THE CENTRE LATHE
The Centre Lathe is used to manufacture cylindrical shapes from a range of materials including; steels and plastics. Many of the components that go together to make an engine work have been manufactured using lathes. These may be lathes operated directly by people (manual lathes) or computer controlled lathes (CNC machines) that have been programmed to carry out a particular task. A basic manual centre lathe is shown below. This type of lathe is controlled by a person turning the various handles on the top slide and cross slide in order to make a product / part.
The headstock of a centre lathe can be opened, revealing an arrangement of gears. These gears are sometimes replaced to alter the speed of rotation of the chuck. The lathe must be switched off before opening, although the motor should automatically cut off if the door is opened while the machine is running (a safety feature).
The speed of rotation of the chuck is usually set by using the gear levers. These are usually on top of the headstock or along the front and allow for a wide range of speeds.
However, sometimes the only way to set the lathe to a particular speed is to change the gear arrangement inside the headstock. Most machines will have a number of alterative gear wheels for this purpose. :THE CENTRE LATHE - 'FACING OFF'
TURNING A SHORT TAPER
A very basic operation is called ‘facing off’. A piece of steel has been placed in the chuck and the lathe cutting tool is used to level the end. This is done by turning the cross-slide handle so that the cross-slide moves and the cutting tool cuts the surface of the steel.
Only a small amount of material should be removed - each pass of the cross slide. After each pass of the cutting tool the top slide can be rotated clockwise to move the tool forward approximately 1mm. This sequence is repeated until the steel has been levelled (faced off).
When using a centre lathe it is always advisable to work patiently and safely. Do not attempt to removed too much material in one go. At best this will caused damage to the steel being worked on and to the expensive cutting tool being used. At worse an accident will occur.
When turning a short taper the topslide is set a the required angle. This is normally done by loosening two small allen screws and then rotating the topslide to the angle and tightening back up the two allen screws.
When the chuck is rotating the topslide handle can be rotated slowly by hand in a clockwise direction. A small amount of metal is removed each time until the taper is formed. If too much steels stands out from the chuck the steel will vibrate and the surface finish will be very poor.[/center]TURNING A LONG TAPER
V. Ryan © 2003
Turning a taper on a long piece of material is best achieved by offsetting the tailstock. The material is held between two centres as a normal three jaw chuck cannot be used. A three jaw chuck will not hold the material safely or accurately when a long taper is being turned. Diagram ‘A’ below shows the side view of the two centres, the material and the toolpost.
Diagram ‘B’ shows a birds eye view (plan view) of the same equipment. However, this time it can be seen that the tailstock has been adjusted so that it is off centre. This means that a long taper can be turned. A small amount of material must be removed with each pass of the cutting tool. Also the lathe should be setup so that the cutting tool feeds automatically along the bed of the lathe. This will lead to a better finish to the surface of the material. The animation below shows a 'speeded up version' of taper turning. Normally taper turning takes along time.
Diagram ‘C’ shows the scale at the back of the tailstock. To adjust the tailstock so that it is off centre usually means loosening a number of allen screws, offsetting the tailstock and then tightening the screws back in position.[/center]DRILLING WITH THE CENTRE LATHE
V. Ryan © 2003
USING THE TAILSTOCK FOR DRILLING
The tailstock of a lathe can be used for drilling, with the aid of a drill chuck attachment. The drill chuck has a morse taper shaft which can be push into the shaft of the tailstock, locking it in position.
The usual starting point for drilling with a centre lathe is to use a countersink bit. This is used to drill slightly into the material and creates a starting point for other drills that are going to be used. Attempting to drill with a traditional drill bit without countersinking first will lead to the drill bit slipping straight away. It is not possible to drill a hole successfully or safely with out using a centre drill first.
If a long piece of material has to be turned on a lathe then a centre drill is used to produce the hole at one end. This allows the drilled end to be supported by the tailstock centre.
Once a hole has been produced by a centre drill, machine twist drills can be used to enlarge the hole and if necessary to drill all the way through. If a large diameter hole is needed then a small hole is drilled first (eg. 4mm dia). Then the hole is enlarged approximately 2mm at a time. Trying to drill a large diameter hole in one go will inevitably lead to the drill bit over heating and then jamming in the material. This is potentially dangerous.
When drilling, it is very important to use soluble oil as a coolant. This should be constantly fed onto the drill bit to keep it cool. This will help prevent jamming and over heating. Over heating will blunt the drill bit quickly.:HOW TO CENTRE THE CUTTING TOOL
V. Ryan © 2003
Before any turning takes place it is common practice to check that the point of the lathe tool is centred. This means that the lathe tool point should be the same height as the tip of the tailstock centre. If this is not done and the tool point is either above or below the centre point - usually the finish to the steel will be poor. Also, a significant amount of vibration could take place during turning.
The best lathe cutting tools are made from high speed steel. Diagram ‘A’ shows a typical solid lathe tool. The shank is clearly shown, this is the part that is fixed into the toolpost. Diagram ‘B’ shows a second type. This is a tool holder. A small lathe tool made of high speed steel is tightened into the cast steel tool holder. The advantage of this type is that the smaller lathe tools are cheaper to buy.TWO TYPES OF LATHE CUTTING TOOLS
[center]A SELECTION OF LATHE CUTTING TOOL PROFILESHOW TO USE A KNURLING TOOL
V. Ryan © 2003
A knurling tool is used to press a pattern onto a round section. The pattern is normally used as a grip for a handle. Apprentice engineers often manufacture screwdrivers. These have patterned handles, to provide a grip and this achieved through the technique called knurling. The pattern produced is called a ‘knurled pattern’.
This diagram shows the knurling tool pressed against a piece of round section steel. The lathe is set so that the chuck revolves at a low speed. The knurling tool is then pressed against the rotating steel and pressure is slowly increased until the tool produces a pattern on the steel.
The automatic control lever is engaged which starts the automatic traverse of the saddle. As the saddle moves along the bed of the lathe the knurled pattern is pressed into the steel along its length.
If the traverse of the lathe is stopped and then reversed a diamond pattern is produced.
Depending on the knurling tool selected, a variety of knurled patterns can be produced. Three typical patterns are seen opposite[/center]