Fillet

Use Fillet to round an edge. Select one or more edges, press F, and type a radius.

One radius, many edges

A fillet feature carries a list of edges and a single radius. Every edge in it gets the same number. Two different radii on one part are two fillet features, and splitting them that way lets you change one without touching the other.

The radius accepts anything a dimension does: a number, a parameter, an expression. It has to be above zero and no more than 100 mm.

Every edge must belong to the same body. Select edges across two bodies and the feature refuses rather than guessing which one you meant. Pick again inside one body, and make a second fillet for the other.

Re-open a finished fillet and its Edges row starts holding the edges it already has, so the next click adds one more instead of starting the list over. Inside that pick no modifier is wanted: a plain click adds an edge, a second click on the same edge takes it back off, and a box dragged across the model adds every edge it catches. That box is how a long edge list gets filled in one gesture.

Why fillets fail, and what to do

A fillet is the operation most likely to fail on a real part, and nothing under the radius field tells you how large a radius will fit. There is no maximum printed there, on purpose: the only trustworthy answer comes from asking the kernel to build the thing, and that question is asked once the build has already failed. When it does, the failure comes back with a number in it:

A 5 mm fillet does not fit here — the largest that fits is 4.6 mm

That number is not an estimate. The failure path re-runs the same fillet on the same edges at halved radii until it finds the largest one the kernel accepts, and puts it under the field as a one-click Use 4.6 mm button. The usual cause is exactly what it says: a radius bigger than the material around the edge. A 5 mm round on a 6 mm wall has nowhere to go.

Two other verdicts come back in place of it. Each edge takes the radius on its own, but together the rounds collide — two fillets sharing a 2 mm face — reads "The selected fillets run into each other — the largest that fits all of them is 1.8 mm", and the way out is that number or fewer edges in this one feature. The edge is already smooth, meaning the two faces meeting along it are tangent and there is no corner there to round, reads "The selected edge is already smooth"; no radius will ever help, and that edge should come out of the list.

When the diagnosis cannot be run, the older and vaguer message survives:

Fillet geometry failed — common causes: radius too large, edges interfere, edges not connectable, tangency problem. Try a smaller radius or fewer edges.

Then the route out is the one it suggests: halve the radius and rebuild. If it works, the problem was size. If it still fails, split the feature. Round the edges that work in one fillet, and deal with the awkward one separately.

When an edge disappears

A fillet tracks the edges you picked rather than the spot where you clicked, so it keeps hold of them through most upstream edits. When some of them genuinely stop existing, the feature rounds the ones that are left and carries on — and it says so. The step keeps its own colour and marks its corner, and its tooltip reads "1 of 4 edges could not be re-identified". If every edge is gone the fillet fails outright, and the message names the step that should have made them and what became of it: "3 of 3 referenced edges no longer exist — created by Cut 2, which was deleted".

There is one edit that causes this more often than you would expect, and it is not an edit you make. Every finished body is tidied before it is handed on: coplanar faces and colinear edges that are artefacts of how the shape was built — the seam a fuse draws where two solids met, the ring a cut leaves where a tool wall crossed a face — are collapsed away, because they are not part of the model you drew. A fillet aimed at one of those has nothing left to hold. It resolves onto a neighbouring edge and marks itself uncertain, or it fails with the message above. What it never does is quietly round a different edge.

After any edit that changes the shape a fillet sits on, look at the step for that warning and re-pick whatever it lost.

See also

• Chamfer: a flat bevel instead of a round
• Shell: hollowing a part after you have rounded it
• Working with Several Bodies: why order keeps fillets alive