Buckling of punches
The punches in a press tool are subjected to compressive stresses during the cutting operation.If these stresses are overlooked thin punches in the tool may fail by buckling.The maximum force that a punch can withstand with out buckling can be caluculated from the following formula.
FB=pie square EI/Ip square
Fb=Maximum force beyond which buckling occurs
E=Modulus of Elasticity of punch material (in GN/mm square)I=Moment of inertia in mm 4
Ip=length of punch in mm
The cutting force coming on the punch should be less than or equal to the buckling force.
Worked out examples:Is it possible to punch 1mm brass sheet with a5mm square punch.
Length of punch=60mm,br>T max=320N/mm square
l=cut length in mm
s= sheet thickness in mm
T max=shear force in N/mm square
Shear force=20*1*320=6400N=6.4kN
Buckling force=pie square E I/Ip square
E=210 GN/mm square
I=a^4/12=5^4/12=52.08mm^4
Ip=60mm
Buckling force=(3.14^2*210*52.o8*10^-12/0.06^2)=29.9835kN
The punch can withstand a force of 29.9835kn.The force coming on the punch is only 6.4kN.Therefore it is possible to use the punch.
