So manufacturers might slow the feed rate or tighten the slack in the feed gears to avoid hourglass-shaped holes. When the hole has the opposite geometry, a concave or ball shape, chip packing is usually the culprit. When chips cannot exit the hole, they pack in the flutes and gall the walls of the hole. Causes include cold welding along the cutting edges and margins, narrow or rough flutes, and insufficient coolant for correct flushing. Coating the tool to make it more lubricious, using through-tool coolant to blast the chip out, or specifying a wider or parabolic flute, or a different helix to create more room, are all solutions to this problem.
Crooked holes are usually because of a misaligned spindle or too high a feed rate.
An incorrectly aligned spindle points the drill in the wrong direction from the outset, and too high a feed rate can make the tool buckle. Both conditions cause the drill to walk, making the hole oblong and oversized. Once a drill starts to walk in a certain direction, it tends to continue that way, said White. Corrective action includes using a shorter, more rigid drill and slowing the feed rate to avoid bending the drill. For deep holes where a long drill is necessary, start the hole with a shorter, more rigid drill first, using a fixture with a drill bushing, or both.