When designing for self-tapping screws, a number of factors are important:
Boss Hole Dimension: For the highest ratio of stripping to driving torque, use a hole diameter equal to the pitch diameter of the screw.
Boss Outside Dimension: The most practical boss diameter is 2.5 times the screw diameter. If its wall is too thin, a boss may crack. Higher stripping torques are not achieved with thicker bosses.
Effect of Screw Length: Stripping torque increases rapidly with increasing length of engagement and levels off when the engaged length is about 2.5 times the pitch diameter of the screw.
Strip-To-Drive Ratio: The torque-turn curve in Figure 1 shows how a self-tapping screw responds to applied torque. Up to point "A", driving torque is applied to cut or form a thread and to overcome sliding friction on the threads. Successive turns require more torque as the area of thread engagement increases. At point "A", the head of the screw seats. Any further application of torque - now referred to as stripping torque ....