The centre lathe or metal turning lathe is a machine used to turn metal or plastic bar into cylindrical shapes. In addition there are my other operations that can be done on this type of lathe.
The work, which usually rotates towards the operator, can be griped securely in a chuck, bolted to a face plate or set up between two centres. The cutting tool, mounted on a tool post on the top of the carriage can be moved along the bed or at right angles to it. The length ways movement (or traverse) of the cutting tool produces a circular surface on the workpiece and the cross transverse produces a flat surface.
In addition to these operations the centre lathe is used in the school workshop for producing tapered work (with the compound slide),for drilling and boring holes, parting off, knurling and screw cutting.
The size of the lathe is specified by (a) the maximum distance between centres and (b) the height of the spindle centre above the lathe bed.
The main parts of the centre lathe are:
- The Bed
- The Headstock
- The Tailstock
- The Carriage
The bed of the lathe is made from cast iron for strength and rigidity. Its surfaces are machined true and accurate to ensure smooth movement of the tailstock and carriage and perfect alignment of these components with each other with the headstock.
Beds are usually either flat-bed or prismatic v-type section or sometimes a combination of both. Note the capacity of a lathe can be increase by the inclusion of a Gap Bed situated immediately in front of the headstock. This allows large diameter work to be turned.
The headstock is securely fixed onto the left hand end of the bed and carries the drive gearing and main spindle. This spindle is hollow to take tapered centres and allow metal and plastic of suitable diameter to pass through. It is also screwed at the right hand end to enable chucks and face plates to be easily attacked and removed. This spindle, known as the ‘live’ centre because it rotates with the work, is driven by a series of gear wheels whose power is transmitted by means of v-belts from an electric motor.
The tailstock has a hollow spindle designed to take tialstock (or dead) centres and drill chucks. In use the tailstock centre supports the ‘free’ end of the work being turned whilst the drill chuck is used when drilling or reaming work that is held in a chuck or face plate.
The tailstock can be located in the desired position by sliding it smoothly along the bed and clamping securely with the tailstock bed clamp. Fine adjustment, to bring the ‘dead’ centre up to the work, is made with the hand wheel. The outer surface of the spindle is graduated in millimetres to allow holes of an exact depth to be drilled. The spindle is clamped in position using the spindle clamp.
The carriage unit consists of the saddle, cross slide, compound slide (or rest), tool post and apron.
The Saddle, or carriage base, spans the bed of the lathe and moves smoothly on the bed guides, when the apron hand wheel is turned or when automatic fed is engaged.
The Cross Slide, mounted on a dovetail shapes section on top of the saddle, can be moved at right angles to the bed length by using the cross feed handle.
The Compound Slide, which carries the tool post and cutting tool, is fed by hand only. It is attached to the top of the cross slide by a swivel pin and can be pivoted and locked at any desired angle when turning short tapers, boring or screw cutting.
The Apron hangs down from the front edge of the saddle. The lead screw, or feed shaft, passes through the apron and when engaged by the appropriate gearing and controls cause the Whole carriage to move smoothly along the bed. The apron also contains the feed mechanism and control for the automatic movement of the cross slide.
Before turning the workpiece to shape the ends, should be turned square. This process is known as Facing Off. The cutting tool moves across the end of the workpiece and at right angles to the axis of rotation to produce a flat, smooth surface. Facing off should be done at high speed.
If the cutting tool is not set to the correct height, a small conical pip of metal will be left. The cutting tool should be reset to the correct height to remove this.
For accurate drilling on the centre lathe it is necessary to start the hole using a centre drill. This drill, also known as a combination or slocombe drill, combines a drill and 60 counter sink to match the point angles of centre lathes.
The work to be drilled is held in a three jaw chuck whilst the centre drill is secured in a Jacobs (or drill) chuck fitted into the tailstock spindle. This gives a safe and accurate start for the twist drill as shown.
A tap, held securely in a drill chuck, can be used to accurately start and cut a thread in a previously drilled hole in the work piece. The motor must be isolated and the drill chuck rotated by hand.
When the cutting tool moves parallel to the axis of rotation of the work piece a cylindrical shape is formed. On lathes used in schools the maximum depth of cut should be no greater than 1 mm for roughing and about 0.25 mm for finishing cuts.
The cut should be started by hand before engaging automatic feed. Light cuts should be done at high speed whilst deeper or rough cuts should be done at low speeds.
To turn very short tapers such as chamfers a form tool (i.e. A specially ground tool) is used.
Short tapers can be cut using th compound slide as shown. The slide is swiveled to the correct angle then clamped and the taper cut by feeding the tool into the workpiece with the cross slide and compound slide handles.
Longer tapers can be cut using the power feed by setting the tailstock out of line with the head stock and positioning the work piece between centres.
Turning between Centres
The sketch shows a work piece securely fitted between centres and ready for turning. If the work is too long it will tend to bend as the cutting tool nears the centre. To prevent this it is usual to use a fixed steady or a travelling steady for support. A brief outline of the process for turning between centres is as follows.
- Face off and centre drill both ends.
- Screw driving plate into headstock spindle, fit soft centre. Fit revolving centre into tailstock as shown.
- Fit lathe carrier (driving dog) on to the work piece as shown.
- Securely set up work between centres.
The work, securely held in a chuck, can be cut or parted off t the required length by using a parting tool as shown. The lathe must run at a slow speed and the cut made as close to the chuck jaw as is practical for maximum support.
The parting tool should be supplied with an even flow of cutting fluid and fed at right angles into the work with a slow and even pressure.
Knurling is the process of impressing a straight or diamond shapes pattern onto the surface of a workpiece such as a centre punch to give a better grip. Coarse, medium and fine knurling tools are available. The lathe should run at the slowest speed and the knurling tool slowly fed in at right angles to the axis of rotation of the work. Automatic feed should then be engaged and the knurling toll fed across the face of the work from right to left.
When the knurling tool reaches the left hand end, the fed should be disengaged; the tool fed a further 1/2 mm into the work and then automatically fed back to the start. Plenty of oil should be used and the process repeated until the knurling operation is complete.
Boring is the process of machining a cylindrical shape inside the previously drilled hole. Take light cuts only.