To obtain good results on the lathe, a knowledge of the variable factors of speeds, feeds, cutting oils, and materials is necessary, as well as the mechanical application of the cutter bits to the workpiece. Cutting speeds, feeds, depth of cut, and cutting oils, if used, are different for each type of material and each operation; failure to keep these factors within proper limits may cause damage to the lathe or lathe tools, and can result in spoiling the workpiece. The methods of operation described in this lesson are general in nature. With experience, the machinist will be able to vary certain procedures with success for individual operations.
Setting Up the Lathe
Before starting a lathe machining operation, always ensure that the machine is set up for the job that is to be accomplished. If the workpiece is to be mounted between centers, check the alignment of the headstock center with the tailstock center (live or deadcenter) and make the necessary changes as needed. Ensure that the toolholder and the cutter bit are set at the proper height and angle. Check the workholding accessory to ensure that the workpiece is held securely. Use the center rest or follower rest to support long workpieces.
The first step in preparing the centers is to see that they are accurately mounted in the headstock and tailstock spindles. The centers and the tapered holes in which they are fitted must be perfectly clean. Chips and dirt left on the contact surfaces will impair accuracy by preventing a perfect fit of the bearing surfaces. Be sure that there are no burrs in the spindle hole. If burrs are found, they must be removed by carefully scraping or reaming the surface with a Morse taper reamer. Burrs will produce the same inaccuracies as chips or dirt.
Center points must be accurately finished to an included angle of 60°. Check the angle of the center with the use of a center gage. If the test shows that the point is not perfect, true the point in the lathe by taking a cut over the point with the compound rest set at 30°. The hardened tailstock center must be annealed before it can be machined or set up in the lathe and ground with the use of a toolpost grinder.