CENTRE OF PRESSURE
When the shape of blank to be cut is irregular, the summation of shear forces about the centre line of press ram may not be symmetrical. Due to this, bending moments will be introduced in the press ram, producing misalignment and undesirable deflections. To avoid this the “centre of pressure” of the shearing action of the die must be found and while laying out the punch position on the punch holder, it should be ensured that the centre line of press ram passes exactly through the centre of pressure of the blank. This “centre of pressure” is the centroid of the line perimeter of the blank. It should be noted that it is not the centroid of the area of the blank.
The centre of pressure can be found out by the following procedure:-
1.An outline of the piece part is drawn.
2.The X and Y axes are placed on it in a convenient position.
3.The outline of the piece part is divided into convenient line elements. These are numbered as 1,2,3 and so on.
4.The lengths l1,l2,l3 etc. of these line elements are determined.
5.The centroids of these line elements are determined.
6.The distance of the centroids from the X and Y axes is determined. Let x1,x2,x3etc. and y1,y2,y3 etc. be the distance of centroids of line elements l1,l2,l3 etc, from the X and Y axes respectively.
7.The distance of the centre of pressure from each axis is determined by the method of centroids. i.e.,
X =l1x1+l2x2+l3x3+………/l1+l2+l3+………………..
And
Y = l1y1+l2y2+l3y3+……/ l1+l2+l3+……………..
Where,
X = x distance to centre of pressure
Y = y distance to centre of pressure
Example for how to calculate centre of pressure
When the shape of blank to be cut is irregular, the summation of shear forces about the centre line of press ram may not be symmetrical. Due to this, bending moments will be introduced in the press ram, producing misalignment and undesirable deflections. To avoid this the “centre of pressure” of the shearing action of the die must be found and while laying out the punch position on the punch holder, it should be ensured that the centre line of press ram passes exactly through the centre of pressure of the blank. This “centre of pressure” is the centroid of the line perimeter of the blank. It should be noted that it is not the centroid of the area of the blank.
The centre of pressure can be found out by the following procedure:-
1.An outline of the piece part is drawn.
2.The X and Y axes are placed on it in a convenient position.
3.The outline of the piece part is divided into convenient line elements. These are numbered as 1,2,3 and so on.
4.The lengths l1,l2,l3 etc. of these line elements are determined.
5.The centroids of these line elements are determined.
6.The distance of the centroids from the X and Y axes is determined. Let x1,x2,x3etc. and y1,y2,y3 etc. be the distance of centroids of line elements l1,l2,l3 etc, from the X and Y axes respectively.
7.The distance of the centre of pressure from each axis is determined by the method of centroids. i.e.,
X =l1x1+l2x2+l3x3+………/l1+l2+l3+………………..
And
Y = l1y1+l2y2+l3y3+……/ l1+l2+l3+……………..
Where,
X = x distance to centre of pressure
Y = y distance to centre of pressure
Example for how to calculate centre of pressure
All dimensions are in cm
Elements l x y lx ly
1 10 5 0 50 0
2 1.25 10 0.625 12.5 0.781
3 6.25 6.875 1.25 42.969 7.813
4 7.5 3.75 5 28.125 37.5
5 1.25 3.125 8.75 3.906 10.937
6 7.5 2.5 5 18.75 37.5
7 2.5 1.25 1.25 3.125 3.125
8 1.25 0 0.625 0 0.781
--------------- ----------- -----------
37.5 159.375 98.437
∑l = 37.5
∑lx = 159.375
∑ly = 98.437
There fore, X = ∑lx/∑l = 159.375/37.5 = 4.25
Y = ∑ly/∑l = 98.437/37.5 = 2.625
