Threadcutting: different ways to
tap screw thread explained
Threadcutting is a method that is being used to create screw thread on and in metal products. This process cuts metal, which creates a spiral shaped furrow. This is the screw thread. Threadcutting is being used in production centers, maintenance and installation frequently. For instance, the screw thread is applied to a bolt and in a nut, which makes these connectable. You can make as well internal as external screw thread with threadcutting tools. For making internal screw thread (in a hole), tapping thread is the most common method. These taps can be hand taps as well as machine taps.
Hand taps always come in a set of three, where the first tap is being used to gradually cut (the starter tap), the second tap cuts deeper (the middle tap) and the third tap cuts the thread completely (the finishing tap). The first tap is the most conical one, and per tap it gets less conical. The fact that you need three taps instead of one, is because otherwise too much strength would be needed to cut the screw thread in one time, which could cause the tap to break and it would not benefit the quality of the screw thread. Hand taps are mostly used for maintenance work or in places where it is impossible to work with a machine tap. Usually a tap wrench is being used to twist a hand tap in the material. This is being placed around the square part of the tap, whereupon twisting the tap wrench with two hands makes the tap cut itself in the material. Besides an adjustable tap wrench, a T-handle tap wrench with ratchet can also be used for this purpose. This solution is usually picked when there is not enough room to work with an adjustable tap wrench. The T-handle tap wrench only needs one hand to be operated.
Machine taps don’t come in a set, because there is only one tap needed in this case. Machine taps can be categorized in two categories, namely taps for through holes and taps for bottoming holes. The machine taps for bottoming holes have spiralized flutes, so the chip is transported out of the hole. These sorts of threadcutters are available in small sizes (with strengthened shank) and large sizes (with through shank), in HSS (High Speed Steel) Fully Ground and in HSS M35 5% Cobalt. The high speed steel is very suitable for cutting tools because of its high resistance to heat where the hardness is preserved. The Cobalt machine taps are suitable for tapping in high alloy content steel types such as inox, the Fully Ground taps are for tapping in most common steel types. The machine taps are being placed in a special tapping machine or in a stationary drilling machine, as long as these can turn as well clockwise as counterclockwise. The square part of the machine tap is being used to transfer the rotating strength to the tap.
To cut external screw thread, a round die or hexagonal die is being used. To use a round die, the die is being placed in a die stock. By twisting the die stock, it cuts away the material and a precise screw thread is being generated in one go. The hexagonal die is twisted on screw thread with a ring spanner or socket wrench.