... Plates and Drilling . ... We drilled 22 plates. The plates were 3.5" diameter 1/8" thick disks of aluminum. 50 holes were drilled in each test plate, 10 each in five concentric circles. ... Five plates (#12-16) were drilled using a carbide spade drill. ... The spade drills were shorter for the current tests, which we thought might improve position accuracy and reduce diameter variation; but the twist drills had the same geometry as in the 3/93 tests, so should have yielded similar results. ...