after you make a couple passes back and forth, if you rub your finger across the passes perpindicularly to the direction of the passes, can you feel them, or are you just seeing them.. being new to all of this myself, the first few times that i did a surfacing pass back and forth back and forth, i looked at it and it looked like it was rough, but when i rubbed my finger across the passes they were silky smooth, it was just the light playing tricks on my eyes...
if your finger feels the difference in the passes, is it like this -_-_-_ or like this / / / /
if its the first then there might be some slack or backlash in the machine, if its the second then the endmill isnt perfectly straight up and down, and there may be some adjustments you need to make....
how deep and how fast are you cutting, and in what material with what endmill? going too agressive with your cuts might reveal some flexing in the machine, and the only way that i know to fix that is to take less agresive cuts...
just my .02
Grizzly X3, CNC Fusion Ballscrew kit, 3 500oz-in bipolar steppers, 3 203v Gecko's, Linear power supply from Hubbard CNC, Mach 3, BOBcad Pro Art V22, Rhino.