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Adding CNC router reliefs into corners

Question asked by J. R. on Jan 1, 2020
Latest reply on Jan 3, 2020 by J. R.

I am modeling 4mm thick sheet parts that will be machined with a CNC, and then assembled with tab-and-slot joints, like here:



The CNC bit that will be used to machine these parts has a diameter of 3.78mm, and these slots are 4mm wide, making it possible to machine them, except the inner 90° corners. My CNC machinist advised that if I provide a DXF that has simple rectangular cut-outs on my parts, the CNC machine will leave these inner corners rounded, making it impossible to assemble the parts later on:



The best solution for this would be to create reliefs in the corners - rounds of 3.78mm (or slightly more), to ensure that the tab will be able slide in without interference, resulting in a bone shaped slot:


(The construction lines represent the shape of the tab that will be inserted into this slot)


The extra gap left around these corners is of no concern. It will be filled during construction with other materials.


Now, my question is, what is the quickest and most convenient way to apply this corner treatment to all the slots in every part? Please note that these slots aren't always the same shape. They might be 4mm wide, but they might be wider (because parts will meet at angles other than 90°). Slot length varies as well. Features used to produce them vary too - some are simple cut-extrudes (in which case I could use the bone-shape slot in the first place), but most of them are produced with other features, and therefore require post-treatment. To produce the example above, I created a sketch block with a circle and some construction lines to help align the cutting circle to each corner with only two coincident relations:



Then copied this sketch block to every corner, and applied cut-extrude or surface trim, depending on the type of the part. The problem is - each slot has 4 corners, and there are nearly a thousand of these slots in my parts. Very few of these slots are alike in their shape and size, so I can't make any kind of block that would treat all 4 corners at the same time.


So, is there a better approach to solving this problem? What is the quickest and most convenient way to apply this kind of corner treatment to already existing slots? Since all my parts are flat, it can be either solid or surface based solution.