Centre-lathe operations

Accurate turning of plain diameters and faces can be simply carried out on a centre lathe. Wherever possible, diameters should be turned using the carriage movement, as the straightness of the bed guideways ensures parallelism of the workpiece and power feed can be used. Avoid using the top slide for parallel diameters, since it is adjustable for angle and difficult to replace exactiy on zero without the use of a dial indicator. It has also to be hand fed. 

When a number of diameters are to be turned on a workpiece, they should be produced at one setting without removing the workpiece from the chuck, in order to maintain concentricity between them. Accuracy is lost each time the workpiece is removed and put back in the chuck. Accurate sizes can he produced by measuring the workpiece when the final size is almost reached, then using the graduated dial on the hand-wheel to remove the required amount. 

Where only diameters are being turned and a square shoulder is required, a knife tool is used. Fig. 9.23A, which cuts in the direction 

Where a relief or undercut at the shoulder is required, e.g. where a thread cannot be cut right up to the shoulder, an undercut tool is used. This tool is ground to the correct width, the face parallel to the work axis, and is fed in the direction shown. Fig. 9.23C. 

Work produced from bar can be cut to length in the lathe, an operation known as parting off . The face of the parting-off tool is ground at a slight angle, so that the work-piece is severed cleanly from the bar. Fig. 9.23D. 

Where only diameters are being turned and a square shoulder is required, a knife tool is used. Fig. 9.23A, which cuts in the direction shown. Where facing and turning are being carried out in the same operation, a turning and facing tool is used, Fig. 9.23B. The slight radius on the nose produces a better surface finish, but the radius will be reproduced at the shoulder. 

Where a relief or undercut at the shoulder is required, e.g. where a thread cannot be cut right up to the shoulder, an undercut tool is used. 

This tool is ground to the correct width, the face parallel to the work axis, and is fed in the direction shown. Fig. 9.23C. 

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Workpieces can be held in a centre lathe by a variety of methods depending on the shape and the operation being carried out. 

Imperial threads are designated not by their pitch but by the number of threads per inch (tp.L). The leadscrew of a metric centre lathe has a pitch of 6mm and, since the number of t.p.i. cannot be arranged as a multiple of the leadscrew pitch, the split nut, once it is engaged, must never be disengaged during the thread-cutting operation. This also means that the thread indicator dial is of no use when cutting imperial threads on a metric lathe. 

Engineering workpieces cannot be consistently produced to an exact size. This is due to a number of reasons such as wear on cutting tools, errors in setting up, operator faults, temperature differences or variations in machine performance. Whatever the reason, allowance must be made for some error. The amount of error which can be tolerated - known as the tolerance - depends on the manufacturing method and on the functional requirements of the workpiece. For example, a workpiece finished by grinding can be consistently made to finer tolerances than one produced on a centre lathe. In a similar way, a workpiece required for agricultural equipment would not... 

Give two reasons why a boring operation would be carried out on a centre lathe. 

These are machine tools designed to rotate glass tubes so that they can be uniformly heated to such a temperature that joins can be made, bulbs blown and internal seals fabricated. To effectively carry out such operations a glassworking lathe must be of precise and robust construction. It must be fitted with two chucks, rotating at exactly the same speed about a common centre line, and at an easily variable distance apart. 

Drilling is carried out on a lathe by holding the drill in a chuck or mounting it directly in the quiU of the tailstock, which contains a Morse taper for this purpose. As with all drilling operations, some guide is required to enable the drill to start central, and a centre drill. Fig. 9.26, is commonly used. 

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