Dog-Bone and T-Bone Fillets: Square-Fitting Inside Corners on a CNC Router

Draw a square slot, cut it on a CNC router, and push a square-cornered tab into it: the tab binds at the corners or stops short. Routed inside corners always come out round, and dog-bone and T-bone fillets — small reliefs cut just past the corner — let a square part seat anyway. This guide covers when they matter, which to choose, how big to draw them and how to get them into your file, for makers, cabinet shops, FIRST robotics teams and furniture designers alike.

Why a router can’t cut a sharp inside corner

A router bit is round and cuts with its side, so wherever your outline turns inward the bit leaves a curve the size of itself: at least 1/16″, the radius of the shop’s smallest 1/8″ bit, and more from a bigger bit. Outside corners are different — the bit swings around the point and leaves it crisp. Drawing an inside corner sharp doesn’t change the result. The corners of every cutout, pocket and mortise, the bottom of every notch and the end of every stopped dado are cut round.

When a sharp corner matters, and when it doesn’t

A rounded corner only causes trouble when something square has to sit in it. Plan relief for:

• Tabs through slots and mortises in slot-together furniture, displays and robot frames.
• Pockets that take a square part, such as an inlay or a flush plate.
• Shelves in stopped dados, where square front corners meet a rounded dado end.
• Box and drawer joints whose fingers must close fully.
• Half-lap (egg-crate) slots, with two rounded corners at the bottom of each.

Decorative cutouts, lettering, openings nothing fits into, dados that run edge to edge and all outside corners can stay as drawn. You can also skip relief by rounding the part that goes in: a tab whose outside corners have at least the bit’s radius slides into a plain slot.

Dog-bone or T-bone: where the relief goes

Both reliefs cut a little past the corner so the mating part’s square corner has room. They differ in where the extra cut lands:

• Dog-bone: a circle on the line that splits the corner in half, its edge passing through the corner point. It bites slightly into both walls and removes less material than a T-bone; from the face it shows as a small round ear at each corner — the bone-end look behind the name.
• T-bone: a circle that cuts straight into one wall, as deep as its radius, and leaves the other wall straight right into the corner. It takes more material, but you choose which wall carries it.

Use dog-bones by default — they’re compact and leave the most material in narrow bridges. Use T-bones where a relief would show: on a through-slot, point them along the slot’s length, and on the face the mating part sits against, each one hides under a shoulder of that part while the long edges beside it stay straight. Avoid them where the wall they cut into is already thin.

How big to draw the relief

The bit has to fit inside the relief, so size it to the bit. A relief smaller than the bit can’t be cut as drawn — the bit clears only what it reaches, and the corner can still hold the part out.

• Radius: at least 1/16″, the 1/8″ bit’s radius, plus a few thousandths of an inch so the cutter isn’t fitting the circle exactly.
• Dog-bone: center the circle on the line that bisects the corner, one radius in from the corner point.
• T-bone: center it on the wall it cuts into, one radius along that wall from the corner.
• Pockets: the relief must reach the floor, or the part sits on a small ledge in each corner. Drawn on the pocket’s outline, it’s cut to the pocket’s depth; in a 3D model, cut it down to the floor.

Bigger isn’t better: an oversized relief shows more and thins nearby bridges without improving the fit.

Reliefs in slot-together parts

Slot-together furniture, displays and robot structures are where most reliefs get drawn, and where one point gets missed: relief clears the corners but does nothing for a slot’s width. Plywood seldom measures what it’s sold as — thickness varies between sheets and deliveries, and Baltic birch is made to metric sizes — so size each slot from a measured sheet plus clearance, as the tab-and-slot design guide explains, and check how snug a joint can be in the CNC routing tolerances guide. MDF and HDPE vary less in thickness than plywood; acrylic cracks from tight corners, so give it generous reliefs.

Drawing reliefs in CAD and CAM

A relief is ordinary geometry — and despite the name, a dog-bone fillet isn’t made with the fillet command, which rounds a corner instead: useful on a tab’s outside corners, no help in a slot. Ways to draw one:

• 2D CAD: place a circle at each corner, then trim the circles and walls, or union them, into one closed outline.
• Illustrator, Inkscape and other vector programs: draw the circles over the corners and unite them with the slot.
• 3D models: cut the relief into the solid, through for a slot or down to the floor for a pocket, then export the STEP.
• Dog-bone add-ins exist for several CAD programs, and some CAM programs add dog-bone or T-bone corners as they make toolpaths. Toolpaths don’t travel with your upload, though — the quote reads geometry, so the relief must be in the file you export.

Cutting a downloaded design? Check whether reliefs are already drawn, and for what bit.

One closed outline per cut

Each slot and its reliefs should leave your software as one closed outline, like every cut in the CAD file prep guide. The importer joins lines and arcs that meet end to end, so walls and arcs drawn separately still arrive as one cut. Loose circles laid over a slot’s corners don’t: depending on where they sit, they’re left out — the cleanup notes list them — or read as separate round holes through the sheet, even beneath a pocket.

Keep each relief on its cut’s layer: on a POCKET layer, a relieved outline is pocketed to the layer’s depth, reliefs and all. A centerline on a DADO layer becomes a plain channel, and where it stops inside the part the bit leaves a rounded end, so draw a stopped dado as an outline with its reliefs. The layer-naming guide has the names.

What the instant quote does with your corners

The routing quote cuts the outline you upload and adds no reliefs of its own. It doesn’t flag an ordinary sharp inside corner either — the bit cuts it with its radius, so a square corner in the drawing arrives round. What it checks is room for the bit: every cutout, pocket and dado inside a part is measured against the 1/8″ bit, and one narrower than the bit, or with a stretch narrower than it — a thin channel, a long needle-sharp point — is named as too small for the smallest tool. That part still gets a price, but online payment waits for the shop’s review; widen the cut or switch it to Engrave in the feature report.

Reliefs aren’t measured, so one drawn smaller than the bit passes without a note. A slot that opens onto the part’s edge, such as a half-lap, is part of the outline — draw it at least the bit’s width plus clearance. After upload, a relieved slot appears in the feature report as one cutout whose cut length includes its reliefs, and the Order step’s “What to expect” box repeats that routed inside corners keep the cutter’s radius. For a critical fit, choose “Something’s wrong — send to an engineer” on the part and describe the joint.

Laser-cut metal: much sharper corners

The fiber laser cuts with a beam whose kerf is a small fraction of a router bit’s width, so inside corners in laser-cut metal come out nearly sharp, and a square tab in a laser-cut slot needs clearance far more than relief. How to prepare a DXF or SVG for laser cutting covers the laser’s hole, slot and web rules. Sharp inside corners in hardened tool steel, such as punches and die sections, are wire EDM work, quoted on request.

Checklist before you upload

• Every corner a square part must reach has a dog-bone or T-bone, or the mating corners are rounded.
• Relief radius at least 1/16″, plus a few thousandths.
• Each relief joined into its cut as one closed outline, on that cut’s layer.
• Pocket reliefs down to the pocket floor; T-bones pointed where they’ll be hidden.
• Slots sized from the measured sheet plus clearance; edge slots at least the bit’s width.
• After upload: one cutout per relieved slot, and no cleanup note about your reliefs.

Then upload to the routing quote — it reads every cut, names any cutout, pocket or dado too narrow for the bit and prices each part. Sheets, sizes and design rules are on the CNC routing page.