Geometrical Analysis of Hole Sizes for Thread-Forming Screws

The practicability of specific hole sizes is known to be influenced by power tool dynamics, material characteristics, finishes, and screw type. Less attention has been paid to resulting internal thread geometry. Historically the engineer controlled tapped hole thread geometry by appropriate thread form specifications. The advent of thread-forming screws has seriously limited his direct knowledge of internal thread geometry. This paper derives the important geometrical parameters for internal threads produced by thread-forming screws. Equations relate internal thread geometry to external thread form and hole size. Without complete standards for thread-forming screws, the technique is illustrated for a Type C and a sharp crested nonstandard thread form. Percent thread and percent volume filled between threads are given by charts.