Fig. 5. Tools for Boring and Reaming Holes
Boring Operations.—There are several methods of machining holes when using a vertical boring mill. Ordinarily, small holes are cored in castings and it is simply necessary to finish the rough surface to the required diameter. Some of the tools used for boring and finishing comparatively small holes are shown in [Fig. 5]. Sketch A shows a boring tool consisting of a cutter c inserted in a shank, which, in turn, is held in the tool slide, or in a turret attached to the tool slide. With a tool of this type, a hole is bored by taking one or more cuts down through it. The tool shown at B is a four-lipped drill which is used for drilling cored holes preparatory to finishing by a cutter or reamer. This drill would probably finish a hole to within about 1/32 inch of the finish diameter, thus leaving a small amount of metal for the reamer to remove. The tool illustrated at C has a double-ended flat cutter c, which cuts on both sides. These cutters are often made in sets for boring duplicate parts. Ordinarily, there are two cutters in a set, one being used for roughing and the other for finishing. The cutter passes through a rectangular slot in the bar and this particular style is centrally located by shoulders s, and is held by a taper pin p. Some cutter bars have an extension end, or “pilot” as it is called, which passes through a close-fitting bushing in the table to steady the bar. Sketch D shows a finishing reamer. This tool takes a very light cut and is intended to finish holes that have been previously bored close to the required size. Sometimes a flat cutter C is used for roughing and a reamer for finishing. The reamer is especially desirable for interchangeable work, when all holes must have a smooth finish and be of the same diameter. When a reamer is held rigidly to a turret or toolslide, it is liable to produce a hole that is either tapering or larger than the reamer diameter. To prevent this, the reamer should be held in a “floating” holder which, by means of a slight adjustment, allows the reamer to align itself with the hole. There are several methods of securing this “floating” movement. (See “[Floating Reamer Holders].”)
Fig. 6. Boring with Regular Turning Tools
Large holes or interior cylindrical surfaces are bored by tools held in the regular tool-head. The tool is sometimes clamped in a horizontal position as shown at A, [Fig. 6], or a bent type is used as at B. Cast iron is usually finished by a broad flat tool as at C, the same as when turning exterior surfaces. Obviously a hole that is bored in this way must be large enough to admit the tool-block.