Lathe Operations: Facing

Facing Operations Facing is the process of removing metal from the end of a workpiece to produce a flat surface. Most often, the workpiece is cylindrical, but using a 4-jaw chuck you can face rectangular or odd-shaped work to form cubes and other non-cylindrical shapes.

When a lathe cutting tool removes metal it applies considerable tangential (i.e. lateral or sideways) force to the workpiece. To safely perform a facing operation the end of the workpiece must be positioned close to the jaws of the chuck. The workpiece should not extend more than 2-3 times its diameter from the chuck jaws unless a steady rest is used to support the free end. Cutting Speeds

If you read many books on machining you will find a lot of information about the correct cutting speed for the movement of the cutting tool in relation to the workpiece. You must consider the rotational speed of the workpiece and the movement of the tool relative to the workpiece. Basically, the softer the metal the faster the cutting. D…

Precautions to be Observed while Shaping Jobs between Centres

  • The alignment of the centre is to be checked (whether it is parallel to the movement of the ram) and adjusted before starting the work.

  • The moving tool should not come and dash against the head stock.

  • For long work pieces, proper support should be given (A jack is suitable for it)

  • The work is to be guarded against turning centres, while the tool cuts, with proper locking device.

Checking the Vice Jaws for Squareness and ParallelismChecking the Vice Jaws for Squareness and Parallelism

For checking the vice jaws for its squareness, place dial test indicator in the tool holder so that the indicator touches the finished surface. Then move the table with the help of cross feed back & forth and note the variation on the indicator.

Setting the Length of Stroke

The following procedure is to be adopted while setting the length of the stroke.Setting Length of Stroke

  • Bring the ram to the rear most position.

  • Loosen the knurled nut.

  • Turn the crank (stroke adjusting crank) till the desired length of the stroke {length of job + 3/4” (1/4” front clearance+1/2” back clearance)}.

  • Tighten the knurled nut.

Position of Tool.   The following procedure is to be adopted while positioning the tool:

  • Bring the ram to the rear most position.

  • Loosen the ram clamp.

  • Turn the crank (until the tool is a little distance away from the work)

  • Tighten the ram clamp.

Normally the tool clearance should be more at the rear end of the job than of the front end so that the tool block is enabled to be set properly for the next cut. The tool holder is to be set so that the tool bit does not extend more than about 2” from the tool post. The over hanging of the tool and tool slide should be as less as possible for rigidity. The tool head and the cutting tool should be vertical and the clapper box turned away from the direction to which it is being fed .This position of the clapper box prevents the tool from digging into the work and it will swing away from the finished surface on the return stroke.

Transmission of Power and Quick Return Motion

The power is transmitted through a driving shaft and a pinion to a large gear known as bull gear. On the face of bull gear there is a dovetail sliding way in which the crank block AB is carried. The sliding block B slides in the slot of the rocker arm CD. The crank block and the sliding block are linked by means of a crank pin. One end of the rocker arm CD is pivoted to the frame at C and the upper end is coupled to the machine ram at D through the small link lever DE. The crank block and the crank pin can be moved across the face of the Bull gear either towards or away from its centre to vary the length of the ram stroke.

Principle of Quick Return Motion

Bull gear, which receives drive through the pinion, revolves at a constant speed about its centre. As the Bull gear revolves, the crank block also revolves about the centre and causes the sliding block through the crank pin to slide up and down in the slot of rocker arm. It circumscribes an imaginary circle of its path of revolution. This causes the rocker arm to oscillate about its pivoted point. As this is coupled to the machine ram at the upper end, its oscillating movement is transferred to the ram as a reciprocating motion. Ram, therefore, moves forward and backward in the dovetail slide ways machined on the top of the frame.

When the ram is either in its foremost position or in its rear most position, the position of the rocker arm is found to be tangential to the imaginary circle circumscribed by crank block. This causes the crank block to fall below the centre of the bull gear.

When the rocker arm is in the position of CD1 tangential to the imaginary circle the ram will be at the extreme backward position of stroke, and when it is at CD2 the extreme forward stroke position would be reached.

The forward stroke takes place when the crank rotates through the angle B1KB21), and the return occurs when it rotates through the angle B1LB2 2). So the peripheral area of the imaginary circle covered during forward or cutting stroke is more than the peripheral area covered during the backward or non-cutting stroke. Hence to cover more area it needs more time and to cover lesser area it takes less time. But at the same time the displacement of ram is equal to both the ways.

The direction of rotation is so arranged that the forward stroke takes more time than the return stroke. Hence time lost is very much minimized by making the return stroke faster.


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