As the name implies, machine taps are intended for screw machines, tapping machines, and lathes. They are held in chucks or collets by their-shanks, and are supported firmly. Consequently the lands may be narrower than those of hand tap3 to make them offer less surface to the work, thereby reducing the amount of frictional resistance. Also, they may be relieved between the teeth, by filing with a sharp-cornered three-square file, commencing at the heel of the tooth and filing nearly to the cutting edge. It is not good practice to relieve the teeth very much, because chips may be drawn between the work and the lands when backing out of the work. When taps are to be used in an automatic tapping machine without reverse motion, the shanks are left long as shown in Fig. 107, in order that the nuts may pass over the thread and on to the shank. When this is full, the tap is taken from the machine and the nuts removed. This can be readily done, as they will pass over the end of the shank.
If a tap is to be used on nuts whose holes are punched to size, much better results are obtained by using a tap with five flutes, Fig. 108, instead of four. The uneven number of cutting edges reduces the likelihood of an imperfectly tapped hole, while the extra land furnishes additional support.
Fig. 108. Tap with Five Flutes.