Unit 3 TURNING The lathe And lts Construction a lathe is a machine tool used primarily for producing surfaces of revolution and flat edges. Based on their purpose, construction, number of tools that can simultaneously be mounted and degree of automation lathes-or, more accurately, lathe-type machine tools-can be classified as follows 1)Engine lathes 2)Toolroom lathes 3)Turret lathes 4)Vertical turning and boring mills 5 )Automatic lathes sPecial purpose lathes In spite of that diversity of lathe type machine tools, they all have common features with respect to construction and principle ofoperation These features can best be illustrated by considering the commonly used representative type, the engine lathe. Following is a description of each of the main elements of an engine lathe, which is shown in Fig. 3. I
The Lathe And Its Construction A lathe is a machine tool used primarily for producing surfaces of revolution and flat edges. Based on their purpose, construction, number of tools that can simultaneously be mounted and degree of automation, lathes-or, more accurately, lathe-type machine tools-can be classified as follows: 1 )Engine lathes 2)Toolroom lathes 3)Turret lathes 4)Vertical turning and boring mills 5 )Automatic lathes 6)Special purpose lathes In spite of that diversity of lathe type machine tools, they all have common features with respect to construction and principle of operation. These features can best be illustrated by considering the commonly used representative type, the engine lathe. Following is a description of each of the main elements of an engine lathe, which is shown in Fig. 3. 1. Unit 3 TURNING
Headstock assembly, Tool post Face plate Compound rest mounted on the Center Cross slide Tailstock quill Tailstock assembl Chip pan Pig. 3.1 The engine lathe (Courtesy of Clausing Industrial, Ine. Kalamazoo Mich. H
Lathe bed The lathe bed is the main frame, involving a horizontal beam on two vertical supports. It is usually made of grey or nodular east iron to damp vibrations and is made by casting. It has guideways to allow the carriage to slide easily lengthwise. The height of the lathe bed should be appropriate to enable the technician to do his or her job easily and comfortably ■ Headstock The headstock is fixed at the left hand side of the lathe bed and includes the spindle whose axis is parallel to the guideways(the slide surface of the bed). The spindle is driven through the gearbox, which is housed within the headstock. The function of the gearbox is to provide a number of different spindle speeds usually 6 up to 18 speeds). Some modern lathes have headstocks with infinitely variable spindle speeds, which employ frictional, electrical, or hydraulic drives
◼ Lathe bed. ◼ Headstock. The lathe bed is the main frame, involving a horizontal beam on two vertical supports. It is usually made of grey or nodular east iron to damp vibrations and is made by casting. It has guideways to allow the carriage to slide easily lengthwise. The height of the lathe bed should be appropriate to enable the technician to do his or her job easily and comfortably. The headstock is fixed at the left hand side of the lathe bed and includes the spindle whose axis is parallel to the guideways(the slide surface of the bed). The spindle is driven through the gearbox, which is housed within the headstock. The function of the gearbox is to provide a number of different spindle speeds ( usually 6 up to 18 speeds). Some modern lathes have headstocks with infinitely variable spindle speeds, which employ frictional, electrical, or hydraulic drives
The spindle is always hollow, i. e, it has a through hole extending lengthwise. Bar stocks can be fed through that hole if continuous production is adopted. Also, that hole has a tapered surface to allow mounting a plain lathe center. The outer surface of the spindle e ls threaded to allow mounting of a chuck, a face plate, or the like Tailstock The tailstock assembly consists basically of three parts, its lower base, an intermediate part, and the quill. The lower base is a casting that can slide on the lathe bed along the guideways, and it has a clamping device to enable locking the entire tailstock at any desired location, depending upon the length of the workpicce. The intermediate part is a casting that can be moved transversely to enable alignment of the axis of the tailstock with that of the headstock. The third part, the quill, is a hardened steel tube, which can be moved longitudinally in and out of the intermediate part as required. This is achieved through the use of a handwheel and a screw, around which a nut fixed to the quill is engaged. The hole in the open side of the quill is tapered to enable mounting oj
◼ Tailstock. The spindle is always hollow, i. e. , it has a through hole extending lengthwise. Bar stocks can be fed through that hole if continuous production is adopted. Also,that hole has a tapered surface to allow mounting a plain lathe center. The outer surface of the spindle is threaded to allow mounting of a chuck,a face plate,or the like. The tailstock assembly consists basically of three parts,its lower base,an intermediate part, and the quill. The lower base is a casting that can slide on the lathe bed along the guideways, and it has a clamping device to enable locking the entire tailstock at any desired location, depending upon the length of the workpicce. The intermediate part is a casting that can be moved transversely to enable alignment of the axis of the tailstock with that of the headstock. The third part,the quill,is a hardened steel tube,which can be moved longitudinally in and out of the intermediate part as required. This is achieved through the use of a handwheel and a screw, around which a nut fixed to the quill is engaged. The hole in the open side of the quill is tapered to enable mounting of lathe centers or other tools like twist drills or boring bars. The quill can be locked at any point along its travel path by means of a clamping
■ The carriage. The main function of the carriage is mounting of the cutting tools and generating longitudinal and/or cross feeds. It is actually an H-shaped block that slides on the lathe bed between the headstock and tailstock while being guided by the v-shaped guideways of the bed. The carriage can be moved either manually or mechanically by means of the apron and either the feed rod or the lead screw. When cutting screw threads, power is provided to the gearbox of the apron by the lead screw. In all other turning operations, it is the feed rod that drives the carriage. The lead screw goes through a pair of half nuts,which are fixed to the rear of the apron. When actuating a certain lever, the half nuts are clamped together and engage with the rotating lead screw as a single nut, which is fed, together with the carriage, along the bed. When the lever is disengaged, the half nuts are released and the carriage stops. On the other hand, when the feed rod is used, it supplies power to the apron through a worm gear: The latter is keyed to the feed rod and travels with the apron along the feed rod, which has a keyway extending to cover its whole length
◼ The carriage. The main function of the carriage is mounting of the cutting tools and generating longitudinal and/or cross feeds. It is actually an H-shaped block that slides on the lathe bed between the headstock and tailstock while being guided by the V-shaped guideways of the bed. The carriage can be moved either manually or mechanically by means of the apron and either the feed rod or the lead screw. When cutting screw threads, power is provided to the gearbox of the apron by the lead screw. In all other turning operations, it is the feed rod that drives the carriage. The lead screw goes through a pair of half nuts,which are fixed to the rear of the apron. When actuating a certain lever, the half nuts are clamped together and engage with the rotating lead screw as a single nut,which is fed, together with the carriage, along the bed. When the lever is disengaged,the half nuts are released and the carriage stops. On the other hand,when the feed rod is used,it supplies power to the apron through a worm gear. The latter is keyed to the feed rod and travels with the apron along the feed rod, which has a keyway extending to cover its whole length
A modern lathe usually has a quick-change gearbox located under the headstock and driven from the spindle through a train of gears. It is connected to both the feed rod and the lead screw and enables selecting a variety of feeds easily and rapidly by simply shifting the appropriate levers. The quick-change gearbox is employed in plain turning, facing and thread cutting operations. Since that gearbox is linked to the spindle, the distance that the apron(and the cutting tool) travels for each revolution of the spindle can be controlled and is referred to as the feed. LATHE CUTTING TOOLS The shape and geometry of the lathe tools depend upon the purpose for which they are employed. Turning tools can be classified into two main groups, namely, external cutting tools and internal cutting tools. Each of these two groups include the following types of tools
A modern lathe usually has a quick-change gearbox located under the headstock and driven from the spindle through a train of gears. It is connected to both the feed rod and the lead screw and enables selecting a variety of feeds easily and rapidly by simply shifting the appropriate levers. The quick-change gearbox is employed in plain turning, facing and thread cutting operations. Since that gearbox is linked to the spindle,the distance that the apron (and the cutting tool) travels for each revolution of the spindle can be controlled and is referred to as the feed. LATHE CUTTING TOOLS The shape and geometry of the lathe tools depend upon the purpose for which they are employed. Turning tools can be classified into two main groups, namely, external cutting tools and internal cutting tools. Each of these two groups include the following types of tools:
■ Turning tools Turning tools can be either finishing or rough turning tools. Rough turning tools have small nose radii and are employed when deep cuts are made. On the other hand, finishing tools have larger nose radii and are used for obtaining the final required dimensions with good surface finish by making slight depths of cut. Rough turning tools can be right- hand or left-hand types, depending upon the direction offeed. They can have straight, bent, or offset shanks a Facing tools. Facing tools are employed in facing operations for machining plane side or end surfaces. There are tools for machining left-hand-side surfaces and tools for right-hand-side surfaces. Those side surfaces are generated through the use of the cross feed, contrary to turning operations, where the usual longitudinal feed is used
◼ Facing tools. ◼ Turning tools. Turning tools can be either finishing or rough turning tools. Rough turning tools have small nose radii and are employed when deep cuts are made. On the other hand,finishing tools have larger nose radii and are used for obtaining the final required dimensions with good surface finish by making slight depths of cut. Rough turning tools can be righthand or left-hand types, depending upon the direction of feed. They can have straight, bent, or offset shanks. Facing tools are employed in facing operations for machining plane side or end surfaces. There are tools for machining left-hand-side surfaces and tools for right-hand-side surfaces. Those side surfaces are generated through the use of the cross feed, contrary to turning operations，where the usual longitudinal feed is used
■ Cutoff tools Cutoff tools, which are sometimes called parting tools, serve to separate the workpiece into parts and/or machine external annual grooves Thread-cutting tools Thread-cutting tools have either triangular, square, or trapezoidal cutting edges, depending upon the cross section of the desired thread Also, the plane angles of these tools must always be identical to tho ose of the thread forms. Thread-cutting tools have straight shanks for external thread cutting and are of the bent-shank type when cutting internal threads
◼ Cutoff tools. Cutoff tools, which are sometimes called parting tools, serve to separate the workpiece into parts and/or machine external annual grooves. ◼ Thread-cutting tools Thread-cutting tools have either triangular, square, or trapezoidal cutting edges, depending upon the cross section of the desired thread. Also, the plane angles of these tools must always be identical to those of the thread forms. Thread-cutting tools have straight shanks for external thread cutting and are of the bent-shank type when cutting internal threads
Form tools Form tools have edges especially manufactured to take a certain form, which is opposite to the desired shape of the machined workpiece. An HSS tool is usually made in the form of a single piece, contrary to cemented carbides or ceramic, which are made in the form of tips. The latter are brazed or mechanically fastened to steel shanks. Fig. 3. 2 indicates an arrangement of this latter type, which includes the carbide tip, the chip breaker, the pad, the clamping screw ( with a washer and a nut), and the shank. As the name suggests, the function of the chip breaker is to break long chips every now and then, thus preventing the formation of very long twisted ribbons that may cause problems during the machining operation The carbide tips ( or ceramic tips)can have different shapes, depending upon the machining operations for which they are Clamp Chi p to be employed. The tips can either be breaker Shank tool holder) solid or with Carbide pending a central through hole, Seat whether brazing O mechanical clamping is employed for mounting the tip on the shank Fig. 3. 2 A carbide tip fastened to a tool holder
◼ Form tools. Form tools have edges especially manufactured to take a certain form, which is opposite to the desired shape of the machined workpiece. An HSS tool is usually made in the form of a single piece, contrary to cemented carbides or ceramic , which are made in the form of tips. The latter are brazed or mechanically fastened to steel shanks. Fig. 3. 2 indicates an arrangement of this latter type, which includes the carbide tip, the chip breaker, the pad, the clamping screw (with a washer and a nut), and the shank . As the name suggests, the function of the chip breaker is to break long chips every now and then, thus preventing the formation of very long twisted ribbons that may cause problems during the machining operation. The carbide tips (or ceramic tips)can have different shapes, depending upon the machining operations for which they are to be employed. The tips can either be solid or with a central through hole, depending whether brazing or mechanical clamping is employed for mounting the tip on the shank
LATHE OPERATIONS R In the following section, we discuss the various machining operations that can he performed on a conventional engine lathe. It must be borne in mind, however, that modern computerized numerically controlled lathes have more capabilities and can do other operations, such as contouring, for example. Following are conventional lathe operations Cylindrical turning Cylindrical turning is the simplest and the most common of all lathe operations. A single full turn of the workpiece generates a circle whose center falls on the lathe axis, this motion is then reproduced numerous times as a result of the axial feed motion of the tool. The resulting machining marks are, therefore, a helix having a very small pitch, which is equal to the feed. Consequently the machined surface is always cylindrical The axial feed is provided by the carriage or the compound rest, either manually or automatically, whereas the depth of cut is controlled by the cross slide. In roughing cuts, it is recommended that large depths of cuts(up to 1/4in. or 6mm, depending upon the workpiece material) and smaller feeds be used On the other hand, very fine feeds, smaller depths of cut(less than 0.05in. or 0.4 mm), and high cutting speeds are preferred for finishing cuts
LATHE OPERATIONS ◼ Cylindrical turning. In the following section, we discuss the various machining operations that can he performed on a conventional engine lathe. It must be borne in mind,however,that modern computerized numerically controlled lathes have more capabilities and can do other operations, such as contouring, for example. Following are conventional lathe operations. Cylindrical turning is the simplest and the most common of all lathe operations. A single full turn of the workpiece generates a circle whose center falls on the lathe axis;this motion is then reproduced numerous times as a result of the axial feed motion of the tool. The resulting machining marks are,therefore,a helix having a very small pitch,which is equal to the feed. Consequently ,the machined surface is always cylindrical. The axial feed is provided by the carriage or the compound rest,either manually or automatically, whereas the depth of cut is controlled by the cross slide. In roughing cuts,it is recommended that large depths of cuts(up to 1/4in. or 6mm,depending upon the workpiece material) and smaller feeds be used. On the other hand, very fine feeds,smaller depths of cut(less than 0.05in. or 0.4 mm) ,and high cutting speeds are preferred for finishing cuts