A hydraulic metal shear does not really cut steel. It fractures it, along a line controlled to within a fraction of a millimetre. The cycle begins when a row of hydraulic hold-downs clamps the sheet against the lower blade. Twin cylinders then drive the upper blade downward, and the material passes through three distinct phases: - Rollover, where the blade indents the surface and the edge deforms plastically before any separation occurs - Burnish zone, where the metal flows in pure shear and leaves a smooth, polished band, often around one third of the thickness - Fracture zone, where cracks initiate at both blade edges, propagate through the section and meet, leaving a rough, granular face Whether those cracks meet cleanly depends mostly on blade clearance. For mild steel, typical practice sets the gap at roughly 5 to 10 percent of sheet thickness. Too tight and secondary shearing appears, doubling the burnish band and accelerating blade wear. Too loose and the sheet is dragged int...