Originally Posted by
atwooddon
My guess is your end mill is not perpendicular to the XY axis. Basically, it is leaning slightly which makes one edge of the bottom of the end mill lower than the other. When you move the bit back and forth on the bottom of your pocket you get 'grooves' where the low side of the end mill cuts deeper than the high side. You need to 'tram' your Z axis to ensure the end mill is perpendicular. To do this, you can simply bend a piece of wire so it has short piece that can be mounted in your router collet, then bends approx 45 degrees and extends out maybe 3-4 (or more) inches, then bend it again so it has a short vertical tail down toward the table of the machine. Mount the wire in the collet then lower the Z axis till the wire just clears the table. Now rotate the collet slowly and see if the gap between the bottom of the wire and the table stays the same. If it changes as you rotate the collet (higher on one side than the other then your router is not mounted perpendicular to the table. If you watch carefully to see where the clearance is different, you can tell which way the router is 'leaning'.
Don