Precautions to be Observed while Shaping Jobs between Centres

Shaper Centres These are a pair of centres similar to the head stock and tail stock of a lathe. The tail stock centre is used for supp... thumbnail 1 summary
Shaper Centres

These are a pair of centres similar to the head stock and tail stock of a lathe. The tail stock centre is used for supporting one end of the job and the head stock is for rotating the work.

There are some classes of work those cannot be completed on a lathe or a milling machine and for finishing such work the shaper centres are used. In setting up the job, which is roughly machined in a lathe is placed between the shaper centres using the same centre holes on the work.

Precautions to be Observed while Shaping Jobs between Centres
(a) 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.
(b) The moving tool should not come and dash against the head stock.
(c) For long work pieces, proper support should be given (A jack is suitable for it)
(d) The work is to be guarded against turning centres, while the tool cuts, with proper locking device.

Checking 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 feedback & forth and note the variation on the indicator.
Checking the Vice Jaws for Squareness and Parallelism
Checking the Vice Jaws for Squareness and Parallelism
Setting the Length of Stroke
The following procedure is to be adopted while setting the length of the stroke.

(a) Bring the ram to the rear most position.
(b) Loosen the knurled nut.
(c) 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)}.
(d) Tighten the knurled nut.

Position of Tool
The following procedure is to be adopted while positioning the tool:
(a) Bring the ram to the rear most position.
(b) Loosen the ram clamp.
(c) Turn the crank (until the tool is a little distance away from the work)
(d) 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 overhanging 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.

Arc B2 K B1 Cutting Stroke, Arc B2 L B1 Non-Cutting Stroke, B2,    B1,    K,    L    Imaginary circle made by sliding block

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 B1KB2 (θ1), and the return occurs when it rotates through the angle B1LB2 (θ2). 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.

Causes of Chattering in Shaper
(a) The tool being suspended too far from the tool holder.
(b) Job not being rigidly held either on the vice or on the table
(c) Ram gib not adjusted properly.
(d) Too much of front clearance of the tool.

Cutting Speed and Feed
In a shaper, the cutting speed is the rate at which the metal is removed by the cutting tool. This is expressed in meters per minute. In a lathe as cutting action is continuous, the cutting speed is expressed by the peripheral speed of the work. But in a shaper the cutting action is intermittent. In shaper the cutting speed is considered only during the forward cutting stroke.

Cutting Speed Calculations - The cutting speed in a shaper is expressed by the formula,

In practice, to calculate the cutting speed it is difficult to measure exactly the time taken during the forward cutting stroke. The ratio between return time to cutting time and the number of double strokes per minute or RPM of the bull gear should be know the cutting speed may be obtained from the equation --- (I)
Let,      L = the length of cutting stroke in mm
            m = the ratio between return time to cutting time
            n = the number of double stroke of the ram per minute or rpm of the bull gear
            V = Cutting speed expressed in m/min From the equation number (I)

The cutting speed so calculated is the average cutting speed as it has been assumed that the cutting stroke is completed at the uniform speed. But, in reality, in a crank driven shaper cutting speed and return speed are not uniform.

Feed
Feed is the relative movement of the tool or work in a direction perpendicular to the axis of reciprocation of the ram per double stroke and is expressed in mm. The feed is always given at the end of the return stroke when the tool is not cutting the metal. The selection of feed is depending upon the kind of metal, type of job, etc.

Depth of Cut
Depth of cut is the thickness of metal that is removed in one cut. It is perpendicular distance measured between machine surface and non-machine surface of the work piece.

Machining Time
If the length of cutting stroke, breadth of job, feed and cutting speed is known, the time required to complete the job may be calculated as follows:  Let
L = the length of stroke in mm B = the breadth of work in mm
s = feed expressed in mm/ double stroke m = the ratio of return time to cutting time
v = the cutting speed is meter / minute Then from equation -------------- (II),

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