the usual case, it is exactly what you set. On a wedge or a cone, expect the fit to be slacker than the number suggests, and tune down rather than up.
Typical starting points on an FDM printer: 0.1 to 0.2 mm for a part that should slide out, 0.3 to 0.4 mm where it has to go in easily or the surfaces are rough. Real printed clearances in the printing track covers how to measure your own machine instead of guessing.
The insertion direction
The channel is swept along the axis you pick, so the part is understood to slide in against it. If the pocket opens out of the wrong side of your part, switch on Flip direction. A picked edge carries no direction of its own, so the tool cannot know which end you meant.
The sweep runs the full depth of both bodies plus twice the clearance, so it always reaches daylight. Over-sweeping into air costs nothing.
Which route your part takes
A part of up to a couple of hundred faces takes the exact route: the part is grown, then every one of its faces is swept, so a stepped part carves a stepped channel rather than one big prism.
Past 200 faces the part takes a different route, computed on a mesh of it, because the exact one gets slower than anyone will wait for. A mesh import crosses that line immediately, and so does a busy Kapy or STEP part, one that has been shelled, filleted and engraved. Curved surfaces on that path are approximated slightly on the generous side, so the pocket comes out a touch loose rather than a touch tight, which is the right way round for a thing meant to fit.
Work out which route you are on before you tune the fit. A part you modelled from a handful of extrudes stays well under 200 faces and takes the exact route, so the Clearance value is the gap you get on every square face. A mesh import, or a part carrying fillets, a shell and engraving, is over the line, so set a smaller Clearance than you would otherwise and expect the pocket to run slightly loose.
If a printed part rattles in its pocket, lower Clearance in small steps and rebuild. Check the sloped faces first: on a wedge or a cone the extra slack comes from the angle, not from the number you typed.
See also
- Subtract: a cut with no clearance and no way in
- Offset Body and Offset Faces: tuning a fit face by face
- Working with Several Bodies: keeping the part you cut with