Large CNC shear machine: how a straight cut is actually made

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  What a large CNC shear machine does to the plate
• 2
  Rake angle and why large plate changes the rules
• 3
  Hold-down force, back gauge, and cut accuracy
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  How material choice changes shear behaviour
• 5
  What the sheared edge means for the next operation
• 6
  Shearing versus other plate cutting methods

Shearing is not sawing and it is not laser cutting. The upper blade descends and forces the plate past a fixed lower blade. The metal does not turn into chips. It fractures along a narrow band once the local stress passes the material's shear strength. The fracture runs from the top edge down toward the lower blade, so the cut face has a small rollover at the top, a burnished band in the middle, and a rougher fracture zone near the bottom.

That band structure is why the cut face looks different from a milled edge. A sheared edge on 6061-T6 aluminium typically shows rollover of 5–15% of thickness and a burnished zone of 40–60%. The fracture zone takes the rest. On 304 stainless the burnished band is narrower and the fracture zone rougher, which matters if the edge will be visible.

Blade clearance is the variable that moves those numbers most. Set it too tight and the blades rub, wearing the edge and raising tonnage. Set it too wide and the plate tears instead of shearing, leaving a burr you have to grind off. A practical starting range is 6–10% of plate thickness for mild steel, 8–12% for stainless, and 5–8% for aluminium.

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  RolloverRounded top edge, 5–15% of thickness
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  Burnished bandSmooth middle zone, 40–60%
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  Fracture zoneRough lower band, the rest
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  BurrControlled by clearance and blade sharpness

The upper blade is not level. It sits at a rake angle, usually 1–3° on a large CNC shear machine. The blade contacts the plate progressively instead of all at once. That single geometry choice cuts the required force a lot. A level blade on a 4,000 mm cut would need tonnage that no practical frame can carry.

The trade-off is angular distortion. The cut edge is not perfectly straight in the vertical plane; it leans slightly, and the plate can bow as the blade walks across it. On short cuts you never see it. On a 3,000 mm cut in 6 mm mild steel, a 2° rake can push the far end of the plate down by a measurable amount.

Thin plate suffers more. A 1.5 mm sheet has little bending stiffness, so the rake pushes it into the lower blade gap and the edge curls. This is why thin-gauge work often moves to a guillotine with a lower rake, or to a different process entirely.

Thick plate has the opposite problem. Required tonnage rises roughly with the square of thickness, so a 12 mm plate on a 4,000 mm bed is near the limit of many machines. Check the tonnage chart before quoting the job, not after.

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  1–3° rakeTypical range on large beds
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  Lower rakeLess distortion, more tonnage
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  Higher rakeLess tonnage, more edge lean

The hold-down bar clamps the plate against the bed before the blade moves. Without it the plate lifts, slides, or rotates, and the cut wanders. Hydraulic hold-downs apply pressure across the full width, usually in zones so a narrow strip is not crushed.

Back gauge positioning sets the cut length. On a large CNC shear machine the back gauge runs on ballscrews with servo drives, and repeatability of ±0.05 mm is normal. That is the positioning number, not the edge quality number. Do not confuse them.

Squareness depends on the plate being seated against the back gauge and the squaring arm. A plate that is not fully seated cuts out of square by 0.5–2 mm over a metre, which is far more than any positioning error. Operators check this with a light gap, not with a caliper.

Repeat runs of the same part benefit from a program. Store the rake, clearance, back gauge position, and hold-down pressure for each material and thickness. Changing material without changing clearance is the most common cause of a sudden burr.

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  Hold-downPrevents lift, slide, and rotation
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  Back gauge±0.05 mm repeatability typical
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  SeatingSquareness starts at the squaring arm

Mild steel shears cleanly across a wide thickness range. A36 and 1018 behave predictably, and clearance charts for them are well established. This is the easiest material to plan around.

Stainless steel work-hardens at the cut edge. The burnished band is narrower, the fracture zone rougher, and the burr harder to remove. Blades wear faster. If the part needs a clean edge, plan a machining pass or a finishing operation rather than expecting the shear to deliver it.

Aluminium shears easily but galls. Soft tempers like 5052 and 6061 in the O condition tend to pick up on the blade and leave a smeared edge. Higher tempers like 6061-T6 and 7075 cut more cleanly. Keep blades sharp and clearance on the tight side.

Titanium and high-nickel alloys are rarely sheared in production. They need high tonnage, they spring back, and the edge quality is poor. For TC4 or Inconel, plan on milling or waterjet instead.

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  Mild steelPredictable, wide clearance window
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  StainlessWork-hardens, faster blade wear
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  AluminiumGalls when soft, cuts well in T6
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  TitaniumAvoid shearing in production

A sheared edge is not a finished edge. It carries a burr, a rolled top corner, and a work-hardened band roughly 0.1–0.3 mm deep. If the part is a blank that will be milled, that zone disappears in the first pass and the shear was the right choice.

If the edge stays visible, the burr has to go. Options are grinding, deburring, tumbling, or a light milling pass. Each adds cost and time. Decide this at quoting, because it changes the routing.

Flatness is the other handoff issue. Rake-induced bow shows up after shearing and can be worse after stress relief. A blank that is 0.5 mm out of flat may not seat in a fixture. For parts with tight flatness callouts, shear first then face both sides.

At GreatLight we run shearing as a first operation and then finish on 3-axis, 4-axis, or 5-axis machining centers, with tolerance held to ±0.005 mm and surface finish from Ra 0.2–0.8 μm when the drawing calls for it. The shear sets the blank; the mill sets the tolerance.

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  BlankingShear, then machine all faces
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  Visible edgeAdd deburr or finish pass
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  FlatnessShear, then face both sides