What Is CNC Shearing Machine?

• 1
  How a shear makes a cut
• 2
  Blade clearance and the other settings that matter
• 3
  What the cut edge actually looks like
• 4
  Where a shear sits in a fabrication sequence
• 5
  Thickness, width, and when a shear is the wrong call
• 6
  Shear vs laser vs punch for flat blanks

A shear works on the guillotine principle. A moving upper blade, called the ram, travels down and pushes the sheet past a fixed lower blade. The two edges are offset by a small gap, so the metal does not tear. It fractures along a controlled line. The cut is straight, full width, and finished in one stroke.

The CNC part is the back gauge. A servo-driven finger stops the sheet at a programmed distance from the blade, so the cut width is set by numbers, not by an operator reading a scale. Position repeatability on a modern back gauge sits around ±0.05 mm. That is enough for most blank work, though not for a finished tolerance callout.

The controller also sequences the hold-down clamps, the ram stroke, and the return. On hydraulic machines the stroke length and pressure are set in the program. On mechanical machines the clutch is engaged per stroke. Either way, the operator loads a program and repeats it.

Cutting does not remove material. There is no kerf. The sheet splits along a shear plane, and the two halves are the same length they were before. That is why a shear is fast and cheap per cut, and why it cannot cut a curve, a hole, or a slot.

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  RamUpper blade holder; travels down along linear guides or a pivot.
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  Back gaugeProgrammed stop that sets cut width; repeatability about ±0.05 mm.
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  Hold-downsHydraulic pads that clamp the sheet just behind the cut line.

Blade clearance is the gap between the upper and lower cutting edges. It is the single setting that decides whether the edge is clean or torn. For mild steel the working range is roughly 5% to 10% of sheet thickness. A 2 mm mild steel sheet runs about 0.10-0.20 mm clearance. Stainless needs slightly more because it work-hardens at the cut.

Set clearance too tight and the blades rub. You get a burr on the top edge, accelerated blade wear, and on thick plate the ram can stall. Set it too wide and the sheet tears instead of shearing. The cut face shows a rough break zone and a pronounced roll-over on the top edge. Both faults show up in the same part if clearance drifts across the bed.

Rake angle is the angle of the upper blade to the sheet. A rake of 1° to 2° means the blade contacts the sheet progressively rather than all at once. This lowers the peak force a lot, which is why a 6 mm capacity machine can cut a 3,000 mm wide sheet without a massive frame. The trade-off is that the cut edge bows slightly. Short rake angles give straighter edges but need more tonnage.

Stroke speed and hold-down pressure also matter. Fast strokes on thin sheet can whip the offcut. Low hold-down pressure lets the sheet slip, and a slipped sheet cuts oversize or leaves a step where the ram re-enters. On our hydraulic machines the hold-downs are set per material: light for aluminium, heavier for stainless and steel plate.

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  Clearance5-10% of thickness for mild steel; a little more for stainless.
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  Rake angle1-2° lowers cutting force but adds slight edge bow.
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  Hold-down pressureSet per material; too low and the sheet slips forward.
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  Back gauge positionSets cut width; re-zero after any blade change.

A sheared edge has four zones, and you can read them with a magnifier. At the top there is roll-over, where the blade pushed the metal down before it bit. Below that is the burnished zone, a smooth band where the blade slid through. Then the fracture zone, a dull rough band where the metal tore. At the bottom, a burr if clearance is off or the blade is dull.

A good setup gives a roll-over of maybe 10-20% of thickness, a wide burnished band, and a small burr you can remove with a light deburr pass. A bad setup flips that: little burnish, a tall fracture zone, and a burr that stands up 0.1 mm or more on thin sheet. That burr matters if the blank feeds into a press brake or a welding fixture.

As-machined sheared edges land around Ra 1.6-3.2 μm on the burnished band, but the fracture zone is much rougher. If a drawing calls a surface finish on a sheared face, flag it. Shear is a blanking process, not a finishing process. If the edge is a functional sealing face or a bearing seat, plan a machining pass after shear.

Harder materials behave differently. Stainless 304 and 316 work-harden at the cut, so the burnished band is narrower and the burr is tougher. Titanium and Inconel shear with more springback and more blade wear, so clearance moves toward the high end of the range. Aluminium 6061 shears cleanly but marks easily under the hold-downs.

The shear is a first-operation machine. It turns a standard sheet into a blank of the right outside size. Everything after that works on the blank: laser or punch for holes and profiles, press brake for bends, then welding and finishing. If the blank is wrong, every downstream step inherits the error.

The strength of shear is speed on long straight cuts. A 3,000 mm cut takes one stroke, a few seconds, with no consumable and no kerf. A laser cutting the same line takes longer and costs gas and power. For a job that is mostly rectangular blanks, shear first then laser the holes is usually the cheaper route.

The weakness is geometry. A shear cannot cut an inside corner, a radius, or a hole. It cannot nest parts to save material the way a laser can. It also needs a straight edge to register against, so the first cut on a raw sheet is a trim cut to establish that edge.

At GreatLight we run shearing as the entry step in sheet metal fabrication, then move blanks into 5-axis machining, forming, and finishing. A stainless blank sheared to size can go straight to a mill for edge profiling, or to a press brake for a bend line. Tolerances on sheared blanks are set by the back gauge and the blade condition, so we re-check the gauge after every blade change.

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  Trim cut firstEstablishes a straight reference edge on the raw sheet.
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  Then holesLaser, punch, or drill after the blank is sized.
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  Then formPress brake bends the sheared blank along marked lines.

Capacity is rated by thickness times width, and the rating assumes mild steel. A machine rated 6 mm × 3,000 mm will not cut 6 mm stainless across the full bed. Stainless needs roughly 1.5 times the force of mild steel, so the practical stainless capacity on the same machine is around 4 mm at full width. Always ask for the derated capacity, not the headline number.

Thickness also sets the minimum cut width. Thin sheet under about 1 mm can bow or fold into the blade gap if the offcut strip is narrow. As a rule, keep the offcut at least 8-10 times the thickness, or use a support and a slower stroke. Below 0.5 mm, shearing gets risky and laser or punch is the safer route.

Angles are out. A shear cuts straight lines only, parallel or perpendicular to the back gauge. If a part needs a 30° corner, that corner comes from laser, punch, or a notcher, not from the shear. Some machines offer an angled back gauge for taper cuts, but the blade still cuts a straight line.

Finally, shear is not a precision finishing process. If a drawing calls ±0.05 mm on a blank length, plan to shear oversize and machine the edge. Shear gets you close, fast, and cheap. Machining gets you exact.