(a) Facing. The tool is fed radially into the rotating work on one end to create a flat surface on the end.
(b) Taper turning. Instead of feeding the tool parallel to the axis of rotation of the work, the tool is fed at an angle, thus creating a tapered cylinder or conical shape.
(c) Contour turning. Instead of feeding the tool along a straight line parallel to the axis of rotation as in turning, the tool follows a contour that is other than straight, thus creating a contoured form in the turned part.
(d) Form turning. In this operation, sometimes called forming, the tool has a shape that is imparted to the work by plunging the tool radially into the work.
(e) Chamfering. The cutting edge of the tool is used to cut an angle on the corner of the cylinder, forming what is called a ‘‘chamfer’’.
(f) Cutoff. The tool is fed radially into the rotating work at some location along its length to cut off the end of the part.
(g) Threading. A pointed tool is fed linearly across the outside surface of the rotating workpart in a direction parallel to the axis of rotation at a large effective feed rate, thus creating threads in the cylinder.
(h) Boring. Asingle-point tool is fed linearly, parallel to the axis of rotation, on the inside diameter of an existing hole in the part.
(i) Drilling. Drilling can be performed on a lathe by feeding the drill into the rotating work along its axis. Reaming can be performed in a similar way.
(j) Knurling. It is a metal forming operation used to produce a regular crosshatched pattern in the work surface.
Engine Lathe Technology. Figure 5.3 is a sketch of an engine lathe showing its principal components. The headstock contains the drive unit to rotate the spindle, which rotates the work. Opposite the headstock is the tailstock, in which a center is mounted to support the other end of the workpiece.
The cutting tool is held in a tool post fastened to the cross-slide, which is assembled to the carriage. The carriage is designed to slide along the ways of the lathe in order to feed the tool parallel to the axis of rotation. The ways are built into the bed of the lathe, providing a rigid frame for the machine tool.
The carriage is driven by a leadscrew that rotates at the proper speed to obtain the desired feed rate. The cross-slide is designed to feed in a direction perpendicular to the carriage movement. Thus, by moving the carriage, the tool can be fed parallel to the work axis to perform straight turning; or by moving the cross-slide, the tool can be fed radially into the work to perform facing, form turning, or cutoff operations.
The size of a lathe is designated by swing and maximum distance between centers. The swing is the maximum workpart diameter that can be rotated in the spindle, determined as twice the distance between the centerline of the spindle and the ways of the machine. The actual maximum size of a cylindrical workpiece that can be accommodated on the lathe is smaller than the swing because the carriage and cross-slide assembly are in the way. The maximum distance between centers indicates the maximum length of a workpiece that can be mounted between headstock and tailstock centers.
(a) facing, (b) taper turning, (c) contour turning, (d) form turning, (e) chamfering, (f) cutoff, (g) threading, (h) boring, (i) drilling, and (j) knurling.
Figure 5.2 – Machining operations other than turning that are performed on a lathe.
Figure 5.3 – Diagram of an engine lathe, indicating its principal components
Methods of holding the work in a lathe. There are four common methods used to hold workparts in turning. The methods, illustrated in Figure 5.4, are (a) mounting the work between centers, (b) chuck, (c) collet, and (d) face plate.
(a) mounting the work between centers using a dog, (b) three-jaw chuck, (c) collet, and (d) faceplate for noncylindrical workparts.
Figure 5.4 – Four workholding methods used in lathes.
Holding the work between centers refers to the use of two centers, one in the headstock and the other in the tailstock, as in Figure 5.4(a). This method is appropriate for parts with large length-to-diameter ratios. At the headstock center, a device called a dog is attached to the outside of the work and is used to drive the rotation from the spindle.
The chuck, Figure 5.4(b), is available in several designs, with three or four jaws to grasp the cylindrical workpart on its outside diameter. The jaws are often designed so they can also grasp the inside diameter of a tubular part. A self-centering chuck has a mechanism to move the jaws in or out simultaneously, thus centering the work at the spindle axis. Other chucks allow independent operation of each jaw.
A collet consists of a tubular bushing with longitudinal slits running over half its length and equally spaced around its circumference, as in Figure 5.4(c). The inside diameter of the collet is used to hold cylindrical work such as barstock. Owing to the slits, one end of the collet can be squeezed to reduce its diameter and provide a secure grasping pressure against the work.
A face plate, Figure 22.8(d), is a workholding device that fastens to the lathe spindle and is used to grasp parts with irregular shapes.