CNC Slots

The CNC cutting head is round, and can only cut round holes. It can cut a line by sliding the circle along a line, but it can only do this for one of the two lines that meet at an inside corner.

Some of you have asked what you can do about the “Mickey Mouse” ears.


Some options are:

  1. Put the tool clearance heads in line with the horizontal edges of the slot. The slot is wider than it needs to be; inserting the matching piece (shown in gray) leaves a gap on the left.
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  3. Put the tool clearance heads in line with the left edge of the slot. This leaves half-circle holes on the top and bottom of the left edge.
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  5. Leave round corners in the slot during the CNC milling step, as in solution #1. Then use a chisel or file to remove the extra material (shown in red below), to turn the slot into a rectangle.
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  7. Leave the round corners in the slot, as in solution #1. Round the edges of the piece that goes in the slot (add a fillet to it).
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If you choose this last solution – of rounding the edges of the piece that goes in the slot – you have these options for adding fillets:

  • Use a file or rasp to fillet the edges.
  • Use the (hand held) router, with a roundover bit.
  • Use the CNC to fillet the top two edges. Then flip the piece over, and use a file or rasp to round over the other two edges.

It is very difficult to use the CNC to fillet the correct edges of the piece that goes into the slot. You want to create a shape such as that on the left.

The CNC can most easily round the corners that it is cutting, to create the shape in the middle. The CNC can fillet the top edge – either by using a roundover bit, or by sculpting the side of the wood – cutting different parts of the stock to different depths. (The roundover bit is faster, and creates a smoother surface.)

Once you remove the tabs and flip the piece over, it is very difficult to clamp it and to align it so that the CNC can follow the edge that it cut from the other side.