METAL CUTTING
Most internal threads produced today are made with taps. These taps are available in a variety of styles, each one designed to perform a specific type of tapping operation efficiently. Taps are used to cut internal threads in holes. The actual cutting process is called tapping and can be performed by hand or with a machine. A tap wrench or a T-handle tap wrench attached to the tap is used to provide driving torque while hand tapping. To obtain a greater accuracy in hand tapping, a hand tapper is used. This fixture acts as a guide for the tap to ensure that it stays in alignment and cuts concentric threads.
Holes can also be tapped in a drill press that has a spindle reverse switch, which is often foot operated for convenience.
Drill presses without reversing switches can be used material is usually selected by the designer, but the machinist can often control the percentage of thread produced and the depth of the thread. The percentage of thread produced is dependent on the diameter of the drilled hole.
TAPPING PROCEDURE, HAND TAPPING
Step 1: Determine the size of the thread to be tapped and select the tap.
Step 2: Select the proper tap drill with the aid of a tap drill chart. Choose a taper tap for hand tapping; or if a drill press or tapping machine is to be used for alignment, use a plug tap.
Step 3: Fasten the workpiece securely in a drill press vise. Drill the hole using the recommended coolant. Check the hole size.
Step 4: Countersink the hole entrance to a diameter slightly larger than the major diameter of the threads. This allows the tap to be started more easily, and it protects the start of the threads from damage.
Step 5: Mount the workpiece in a bench vise so that the hole is in a vertical position.
Step 6: Tighten the tap in the tap wrench.
Step 7: Cup your hand over the center of the wrench and place the tap in the hole in a vertical position. Start the tap by turning two or three turns in a clockwise direction for a right-hand thread. At the same time, keep a steady pressure downward on the tap. When the tap is started, it may be turned.
Step 8: After the tap is started for several turns, remove the tap wrench without disturbing the tap. Place the blade of a square against the solid shank of the tap to check for squareness. Check from two positions 90 degrees apart. If the tap is not square with the work, it will ruin the thread and possibly break in the hole if you continue tapping. Back the tap out of the hole and restart
Step 9: Use the correct cutting oil on the tap when cutting threads.
Step 10: Turn the tap clockwise one-quarter to one-half turn and then turn it back three-quarters of a turn to break the chip. Do this with a steady motion to avoid breaking the tap.
Step 11: When tapping a blind hole, use the taps in the order starting, plug, and then bottoming. Remove the chips from the hole before using the bottoming tap, and be careful not to hit the bottom of the hole with the tap.
Step 12: A 60-degree point center chucked in a drill press to align a tap squarely with the previously drilled hole. Only very slight follow-up pressure should be applied to the tap. Too much downward pressure will cut a loose, oversize thread.
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