Theoretical Tapping Hole Size Cold Form Threads

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Theoretical Tapping Hole Size Cold Form Threads

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Conventional tap drill sizes should not be used for cold form tapping because the metal is displaced to form the thread. The cold formed thread is stronger than the conventionally tapped thread, so the thread height can be reduced to 60 percent without much loss of strength; however, the use of a 65 percent thread is strongly recommended. The following formula is used to calculate the theoretical hole size for cold form tapping:

T_{h s} = T_{o} - \frac{0.0068 \cdot F_{t} / 100}{T_{i n}}

where

T_hs = Theoretical hole size (in)
T_o = basic tap O.D. (in)
F_t= full thread percent (%)
T_n = Number of Threads per Inch

Sharp drills should be used to prevent cold working the walls of the hole, especially on metals that are prone to work hardening. Such damage may cause the torque to increase, possibly stopping the machine or breaking the tap. On materials that can be die cast, cold form tapping can be done in cored holes provided the correct core pin size is used. The core pins are slightly tapered, so the theoretical hole size should be at the position on the pin that corresponds to one-half of the required engagement length of the thread in the hole. The core pins should be designed to form a chamfer on the hole to accept the vertical extrusion.

Reference:

ANSI/ASME B1.1