CNC Shearing Machine: How Precision Cutting Actually Works

• 1
  What a CNC shearing machine controls between the blades
• 2
  Tonnage, hold-down force, and why sheet bows after the cut
• 3
  Which sheet metal suits CNC shearing and which does not
• 4
  Reading the cut edge as a process record
• 5
  CNC shearing machine against laser, waterjet, and punch
• 6
  Process comparison for flat sheet cutting

A shear cuts by trapping sheet metal between an upper blade and a fixed lower blade. The upper blade travels down and pushes the metal past its shear strength, so the material fractures along a line instead of being sawed away. There is no kerf, no chips, and no heat-affected zone. The cut face shows two zones: a smooth burnished band at the top and a rougher fractured band below it.

On a manual shear the operator sets back gauge position and presses a pedal. A CNC shearing machine adds programmed control of the back gauge, the blade gap, the rake angle on the upper blade, the hold-down force, and the stroke depth. Each cut is repeatable, so a 500-piece run holds the same blank length from the first part to the last. That repeatability is the whole point, not raw speed.

Blade gap is the parameter that decides edge quality. Too wide and the sheet bends into the gap, producing a rolled edge and a burr. Too narrow and the blades rub, which loads the hydraulics and shortens blade life. As a working range, gap runs from about 6% to 10% of sheet thickness for mild steel, and toward the lower end for stainless and aluminium. A 2 mm mild steel sheet lands near 0.15 mm; the same thickness in 304 stainless runs closer to 0.12 mm.

Rake angle matters on thick plate. A rake of 1° to 2° spreads the cutting force across the stroke, which lowers peak tonnage and lets a smaller machine cut thicker sheet. The trade-off is that the offcut curls downward as the blade descends. On thin sheet, a low rake angle keeps the cut line straight and the distortion small.

• 1
  Blade gap6–10% of thickness for mild steel; tighter for stainless and aluminium.
• 2
  Rake angle1–2° for thick plate to cut peak tonnage; near flat for thin sheet.
• 3
  Hold-down forceClamps the sheet so it cannot lift or slide during the stroke.
• 4
  Back gaugeSets blank length; CNC positioning holds it across the run.

Shear tonnage follows a simple relationship: cutting force rises with sheet thickness, cut length, and the tensile strength of the alloy. Doubling thickness roughly doubles the force, because the shear area doubles. That is why a machine rated for 4 mm mild steel will not cut 4 mm 304 stainless without derating. Stainless work-hardens and needs noticeably more force per square millimetre than low-carbon steel.

Hold-down force is the parameter most often set too low. The cylinders clamp the sheet just behind the cut line. If the clamp pressure is light, the sheet lifts as the blade enters and the cut edge bows upward. The blank still measures right at the back gauge, but the edge is not square in the vertical plane. On thin sheet this shows up as a visible curve along the cut.

Springback is the other source of error. Metal bends elastically before it fractures, so the cut edge relaxes slightly once the blade clears. On thin, soft sheet the effect is small. On 3 mm and above it can shift the edge angle by a few tenths of a degree, which matters when the blank feeds a press brake next.

Cut quality also depends on blade sharpness. A dull blade increases the fractured zone and the burr height. Most shops rotate or regrind blades on a schedule tied to cut count and material, not to the calendar. Cutting abrasive or hard material shortens that interval.

• 1
  Force scales with areaThickness × cut length × tensile strength.
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  Light clamp pressureShows up as a bowed edge, not a wrong length.
• 3
  SpringbackA few tenths of a degree on 3 mm sheet and above.

Shearing works best on flat sheet from roughly 0.5 mm to 6 mm, depending on the machine. Mild steel, aluminium, brass, and copper cut cleanly. Stainless cuts fine but needs more tonnage and a tighter gap. Material hardness matters more than thickness alone: 6061 aluminium shears easily, while a 7075 sheet at the same thickness needs more force and can crack along the cut line if the gap is wrong.

Straight-line blanks are the natural fit. Panels, brackets, mounting plates, enclosure sides, and blank stock that will be formed later all suit shearing. If the part is a rectangle with square corners and no internal cutouts, shearing is usually the fastest and cheapest way to make it.

Shearing is the wrong process for profiles, holes, slots, and curves. A shear cannot start a cut in the middle of a sheet, so internal cutouts need a punch, laser, or mill. Tapered or curved outlines also fall outside what a straight blade can produce.

Thin gauge is another boundary. Below about 0.5 mm, sheet tends to tear rather than shear cleanly, and the burr becomes hard to control. Very thick plate above the machine rating is simply out of range, and the shop will route it to plasma, waterjet, or a band saw.

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  Good fitFlat rectangular blanks, 0.5–6 mm, low to medium hardness.
• 2
  Poor fitInternal cutouts, curves, tapers, holes, and formed profiles.
• 3
  Hard alloysNeed more tonnage and a tighter gap; watch for edge cracking.

A sheared edge tells you how the setup ran. A narrow burnished band with a small, even burr means the gap was close to ideal. A wide, rough fractured zone with a heavy burr means the gap was too wide or the blade was dull. A rolled or rounded top edge means the blade was not sharp or the gap let the sheet bend into the opening.

Burr direction is predictable. The burr forms on the side facing the lower blade, so flipping the sheet changes which face carries it. If the next operation is a press brake bend and the burr sits on the outside of the bend, it can act as a stress riser and start a crack. Shops often deburr before forming, or orient the blank so the burr lands on the inside.

Edge finish from shearing is not a machined finish. It will not match Ra 0.8–1.6 μm from a mill or a surface grinder. If the drawing calls for a fine finish on the cut face, plan a secondary operation such as milling, grinding, or bead blasting rather than expecting the shear to deliver it.

Because shear edges are cut cold and fast, the material near the edge is work-hardened. On stainless and some aluminium alloys this makes the edge harder than the parent sheet. That is usually harmless, but it does affect how the edge behaves during welding and forming.

• 1
  Tight gapNarrow burnished band, small even burr.
• 2
  Wide gapRough fractured zone, heavy burr, rolled top edge.
• 3
  Burr sideForms toward the lower blade; flip the sheet to choose the face.

The four processes overlap but do not compete on the same jobs. Shearing wins on straight cuts in flat sheet at volume, where the cost per cut drops as the run grows. Laser wins on profiles, holes, and nested layouts, and it cuts far more shapes per sheet. Waterjet wins on thick plate, heat-sensitive alloys, and parts where a heat-affected zone is unacceptable. Punching wins on repeated hole patterns in thin sheet.

Cut edge quality differs by process. Laser leaves a narrow heat-affected zone and a slightly oxidised edge on some alloys. Waterjet leaves a matte, near-machined edge with no thermal damage. Punching leaves a shear-and-fracture edge similar to shearing. Shearing leaves the cleanest straight edge of the three for thin sheet, but only along one straight line.

Accuracy follows the same split. A CNC shearing machine holds blank length by the back gauge, so repeatability is good across a run. Laser and waterjet hold position across a two-dimensional profile, which is what a cutout or curve needs. If your drawing has a 25 mm × 300 mm blank with square ends and nothing inside it, the shear is the right tool.

At GreatLight we route sheared blanks into the machining side of the shop when a part needs holes, pockets, or tighter tolerances. A sheared blank at ±0.005 mm is not achievable on the shear alone, but the blank gives the mill a clean, square start and removes roughing time.

• 1
  Choose shearingStraight blanks, flat sheet, medium to high volume.
• 2
  Choose laserProfiles, holes, nesting, thin to medium sheet.
• 3
  Choose waterjetThick plate, heat-sensitive alloys, no heat-affected zone.