A boring tool holder suitable for holes of from 2 to 4 or 5 inches is shown in [Fig. 1118], in which a represents a round bar shaped at the end b to fit into the tool post of the slide rest, and having a groove across the diameter of the end c d to receive a short tool. The slot and tool may be either square or V-shaped, the tool being locked by a wedge. It is obvious that instead of shaping the end b as shown, the bar may be held (if the slide-rest head is provided with a clamp instead of a tool post) by two diametrically opposite flat faces.
Fig. 1119.
Fig. 1120.
For holes of a greater diameter a holder such as shown in [Fig. 1119] should be used, the body being a square bar, and the tool being held in the box a a by two set screws b. For holes of small diameter, as, say, less than 11⁄2 inches, a tool holder is especially desirable, because when a boring tool is forged out of a piece of tool steel, its length is determined, and in order to have tools suitable for various depths of hole a number of tools of varying lengths are requisite. Suppose, for example, that a piece of steel be forged into a boring tool suitable for a hole of an inch diameter, and 4 inches deep, then the steel must be forged round for a distance of at least 4 inches from the cutting end, and if such a tool were applied to a hole, say, two inches deep, the cutting edge would stand out from the tool post at least two inches more than is necessary, which would cause the employment of a tool weaker than necessary for the work. To enable the use of one tool for various depths of work, and yet hold it in each case as close to the tool post as the work depth admits, tool-clamping devices, such as in [Fig. 1120] (which are extracted from The American Machinist), are employed. 1 and 2 are pieces of steel fitting in the tool post and clamping the tool, which for very small holes is made of octagon or round forged steel. The tool may be passed to any required distance through the clamp, so as to project only to the amount necessary for the particular depth of hole requiring to be bored. These clamping pieces 1 and 2 should bed upon the tool fairly along their full length; or, what is better, they may bed the firmest at their extremities, which will insure that the tool is gripped firmly as near to the cutting edge as possible.
