Identify and explain function, and safely machine components while using a milling machine and lathe

SUBJECT OUTCOME
4.1 Use a centre lathe to produce a component
Identify and explain
 
The function of the centre lathe with respect to energy flow.  Video
 
The lathe spindle is the main component on the lathe.  Power transmission begins at the motor shaft. The shaft is either keyed to a pulley for belt driven lathes or to a gear which is engaged with a system of gears (like in a car gear box).

Belt driven lathes will have to be stopped to move belt position when changing speeds. Reversing spindle rotation requires the use of an idler. So, using the reverse lever in a lathe engages the idler which reverses the spindle rotation.

Gear driven lathes use dog gears to engage different sets of gears for varying speeds. Power transmission is a bit more complex in these lathes and special care has to be taken to ensure metal chips and debris do not enter the gear box. Maintenance on these machines is also a bit more expensive but the power transmission is smooth, compact and easy.

The feed shaft and lead shaft is also driven from the gearbox. Feed can be adjusted from the gearbox as well pitch for cutting screw thread with the lead shaft.

It must be noted that spindle speeds cant be changed before the motor have not come to a complete stand still. Care must also be taken not to change spindle direction before the motor have come to a complete stand still as failure to do this can lead to severe damage to the gearbox.

Chuck can be rocked forward and backwards by hand to facilitate easy gear engagement.  Never force gears into position.

 
Spindle speed selection Video    
     
The lathes are designed to operate at various spindle speeds for machining of different materials. There speeds are measured in RPM (revolutions per minute) and are changed by the cone pulleys or gear levels. One a belt-driven lathe, various speeds are obtained by changing the flat belt and the back gear drive.

Steps to be followed setting spindle speed:

  • Calculate the RPM according to work piece material, diameter and machining operation
  • Make sure that machine is at a complete standstill before attempting to change speeds
  • On speed table, select the closed speed to calculated speed
  • Move selector levers to correct position according speed table
  • Never force levers, rock chuck forward or backward for smooth engagement
  • On large centre lathe, use jock switch for assistance on engagement.

The function and operation of the components that make up a centre lathe
 
Clamping devices:
Thee jaw chuck Video    
Three-jaw universal chuck is used to hold round and hexagonal work. It grasps the work quickly and within a few hundredths of a millimetres or thousandths of an inch of accuracy, because the three jaws move simultaneously when adjusted by the chuck wrench.

Three Jaw, self centring chuck. One jaw has been removed and inverted to show the teeth that engage with the scroll plate and thereby allow movement. Underneath the image is the chuck key used to rotate the scroll and therefore operate the chuck.

 

 

 Advantages of three jaw chuck:
  • Can hold hex and round bar-stock
  • Quick and easy to use
  • Self centring
  • Parallel, facing and boring operations can be done
  • Internal and external jaws available

Disadvantages of three jaw chuck:

  • Can't hold square bar-stock
  • Run-out/off-centre can't be easily fixed
  • Can't hold irregularly shaped work
  • Can't run off-centre
Four jaw chuck    
This four- jaw independent chuck has four jaws; each of which can be adjusted independently by a chance wrench.

They are used to held wound, square, hexagonal, and irregular-shaped work pieces. The jaws can be reversed to hold work by the inside diameter.

 

 

 

Advantages of four jaw chuck:
  • Work can be centred to high precision
  • Can handle square/rectangular bar
  • can turn work off-centre
  • slightly more grip on round stock
  • Internal and external jaws available

Disadvantages of four jaw chuck:

  • Slower and difficult to mount work (dial-indicator required)
  • can't hold hex-stock
Faceplate Video 1  Video 2    
A lathe faceplate is a basic work holding accessory for a centre lathe. It is a circular metal (usually cast iron) plate which fixes to the end of the lathe spindle. The work piece is then clamped to the faceplate, typically using t-nuts in slots in the faceplate, or less commonly threaded holes in the faceplate itself.

It may be necessary to use counter balancers when there is vibration.

An angle pate can also be used on faceplate when a part such as a pedestal bearing has to be machined.

     
Collet chuck Video    
The three-jaw powered chuck is the standard work holding device for most  lathe users. This type of chuck is versatile enough to be used in a wide range of turning applications. However, it's not the best chuck for all jobs. The Collet chuck is an alternate work holding device that, like the jaw chuck, also uses mechanical force to hold the part being turned. While a Collet chuck lacks the capacity for the same wide range of work piece sizes that a jaw chuck can accommodate, it offers advantages related to speed, accuracy and productivity that may be crucial for certain jobs.

Collet chucks cater for standard stock material such as 6 mm, 8 mm etc round bar.

 

 Collet Chuck Advantages
  • Light weight
  • Fast acceleration
  • Less affected by centrifugal force
  • Tight concentricity
  • Fast clamping
  • Fast Collet change for part changeover

Collet Chuck Drawbacks

  • Limited range of work piece sizes
  • Large axial dimension
  • Better suited to small parts
  • Better suited to work pieces with consistent diameter

Mandrels Additional website Video    
Sometimes it is necessary to machine the outer surfaces of cylindrical works accurately in relation to a hole concentric that has been previously bored in the centre of the work. In such cases the work is mounted on a device known as a mandrel.

Lathe mandrels are devices used to hold the job for machining on lathes. They are mainly used for machining outside diameters with reference to bores which have been duly finished by either reaming or boring on a lathe.
 


 

 Types of mandrels

 

 

 
Driving Plate (Catch pate)    
Catch plates are either screwed or bolted to the nose of the threaded, stock spindle. A projecting pin from the dog fits into the slot provided in catch plate. This imparts a positive drive between the lathe spindle and work piece.