I make dog urns. They are small. roughly 7 3/4 x 5 3/4. I need to cut miters against the grain to join the pieces together. I built a miter sled for my table saw. When I make the cut, I always seem to get a slight curve in the middle edge of the piece. It doesn't occur if I cut with the grain. I use a block on the end to keep it from sliding. Is this normal, or am I doing something wrong?