Fig. 369.
A second example of uniform tapping is shown in [Figs. 366], [367], and [368]. The piece, [Fig. 366], is to have its bore a tapped in line with the slot c, and the thread is to start at a certain point in its bore. In [Fig. 367] this piece is shown chucked on a plate d. f is a chuck having a lug e fitting into the slot (c, [Fig. 366]) of the work. This adjusts the work in one direction. The face d of the plate adjusts the vertical height of the work, and the alignment of the hole to the axis of the tap is secured in the construction of the chuck, as is shown in [Fig. 369]. A lug k is at a right angle to the face b of the chuck and stands in a line with lug e, as denoted by the dotted line g g, and as lug k fits into the slot g, [Fig. 367], the work will adjust itself true when bolted to the plate.
[Fig. 368] shows a method of tapping or hobbing four chasers (as for a bolt cutter), so that if the chasers are marked 1, 2, 3 and 4, as shown, any chaser of No. 1 will work with the others, although not tapped at the same operation. c is a chuck with four dies (a, b, c, d) placed between the chasers. By tightening the set-screws s, the dies and chasers are locked ready for the tapping. n is a hub to receive a guide-pin p, which is passed through to hold the chasers true while being set in the chuck, and it is withdrawn before the tapping commences; d e f are simply to take hold of when inserting and removing the dies. It is obvious that a chuck such as this used upon a plate, as in [Fig. 365], with the hob guided in the head h there shown, would tap each successive set of chasers alike as a set, and individually alike, provided, of course, that the hob guide or head h is at each setting placed the same distance from the face of the chuck, a condition that applies to all this class of work. In the case of work like chasers, where the tap or hob does not have much bearing to guide it in the work, a three-flute hob should be used for four chasers, or a four-flute hob for three chasers, which is necessary so that the hob may work steadily and tap all to the same diameter.
Chapter V.—FASTENING DEVICES.
Bolts are usually designated for size by their diameters measured at the cylindrical stem or body, and by their lengths measured from the inner side of the head to the end of the thread, so that if a nut be used, the length of the bolt, less the thickness of the nut and washer (if the latter be used), is the thickness of work the bolt will hold. If the work is tapped, and no nut is used, the full length of the bolt stem is taken as the length of the bolt.
A black bolt is one left as forged. A finished bolt has its body, and usually its head also, machine finished, but a finished bolt sometimes has a black head, the body only being turned.
A square-headed bolt usually has a square nut, but if the nut is in a situation difficult of access for the wrench, or where the head of the bolt is entirely out of sight (as secluded beneath a flange) the nut is often made hexagon. A machine-finished bolt usually has a machine-finished and hexagon nut. Square nuts are usually left black.