shear

A shear is a powered cutting machine that forces two hardened blades past each other to cut sheet metal, plate, or bar stock in a single clean action. The upper blade is fixed to a ram that drives downward, while the lower blade sits stationary on the bed. Material is placed between them, and hydraulic or mechanical force brings the ram down with enough pressure to sever the workpiece cleanly without distortion.

Sheet metal shears come in several configurations. A guillotine shear cuts across the full width of the blade in one stroke, best for straight cuts on rectangular sheet. A rotary shear uses rotating blades like powered scissors and handles curved cuts or longer material. Circle shears feature a small blade assembly that rotates around a pivot, used for cutting discs and tight shapes. Industrial machines range from bench-mounted units taking 16-gauge mild steel up to massive hydraulic shears rated for 1/2-inch plate across a 12-foot width.

The critical specification is shear capacity, expressed as thickness and material grade. A machine rated for 1/4-inch mild steel cannot safely cut 1/4-inch stainless at the same amperage; different alloys require different loads. Blade gap, or clearance, is equally important. Too large a gap causes the material to bend and buckle rather than separate cleanly. Too small a gap dulls blades fast and requires excessive force. Most production shears allow adjustment of this clearance to within 0.010 inch.

Dull or chipped blades produce the most common problems: rough, distorted edges and increased energy consumption. Blade life depends entirely on material composition and thickness. Mild steel generates far less blade wear than stainless or aluminum alloys. The machine should never be forced past its rated capacity, as this breaks blades and can damage the ram or frame.

The term "shear" comes from the shearing action itself, the same fundamental cutting principle found in scissors or pruning shears, scaled to industrial proportions. In fabrication shops, shearing is often the first operation after material arrives from supply, preceding bending, welding, or stamping. It competes with plasma cutting for rough stock preparation, though shearing produces no heat distortion and no need for secondary edge finishing on clean cuts.