Blind hole

A Blind Hole refers to a hole that is reamed, drilled, milled etc., to a specified depth. The etymology is that you cannot see through a Blind Hole. Other types of holes also include Through Holes, and clearance holes. In this instance Blind may also refer to any feature that is taken to a specific depth. More specifically referring to internally threaded holes (tapped holes).

Threading blind holes

There are three accepted methods of threading blind holes:

1. Conventional Tapping
2. Single-point threading, where the workpiece is rotated, and a pointed cutting tool is fed into the workpiece at the same rate as the pitch of the internal thread.
3. Circular interpolation, where the workpiece stays still, held by a jig or fixture and the cutting tool is rotated on two parallel axes to mill the thread. The cutting tool is introduced to the bottom of the blind hole and, while turning at high speed, is rotated into the workpiece at the same rate as the pitch of the internal thread.

Machine tools used for threading blind holes

At least two U.S. tool manufacturers have manufactured tools for circular interpolation of blind holes, Ingersoll Cutting Tools of Rockford, Illinois, and Tooling Systems of Houston, Texas, who introduced the ThredMil in 1977, a device that circularly interpolated large internal threads in the blind holes of Oil Well Blowout Preventers.