POSSIBLE MOVEMENTS OF WORK PIECE

3. POSSIBLE MOVEMENTS OF WORK PIECE

3-2-1 METHOD OF LOCATION:

A work piece can be positively arrested by means of six pins so positioned that they collectively they restrict the work piece in nine of its degrees of freedom. This is known as Six point Location method.

Out of the six pins, three of these pins are in the first plane and two pins are in the 2nd plane perpendicular to the first and one pin is in the plane which perpendicular and adjacent to both 1st and 2nd planes. Thus this method of location is also known as 3-2-1 method of location.

The figure-3.1 shows the prism resting on the three pins ABC. The faces of three pins supporting the prism form a plane parallel to the plane that contains the x and y axes. The prism. Cannot rotate about the x and y-axes and it cannot move downward in the direction of freedom 5. Therefore freedoms 1,2,3,4, and 5 have been restricted. In the figure given two additional pins D and E whose faces are in a plane parallel to the plane containing the x and z-axes prevent rotation of the prism about the z-axis. It is not free to move to the left in the direction of 8. Therefore, 6, 7, 8 have been restricted and the prism cannot rotate. The addition of pin F as shown in figure, freedom 9 is restricted. Thus by means of six locating points three in a base plane, two in a vertical plane and one in a plane perpendicular to the first two, nine degrees of freedom have been controlled. The remaining three freedoms are controlled by means of damping devices which can take care of any variation in dimensions of work piece in the direction of freedoms 10.11 and 12. Hence 3-2-1 principle should be used to locate work pieces. The reason for placing 3 point in the first plane is that this is the smallest number of point or which a work can seat and a plane can be drawn through three points. If a body rests on two points only, it can lean over to one side. It is not, also advisable to use more than three fixed points in the first plane as any extra point of support become redundant. This is clear from the fact that three legged chair can be easily placed on any type of floor, whereas a four legged chair will not contact the floor if, is not absolutely plane.

The six (3-2-1) point location principle may be summered as 'To locate a work piece fully, place and hold it against three point in a base plate, two points in a vertical plane and one point in a plane square with the first two planes. The three point of support in the first plane should be selected that the weight of the work piece is evenly distributed on them and the Centre .of gravity of the work piece is positioned property with respect to them. And the six points suitably chosen and located, the work piece is held firmly and any machining done on it will not spring, strain or distort it.

SIX POINT LOCATION PRINCIPLE

Fig: 3.1

IN CONJUNCTION WITH THE CLAMPING SYSTEM:-

Pads 1, 2 & 3 (A, B, C) - Constrain work piece along Z-Z’, About Y-Y' and X-X'

Pads 4 & 5 (D; E) - Constrain work piece along Y-Y' About Z-Z'

Pad 6 (F) - Constrain work piece along X-X' About Z-Z'

THE WORKPIECE IS THEREFORE FULLY CONSTRAINED.

SIX POINT LOCATION OF A THREE LEGGE.O OBJECT:

The principle of locating a three legged object is illustrated in Fig.3.2 (a). The object is resting on three .pointed legs on: a flat surface. It is thus prevented from moving Along CO and restrained from rotating along EF and AB axis. The AB, CD and EF Are axis. In the Fig.3.2 (b), the front legs are made ball ended and are made to rest on a V- groove. The spherical balls make a perfect mating surface with the V. The object is now restrained 'to move along. AB and is prevented from rotation About CD. In the Fig.3.2(c), the rear leg also made ball ended and is made to rest on a separate V-groove set at right angles to the first one. The body is now perfectly restrained against any directional movement and is correctly located on the base Plate.

LOCATION OF A CYLINDER ON AV-BLOCK

The analysis of the principle of location of a cylinder mounted on a V-block is illustrated in Fig.3.3. The cylinder is restrained from any movement along CD and EF, and is prevented from rotation about the axis CO and EF. The body is free to Move along AB and can also rotate about the axis AB. 'the free movements of the Work are shown by dotted arrows. In order to locate the completely, certain other Locating arrangements must be incorporated in addition to the V-block.

 

 

Figure 3.2 Six point location of a three legged object

Figure 3.3 Location of a cylinder on a V-block

THREE PINS RESTRICT FIVE DEGREES OF FREEDOM. PART IS SINGLE DEFINED

FIVE PINS RESTRICT EIGHT DEGREES OF FREEDOM. PART IS DOUBLE DEFINED

SIX PINS RESTRICT NINE DEGREES OF FREEDOM.PART IS FULLY DEFINED

Fig: 3.4