I wonder if the "break-through" disk is what is breaking your drills? In mid stack, if one of these disks happens to get caught between the bottom of the hole and the bit, it effectively caps the cutting edges and snap!
I would try an even more acute angle to the drill, which should be of split point grind, in an effort to make sure it penetrates the next sheet before it has a chance to break out the "breakthrough disk" of the current sheet.
Maybe try not pecking at all, too.
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(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management) |