I need to enlarge 8 holes in a Delta RAS base as part of my renovation of an early 50's saw (sneak pre-gloat). The holes are about 3/8 inch, originally used to mount legs to the base but I'm going to use 1/2 inch bolt supports so I can adjust the whole table to be the same height as my shaper which sits nearby. When I try just drilling them out with a 1/2 inch bit, it chatters and they become irregularly shaped. Suggestions?