Thread Cutting

Thread Cutting

Thread Cutting - Taps are used to cut internal threads. A tap is a cylindrical bar of steel with threads formed around it and grooves or flutes running lengthwise in it, intersecting with the threads to form cutting edges. The cutting of an internal thread by means of a multiple point tool is called thread tapping, and the tool is called a tap.

In order to cut an internal thread by tapping, a hole of diameter slightly larger than the minor diameter of the thread must already exist, made by drilling, boring, or casting. Solid taps, such as are shown in Fig, usually are used for small diameters.

These are similar to threaded bolts with four flutes to provide cutting edges. Such taps are made of either carbon or high speed tool steel, and the flutes can be either straight helical or spiral pointed.

Spiral Flute Tap

Spiral Point Straight Flute Tap

With respect to the tapping method, taps are divided into machine taps, used in tapping screw threads in machine tools, and hand taps used in bench work to tap a hole by hand with a tap wrench.

Thread is tapped by hand in two or three passes depending upon the size of the thread and corresponding to the number of taps in the set. Machine taps usually cut the thread in one pass with a single tap. All machine tapping is done, as a rule, in one pass.

Two taps are used only for long threads or in blind holes. After being tapped in a machine, precise screw threads may be additionally tapped by hand or in a machine by a master, or sizing, tap. Machine taps are used for cutting threads in either through or blind holes.

This work may be performed in tapping machines, drill presses, turret lathes, automatics and semiautomatics. Blind holes are drilled somewhat deeper (approximately 3 to 4 thread pitches) than the required depth of the thread. This makes tapping easier, ensures a full thread form to the specified depth, and prevents tap breakage.

A condition for successful machine tapping (except when collapsible taps are employed) is rapid reversal of the tap when it has cut the thread to the specified depth. In tapping thread in a blind hole, the machine tool must have some device for disengaging working travel and reversing the tap when it reaches its final position.

Tap feed and rotation may also be disengaged by means of a safety tap chuck. Spindle reversal is required even when a safety tap chuck is used except in special nut tapping machine. Various types of tap chucks are used:

(1) solid chucks,

(2) floating chucks,

(3) safety chucks operated by a stop, and

(4) torque operated safety chucks.

Taps operate under very severe conditions, both because of the severe friction involved and the difficulty of chip removal. Also, taps are relatively fragile. Spiral fluted taps, illustrated in, are very helpful in providing better removal of chips from a hole particularly in tapping materials, which produce long, curling chips.

They also are helpful in tapping holes where the cutting action is interrupted by slots or keyways. In tapping through holes or blind holes that are not tapped to the bottom, taps having a spiral point; as illustrated in, are very useful. These project the chips ahead of the tap so that they do not interfere with the cutting action and the flow of cutting fluid into the hole.

Care must be exercised in using taps, particularly in tapping by hand. Proper cutting fluid should be used and the tap should be reversed a partial turn after each two or three forward turns to assist in clearing the chips.

Solid taps also are used in tapping operations in machine tools, such as lathes, drill presses, and special tapping machines. In tapping on a drill press, a tapping attachment frequently is used. These devices rotate the tap slowly when the drill press spindle is fed downward against the work.

When the tapping is completed and the spindle raised, the tap is automatically driven in the reverse direction at a higher speed to reduce the time required to back the tap out of the hole. Some modern machine tools provide for extremely fast spindle reverse for backing taps out of holes.

 When solid taps are used on a screw machine or turret lathe, a special holder is employed in which a pin prevents the tap from turning while it is being fed into the work. As the tap reaches the end of the hole, it pulls the pin away from its stop so that the tap is free to rotate with the work. The rotation of the work is then reversed and the pin again prevents the tap from turning while it is backed out of the hole.