Plates Shear Machine: Principle of Work and Machine Categories

• 1
  What a plates shear machine actually does to the metal
• 2
  Blade angle, rake, and clearance in the plates shear machine cutting process
• 3
  The main categories of a plates shear machine
• 4
  What the cut edge tells you about the setup
• 5
  Which shear category fits which job

A plates shear machine does not remove material the way a mill or a laser does. It forces two edges past each other and lets the sheet fracture along a narrow band. One blade is fixed to the bed. The other moves down or across, driven by a hydraulic cylinder or a mechanical crank. The metal between them is squeezed past its yield point in a fraction of a second.

Two forces act at the same time. The blade edges push down with a vertical force, and the gap between the blades creates a bending moment. The sheet cracks from both top and bottom edges inward. If the clearance is right, the two cracks meet and the cut face shows a clean shear zone with only a small rollover at the top.

The blank holder matters as much as the blade. Without downward clamping near the cut line, the sheet lifts and rotates instead of fracturing. The result is a bowed edge, an oversized part, or a blade chip. On thin gauge work the holder force is light. On 6 mm plate it can exceed several tonnes.

Cutting speed on hydraulic shears is usually in the range of 20 to 60 strokes per minute for short beds. The number depends on stroke length and oil flow, not on material hardness. Heavier plate simply needs more tonnage per stroke, not more speed.

• 1
  Fracture, not cuttingThe edge separates by crack propagation, so clearance controls the edge more than blade sharpness.
• 2
  Clamping is not optionalBlank holders stop the sheet lifting and keep the cut line straight.
• 3
  Tonnage scales with thicknessDoubling sheet thickness roughly doubles the required force.

The cutting edge itself is ground to a small angle, typically 75° to 85° included. A sharper edge wears faster, so shops running mild steel all day tend to keep the edge blunter than a shop cutting 0.8 mm stainless. Edge radius grows with use, and once it passes roughly 0.05 mm the sheet starts to tear instead of shearing cleanly.

Rake angle is the tilt of the moving blade across the bed. A flat, parallel blade contacts the full sheet length at once, so it needs very high tonnage. Tilting the blade to 1°–3° concentrates the load at one point. A 4,000 mm bed at 2° rake can cut the same sheet with a fraction of the peak force, because only a short section of edge is engaged at any instant.

Clearance is the horizontal gap between the blades. For mild steel the usual value is 5% to 8% of sheet thickness. Stainless and aluminum sit lower, around 4% to 6%, because they work-harden and crack less readily. Too much clearance pulls material into the gap and produces a heavy burr. Too little clearance raises the force and can chip the edge.

Rake has a cost. The blade travels further to finish the cut, so the sheet bends downward more. Long thin parts can come off with a visible bow. If flatness matters more than tonnage, a smaller rake angle is the better trade.

• 1
  Edge angle 75°–85°Blunter edges last longer on mild steel, sharper edges suit thin stainless.
• 2
  Rake 1°–3°Lower peak tonnage, but more downward deflection on the offcut.
• 3
  Clearance 5%–8% of thicknessThe single setting that most affects burr height and edge quality.

Three categories cover most shop floor work. A guillotine shear uses a straight moving blade and a long bed, and it is the standard machine for straight cuts in sheet from 0.5 mm up to 6 mm or more. Bed lengths commonly run from 2,000 mm to 4,000 mm. It handles mild steel, stainless, and aluminum, and it is the machine most people mean when they say plates shear machine.

A swing beam shear carries the moving blade on two pivoting arms instead of vertical guides. The blade follows an arc, which gives a slight rake through the stroke and a naturally progressive cut. Swing beam machines are common for thinner sheet, and they are cheaper to build because the frame sees less bending load. They cut accurate straight lines but the arc limits how thick the sheet can be.

A variable rake shear lets the operator change the rake angle between jobs. Thin sheet runs at high rake for speed and low tonnage. Thick plate runs at low rake to control bow and keep the edge square. This is the most flexible category and the most expensive. For a shop that moves between 1 mm and 8 mm work on one machine, the variable rake pays for itself.

Other categories exist for specific work. Alligator shears cut bar and scrap. Rotary and disc shears cut circles and curves in thin sheet. None of them replace a guillotine for straight blanking. Plate shears are also distinct from press brakes, which bend rather than cut, and from ironworkers, which punch and shear with separate stations.

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  GuillotineStraight blade, long bed, thick and thin sheet, the general-purpose choice.
• 2
  Swing beamPivoting arms, lower cost, best on thin sheet with straight cuts.
• 3
  Variable rakeAdjustable angle for mixed thickness work, highest cost but widest range.

A correct shear cut has four zones on the cut face, read from top to bottom: rollover, shear zone, fracture zone, and burr. Rollover depth is usually 10% to 20% of thickness. A deep shear zone means clearance is on the tight side. A wide fracture zone with a rough surface means clearance is too large. Reading these zones is faster than measuring burr height with a caliper.

Burr height is the practical acceptance criterion for most sheet metal work. On 2 mm mild steel with correct clearance, burr height typically lands between 0.05 mm and 0.15 mm. If it climbs past 0.2 mm, check clearance first, then blade edge condition, then the blank holder force. Changing blade material before checking clearance wastes money.

Squareness is the other limit. A shear cuts a straight line relative to the back gauge, not relative to the sheet edge. If the back gauge is out of square by 0.1 mm over 1,000 mm, every blank inherits that error. Operators should check squareness with a try square on the first part of a run, not halfway through the stack.

Shearing has hard boundaries. It cannot cut curves, holes, or tapers in one pass without a nibbling step. It leaves a work-hardened edge on stainless, which matters if the part is later bent. It also cannot hold the ±0.005 mm tolerance that a mill or a grinder can. For profile accuracy on a finished part, shearing is a blanking step, not a finishing one.

• 1
  Read the four zonesRollover, shear, fracture, and burr tell you if clearance is right.
• 2
  Burr 0.05–0.15 mmTypical on 2 mm mild steel with correct clearance.
• 3
  Square to the back gaugeSheet edge squareness does not carry into the cut line.