What is the recommended included angle for a twist drill used on hard metal?

The recommended included angle for a twist drill used on hard metal is 118 degrees. This angle is optimal for providing a balance between cutting efficiency and chip removal when drilling into harder materials. A drill with a larger angle, such as 135 degrees, may reduce cutting efficiency by being less aggressive, which is not ideal for harder metals that require stronger cutting edges to penetrate effectively.

Similarly, angles like 90 degrees and 115 degrees are less common for hard materials; the 90-degree angle might be too acute, leading to faster wear and ineffective cutting, while 115 degrees doesn’t optimize the chip clearance and cutting action as effectively as the 118-degree angle. Overall, the 118-degree included angle strikes a proper balance that allows for effective penetration and good chip removal, making it the best choice for drilling into hard metals.