3D printed inlays that actually fit
This guide was drafted with AI assistance.
An inlay fits when its side walls run straight along one direction, the direction it slides in, with a flat floor across it. Give the pocket 0.2 mm per side and only 0.05 mm under the floor, print the inlay floor-down with its visible face up, and glue it in with a sliding fit, never a press fit. On a curved surface, the floor goes below the inlay's deepest point. The 3MF color splitter builds inlays this way from paint.
What an inlay is, and when you want one
An inlay is a small part in one color that sits in a matching pocket in a bigger part in another color, flush with the surface. Eyes on a figure, spots on a mushroom, a logo on a box lid, a stripe on a toy. It is how you get a second color onto a surface without a multi-material printer (more on that here), and even with one it can be the better print: no purge on every layer, and a sharp edge where two filaments would otherwise bleed into each other.
Most inlays that do not fit fail for one of three reasons: walls that are not parallel, a clearance chosen by guesswork, or a floor that leaves the edge of the inlay paper-thin. Each has a simple fix.
Straight walls along one direction
Pick the direction the inlay goes in, and make every side wall of the inlay, and of the pocket, parallel to it. A shape with straight walls like that is a prism: slide it along its axis and it never catches.
It is tempting to let the walls follow the surface instead, square to the model everywhere, because that is how the color looks when painted. On a flat face that is the same thing. On anything curved it is not: walls that lean different ways lock against each other, and the inlay only goes in if it is pushed in pieces. One direction for the whole inlay, and the floor square to it.
Clearance: side and floor are different numbers
| Gap | Value | Why |
|---|---|---|
| Side, per side | 0.2 mm | Slides in by hand on a typical FDM printer. 0.3 mm shows as a visible line round the inlay; 0.1 mm fits only on a well-tuned printer. |
| Side, if the walls print as steps | 0.3 mm | A wall printed as a staircase eats into the gap. |
| Floor | 0.05 mm | The inlay rests on the floor, so the whole floor gap is how far it sinks below the surface. |
The floor number surprises people. It is not a fit, it is a depth: an inlay stops when it touches the floor, so a 0.2 mm floor gap leaves it 0.2 mm below the surface, which you can see and feel. Keep the floor nearly tight and the side loose.
Inlays on a curved surface
An eye on a round head, a spot on a mushroom cap: the painted patch curves, but the inlay still needs one straight direction. Use the average direction the patch faces, and square the floor to it.
- Put the floor below the deepest point. A curved patch dips away from its own middle. Measure the thickness from the lowest point of the patch, not from its centre, or the edge of the inlay tapers to nothing and snaps.
- Accept a wedge. The inlay ends up thicker in the middle than at the edge. That is fine: the side walls hold it, not its thickness.
- Watch the wrap. If a patch wraps more than about a quarter of the way round a curve, some of its surface faces away from any one direction, and the inlay cannot slide straight out of its pocket. Split the patch in two, or print that color as paint.
- Clean the outline. A painted border is a staircase of tiny triangles. Rounding it off by a few tenths of a millimetre before building the walls gives the inlay and the pocket smooth sides that slide instead of catching.
Printing the inlay and the pocket
Print the inlay floor-down, visible face up. The side walls then stand vertical on the bed and come out smooth, which is what the fit depends on. A flat visible face prints perfectly on top. A curved one shows layer steps; fix that with a finer layer height for the inlay, not by turning it over, because a curved face on the bed rests on a point.
Print the part with the pocket the way it prints best anyway; the pocket walls are just more side walls. A lattice infill (grid, cubic, gyroid) under a pocket gives its wall something to lean on; a concentric or spiral pattern runs along the wall and supports it poorly.
Fitting and gluing
- Sliding fit plus glue, never a press fit. Pushing an oversized inlay in forces the pocket wall outward across its layer lines, the weakest direction of any FDM print, and it cracks along a layer.
- Dry-fit first. If an inlay is tight, sand its sides lightly. Never sand the top; that is the visible face.
- A drop, not a bead. A small drop of CA glue on the floor of the pocket is enough. The side clearance is a gap glue will happily fill and squeeze out of.
- Thin necks break. An inlay with a narrow waist can snap there when you pull it off the bed. Make it thicker rather than wider: the width is the design, the thickness is hidden in the pocket.
Common questions
What clearance should a 3D printed inlay have?
About 0.2 mm on each side for a sliding fit on a typical FDM printer. At 0.3 mm the gap shows as a line around the inlay; at 0.1 mm it only fits on a well-tuned printer. The floor needs far less, about 0.05 mm, because every bit of floor gap is how far the inlay sinks below the surface.
Should a printed inlay be a press fit?
No. A press fit pushes the pocket wall outward across its layer lines, the weakest direction of an FDM print, and it tends to split. Use a sliding fit and a drop of glue.
How do I make an inlay on a curved surface?
Give the whole inlay one pull direction, the average direction its surface faces, with side walls parallel to it and a flat floor across it. Put the floor below the inlay's deepest point, so its thin edge still has real thickness. If the patch wraps too far round the model, no single direction pulls it out, and it has to be split or kept as paint.
Made by meshcast.app. The clearance and joint rules follow the notes published with bambu-3mf-tools (MIT). Your printer may want 0.05 mm more or less: print one small test inlay before a big job.