Up to this point, you have studied the preliminary steps leading to the performance of machine work in the lathe. You have learned how to mount the work and the tool and which tools are used for various purposes. Now, you need to consider how to use the proper tools in combination with the lathe to perform various machining operations.

FACING

Facing is the machining of the end surfaces and shoulders of a workpiece. 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.

Figure 9-26 shows the facing of a cylindrical piece. The work is placed between centers 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 crossfeed) from the center toward the outside. Take one or two light cuts to remove enough stock to true the work Then reverse the workpiece, 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 crossfeed 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 workpiece 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 workpiece.

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.

Figure 9-28 shows the position of the tool for taking a heavy cut on large work. Set the tool so that if anything

Figure 9-28.Position of the tool for a heavy cut.

Figure 9-29.Machining to a shoulder.

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 are 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 workpiece 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.