Threading - Internal Threads

Internal Threads

To achieve an internal screw thread, a hole has to be drilled first and then a tool called a TAP is used to cut a thread within the hole. TAPS are made from high speed steel (HSS). The top of the tap is square which enables the tap to be held securely in a TAP WRENCH (right).

Taps are generally available in sets of three and are used in the following order:-

1. Taper Tap: This tap has much smaller teeth at the bottom than the Second or the Plug taps which allows it to get started by making a shallower thread cut
2. Second Tap: This tap follows the taper and has slightly more teeth and cuts the thread slightly deeper.
3. Plug Tap: This is the final tap and has fully formed thread cutting teeth almost to the end.

Blind Hole

the Plug Tap is especially important when threading a blind hold. A blind hole has a bottom to it. If a blind hole is to be threaded it is very important to ensure that the depth of the hole is correct before starting to thread the hole. A piece of tape attached to the tap can indicate when the bottom of the hole has been reached; if the tap is forced into the bottom of the hole it could be broken.

Tapping Drill sizes

When tapping a thread in an internal hole the actual diameter of the hole to be drilled must be smaller than the actual overall size of the thread to be cut otherwise there will be nothing for the tap to cut into. this drill is call the tapping drill and different sizes are needed for different threads as shown in the table. If a hole on a drawing is shown as an M5 diameter, this means it will be threaded with a Metric thread and that tapping hole should be 4.2mm. Threads have different diameters and pitches, it is important to make sure the correct thread is cut using the correct tool so that parts fit together.

More details on the process of cutting an internal thread can be found in this document.

A more detailed description can be found by following this link:

Proper Tapping Technique.