For a tap to create a thread, it must have a chamfer. These are the tapered or incomplete threads at the front of the tap. The major diameter of the threads are ground to a smaller diameter at the front. This diameter, known as the point diameter, is generally slightly smaller than the pre-drilled hole size or tap-drill size.
When the tap enters the hole and begins to cut, each tooth in the chamfer gradually enlarges the thread in the part. Only the first full thread behind the chamfer produces the finished size of the thread. The teeth beyond the first full thread serve to guide and support the tap as it creates the desired complete threaded depth of the tapped hole.
There are different lengths of standard chamfers. The most common are taper, plug, and bottoming. The taper chamfer has 7 to 10 threads tapered, the plug chamfer has 3 to 5 threads tapered, and the bottoming chamfer has 1 to 2. Generally a taper chamfer, often referred to as a starter tap, is used for roughing or sometimes heat-treated material. Plug is the preferred chamfer for most tapping applications and are most common. Bottoming chamfers are used when there isn't enough depth of hole for the taper or plug.
Chamfer lengths are selected based upon the type of hole to be tapped. If the hole goes completely through the part or the drill depth is considerably deeper than the required thread depth, a taper or plug chamfer is used. A bottoming chamfer should only be used when the threads must come close to the bottom of the drilled hole and should be avoided whenever possible. A bottoming tap creates the greatest amount of tapping torque, requires slower speed (RPM), produces a rougher finish, and substantially reduces tool life.
In addition to the three most common chamfers, semi-bottoming and modified bottoming chamfers are found on specialty taps such as high performance taps and some cast iron taps used in the automotive industry. Semi-bottoming are usually 2 to 2-1/2 threads long, and modified bottoming range between 2-1/2 and 4 threads long depending upon the style of tap. The additional length is used to reduced chip load, add tool life in difficult to machine materials and to allow higher tapping speeds.
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