CNC shearing: power and precision in sheet metal cutting

• 1
  Key takeaways
• 2
  How a CNC shearing cut actually forms
• 3
  Rake angle, backgauge and hold-down force
• 4
  What the cut edge tells you about the setup
• 5
  Which materials shear well and which fight back
• 6
  When shearing beats laser, punch or milling
• 7
  CNC shearing compared with other sheet processes

CNC shearing does not cut with a saw tooth or a focused beam. It drives a straight upper blade down past a fixed lower blade, and the sheet fails in shear along one line. The CNC part is not the cutting itself; it is the control of where the sheet stops, how hard it is clamped and how far the blade travels on each stroke.

Two blades pass each other with a small lateral clearance, usually 5–10% of sheet thickness. If the clearance is too tight, the blades rub and the edge tears. If it is too loose, the sheet bends into the gap instead of shearing and you get a rolled-over edge with a burr on the underside.

The fracture starts at the blade contact point and propagates across the sheet. That is why cut quality depends on material shear strength, thickness and blade sharpness more than on machine tonnage alone. A 4,000 mm machine and a 1,300 mm machine cut the same way; only the length of the straight line changes.

One consequence matters for design: the cut is always straight. Curves, slots, corner radii and holes are not produced on a shear. Those come from laser, punch or a mill. Shearing gives you a square edge on a flat blank, fast.

Rake angle is the tilt of the upper blade relative to the lower one. A larger rake, roughly 1–3°, spreads the cut across the sheet progressively and lowers peak force. That lets a lighter machine cut thicker plate. The trade-off is angular distortion: the offcut curls and the blank can bow slightly, so a 3° rake on 1 mm stainless will not hold the same flatness as a near-flat 0.5° setup.

The backgauge is the stop that positions the sheet before each stroke. On a CNC shear it is a servo-driven finger set with a programmable position, and it is where the length tolerance comes from. Repeatability of ±0.1 mm on cut length is normal on a well-maintained machine; the first part off a fresh setup may need one trim cut before the run is steady.

Hold-downs clamp the sheet against the table just behind the cut line. They must be set so the sheet cannot lift or slide during the stroke, but not so hard that soft aluminium is marked. On thin gauge, spring-loaded or urethane-faced hold-downs are used for exactly that reason.

Blade gap is set per material and thickness, then checked with a feeler gauge. A shear that ran 1.5 mm mild steel yesterday needs a different gap for 1 mm 5052 aluminium today. Skipping that change is the most common cause of a burr complaint on a job that ran fine last month.

A correct shearing cut shows four zones on the edge: a small rollover at the top, a shiny burnished band, a dull fracture zone, and a burr at the bottom. The proportion between them is diagnostic. A wide burnished band with almost no fracture zone means the blade is dull or the clearance is too tight, and the blade is rubbing its way through.

A large rollover with a heavy burr usually means clearance is too wide or the hold-downs are not gripping. On 1.5 mm cold-rolled steel you would expect a burnished band of roughly 30–50% of thickness and a small burr. On 3 mm 304 stainless the fracture zone grows because the material work-hardens as it shears.

Burr height is measured, not eyeballed. For most industrial work a burr under 0.05 mm on thin sheet is acceptable without secondary work. Above that, parts need deburring, and on a high-volume run that cost has to be in the quote from the start.

If the edge will be welded, a light burr is usually harmless because the weld pool consumes it. If the edge is a visible face or a sliding contact surface, plan a finishing step: belt sanding, tumbling or a machined chamfer after forming.

Mild steel, low-carbon steel and most aluminium alloys shear cleanly. 5052 and 6061 aluminium cut easily and hold a straight edge. 1018 and A36 steel at 1–6 mm are the everyday workhorses of a shear. Copper and brass also shear well, though soft copper can smear if clearance is too tight.

Austenitic stainless is harder work. 304 and 316 work-harden at the cut line, so the fracture zone is rougher and burr is more likely. Higher tonnage and a slightly wider clearance help. 17-4PH in the hardened condition is normally not a shear job at all.

Titanium and Inconel sit outside normal sheet shearing practice. Ti-6Al-4V sheet can be sheared in thin gauges with sharp blades and high hold-down force, but edge cracking is a real risk and the parts usually go to laser, waterjet or a mill instead.

Thickness is the other gate. Most hydraulic CNC shears in a job shop handle up to 6 mm mild steel, less for stainless. Beyond that, the force required and the distortion risk both climb, and a different process is usually cheaper than forcing the shear.

Shearing wins on one specific geometry: a straight cut across a full sheet or a rectangular blank. It is faster per cut than laser on long straight edges, it leaves no heat-affected zone, and it does not consume gas or lens time. For a run of flat blanks that will be formed later, shearing the perimeter is often the cheapest first operation.

Laser wins as soon as the cut is not straight. Holes, radii, nested profiles, tabs and any contour come off a laser or punch in one setup. A shear would need a separate operation for each edge, and the part would still need the contour cut somewhere else.

Punching wins on repeated small features in thin sheet: louvers, slots, dimples and hole patterns at high rate. It is a tooling investment, so it pays on volume, not on a one-off prototype.

Milling wins when the edge needs a machined finish, a tight perpendicularity callout, or a thickness step. A sheared edge has a slight rollover and a burr; if your drawing calls for a square, burr-free edge at ±0.05 mm, plan a milling or grinding pass after shearing. At GreatLight we run shearing for blanks and pair it with 3-axis and 5-axis machining when the edge itself is a functional surface.