I had to drill holes for deadbolt locks in some steel doors that had foam cores, and I almost destroyed the first couple of doors using a standard hole saw because the 1/4-in. pilot bit would wander off course in the soft foam. To remedy the situation, I removed the pilot bit and replaced it with a piece of 1/4-in. mild-steel rod, as shown in the drawing. To use the modified saw, I first drilled a 1/4-in. pilot hole through the door and a 2x block clamped to the door. Using the pilot hole to guide the steel rod, I cut my holes with no wobble and no tearout. Incidentally, for this work I found that a bi-metal hole saw held its edge better than one made of steel.
E. F. Bott, Jr., Troy, NY