After the thread is cut to size and the end chamfered, the tap is ready to be grooved in the milling machine. The tap is held between centers, and the grooves cut with a cutter especially adapted to the size and style of tap. While the grooves are best cut with a milling-machine cutter, it is possible to cut them in a planer or a shaper, using a tool of the proper shape. Great care must be used not to stretch the tap by heavy chips, or by using a dull tool.
Fig. 98. Threading Tool for Hand Taps.
Courtesy of Armstrong Brothers Tool Company, Chicago.
Fig. 99. Chaser Blade.
The grooves cut in taps are ordinarily termed flutes. When making taps for the market, it is usual to cut four flutes in all taps up to and including those 2 1/2 inches in diameter. But when taps are made in the shop where they are to be used, the number and shape of the grooves depend on the nature of the intended work. A tap that is to run through the work without any backing out can have a flute of a shape different from one that is to tap a deep hole in a piece of steel where it is necessary to reverse the motion of the tap every two or three revolutions to break the chip, and also to allow the lubricant to reach the cutting lips.
While all taps up to and including those 2 1/2 inches in diameter are usually given four straight flutes, spiral flutes are sometimes desirable, especially with small taps, for some classes of work. With spiral flutes, it is generally necessary to cut a smaller number than with straight flutes, and, as taps are not ground after hardening, there is no objection to giving an odd number of teeth, as in the case of a reamer. Three spiral flutes are often cut.
If a tap one inch in diameter, having four flutes of the regulation width, were used to tap tubing having thin walls, the tubing between the lands would have a tendency to close into the flutes of the tap and might break the tubing or the tap. In such a case there should be double the number of flutes, in order to provide enough lands to hold the tubing in shape. If the hole to be tapped has part of its circumference cut away, as shown in Fig. 100, more than four lands are necessary. For general machine-shop work, however, four flutes work well in hand taps up to and including those 2 1/2 inches in diameter. For larger sizes, some tool-makers advocate six flutes; others claim best results from taps having four flutes, regardless of size. The class of work and the stock used in the individual shop must determine this.
Fig. 100. Threading Hole Where Part of Wall ls Cut Away.