INSTRUCTIONS TO LEARN HOW TO USE ALATHEThe lathe is a machine tool used principally for shaping pieces of metal (and sometimeswood or other materials) by causing the workpiece to be held and rotated by the lathewhile a tool bit is advanced into the work causing the cutting action. The basic lathe thatwas designed to cut cylindrical metal stock has been developed further to produce screwthreads, tapered work, drilled holes, knurled surfaces, and crankshafts. Modern lathesoffer a variety of rotating speeds and a means to manually and automatically move thecutting tool into the workpiece. Machinists and maintenance shop personnel must bethoroughly familiar with the lathe and its operations to accomplish the repair andfabrication of needed parts.TYPES OF LATHESLathes can be divided into three types for easy identification: engine lathe, turret lathe,and special purpose lathes. Some smaller ones are bench mounted and semi-portable. Thelarger lathes are floor mounted and may require special transportation if they must bemoved. Field and maintenance shops generally use a lathe that can be adapted to manyoperations and that is not too large to be moved from one work site to another. Theengine lathe (Figure 7-1) is ideally suited for this purpose. A trained operator canaccomplish more machining jobs with the engine lathe than with any other machine tool.Turret lathes and special purpose lathes are usually used in production or job shops formass production or specialized parts, while basic engine lathes are usually used for anytype of lathe work. Further reference to lathes in this chapter will be about the variousengine lathes.
ENGINE LATHESSizesThe size of an engine lathe is determined by the largest piece of stock that can bemachined. Before machining a workpiece, the following measurements must beconsidered: the diameter of the work that will swing over the bed and the length betweenlathe centers (Figure 7-1).CategoriesSlight differences in the various engine lathes make it easy to group them into threecategories: lightweight bench engine lathes, precision tool room lathes, and gap lathes,which are also known as extension-type lathes. These lathe categories are shown inFigure 7-2 Different manufacturers may use different lathe categories.
LightweightLightweight bench engine lathes are generally small lathes with a swing of 10 inches orless, mounted to a bench or table top. These lathes can accomplish most machining jobs,but may be limited due to the size of the material that can be turned.PrecisionPrecision tool room lathes are also known as standard manufacturing lathes and are usedfor all lathe operations, such as turning, boring, drilling, reaming, producing screwthreads, taper turning, knurling, and radius forming, and can be adapted for specialmilling operations with the appropriate fixture. This type of lathe can handle workpiecesup to 25 inches in diameter and up to 200 inches long. However, the general size is abouta 15-inch swing with 36 to 48 inches between centers. Many tool room lathes are used forspecial tool and die production due to the high accuracy of the machine.GAP OR EXTENSION-TYPE LATHESGap or extension-type lathes are similar to toolroom lathes except that gap lathes can beadjusted to machine larger diameter and longer workpieces The operator can increase theswing by moving the bed a distance from the headstock, which is usually one or two feet.By sliding the bed away from the headstock, the gap lathe can be used to turn very longworkpieces between centers.
LATHE COMPONENTSEngine lathes all have the same general functional parts, even though the specific locationor shape of a certain part may differ from one manufacturer The bed is the foundation ofthe working parts of the lathe to another (Figure 7-3).The main feature of its construction are the ways which are formed on its upper surfaceand run the full length of the bed.Ways provide the means for holding the tailstock and carriage, which slide along theways, in alignment with the permanently attached headstockThe headstock is located on the operator's left end of the lathe bed. It contains the mainspindle and oil reservoir and the gearing mechanism for obtaining various spindle speedsand for transmitting power to the feeding and threading mechanism. The headstockmechanism is driven by an electric motor connected either to a belt or pulley system or toa geared system. The main spindle is mounted on bearings in the headstock and ishardened and specially ground to fit different lathe holding devices. The spindle has ahole through its entire length to accommodate long workpieces. The hole in the nose ofthe spindle usually has a standard Morse taper which varies with the size of the lathe.Centers, collets, drill chucks, tapered shank drills and reamers may be inserted into thespindle. Chucks, drive plates, and faceplates may be screwed onto the spindle or clampedonto the spindle nose.The tailstock is located on the opposite end of the lathe from the headstock. It supportsone end of the work when machining between centers, supports long pieces held in the
chuck, and holds various forms of cutting tools, such as drills, reamers, and taps. Thetailstock is mounted on the ways and is designed to be clamped at any point along theways. It has a sliding spindle that is operated by a hand wheel and clamped in position bymeans of a spindle clamp. The tailstock may be adjusted laterally (toward or away fromthe operator) by adjusting screws. It should be unclamped from the ways before anylateral adjustments are made, as this will allow the tailstock to be moved freely andprevent damage to the lateral adjustment screws.The carriage includes the apron, saddle, compound rest, cross slide, tool post, and thecutting tool. It sits across the lathe ways and in front of the lathe bed. The function of thecarriage is to carry and move the cutting tool. It can be moved by hand or by power andcan be clamped into position with a locking nut. The saddle carries the cross slide and thecompound rest. The cross slide is mounted on the dovetail ways on the top of the saddleand is moved back and forth at 90 to the axis of the lathe by the cross slide lead screw.The lead screw can be hand or power activated. A feed reversing lever, located on thecarriage or headstock, can be used to cause the carriage and the cross slide to reverse thedirection of travel. The compound rest is mounted on the cross slide and can be swiveledand clamped at any angle in a horizontal plane. The compound rest is used extensively incutting steep tapers and angles for lathe centers. The cutting tool and tool holder aresecured in the tool post which is mounted directly to the compound rest. The aproncontains the gears and feed clutches which transmit motion from the feed rod or leadscrew to the carriage and cross slide.CARE AND MAINTENANCE OF LATHESLathes are highly accurate machine tools designed to operate around the clock if properlyoperated and maintained. Lathes must be lubricated and checked for adjustment beforeoperation. Improper lubrication or loose nuts and bolts can cause excessive wear anddangerous operating conditions.The lathe ways are precision ground surfaces and must not be used as tables for othertools and should be kept clean of grit and dirt. The lead screw and gears should bechecked frequently for any metal chips that could be lodged in the gearing mechanisms.Check each lathe prior to operation for any missing parts or broken shear pins. Refer tothe operator's instructions before attempting to lift any lathe. Newly installed lathes orlathes that are transported in mobile vehicles should be properly leveled before anyoperation to prevent vibration and wobble. Any lathes that are transported out of a normalshop environment should be protected from dust, excessive heat, and very coldconditions. Change the lubricant frequently if working in dusty conditions. In hotworking areas, use care to avoid overheating the motor or damaging any seals. Operatethe lathe at slower speeds than normal when working in cold environments.SAFETYAll lathe operators must be constantly aware of the safety hazards that are associated withusing the lathe and must know all safety precautions to avoid accidents and injuries.
Carelessness and ignorance are two great menaces to personal safety. Other hazards canbe mechanically related to working with the lathe, such as proper machine maintenanceand setup. Some important safety precautions to follow when using lathes are: Correct dress is important, remove rings and watches, roll sleeves above elbows.Always stop the lathe before making adjustments.Do not change spindle speeds until the lathe comes to a complete stop.Handle sharp cutters, centers, and drills with care.Remove chuck keys and wrenches before operatingAlways wear protective eye protection.Handle heavy chucks with care and protect the lathe ways with a block of woodwhen installing a chuck.Know where the emergency stop is before operating the lathe.Use pliers or a brush to remove chips and swarf, never your hands.Never lean on the lathe.Never lay tools directly on the lathe ways. If a separate table is not available, usea wide board with a cleat on each side to lay on the ways.Keep tools overhang as short as possible.Never attempt to measure work while it is turning.Never file lathe work unless the file has a handle.File left-handed if possible.Protect the lathe ways when grinding or filing.Use two hands when sanding the workpiece. Do not wrap sand paper or emorycloth around the workpiece.TOOLS AND EQUIPMENTGENERAL PURPOSE CUTTING TOOLSThe lathe cutting tool or tool bit must be made of the correct material and ground to thecorrect angles to machine a workpiece efficiently. The most common tool bit is thegeneral all-purpose bit made of high-speed steel. These tool bits are generallyinexpensive, easy to grind on a bench or pedestal grinder, take lots of abuse and wear,and are strong enough for all-around repair and fabrication. High-speed steel tool bits canhandle the high heat that is generated during cutting and are not changed after cooling.These tool bits are used for turning, facing, boring and other lathe operations. Tool bitsmade from special materials such as carbides, ceramics, diamonds, cast alloys are able tomachine workpieces at very high speeds but are brittle and expensive for normal lathework. High-speed steel tool bits are available in many shapes and sizes to accommodateany lathe operation.SINGLE POINT TOOL BITSSingle point tool bits can be one end of a high-speed steel tool bit or one edge of acarbide or ceramic cutting tool or insert. Basically, a single point cutter bit is a tool thathas only one cutting action proceeding at a time. A machinist or machine operator should
know the various terms applied to the single point tool bit to properly identify and grinddifferent tool bits (Figure 7-4). The shank is the main body of the tool bit.The nose is the part of the tool bit which is shaped to a point and forms the cornerbetween the side cutting edge and the end cutting edge. The nose radius is therounded end of the tool bit.The face is the top surface of the tool bit upon which the chips slide as theyseparate from the work piece.The side or flank of the tool bit is the surface just below and adjacent to thecutting edge.The cutting edge is the part of the tool bit that actually cuts into the workpiece,located behind the nose and adjacent to the side and face.The base is the bottom surface of the tool bit, which usually is ground flat duringtool bit manufacturing.The end of the tool bit is the near-vertical surface which, with the side of the bit,forms the profile of the bit. The end is the trailing surface of the tool bit whencutting.The heel is the portion of the tool bit base immediately below and supporting theface.Angles of Tool BitsThe successful operation of the lathe and the quality of work that may be achieveddepend largely on the angles that form the cutting edge of the tool bit (Figure 7-4). Mosttools are hand ground to the desired shape on a bench or pedestal grinder. The cuttingtool geometry for the rake and relief angles must be properly ground, but the overallshape of the tool bit is determined by the preference of the machinist or machineoperator. Lathe tool bit shapes can be pointed, rounded, squared off, or irregular in shape
and still cut quite well as long as the tool bit angles are properly ground for the type ofmaterial being machined. The angles are the side and back rake angles, the side and endcutting edge angles, and the side and end relief angles. Other angles to be considered arethe radius on the end of the tool bit and the angle of the tool holder. After knowing howthe angles affect the cutting action, some recommended cutting tool shapes can beconsidered.Rake angle pertains to the top surface of the tool bit. There are two types of rake angles,the side and back rake angles (Figure 7-4). The rake angle can be positive, negative, orhave no rake angle at all. The tool holder can have an angle, known as the tool holderangle, which averages about 15 , depending on the model of tool holder selected. Thetool holder angle combines with the back rake angle to provide clearance for the heel ofthe tool bit from the workpiece and to facilitate chip removal. The side rake angle ismeasured back from the cutting edge and can be a positive rake angle or have no rake atall.Rake angles cannot be too great or the cutting edge will lose strength to support thecutting action. The side rake angle determines the type and size of chip produced duringthe cutting action and the direction that the chip travels when leaving the cutting tool.Chip breakers can be included in the side rake angle to ensure that the chips break up anddo not become a safety hazard.Side and relief angles, or clearance angles, are the angles formed behind and beneath thecutting ed
LATHE COMPONENTS Engine lathes all have the same general functional parts, even though the specific location or shape of a certain part may differ from one manufacturer The bed is the foundation of the working parts of the lathe to another (Figure 7-3). The main feature of its construction are the ways which are formed on its upper surface and run the full length of the bed. Ways provide the .