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****************** Recommended Reading: Manufacturing Engineering and Technology Marks' Standard Handbook for Mechanical Engineers ******************
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3.1 a) CENTRE LATHE The centre lathe is the simplest of lathes. Fig 3.1.1. is a basic representation of a typical centre lathe. Fig 3.1.1.
Click the image for full size picture The workpiece is held in the chuck and rotates (i). Longitudinal turning and boring is carried out as the tool, held in the tool post, intersects the rotating workpiece as the saddle moves (ii). Faces and shoulders are machined perpendicular to the axis of rotation as the cross slide moves the tool (iii). Drilling operations are carried out by holding a drill in a chuck mounted in the tailstock, which moves (iv). A centre lathe takes a moderately long time to set up and only one tool at a time may be set. The operator controls all aspects of machining – dimensional accuracy and surface finish are therefore variable and only as good as the operator. If a production run of parts is to be produced, each part can be effectively considered a one off. Centre lathes are usually found in toolrooms where productivity is considered less important than flexibility.
3.1. b) TURRET LATHE On a turret lathe a selection of tools required for the job to be set are held in a turret (Fig 3.1.2. a). These tools may perform axial operations on the workpiece such as turning (knee type tool holder Fig 3.1.2. b); tapping – using a ratchet type tap holder (Fig3.1.2. c) and drilling. Fig 3.1.2.
The turret slides directly on the machine bed and is indexed and fed manually up to a stop, removing some operator induced variation. Facing operations are performed by a separate cross slide with tools mounted front and rear. Production cycle times are still considerable and versatility is limited. Surface finish and, to some degree, geometric tolerances are still dictated by the skill of the operator.
3.1. c) CAPSTAN LATHE The capstan lathe is fundamentally similar to the turret lathe. Used mainly for smaller bar work the main differences to the turret lathe are that the turret indexes automatically at the end of it’s travel; bars are usually held in a collet rather than a chuck and the turret moves on a separate slide bolted to the bed. 3.1. d) SINGLE SPINDLE AUTOMATIC Similar to a capstan in configuration all operations on a single spindle automatic are carried out automatically and controlled by an auxiliary camshaft. At the end of each cycle the collet chuck is opened and the bar fed forward to a dead stop and the machining cycle starts again. In addition to the simple cross slide found on a capstan, many automatics have cross slides arranged radially about the spindle axis, these are capable of performing operations such as forming, facing and parting off. 3.1.e) MULTI SPINDLE AUTOMATIC On a multi-spindle automatic, usually six, separate spindles are arranged radially around a central axis. One or two operations may be carried out on the workpiece in each spindle at the same time producing a finished part every time the spindle arrangement indexes one position. On a multi-spindle a complete part can be finished in only the time it takes to carry out the longest operation and not the sum of the time of all operations. These machines may take over a day to set up but are highly productive, lending themselves to very high volume production. 3.1.f) SLIDING HEADSTOCK (SWISS TYPE) AUTOMATIC Originally developed for producing clock and watch parts, these machines are capable of producing very small components with very high length to diameter ratios.
Fig 3.1.3.
Click the image for full size picture Fig 3.1.3. shows the principle behind the sliding headstock machine. The collet, mounted on the headstock, grips the bar and rotates it. The guide bush steadies the bar a very short distance from the tool. As the headstock slides forward it pushes the bar through the guide bush and cutting occurs. Infeed of the tools, and headstock movement are controlled by an auxiliary camshaft. Usually five tools are arranged radially about the spindle axis. End working of the component such as drilling and tapping is carried out by a tailstock attachment. At the end of each cycle the collet opens and the headstock retracts to grip the bar further back and produce the next component. 3.1.g) C.N.C. TURNING By controlling slide movement by Computer Numeric Control (C.N.C.) both fixed and sliding headstock machines become capable of generating very complex profiles using simple single point tools. Live (powered) tooling and a spindle in place of the tailstock, known as a pick-up or sub-spindle, allows radial and off centre drilling, milling and slotting, and machining of both faces of the part, eliminating secondary operations (one hit machining). |
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FIG. 3.1.3 Sliding Head or Swiss Type Automatic Lathe
