Explain the construction of an engine lathe.
Engine Lathe. It is so called because the first of this type of lathe was driven by a steam engine. It is also called. “Centre lathe” because, it has two centres between which the job can be held and rotated. A very high percentage of all lathe work is turned between centres.
The main parts of a centre lathe are : Bed, Head stock, Tail stock, Carriage and the Electric drive, (Fig 1.67)
1. Bed. The bed is the base or foundation of the lathe. It is a massive (heavy) and rigid casting made in one piece to resist deflection and vibrations. It holds or supports all other parts, that is, head stock, tailstock and carriage etc. The top of the bed is planed to form “guides” or “ways”. Ways are accurate rails which support carriage and the tailstock. More expensive lathes have a combination of V ways and flat ways. Less expensive lathes have flat ways. Directly under the front way on the bed is a rack. A pinion gear meshes with the rack for moving the carriage when the handwheel is turned. The bed is usually fastened to steel legs so the lathe can be bolted to the shop floor.
2. Head stock. The headstock assembly is permanently fastened to the left hand end of the lathe. It serves to support the first operative unit of the lathe, that is, the spindle. The spindle revolves in bearings, one at each end of headstock. The spindle is rotated by a combination of gears and cone pulleys or by gears alone. Present day lathes have individual motor drives and most of them have geared headstocks. The steel spindle is hollow to take long bar stock. The spindle has a definite taper at the front end for holding centres and other tools having a tapered shank. The hole through the spindle makes it possible to use a knockout bar to remove such tools. Workholding attachments such as driving plate, face plate or various types of chucks may be mounted on the threaded spindle nose. Some type of work may also be held in a collet which is inserted into the hollow headstock spindle. A taper sleeve fits into taper spindle hole. The headstock or live centre fits into the sleeve. This centre is called live centre because it turns with the work. The centre is a tapered metal part with a pointed end. This supports the end of the workpiece as it turns between the centres. All centre points have a 60-degree included angle.
3. Tail stock. Tail stock is on the other end of the bed from the head stock. Its chief function is to hold the dead centre so that long workpieces (L/D > 4) can be supported between centres. It can be moved along the bed and clamped to the bed at the various desired locations to suit the length of the workpiece.
Tailstock consists of two main parts. The lower part rests directly on the bed ways, and the upper part rests on the lower part. Adjusting screws hold the two together. The upper casting can be moved toward or away from the operator to offset the tailstock for taper turning and to re-align the tailstock centre for straight turning. The body of the tailstock has a bore for the hollow cylindrical sliding member, known as a “quill”. This quill is sometimes called as “tailstock spindle” even though it can not rotate. The quill moves in and out of the tailstock bore when the tailstock handweel is turned. Once set, the quill may be clamped to remain in a desired position. The quill has a taper hole in to which the dead centre is fitted. Drills, reamers, taps and other end cutting tools are held and fed to the work piece by the quill, the shanks of the tools being held in the tapered hole of the quill.
4. Carriage. In between the headstock and the tailstock is the carriage. It is movable on the bed ways and its purpose is to hold the cutting tool and to impart to it either longitudinal or cross feed. It has five major parts :
(a) Saddle. The base of the carriage is the saddle which slides along the way of the lathe bed.
(b) Cross-Slide. The cross-slide is mounted on the saddle. It provides cutting tool motion which is perpendicular to the centre line of the lathe itself. The cross-feed movement may be controlled by manual (with cross slide handle) or by power feed.
(c) Compound Rest. It is mounted on top of the cross-slide. The compound rest has a graduated base and can be swivelled around a vertical axis. In this way, its slide can be set at any angle with the axis of the workpiece. It can be clamped to remain at any angular setting. The range of compound rest is only limited and is used for obtaining angular cuts and short tapers, as well as convenient positioning of the tool to the work. Both the cross slide and the compound rest screws are equipped with micrometer collars. These are used in making accurate adjustments when turning workpieces to close measurements, and when cutting screw threads. There is no power feed for the compound rest.
(d) Tool post. The tool post is mounted on the compound rest and slides in a T-slot. Cutting tool/tool holder is firmly held in it. The tool can be swivelled as well as tilted by means of a rocker and a concave ring collar, (Fig. 1.68)
(e) Apron. The apron is secured underneath the saddle and hangs over the front of the bed. It contains the gears, clutches, and levers for operating the carriage by hand and power feeds. The apron hand wheel can be turned to move the carriage longitudinally by hand. This hand wheel is attached to a pinion that meshes with the rack under the front of the bed. The apron also contains friction clutches for automatic feeds and a splitnut or halfnut. The split nut can be closed over the lead screw threads and is used only when cutting screw threads.