When designing a part that will be cut out with a CNC, it’s important to remember that router bits cannot reach all the way into the corners.
Here is an example. In the picture below on the left, you will see the corners as they will appear when you design them in Sketchup. On the right though, you will see what the corners will look after they are cut out with a CNC. As you can see, the material that the router can’t reach is left behind. The larger the bit’s diameter is, the worse it gets.
Most of the time, another part must fit in the hole that has been cut out, and that extra little bit of material will get in the way. If it is important for the part to look good, then you can just remove it with a chisel or something, but, if that sounds like too much of a hassle, there is another option.
Below, we have the same two corners, but, this time, they have been redesigned so that the router bit will overcut them and remove the material that would otherwise get in the way.
This was done by drawing two circles with a radius slightly bigger than the router bit’s radius in each corner and then deleting the unnecessary lines.
The reason I made the radius slightly bigger is because the CAM program I use for making my parts, CamBam, sometimes thinks that the router bit will not fit into the opening and will just skip over it. Making it just a little bit bigger though will fix this problem.
For example, when designing for a quarter inch router bit, I make the radius 0.13″. The radius of a quarter inch router bit is normally 0.125″, but that extra 0.005″ makes certain that it will work correctly in CamBam.
