Machining to a shoulder is often done by locating the shoulder with a parting tool. Insert the parting tool about 1/32 inch from the shoulder line toward the small diameter end of the work Cut to a depth 1/32 inch larger than the small diameter of the work. Then machine the stock by taking heavy chips up to the shoulder. This procedure eliminates detailed measuring and speeds up production.

Figure 9-29 illustrates this method of shouldering. A parting tool has been used at P and the turning tool is taking a chip. It will be unnecessary to waste any time in taking measurements. You can devote your time to rough machining until the necessary stock is removed. Then you can take a finishing cut to accurate measurement.

Boring

Boring is the machining of holes or any interior cylindrical surface. The piece to be bored must have a drilled or cored hole, and the hole must be large enough to insert the tool. The boring process merely enlarges the hole to the desired size or shape. The advantage of boring is that a true round hole is obtained, and two or more holes of the same or different diameters may be bored at one setting, thus ensuring absolute alignment of the axis of the holes.

Work to be bored may be held in a chuck, bolted to the faceplate, or bolted to the carriage. Long pieces must be supported at the free end in a center rest. When the boring tool is fed into the hole of work being rotated on a chuck or faceplate, the process is called single point boring. It is the same as turning except that the cutting chip is taken from the inside. The cutting edge of the boring tool resembles that of a turning tool. Boring tools may be the solid forged type or the inserted cutter bit type.

When the work to be bored is clamped to the top of the carriage, a boring bar is held between centers and driven by a dog. The work is fed to the tool by the automatic longitudinal feed of the carriage. Three types of boring bars are shown in figure 9-30. Note the center holes at the ends to fit the lathe centers.

Figure 9-30, view A, shows a boring bar fitted with a fly cutter held by a headless setscrew. The other setscrew, bearing on the end of the cutter, is for adjusting the cutter to the work

Figure 9-30, view B, shows a boring bar fitted with a two-edged cutter held by a taper key. This is more of a finishing or sizing cutter, as it cuts on both sides and is used for production work.

The boring bar shown in figure 9-30, view C, is fitted with a cast-iron head to adapt it for boring work

Figure 9-30.Boring bars.

Figure 9-31.Tapers.

of large diameter. The head is fitted with a fly cutter similar to the one shown in view A of figure 9-30. The setscrew with the tapered point adjusts the cutter to the work

TAPERS

Although you will probably have little need to machine tapers, we have provided the following explanation for your basic knowledge.

A taper is the gradual decrease in the diameter of a piece of work toward one end. The amount of taper in any given length of work is found by subtracting the size of the small end from the size of the large end. Taper is usually expressed as the amount of taper per foot of length or taper per inch of length. We will take two examples. (See fig. 9-31.)

Example l.Find the taper per foot of a piece of work 2 inches long. The diameter of the small end is 1 inch; the diameter of the large end is 2 inches.

The amount of taper is 2 inches minus 1 inch, which equals 1 inch. The length of the taper is given as 2 inches. Therefore, the taper is 1 inch in 2 inches of length. In 12 inches of length the taper is 6 inches. (See fig. 9-31.) Example 2.Find the taper per foot of a piece 6 inches long. The diameter of the small end is 1 inch; the diameter of the large end is 2 inches. The amount of taper is the same as in example 1, that is, 1 inch. However, the length of this taper is 6 inches; hence the taper per foot is 1 inch times 12/6, which equals 2 inches per foot (fig. 9-31).