The ratio of actual internal thread height to basic thread height, controlled by the tap drill diameter. Tap drill charts usually aim for about 75 percent, because full thread adds only around five percent strength while taking roughly three times the torque to tap. Tough and work-hardening materials such as stainless are often tapped at 60 to 65 percent to save taps, and thread-forming taps need a larger drill than cutting taps.
Bump the tap drill up a size on the 304, we only need sixty-five percent thread and we keep snapping taps.
Also heard as
- thread percentage
- percentage of thread
Related terms
Minor diameter
The smallest diameter of a screw thread, at the roots of an external thread or the crests of an internal one.
Thread engagement
The axial length over which external and internal threads fully overlap, which sets stripping strength.
Thread stripping
Failure where the threads of a nut, tapped hole or bolt shear off instead of the bolt breaking.
Thread-forming screw
A screw that displaces material to form its own mating thread in a plain pilot hole, without cutting chips.