Boring Rate With a Brace and Bit in White Oak: Torque, Feed and the Resharpen-or-Stop Rule as Functions of Bit Condition

A 25 mm auger bit in a hand brace, boring through 50 mm of air-dry American white oak, is modelled in three bit conditions: sharp, dulled, and with a damaged screw tip. A sharp bit requires 4.69 N m of torque at a feed of 2.0 mm per revolution, completes a hole in 0.96 minutes including handling, and achieves 62.79 holes per hour at a metabolic cost of 3.68 kJ per hole. A dulled bit raises torque to 8.67 N m and metabolic cost to 6.81 kJ per hole, and reduces output to 46.41 holes per hour. A damaged screw tip is far worse than dullness: self-feed falls to 0.70 mm per revolution, 71.4 revolutions are needed per hole, output falls to 16.45 holes per hour, and metabolic cost rises to 18.62 kJ per hole, five times the sharp value. Resharpening a dulled bit for 8 minutes pays back only if at least 25 further holes remain; for fewer holes the arithmetic favours boring on. A damaged screw tip does not meet that test at all, because sharpening the cutting edges does not restore the screw: the correct decision is to stop and repair or replace the bit. Dullness is an economic inconvenience; screw-tip damage is a defect that makes continued boring the most expensive option in the table.