Hand Tapping

The cutting of a thread with a tap is not a difficult operation but requires care in the manipulation. The tap does not need to be forced into the work, since the thread will draw it forward. The tapering of the tap has a two-fold effect. No one thread does all of the work in the removal of the metal; each succeeding thread removes a small amount until the full thread has entered the hole. The second effect is that, as in the case of a reamer, the tap is easily entered and started. Care must always be exercised at this point of the work. The taper of the tap allows it to easily enter the hole and also makes it possible for it to enter at an angle. If it is started in the latter condition, the thread will not be at right angles to the hole. The degree of care needed in the starting of the tap depends. upon the job that is to be done. In the case of tapping a nut, it will usually be quite sufficient to set the tap by the eye. In finer classes of work, however, the tap should be set with a square. Start the tap into the hole and place a square on the surface beside it in two positions at right angles to each other and see that the tap stands parallel to the vertical blade.

Starting the Tap

When holes have been drilled that are to be tapped, a good way of setting the tap is to put a center in the drill spindle. Put the tap into the hole and bring this center down into the center hole in the head of the tap; this will steady the latter while it is being started.

In using the tap, it is well to work it back and forth. This allows the chips to work clear of the cutting edges, and the oil to cover them. In case of heavy work, it is possible to drive the tap with the drill spindle, but when thus driving a tap in a machine, the backing up is impossible.

Use of Bottoming Tap

Sometimes a thread is to be cut down to the bottom of a hole that does not pass entirely through the metal. In this case the bottoming tap is used. This is a tap that is not tapered at the entering end. The method of working is to first cut the thread as far as possible with the plug tap and then use the bottoming tap, which will enter easily and can be driven to the bottom.

Machine Tapping

Machine tapping is best done by using a frictional tapholder, that is, one in which the friction is enough to cut the threads, but which will slip when the tap strikes the bottom of the hole. This will insure the hole being tapped to the bottom and avoid all danger of breaking the tap. To withdraw the tap, the machine is reversed, usually at a higher speed than used in tapping.

Lubrication

When tapping wrought iron and steel, a plentiful supply of lard oil should be used. On brass the use of oil is unnecessary.