Mark - thanks for the info, however the last ones I had to do had hex-only screws in them, no slots! Tried everything on hand, including a 1/4" Makita driver bit holder, only slightly larger in diameter than the 1/4" hole. It still wouldn't go all the way down. Luckily this was at a manufacturing plant, so we went out into the machine shop, grabbed a spade bit, and ground off the center point and made the center concave (covex?). This let us drill down to the screw heads, still leaving enough plactic to be able to reassemble the units.