Let d be the diameter of **** in mm
Area being sheared = π.d.t = πd × 10 = 10πd mm2
Force required to punch the **** = Ultimate shear strength x area sheared
= 350 × 10πd = 3500πd (N)
Cross sectional area of the **** = (π/4)d2 mm2
Compressive stress on the punch
σC = 3500πd/{(π/4)d2} = 14000/d
But σC is limited to 800 N/mm2 and therefore
800 = 14000/d
d = 17.5mm