Dog-bone or T-bone fillets: which should I use on my joints, and where do I put them?

Why you need them

A round bit cannot cut a square inside corner. It leaves a radius equal to the bit radius, and a square-cornered mating part rides up on it. A relief fillet pushes the cut past the corner so the square part seats.

The two relief types

Dog-Bone Fillet. A circular cutout placed on the diagonal so its edge just reaches the original sharp corner. The official documentation warns not to use it when the slot width and the tool are similar in size.

T-Bone Fillet. A circular cutout pushed out along one side only. The official documentation says to use it when the slot width and the tool are similar in size, so the slot can grow out to the side and still leave room. A maker space guide adds the other reason: use it when there is not enough material on both sides of the corner.

Placing them in the software

1. Open Create Fillets on the drawing tab.
2. Enter the radius. A published maker space guide says the minimum is the tool radius (0.125 inch for a 1/4 inch end mill) and that 0.150 inch cuts cleaner. A tooling maker's joinery article gives a similar rule of at least 1.1 times the tool radius.
3. Choose Dog-Bone Fillet or T-Bone Fillet.
4. Hover over an inside corner until the cursor shows a check mark, then click.
5. For a T-bone, clicking directly on the corner puts it on the longest side. Click to one side of the corner to choose that side yourself.
6. To remove one, hover until the cursor shows a cross and click. The corner returns as two straight lines, even if it was originally curved.

Where to put them so they hide

• Put T-bones on the side that the mating part covers. On a slot that receives a panel edge, that is usually the short ends, so the relief extends along the slot and disappears under the panel.
• On a mortise or pocket seen from the face, dog-bones are the smaller visual mark.
• Only inside corners of the female part need relief. Outside corners of a tab are cut square already.

Pro tips

• The Normal Fillet option is for design rounding only. It does not make a joint fit.
• Fillet radius smaller than the real tool radius does nothing. Measure the bit.
• For automatic placement on many parts, see the entry on the dogbone gadget. For typing an exact radius and previewing the fillet in the 3D view, see the entry on adding a fillet with an exact radius.