LATHE OPERATIONS 1. Facing The process of making flat or smooth a surface (usually the end) of a work piece. 2. Turning To make a particular form using a facing tool – generating a symmetrical shape. 3. Drilling create a round hole in a work piece using a drill bit. 4. Boring involves enlarging of an existing hole which may have been made by a drill. The purpose of boring is to make the hole concentric with the axis of rotation. 5. Reaming a hole produced by a twist drill is likely to be out of round, oversize and with a rough finish. This can be overcome by drilling the hole undersize and opening it up with a reamer. It only removes a small amount of metal but leaves a hole of good finish and accurate size. 6. Parting it is the operation by which a section of a work piece is severed from the remainder by means of a cut off bit 7. Knurling produces a regularly shaped, roughened surface on a work piece. 8. Chamfering, threading, forming, profiling, recessing etc CAPSTAN AND TURRET LATHES Both capstan and turret lathes are simply called turret lathes. A simple block diagram of a turret lathe is shown in Figure 1. Figure 2 shows a block diagram of a capstan lathe. They both have a hexagonal turret head and a four way tool post for mounting tools. The operations which are usually performed from the hexagonal turret include: turning, boring, drilling, threading, reaming and taping. The operations which are done from the cross slide include: facing, forming, knurling, chamfering, recessing and parting off. Internal cuts are almost always made by tools mounted on the hexagonal turret. Figure 1: Turret Lathe Figure 2: Capstan Lathe Tools can be mounted and operated so as to achieve three types of cuts: combined cut, multiple cut and successive cut. A combined cut is where tools in both the hexagonal turret and square slide (four way tool post) are made to cut at the same time. A multiple cut is where two or more tools are applied at the same time from the turret station. A successive cut involves making cuts from successive faces of the turret consecutively, i.e., preceding a logical sequence. Major Differences between Capstan and Turret Lathes 1. A turret lathe is relatively more robust and a heavy duty machine. It is suited to heavy duty jobs. On the contrary, a capstan lathe is of lighter construction and suitable for bars of 6 to 65 mm diameter. 2. Capstan lathes generally deal with short or long rod type blanks held in collet, whereas turret lathes mostly work on chucking type jobs held in the quick acting chucks. 3. In capstan lathes, the turret travels with limited stroke length within a saddle type guide block called auxiliary bed, which is clamped on the main bed. In turret lathes, the heavy turret is mounted on the saddle which directly slides with larger stroke length on the main bed. 4. One additional guide rod or pilot bar is provided on the headstock of the turret lathes to ensure rigid axial travel of the turret head. 5. External screw threads are cut in capstan lathes, if required, using a self opening die mounted in one face of the turret, whereas in turret lathes, external threads are cut, if required, using a single point and multipoint chasing tool mounted on the front slide and moved by a short lead screw and a swing type half nut. Process Planning and Tool Layout Tool layout is necessary in machining operations in order to reduce the total production time, improve accuracy and reduce overall production cost. It also eliminates the need for a skilled labourer. The following steps should be followed when planning a machining process tool layout. a. Thoroughly study the job to be produced. - Volume of production - Material to be used, its properties, size and shape - Surfaces to be machined etc b. Select the machine tools required. Consider - Type, size, precision, degree of automation c. Select the blank d. Identify and list all the elementary machining operations required. e. Combine the elementary machining operations as much as possible to reduce time. f. Sequence the operations. g. Select cutting tools. Consider - Type, material, size, geometry - Availability etc h. Do a work schedule/ instruction sheet/ operation chart giving: - Description of machining work; in sequence - Cutting tools required and location - Speed and feed of each operation - Length and travel of tools - Cutting fluid application i. Draw a tool layout – schematically showing the type and configuration of the cutting tools, their location and mounting. An example of how to draw an operation sheet and tool layout is illustrated in class. Refer to class example.
