Dog-bones for plywood furniture
The short answer: a round router bit cannot cut a sharp inside corner, so a square part will not seat in it. A dog-bone is a small extra cut into the corner that gives the part room to go all the way in.
Why you need them
When a CNC router cuts a slot, the corner of the slot ends up rounded with the radius of the bit. A plywood part with a square corner then hits that rounded corner and stops short of the bottom. The joint looks gap-toothed and is weaker.
What a dog-bone is
Instead of cutting only along the corner, the toolpath drives the bit a little further into it, so the corner has a small round notch. The mating part sits in that notch with its square corner. The name comes from the shape of the slot end, which looks like a bone.
Where they go, and where they do not
- Inside corners that hold a square part: yes. These are the slots, the half-laps and the tenon holes.
- Outside corners: no. The bit simply follows the edge.
- Open slots with a round end that you do not mind: not needed.
How big they are
The notch is as big as the bit. A thinner bit leaves smaller, less visible dog-bones. In the plans from this site the dog-bones are calculated from the bit diameter you enter, so when you change the bit the files change too. If the dog-bones look big on a small piece, try a thinner bit.
Alternatives
- Round the corner of the mating part: the part fits a rounded slot, with no visible notch. This needs a change in the design.
- File or chisel the corners square: neat, but it is hand work on every joint.
Dog-bones are the fastest option and the one used by the plans here. They end up hidden inside the joint once the pieces are glued.
In your CAM software
The DXF files separate outside cuts, inside cuts and pockets on different layers, and the dog-bones are already part of the inside outlines. You compensate the bit on the outside for the outline and on the inside for the slots, as in the assembly guide that comes with every download. Check the clearance with a fit test before cutting the whole job.
Frequently asked questions
Do I need dog-bones for every joint?
Only for inside corners where a square-cornered part has to seat all the way. Outside corners and open slots do not need them.
Can I avoid dog-bones?
You can round the corners of the mating part, or file the corners square by hand after cutting. Dog-bones are the fastest option and need no extra work.
How big are the dog-bones in the files?
They are calculated from the bit diameter you enter, so a thinner bit makes smaller, less visible ones.