Taps and tapping operation
Hand taps are used in the engine room to repair damaged internal threads and to make new ones after drilling and reaming operations. Taps are identified by the threads they will machine and form, for example M12 x1.5, where M stands for Metric threads (the type of threads used), 12 means 12 millimeter major diameter, and 1.5 stands for the pitch, which in this case is 1.5 mm. Also they are identified as number 1, 2, and 3 or also as Taper, second, and plug or bottoming.
After a hole has been drilled, the number one or taper tap is inserted in the hole, with a tap holder. In taper tap the last five or six threads are ground out to enable the entry of the tap in the hole easily. Care must be taken that the tap is parallel to the hole, this first step is crucial as it will determine whether at the end you get a correctly threaded hole or if you have to do it again after re-drilling (at a larger size). Tapping must be done in stages, a quarter turn clock wise tapping motion, must be followed by similar motion in anti-clock wise direction, to allow for the cut metal bits to come out. Lubrication with water or cutting oil will make the machining easy and protect the taps.
After tapping with the first tap has been done, the hole is cleaned with compressed air, and then the second tap in inserted and tapping is done in the same way. Frequent to and fro movements must be done, any hard obstruction or too much force required must be avoided and the cause investigated. After the tapping has been done a similar sized bolt must be inserted into the hole and the quality of the threads checked.
In holes which are blind, the second tap would not create threads at the end, for this purpose the third tap or the plug or the bottoming one must be used.