One of the most important machine tools in the metalworking industry is the lathe (Figure I-24).
A lathe is a device in which the work is rotated against a cutting tool.The shape of the workpiece is generated as the cutting tool is moved length wise and crosswise to the axis of the workpiece.
Figure I-25 shows a lathe and its most important parts. A lathe consists of the following major component groups: headstock, bed, carriage, tailstock, quick-change gearbox, and a base or pedestal. The headstock is fastened on the left side of the bed. It contains the spindle that drives the various work-holding devices. The spindle is supported by spindle bearings on each end. If they are sleeve-type bearings, a thrust bearing is also used to take up end play. Tapered roller Figure I-24 The engine lathe.
spindle bearings are often used on modern lathes. Spindle speed changes are also made in the headstock, either with belts or withgears. Figure I-26 shows a geared-type headstock. Speed changes are made in these lathes by shifting gears (Figure I-27) in much the same way as in a standard automobile transmission.
Most belt-driven lathes have a slow speed range when back gears are engaged. Figure I-28 shows a back-geared headstock. Smaller lathes have vee-belt drives and back gears. See Unit Five for operating details on these various drives. A feed reverse lever, also called a lead screw direction control, is located on the headstock. Its function is simply to control the direction of rotation of the leadscrew. This rotation determines the direction of feed and whether a thread cut on the lathe is left-hand or right-hand. The threading and feeding mechanisms of the lathe are also powered through the headstock.
The spindle is hollow to allow long, slender workpieces to pass through. The spindle end facing the tailstock is called the spindle nose. Spindle noses usually are one of three designs: a long taper key drive (Figure I-29), a camlock type (Figure I-30), or a threaded spindle nose (Figure I-31). Lathe chucks and other work-holding devices are fastened to and driven by the spindle nose. The hole in the spindle nose typicallyhas a standard Morse taper.
The bed (Figure I-32) is the foundation and backbone of a lathe. Its rigidity and alignment affect the accuracy of the parts machined on it. Therefore, lathe beds are constructed to withstand the stresses created by heavy machining cuts.On top of the bed are the ways,which usually
consist of two inverted vees and two flat bearing surfaces.
The ways of the lathes are accurately machined by grinding or by milling and hand scraping. Wear or damage to the ways will affect the accuracy of workpieces machined on them. A gear rack is fastened below the front way of the lathe. Gears that link the carriage handwheel to this rack make possible the lengthwise movement of the carriage by hand. The carriage is made up of the saddle and apron (Figure I-33). The apron is the part of the carriage facing the operator; it contains the gears and feed clutches that transmit motion from the feed rod or leadscrew to the carriage and cross slide. The saddle slides on the ways and supports the cross slide and compound rest. The cross slide is moved crosswise at M09_KIBB5087_09_SE_C09.QXD 6/3/09 4:01 AM Page 393 90 degrees to the axis of the lathe by manually turning the crossfeed screw handle or by engaging the cross-feed lever (also called the power feed lever or, on some lathes, the clutch knob), which is located on the apron for automatic feed. On some lathes a feed change lever (or plunger) on the apron directs power from the feed mechanism to either the longitudinal (lengthwise) travel of the carriage or to the cross slide. On other lathes, two separate levers or knobs transmit motion to the carriage and cross slide.A thread dial fastened to the apron (usually on the rightside) indicates the exact place to engage the half-nuts while
UNIT ONE THE ENGINE LATHE 393
The compound rest is mounted on the cross slide and can be swiveled to any angle horizontal with the lathe axis to produce bevels and tapers. The compound rest can be moved only manually by turning the compound rest feed screw handle. Cutting tools are fastened on a tool post that is located on the compound rest.
The tailstock (Figure I-34) is used to support one end of a workpiece for machining or to hold various cutting tools M09_KIBB5087_09_SE_C09.QXD 6/3/09 4:01 AM Page 394 394 SECTION I TURNING MACHINES such as drills, reamers, and taps. The tailstock slides on the ways and can be clamped in any position along the bed. The tailstock has a sliding spindle that is operated by a handwheel and locked in position with a spindle clamp lever. The spindle is bored to receive a standard Morse taper shank. The tailstock consists of an upper and lower unit and can be adjusted to make tapered workpieces by turning the adjusting screws in the base unit.
The quick-change gearbox (Figure I-35) is the link that transmits power between the spindle and the carriage. Different feeds can be selected by using the gear shift levers on the quick-change gearbox. Power is transmitted to the carriage through a feed rod or, as on smaller lathes, through the lead screw with a keyway in it. The index plate on the quick-change gearbox indicates the feed in thousandths of an inch or as threads per inch for each lever position. The base of the machine is used to level the lathe and to secure it to the floor. The motor of the lathe is usually mounted in the base. Figure I-36 shows how the lathe is measured.
ENGINE LATHE MAINTENANCE
The engine lathe, a precision machine tool, is perhaps the most abused of all shop equipment. With proper care the lathe will maintain its accuracy for many years, but its service life will be shortened severely if it is misused. Even small nicks or burrs on the ways can prevent the carriage or tailstock from seating properly. Fine chips, filings, or grindings combine with the oil to form an abrasive mixture that can score and wear sliding surfaces and bearings. Frequent cleaning of way surfaces is helpful, but do not use an air jet, because this will blow the abrasive sludge into the bearing surfaces. Use a brush to remove chips and wipe off with a cloth, then apply a thin film of oil. Nicks or burrs are often caused by dropping chucks and workpieces on the ways and by laying tools such as files across them. This should never be done. Wooden lathe boards are used for handling chucks and heavy work (Figure I-37). Larger boards are often used for tool trays placed on an
unused portion of the lathe way (Figure I-38). Occasionally, it is advisable to clean the lead screw. To do this, loop a piece of string behind the lead screw and hold each end of the string (Figure I-39). With the machine on and the lead screw turning, draw the string along the threads. Never hold the string by wrapping it around your fingers; if the string grabs and begins winding on the lead screw, let it go. Lathes should be completely lubricated daily or before using. Oil cups or holes should be given a few drops of oil (too much just runs out). Apron and headstock reservoirs should be checked for oil level. If they are low, use an oil recommended by the manufacturer, or its equivalent.
Before operating any lathe, wipe the way surfaces, even if they look clean, as gritty dust can settle on them when the machine has been idle for a few hours. Then, give the wiped ways a thin film of way oil, which is specially compounded to