If, on the other hand, the tool be below the horizontal centre, as in [Fig. 1123], the vertical spring will be in a direction to increase the amount of the cut, and thus offset the tapering effect of the increased tool spring due to the wear of the tool. Furthermore, the shaving will be easier bent if the tool be below than if above the horizontal centre, because the metal will be less supported by the metal behind it. It is always desirable therefore to have the cutting edge of a boring tool used on small work below rather than above the horizontal centre of the work. On large work, however, as say, having a bore of 6 inches and over, the curve of the bore in the length of the circumference affected by the cut or bending of the cut is so small, that the height of the tool is of less consequence.
To enable the use of a stout-bodied boring tool, while keeping its cutting edge below the centre, the top face of the tool may be depressed, as shown in [Fig. 1123].
Fig. 1124.
Fig. 1125.
An excellent attachment for boring parallel holes is shown in [Figs. 1124] and [1125], in which there is fixed to the cross slide a the bracket b, which is bored to receive a number of bushes c, whose bores are made to suit varying diameters of boring-bars or reamers d. The hub of the bracket is split on one side to enable it to be closed (by the bolt e) upon the bush c and grip it firmly, the bush also being split at f. The bracket b is provided with a taper pin g, which brings it in position upon the slide so that the bushes c are true with the line of lathe centres. It is also provided with the screws h, which lock it firmly to the cross slide and prevent any spring or movement from play or looseness.
When the bracket is adjusted and the bar fastened up (by screw e), the lathe-carriage feeds the boring tool to the cut in the usual manner. Now suppose that, as shown in our illustrations, a pulley p requires to be bored, and the boring tool or reamer may be set to have its cutting end stand out just as far as the length of the hub requires, and no farther, so that the bar will be held and supported as close to the pulley hub as is possible from the nature of the job. There need not be a separate bush for every size of reamer, because the bodies of several size bars may fit to one size of bush, especially if the set of reamers for every size of bush be made with its smallest size equal to the bore of the bush; because in that case the whole of the set may be adjusted to bore any required depth of hole by sliding the reamer through the bush to the required distance. If there are a number of lathes in a shop, each lathe may have its own bracket b, all these brackets being bored to receive the same bushes, and therefore the same boring-bars or reamers.