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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.

rectangle with internal radii

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.CNC dog bone and t bone fillets

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.CNC tenon

With the  parts assembled, the images below show the appearance of the joints; dog bone above,  and t-bone below.CNC assembled joint

CNC assembled joint

By reducing the pocket depth and shortening the tenons it is possible to create a stub joint with t-bone fillets that are completely concealed. stub mortise and tenon joint

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