what is the centre of gravity of an arc(as opposed to sector) and that of the periphery of triangle(not the area of the triangle)? I'm supposed to use it for calculating blank diameter before drawing and for calculating centre of pressure during blanking.
You can evaluate those shapes with the usual integral for the center of gravity (should be easy to find in any textbook, wikipedia or other web pages). For the triangle, it is easier: Calculate the position for each side, and calculate the weighted average (with mass as weight) of them afterwards.