Taps

Taps are manufactured from high carbon or alloy steel and are fluted to provide cutting edges. They are used for to cut threads on the inside of a hole. Markings on the shank include type of thread, size and the number of tpi. Hand taps are usually available in sets of three taps for each diameter and thread series. They are usually identical in diameter and cross section with the difference only in the amount of taper.

The following are the types of taps.

Taper Tap: It is used to begin the tapping process, because it is tapered from the tip to the sixth thread. The tip diameter is equal to the root diameter of the threads. This tap cuts the full thread in a through hole.

FIGURE

Plug Tap: It has a chamfer length of 3-5 threads and is used when one end of the hole is closed but a full thread is not required all the way to the bottom of the hole.

FIGURE

Bottoming Tap: It is not tapered and is used to cut full threads to the bottom of a blind hole (A hole that does not go completely through the metal).

FIGURE

The following is the procedure for cutting internal threads.

1. Drill a hole that is slightly larger to prevent the tap from binding and breaking. This hole should be the core diameter of the thread.
2. Choose a suitable drill for the task by referring an Engineers pocket book or by measuring the core diameter and taking the nearest drill which is slightly larger than the core diameter or by selecting a drill which can pass through a nut of the correct size or based on the drill size that is specified on the shank of the tap.
3. Enter the tap into the hole perpendicular to the face.
4. Make sure the tap sits in the hole square by using a square.
5. Continue turning in the cutting direction for half a turn and then reverse the direction for a quarter turn, until the tap protrudes from the other side of the hole. This process will break the cutting the chips thus preventing tap strain.
6. Change to the plug tap as required and continue until the thread has been properly cut.
7. Lubricate the work using lubricants such as cutting oil, soapy water, turpentine or paraffin.

It is important to follow proper procedure, as poor engineering practices such as using an incorrect tap size, smaller hole, forcing a tap too far down the blind hole, turning tap clockwise throughout and lack of lubrication.