Fig. 348.
In hand taps the position of the square at the head of the tap with relation to the cutting-edges is of consequence; thus, in [Fig. 346], there being a cutting-edge a opposite to the handle, any undue pressure on that end of the handle would cause a to cut too freely and the tap to enlarge the hole; whereas in [Fig. 347] this tendency would be greatly removed, because the cutting-edges are not in line with the handle. In a three-flute tap it makes but little difference what are the relative positions of the square to the flutes, as will be seen in [Fig. 348], where one handle of the wrench comes in the most favorable and the other in the most unfavorable position. Taps for use by hand and not intended to pass through the work are sometimes made with the shank and the square end which receive the wrench of enlarged diameter. This is done to avoid the twisting of the shank which sometimes occurs when the tap is employed in deep holes, giving it much strain, and also to avoid as much as possible the wearing and twisting of the square which occurs, because in the course of time the square holes in solid wrenches enlarge from wear, and the larger the square the less the wear under a given amount of strain.
Fig. 349.
Brass finishers frequently form the heads of their taps as in [Fig. 349], using a wrench with a slot in it that is longer than the flat of the tap head.
The thickness of the flat head at a is made equal for all the taps intended to be used with the same wrench. By this means one wrench may be used for many different diameters of taps.
