5.2 Turning and Lathe

Turning is one of the most common of metal cutting operations. In turning, a workpiece is rotated about its axis as single-point cutting tools are fed into it, cutting away excess material and creating the desired cylindrical surface. Turning can occur on both external and internal surfaces to produce an axially-symmetrical contoured part. Parts ranging from pocket watch components to large diameter marine propeller shafts can be turned on a lathe.

Apart from turning, several other operations can also be performed on a lathe.

Boring and internal turning. Boring and internal turning are performed on the internal surfaces by a boring bar or suitable internal cutting tools. If the initial workpiece is solid, a drilling operation must be performed first.[1] The drilling tool is held in the tailstock, and the latter is then fed against the workpiece. When boring is done in a lathe, the work usually is held in a chuck or on a face plate. Holes may be bored straight, tapered, or to irregular contours. Boring is essentially internal turning while feeding the tool parallel to the rotation axis of the workpiece.

Facing. Facing is the producing of a flat surface as the result of a tool’s being fed across the end of the rotating workpiece. Unless the work is held on a mandrel, if both ends of the work are to be faced, it must be turned around after the first end is completed and then the facing operation repeated.[2] The cutting speed should be determined from the largest diameter of the surface to be faced. Facing may be done either from the outside inward or from the center outward. In either case, the point of the tool must be set exactly at the height of the center of rotation. Because the cutting force tends to push the tool away from the work, it is usually desirable to clamp the carriage to the lathe bed during each facing cut to prevent it from moving slightly and thus producing a surface that is not flat. In the facing of casting or other materials that have a hard surface, the depth of the first cut should be sufficient to penetrate the hard material to avoid excessive tool wear.

Parting. Parting is the operation by which one section of a workpiece is severed from the remainder by means of a cutoff tool. Because cutting tools are quite thin and must have considerable overhang, this process is less accurate and more difficult. The tool should be set exactly at the height of the axis of rotation, be kept sharp, have proper clearance angles, and be fed into the workpiece at a proper and uniform feed rate.

Threading. Threading can be considered as turning since the path to be travelled by the cutting tool is helical. However, there are some major differences between turning and threading. While in turning, the interest is in generating a smooth cylindrical surface, in threading the interest is in cutting a helical thread of a given form and depth which can be calculated from the formulae. There are two basic requirements for thread cutting. An accurately shaped and properly mounted tool is needed because thread cutting is a form-cutting operation. The resulting thread profile is determined by the shape of the tool and its position relative to the workpiece. The second requirement is that the tool must move longitudinally in a specific relationship to the rotation of the workpiece, because this determines the lead of the thread. This requirement is met through the use of the lead screw and the split unit, which provide positive motion of the carriage relative to the rotation of the spindle.

Many types of lathes are used for production turning. According to purposes and construction, lathe-type machine tools can be classified as follows:

1. Engine lathes;

2. Vertical lathes;

3. Turret lathes;

4. Single- or multiple-spindle automatic or semi-automatic lathes;

5. Contouring lathes;

6. Universal lathes;

7. Special-purpose lathes such as crankshaft lathes, camshaft lathes, car wheel lathes and backing-off lathes, etc.

The engine lathe is the most representative member of the lathe family and is the most widely used, so there is a description of each of the main elements of an engine lathe, which is shown in Fig.5.1.

Lathe bed is the foundation of the engine lathe, which is a heavy, rugged casting made to support the working parts of the lathe. The size and mass of the bed give the rigidity necessary for accurate engineering tolerances required in manufacturing. On top of the bed are machined slideways that guide and align the carriage and tailstock, as they move from one end of the lathe to the other.

Headstock is clamped atop the bed at the left-hand end of the lathe and contains the motor that drives the spindle whose axis is parallel to the guideways through a series of gears housed within the gearbox. The function of gearbox is to generate a number of different spindle speeds. A spindle gear is mounted on the rear of the spindle to transmit power through the change gears to the feeding box that distributes the power to the lead screw for threading or to the feed rod for turning. [3]

The spindle has a through hole extending lengthwise through which bar stocks can be fed if continuous production is used. The hole can hold a plain lathe center by its tapered inner surface and mount a chuck, a face plate or collet by its threaded outer surface.

Carriage assembly is actually an H-shaped block that sits across the guideways and in front of the lathe bed. The function of the carriage is to carry and move the cutting tool longitudinally. It can be moved by hand or by power and can be clamped into position with a locking nut. The carriage is composed of the cross slide, compound rest, tool saddle, and apron.

The cross slide is mounted on the dovetail guideways on the top of the saddle and 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.

The compound rest is mounted on the cross slide and can be swiveled and clamped at any angle in a horizontal plane. The compound is typically used for cutting chamfers or tapers, but must also be used when cutting threads. The compound rest can only be fed by hand. There is no power to the compound rest. The cutting tool and tool holder are secured in the tool post which is mounted directly to the compound rest.

The tool saddle is an H-shaped casting mounted on top of the guideways and houses the cross slide and compound rest. It makes possible longitudinal, cross and angular feeding of the tool bit.

The apron is attached to the front of the carriage and contains the gears and feed clutches which transmit motion from the feed rod or lead screw to the carriage and cross slide. 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.

Tailstock is composed of a low base and the movable part of the tail-stock proper, the transverse adjustments being made with a cross screw furnished with a square head. The two parts are held together by the holding-down bolts which secure the tailstock to the bed. The tailstock is located on the opposite end of the lathe from the headstock. It supports one 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.