| clamp a straight edge on the bottom of the door to guide your circular saw so you get a straight edge. (straight edge = any piece of reasonably straight lumber)
you can also score the top side along the cut line to reduce the splinters. The bottom edge will be fine because of the way the blade turns. (i.e. put the good side of the door, if there is one, face down). If your blade is sharp, it shouldn't be a problem.
As Amricca says, you should be OK to take out 1/2 to 3/4". If you end up having to cut the entire piece of wood at the bottom out, then strip and sand the old veneer from the wood, and reglue it inside the bottom again. |