Inside corners that result in a radius are a commonly overlooked issue when designing a part for machining. As of yet, there isn’t a magic way to cut a corner on a CNC router with a sharp 90º corner. This is because all CNC bits are round. 

Feeling lazy? Read the TLDR.

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The Problem

Depending on your project, internal filleted corners may not be an issue for you. For some, this means your joints won't fit and you need a nice square corner. We have a few options on how to solve this problem.

Let's imagine a common material - 0.75” thick plywood sheet. We need to profile cut (cutting on the edge of a part) that with a 0.5“ bit. We will leave an interior corner radius of 0.25”. To minimize that we can:

2nd Operation: Smaller Diameter Bit

To make that corner a smaller radius, we can cut our profile with a 0.5” bit and come back with a 2nd operation and a smaller bit cut into the corner say 0.25”. (Click to play below)

In Fusion 360, Rest Machining with a smaller bit will only machine that unreachable area from a previous operation.

Cost: Medium
Design Difficulty: None
CAM Setup Time: Low

Problems: Smaller diameter bits need to be shorter with less of a cutting edge. The thicker the material you are cutting the more difficult it is to create a smaller radius as the tool needs to be larger for rigidity and break-resistance.

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Dog Bones

Dog bone corners are a jargon-y way to describe the shape of a corner that is extended outside the cut area to create a perfect 90º corner that looks like the cartoon shape of a canine chew toy. There are a few ways to do a dog bone corner and none are right or wrong.

The only constraint is to use a slightly larger entrance point to the dog bone corner than the size of the router bit diameter to bed used. Something like 15–20% larger should work. E.G. 0.25”ø bit should have at least a 0.2875” opening to the dog bone.

Cost: Low
Design Difficulty: Low-Medium
CAM Setup Time: Medium (creating dog bones)

Manual Squaring

The simplest solution? Break out your pull-saw, standard chisels, or a corner chisel and square those corners yourself. Sometimes this is the best solution as you can get a square corner and not have to dog bone into your work for a clear corner.

Cost: Low (If customer does work)
Design Difficulty: None
CAM Setup Time: None

TLDR:  CNC cut corners leave a radius on inside corners. You can design your corners differently depending on function and aesthetic. Dog-bones are a great way but have a specific aesthetic. If a smaller radius will do, use a 2nd Op with Rest Machining. If you need a clear corner without modifying your cut lines – get a chisel or pull-saw and square that CNC cut radius.

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