Round the corners of a 3D-printed part (and when you need the AI)
Sharp corners on a printed part look unfinished, catch on things and crack first. Rounding them is the one finishing step every part deserves, and on a phone it is a slider. This guide shows the Round tool, what it rounds, what it does not, and the honest line between the hand tool and an AI request.Why round a printed part at all
Three reasons, all practical. A rounded vertical corner prints with less warping, because the extruder does not slow to a stop and change direction at a sharp point; the corner stays stuck to the bed. A rounded outside corner on a bracket or a case does not dig into a hand or a cable. And a rounded inside corner (the bottom of a pocket, the junction of a wall and a floor) spreads the load that would otherwise start a crack along a layer line.
What the Round tool rounds
Tap Round in the rail, then tap a flat face of the part. The card shows a Radius slider and the same number as a field. Press Apply round corners. The tool rounds the corners of that face's outline: the edges that run parallel to the direction you tapped from. On a box or a plate tapped from the top, that is the four vertical corners, the ones that touch the bed. On a pocket you cut with the Cut tool, the Corner radius field in the cut's own card does the same for the pocket's corners.
That is the honest scope of the tool: the corners of a part, and the corners of a pocket through the Cut card. It is also most of what a printed part needs, because the vertical corners are the ones that warp and the ones your hand meets.
The slider goes from 0.5 mm to half the smallest dimension of the part minus a hair, so you cannot round a 20 mm wide bar with a 12 mm radius and end up with nothing. 2 mm is the default and suits most small parts; 4 to 6 mm reads as deliberately soft on a case; on a plate, a radius equal to the plate's thickness looks right.
Rounding the corners of a pocket
A pocket you make with Cut has its own Corner radius field in the cut's card. Set it to 1 or 2 mm before you press Apply cut, or tap the pocket afterwards and change it there. A rectangular pocket with rounded inside corners prints cleaner (the nozzle cannot turn a sharp inside corner anyway) and is far less likely to start a crack. The same number on a battery pocket or a display window also matches the rounded corners the real object has.
What it does not round
A single edge. The top edges of a box, where the walls meet the opening. The edge between a boss and the face it stands on. These are fillets along an edge that is not parallel to the tapped direction, and the tool does not do them. The card says so under the slider: "Rounds the corners of this face's outline (edges parallel to its normal). To round a single edge or the top edges, ask the AI."
Under that line is a button: Ask the AI to round just this edge. It writes "Round just this edge with a 2 mm fillet" (with your radius) into the chat and leaves it there for you to send. Nothing is spent until you send it. When you do, it is one AI request, 2 credits, and the AI rebuilds the part with the fillet. If the part also has hand edits, they stay.
Fillet or chamfer, on an FDM printer
A fillet is a rounded edge; a chamfer is a flat cut at 45°. Both take the sharpness off. Which one prints better depends on where the edge is:
| Edge | Better choice | Why |
|---|---|---|
| Vertical corners (touch the bed) | Fillet | Smooth tool path, no slowdown, no warping at the point |
| Bottom edge, where the part meets the bed | Chamfer, 0.5 to 1 mm | Hides the elephant's foot bulge; a fillet there prints as a ragged first layer |
| Top edges, facing up | Either; fillet looks better | Fine layers at the top, both print cleanly |
| Edge facing down, over air | Chamfer at 45° | Prints without support; a fillet becomes a steep overhang at its start |
| Inside corner of a pocket or a wall-to-floor junction | Fillet | Spreads load, avoids a crack along the layer |
The Round tool makes the first row. The chamfer at the bed, the top-edge fillet and the downward chamfer are AI requests: "chamfer the bottom edge 0.6 mm", "round the top edges 2 mm", "chamfer the underside of the ledge at 45°". Say the number and the AI puts it where you said.
A worked example on a phone
Start from a Plate and set it to 80 by 50 by 4 mm by tapping it and typing the numbers. Tap Round, tap the top face, slide the radius to 5 mm, Apply round corners. Four soft corners. Tap Hole, tap near one corner, choose M4, Through, Apply hole; three more. Now type in the chat: "chamfer the bottom edge 0.6 mm". That one line is 2 credits, and the plate prints without a bulge and without a sharp corner anywhere. The four holes and the rounded corners you made by hand are still numbers on their cards.
What it costs
Shaping a part by hand (a hole, a cut, an added tab, an extruded face, rounded corners) and downloading what you shaped are free. Each AI request spends 2 credits, a download of AI work 4 credits, and G-code 5 more. The rounding and the holes above were free; the chamfer was 2 credits; downloading the plate with the chamfer is 4 credits, and a small change after a test print re-downloads free. Without the chamfer, a plate shaped only by hand downloads free with a free account.
Related reading
Design rules for print-ready parts covers the elephant's foot and overhang angles in more depth, and Shape a part by hand walks through the other five tools.
- 1Open the partStart from a shape (box, plate, tray) or open a part the AI designed. Round works on a part that has a name; the starters and AI designs have them.
- 2Tap Round, then tap a flat faceTap the face whose outline you want rounded: the top of a box or plate to round its vertical corners. The card opens with a Radius slider.
- 3Set the radiusSlide or type the radius in millimetres. 2 mm is the default; the slider stops at half the smallest size of the part so it stays a solid.
- 4Apply round cornersPress Apply round corners. The part recompiles in your browser and the card shows the radius as a number you can change later.
- 5For one edge, ask the AIThe card's button, Ask the AI to round just this edge, writes the request into the chat. Send it when you are ready: one AI request, 2 credits.