Simple Solutions to Internal Radii
There is no way of avoiding a radius to internal corners of parts cut using a CNC spindle. The radius is a result of rotating tools and is equal to half the cutting tools diameter. For example, a 1/4″ or 6.35mm dia tool will create a 1/8″ or 3.175 radius on internal corners. In many circumstances this does not cause any design problems, for example this internal profile cut.
Where parts are assembled together, the internal radii may need to be eliminated so joining features can mate fully. There are two easy ways to achieve this, by adding ‘t-bone’ or ‘dog bone’ fillets to corners.
The t-bones on the left are half circles cut on opposite sides, they can be positioned so that when joined the features are hidden. The dog bones on the right take out the minimum amount of material to create square corners, and these are particularly useful in creating strong mortises. The mating tenon part may also require filleting, depending on it’s orientation to the cutting tool.