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The preferred included angle for a twist drill designed for hard metals is 118 degrees. This angle strikes a balance between providing adequate cutting efficiency and minimizing the tendency to snag or bind in hard materials.

A 118-degree angle allows for effective penetration into the material while maintaining a sharp point capable of cutting through tougher surfaces. The geometry of the drill bit at this angle also helps in reducing the load on the cutting edges, facilitating a smoother drilling process and prolonging the life of the tool.

Higher angles, such as 135 degrees, might be more suitable for softer materials or when a faster cutting action is required, but they typically lead to reduced cutting ability in harder substances due to their tendency to create larger chips and higher stress on the drill bit. Lower angles, like 90 degrees, would not provide the necessary cutting efficiency for hard metals and may lead to premature wear or breakage. Thus, 118 degrees is the optimal choice for drills intended for use in hard metal applications.