CNC Internal Corner Relief: How to Machine Square Inside Corners
A round cutter has a radius. Its centre cannot sit in a geometric corner, so the finished inside corner is round unless you remove extra material on purpose.
Ideal square cornerWhat a round bit leavesRelieved corner
Why CNC inside corners become rounded
Bit radius is greater than zero. On an inside toolpath the cutter centre stays one radius away from the finished edge, so it never reaches the square vertex. The part that should have been a sharp corner is left as a fillet about the size of the bit.
Common solutions
Dogbone — a small symmetric overcut so a square tenon can seat.
T-bone — the same clearance, placed on one wall so the other face stays closer to square.
A smaller bit — less fillet, more machine time, and it still cannot make a true zero radius.
Hand finishing with a chisel or file after the CNC pass.
Same pocket, three results
100 × 70 mm, 6 mm bit. Switch the relief. The geometry is the same engine as the generator.
100 × 70 mm pocket, 6 mm bit. Grey dashes are the square corner. The black line is the shape this style actually cuts. Overcut past the corner is 0.88 mm. Edge use: 4.24 mm on both edges.
When to use a dogbone
Use it for slots, tabs, and joints that are hidden after assembly. The overcut is symmetric, so both walls give up the same length. A 6 mm bit in the minimal style bulges about 0.88 mm past the corner and consumes about 4.24 mm of each adjacent edge.
When to use a T-bone
Use it when one face of the joint will be seen. The cutter still needs a full diameter of room, but that room sits on one wall. The facing edge is consumed by the full bit diameter; the other edge is not. If neither wall is long enough, the generator refuses the export instead of overlapping the two corners.
Bit size
The relief scales with radius. A 1/8″ dogbone and a 1/4″ dogbone are the same construction at two diameters. Open either page and the tool is already set to that bit.