What should be the included angle of a twist drill for soft metals?

The included angle of a twist drill designed for soft metals is typically set at 90 degrees. This angle is optimal because it creates a sharper cutting edge that efficiently penetrates softer materials like aluminum, brass, and copper without causing excessive deformation or additional friction. A 90-degree angle helps in generating less heat while drilling, which is important in maintaining the integrity of the soft metal being worked on.

Using a drill with a larger angle, such as 120 or 135 degrees, is more suited for harder materials. These larger angles provide more strength at the cutting edge, but they are less effective on softer metals where a finer, sharper point is needed for effective cutting. Therefore, the 90-degree angle strikes the right balance for performance when drilling into softer metals.