MACHINING OPERATIONS

MACHINING OPERATIONS

FACING

Facing is the machining of the end surfaces and shoulders of a work piece. In addition to squaring the ends of the work, facing provides a way to cut work to length accurately. Generally, only light cuts are required since the work will have been cut to approximate length or rough machined to the shoulder.

The work is placed between centres and driven by a dog. A right-hand side tool is used as shown. Take a light cut on the end of the work, feeding the tool (by hand cross feed) from the center toward the outside. Take one or two light cuts to remove enough stock to true the work then reverse the work piece, install the dog on the just finished end, and face the other end to make the work the proper length. To provide an accurate base from which to measure, hold another rule or straightedge on the end you faced first. Be sure there is no burr on the edge to keep the straightedge from bearing accurately on the finished end. Use a sharp scribe to mark off the dimension desired. Figure 9-27 shows the use of a turning tool in finishing a shouldered job having a fillet corner. Take a finish cut on the small diameter. Machine the fillet with a light cut. Then use the tool to face the work from the fillet to the outside of the work.

In facing large surfaces, lock the carriage in position, since only cross feed is required to traverse the tool across the work. With the compound rest set at 90° (parallel to the axis of the lathe), you can use the micrometer collar to feed the tool to the proper depth of cut.

TURNING

Turning is the machining of excess stock from the periphery of the work piece to reduce the diameter. In most lathe machining operations requiring removal of large amounts of stock, a series of roughing cuts is taken to remove most of the excess stock Then a finishing cut is taken to accurately "size" the work piece.

Rough Turning

When a great deal of stock is to be removed, you should take heavy cuts to complete the job in the least possible time. This is called rough turning. Select the proper tool for taking a heavy chip. The speed of the work and the amount of feed of the tool should be as great as the tool will stand.

When you take a roughing cut on steel, cast iron, or any other metal that has a scale on its surface, be sure to set the tool deep enough to get under the scale in the first cut. Unless you do, the scale on the metal will dull or break the point of the tool.

Rough machine the work to almost the finished size; then take careful measurements.

Bear in mind that the diameter of the work being turned is reduced by an amount equal to twice the depth of the cuts; thus, if you desire to reduce the diameter of a piece by 1/4 inch, you must remove 1/8 inch of metal from the surface.

The position of the tool for taking a heavy cut on large work. Set the tool so that if anything occurs during machining to change the position of the tool, it will not dig into the work, but rather will move in the direction of the arrow-away from the work

Finish Turning

When you have rough turned the work to within about 1/32 inch of the finished size, take a finishing cut. A fine feed, the proper lubricant, and, above all, a keen-edged tool is necessary to produce a smooth finish. Measure carefully to be sure you are machining the work to the proper dimension. Stop the lathe when you take measurements.

If you must finish the work to close tolerances, be sure the work is not hot when you take the finish cut. If you turn the work piece to exact size when it is hot, it will be undersize when it has cooled.

Perhaps the most difficult operation for a beginner in machine work is to make accurate measurements. So much depends on the accuracy of the work that you should make every effort to become proficient in the use of measuring instruments. You will develop a certain "feel" in the application of micrometers through experience alone; do not be discouraged if your first efforts do not produce perfect results. Practice taking micrometer measurements on pieces of known dimensions. You will acquire skill if you are persistent.

Turning to a Shoulder

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.

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

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 of large diameter.

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.

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. 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.