I'm brand new to CNC and have made a few trays from 4/4 soft maple on my Shapeoko 4 XXL. They have turned out ok but I've noticed that they are becoming more and more warped as the days have gone by. And what I mean by warped is they don't have a flat base anymore - they are able to rock. I made them from 7" wide boards that I got from a reputable hardwood dealer. The boards were planed flat before I bought them. Is this normal? Is there a way to counteract the warping so they have a flat bottom? Any help/insight would be appreciated!