Hey John,
you will understand that easier as soon as you have experience. If your workarea is 1220 x 1220 mm, then you have the outermost positions
Front left: X=0, Y=0,
Front right: X=1220 Y=0,
Rear right: X=1220, Y=1220
Rear left: X=0, Y=1220.
You can move to these positions, and all positions in between, manually or with mdi commands like “G0 X1220 Y1220”. So if you have a bit with 8 mm diameter, which is 4 mm radius, then this will mill additional 4 mm outside the workarea at every edge of the workarea. So your wasteboard could be 1228 x 1228 mm big and you would still be able to surface it.
However if you use a CAD/CAM software, it wants you to enter the bit radius of the bit you use for the toolpath. So if inside the CAM program, the limits of the workarea is set to 1220 x 1220 mm, and you tell the program that you will use a 8 mm diameter bit, then it will stay inside the limits, which means, for generating the toolpath, it will subtract the 4 mm radius from the 3D model automatically in order to honor the bit’s radius which is responsible for the fact that the milling of the outermost 4 mm is already included.
So in fact on a 1220 x 1220 mm workarea, it will mill from positions X=4, Y=4 to X=1216, Y=1216, which are the positions of the center of the bit; and the outer 4 mm are still milled because of the 4 mm radius that surrounds the bit center. The CAM program always takes the radius of the bit into its calculations, to exactly reproduce your virtual 3D model. So it will not go outside of 0,0 and 1220,1220 mm if you have set the limits inside the CAM program this way. Except if you either tell the CAM program that it’s a 8 mm bit and e.g. use a 16 mm bit in real, or if you change the limits values of the workarea dimensions inside the CAM program.