The centre lathe, which will help you to produce and repair a wide range of articles in metal, has been developed from the woodworking lathe. It consists of a power-driven spindle incorporating a holding device for work, a tailstock to support the other end of the work and a cutting tool between the two so its edge is at a fixed height. The tool moves against the work in two directions; one is the adjustment for the depth of cut and the other is the traverse along or across the work.
You may be able to buy a centre lathe secondhand, preferably one which is functioning accurately and has a set of accessories so you can tackle a range of jobs. New lathes come in various makes and sizes; they are usually sold without accessories and sometimes without a motor to drive the spindle — and this you will have to buy separately. Although a lathe is expensive in comparison with other power or hand tools, it will last a very long time if it is handled and maintained correctly. However, the lathe is not a portable machine so you will have to find a permanent position for it.
The lathe consists of a cast-iron bed with a fixed headstock at the left-hand end; this holds the spindle and the driving force for it. The tailstock is at the other end; this provides support for the extreme end of the work and, on occasions, supports tools to be used on the work. A saddle slides along the bed of the lathe between the headstock and tailstock ; this carries the mechanisms for mounting and moving cutting tools used on the work. When screw-cutting, for example, it may be necessary to move the saddle along the bed auto-
maximum length of work you can place on a lathe is given as ‘distance between centres’; the maximum diameter of work a lathe can accept is indicated by the ‘centre height’ or swing of the lathe. A small lathe might have a centre height of 90mm; it will then have a swing of 180mm (or 7in), since the swing is always twice the centre height. You will not always be able to turn work of the maximum specified diameter because in some cases the tool-mounting arrangements you have may restrict available space. A lathe which has a ‘gap bed’ can be used to turn work of a larger diameter, than the swing specified, but then the length of this large diameter work will be restricted by the length of the gap in the lathe bed.
One popular small lathe, often used by model makers, has a swing of 168mm and a distance between centres of 330mm. You can easily mount this lathe on a bench since its overall length and width are only 910mm x 660mm (or 36in x 26in) and it weighs only 65kg (131 1b).
It is possible to turn wood on a centre lathe, but it is not advisable to use one as a substitute for a wood lathe. Wood lathes are designed so sawdust is kept clear of bearings and other parts which move; a build-up of sawdust on the moving parts and bearings of a centre lathe will affect both its accuracy and performance.
Manufacturers describe what their lathes can do by calling them SSSC (sliding, surfacing and screw-cutting) machines. If you move the cutting tool along the length of the work, it will reduce the diameter of the work and produce a cylindrical shape. This tool movement is called a ‘sliding cut’ and the operation itself is called ‘parallel turning’. If you move the cutting tool across the end of the work, it produces a flat surface; this is known as a ‘surface cut’ and the operation is usually described as ‘facing’. You can form a groove (in fact a screw thread) in the work by using a tool with a cutting edge ground to the shape of a vee-thread and taking a sliding cut at a controlled speed. If you use the various lathe accessories available, all these operations may be performed on the inside or outside of the rotating workpiece. If you set the top slide at an angle to the work, you can take a taper cut which will generate a tapered surface. A lathe will also enable you to drill and tap holes to a greater degree of accuracy than you could by hand, as long as the work will fit in the lathe.
The size of lathe is defined by the size of work it can accept — both its length and diameter.
The lathe bed
Usually made from heavily ribbed cast iron, the bed is designed to prevent twisting and deflecting as well as to absorb vibrations which occur during turning. Slideways Sometimes called ‘shears’, these are mounted on the top of the bed; the lathe saddle and tailstock slide along them. Some are flat or machined to a ‘V’ shape. Other lathes have separate ‘V’ shape slideways machined for both saddle and tailstock ; these are very accurate and ensure the path of the cutting tool is always parallel to the centre line of the work being turned. On a gap bed lathe, the slideways do not run right up to the headstock; the gap in front of the headstock allows you to mount work which has a larger diameter than the specified swing of the lathe.
Wear or damage to slideways will affect the accuracy of the lathe so they should always be kept clean and well lubricated. If the slideways become twisted, they will be inaccurate. You can check for twists with two long metal straight-edges by placing them at right-angles to the length of the bed. If the straight-edges are long enough, any twist should be apparent to the eye; but you can use a metal spirit level to check both straight-edges are horizontal.
The saddle produces tool movements along the length or across the width of the work in the lathe — or a combination of these movements. The saddle rests on top of the slideways and part of it, called the apron, extends down the front of the lathe and has a handle so you can move the saddle along the bed. You may need to feed the tool by hand or automatically. The saddle has a feed lever which engages a nut inside the apron with the leadscrew; when the nut is engaged, the saddle will travel automatically along the length of the lathe.
This is mounted on top of the saddle and has a hand wheel so the tool can be moved backwards and forwards at right-angles to the length of the lathe. On larger lathes the cross slide can be driven automatically.
This slide, which is mounted on top of the cross slide, carries the tool holder. With some lathes the top slide may be rotated through 360 degrees and a scale is engraved on its base so the rotation can be measured. This slide can also be moved back and forth with a hand wheel ; the wheel has a micrometer dial which measures the amount the tool is moved forwards or backwards. The top slide can be set to make sliding cuts, surfacing cuts or taper cuts in short pieces of work. The length of cut you can make using the top slide only is usually restricted to about 50mm (2in).
For long surfacing cuts you should use the cross slide ; a long sliding cut can be made by moving the whole saddle using hand or power feed. If you need long taper cuts, you should set up the work in the lathe so its centre line is offset while the tool is moving parallel to the centre line of the lathe.
The main function of the headstock is to hold the lathe spindle, which is itself mounted in bearings; you can usually adjust the bearings to take up any wear or end play in them. The spindle is hollow so you can pass work through it if necessary ; it has an internal (morse) taper in its nose so the centres can be fitted. Some lathes have the outside of the spindle nose threaded so chucks or face plates can be screwed directly onto the spindle ; other models have a flange at this end of the spindle to which chucks or face plates can be bolted.
The spindle is driven by an electric motor which is attached to gears inside the headstock. On small lathes, however, the drive is usually by vee belt with gears: this is called the backgear and is engaged for the lower spindle speeds. Speed of rotation will vary according to the size and make of lathe; a typical speed range is 500-900 revolutions per minute, although it may be up to 2000. Provided with the lathe is a set of gears, called change wheels, which you can use to change the speed of the leadscrew and in some cases the direction of its rotation. There is a guard door at the left-hand side of the headstock which opens to give access to these gears. More expensive lathes have gear boxes which allow the speed of leadscrew rotation to be changed without manually substituting gear wheels.
To check this centre is true, set up a dial test indicator. Insert a centre (see below) into the spindle and set up the indicator so it touches the centre as it revolves; if the reading is constant there is no variation between the centre and the lathe bed.
The tailstock rests on the slideways on top of the lathe bed; it can be slid along the slideways to any convenient place and locked in position. On most lathes you can ‘set over’ the tailstock for a short distance, that is move it in a direction at right-angles to that of the axes of the centres. This facilitates taper-turning; you can also adjust the centre line of the tailstock to line up with the centre line of the headstock spindle for parallel turning. Barrel This is a hollow spindle which is mounted in the tailstock; it can be moved backwards and forwards inside the tailstock by turning a handle. The inside of the barrel has a morse taper so centres or tools with tapered shanks may be fitted. The barrel often has graduations on the outside so its movement can be accurately measured. When the barrel is fully drawn back into the tailstock, any tool inside it will be automatically pushed out. However, on small lathes designed primarily for model-makers, the barrel may have a hole right through it; in this case it is necessary to have a bar which can be inserted in the hole and used as an ejector.
The main function of the tailstock is to hold a centre, which supports the work mounted at the headstock end. You can also use drill and tap holders with a tapered shank, which mate with the taper in the tailstock barrel. Place the drill or tap into the holder and bring it up to the work by moving the barrel forward. You can now rotate the work while the drill or tap remains stationary. Centre alignment Unless you intend to do taper-turning, the centre line of the headstock spindle and tailstock barrel must coincide exactly. Do this by placing a centre in both headstock and tailstock and bringing the centres together; rotate the headstock spindle by hand and watch the centre points to detect any eccentricity.
To measure the tailstock offset, turn down a bar along its length, leaving a collar at each end. Turn down both collars, keeping the tool at the same setting, and measure the two diameters. The differ-en we in diameter is twice the distance by which the tailstock is offset from the headstock; if the collar diameter at the tailstock end is the smaller, the offset is to the front of the lathe and vice versa.