Abstract
The exit burr in drilling is classified into two types according to its formation process or shape. “Type 1” is a burr of a smaller height with an uncut part of the hole bottom shaped like a conical hat. “Type 2” is a larger burr shaped like a petal or crown. Through holes are drilled in a sliced bar of aluminum alloy (thickness 10 mm, A5056 in JIS) without cutting fluid. The drill used in experiment is a parallel shank twist drill (diameter 10 mm, point angle 118 deg, helix angle 30 deg and material SKH51 in JIS). The temperature of workpiece is measured over the burr formation area with an infrared thermal imaging camera right before the drill penetrates the bottom of a hole. Experiments clearly demonstrate that the highest temperature is observed at the center of the hole and that it decreases toward the outside. “Type 2” burr starts to form when the workpiece temperature on the exit burr formation area exceeds around 250 degrees Celsius. The workpiece temperature at the exit of the hole increases with feed speed.
