Basic Knowledge of CNC Shearing Machines
A working guide for engineers and buyers who need flat blanks before machining. It covers how a shear cuts, what blade clearance and rake angle do to edge quality, what the controller actually controls, and where shearing stops being the right process. Read it and you can size a shear, read a cut edge, and pick the right cutting method for a given part.
What a shear does, and what it does not
Shearing is a single straight cut. Everything else on this page follows from that.
What CNC shearing actually is
A CNC shearing machine holds a sheet between a fixed lower blade and a moving upper blade. The upper blade travels down in a straight line and pushes the metal past its shear strength. No chip forms. No heat-affected zone appears. The cut face is left with a small rollover, a burnished band, and a fracture zone, and how those three zones balance tells you whether the setup was right.
The CNC part is not the cut itself. It is the control of blade clearance, rake angle, back gauge position, stroke length, and hold-down pressure for each material and thickness. An operator enters the material type and thickness, and the controller sets the rest. That is the difference between a mechanical guillotine and a modern hydraulic shear: repeatability, not raw force.
This process sits at the front of most sheet metal workflows. A laser or turret punch cuts profiles, but a shear produces flat rectangular blanks fast and at low cost per edge. Those blanks then go to forming, welding, or into 5-axis machining at GreatLight for finishing to ±0.005 mm.
- 1Straight cuts onlyCurves, holes, and notches need another process.
- 2No thermal damageCold cutting keeps material properties intact.
- 3Edge quality is controlledClearance and rake set the rollover and burr height.
How the cut happens at the blade edge
Two blades act against each other. The upper blade descends while the lower blade stays fixed, and the gap between them is the blade clearance. This gap is the single most important number in shearing. Set it too wide and the metal tears; the cut face turns rough and a heavy burr forms. Set it too narrow and the blades rub, load spikes, and edge life drops fast. A common starting point is 5% to 10% of sheet thickness, adjusted by material.
Rake angle is the tilt of the upper blade across the width of the machine. A low rake, around 0.5°, keeps the cut nearly parallel and reduces distortion on thin sheet. A higher rake, up to about 2°, lowers the peak force needed on thick plate but pushes the sheet sideways. That sideways push is why long thin parts can come out curved. Choosing rake is a trade between tonnage and part straightness.
Cutting force scales with thickness, tensile strength, and cut length. A shear rated for 6 mm mild steel will not cut 6 mm of 304 stainless, because stainless work-hardens and needs roughly 1.5 to 2 times the force. Capacity charts assume mild steel at about 450 MPa tensile. Always check the rating against the actual material, not the nominal thickness.
- 1Blade clearance5–10% of thickness for mild steel; tighter for thin sheet.
- 2Rake angleLow rake for flatness, high rake for thick plate.
- 3Capacity is material-specificStainless and high-strength steel need more tonnage.
Key components of a modern CNC shear
The frame carries the cutting load and must resist deflection. On hydraulic shears it is a welded steel structure, stress-relieved and machined flat on the blade mounting surfaces. If the frame flexes under load, the blade clearance changes mid-cut and the edge quality varies along the length of the part.
The upper beam holds the moving blade and is driven by hydraulic cylinders or, on smaller machines, a mechanical linkage. Modern hydraulic shears use proportional valves so the beam speed and force can be tuned. The lower bed holds the fixed blade. Both blades are typically four-edge, so a worn edge can be rotated before regrinding. Blade material is usually a high-chromium tool steel hardened to around 55–60 HRC.
The back gauge is a motorized stop that positions the sheet for each cut. On a CNC shear it moves on ball screws and can be programmed for a sequence of cut lengths. A squaring arm and front supports keep long sheets aligned. Hold-downs clamp the sheet just behind the cut line to stop it lifting or shifting. The controller ties all of this together and stores programs by material and thickness.
- 1FrameWelded, stress-relieved steel to resist deflection.
- 2BladesFour-edge tool steel, 55–60 HRC, rotated when worn.
- 3Back gaugeCNC-driven stop for repeatable cut lengths.
- 4Hold-downsClamp the sheet to prevent lift and movement.
Choosing the right cutting process
Shearing wins on straight edges and speed. It loses on anything curved.
| Process | Best for | Cut edge | Avoid when |
|---|---|---|---|
| CNC shearing | Straight blanks, rectangular parts | Small rollover, light burr | Part has curves or holes |
| Laser cutting | Profiles, holes, tight corners | Narrow kerf, heat-affected zone | Thick plate over 25 mm |
| Plasma cutting | Thick plate, rough blanks | Wide kerf, dross on edge | Tight tolerance needed |
| Waterjet | Thick or heat-sensitive material | Smooth, no heat input | High volume, cost-sensitive |
Where shearing fits in a production chain
Shearing is a blanking step. It produces flat, rectangular or square pieces that feed the next operation. In a sheet metal shop those blanks go to a press brake for bending, a punch for holes, or a welder for assembly. In a machine shop they become stock for milling. GreatLight uses sheared blanks as the starting point for 5-axis machining, where a flat, square edge makes clamping and locating far easier than a rough saw cut.
Common parts made from sheared blanks include brackets, mounting plates, gussets, electrical enclosure panels, and base plates for machinery. The process handles aluminium, mild steel, stainless, copper, and brass with the right blade clearance and tonnage. Thin sheet down to about 0.5 mm can be sheared, though very thin material distorts easily and may need a different approach.
Speed is the main advantage. A hydraulic shear makes a full-width cut in a few seconds, and a CNC back gauge repositions automatically between cuts. For a run of 500 rectangular plates, shearing is often several times faster than laser cutting the same outline. Cost per edge is low, and there is no consumable gas or nozzle to replace.
- 1Blanks for machiningSquare edges simplify clamping and locating.
- 2Bracket and plate runsFast, low cost per part on straight outlines.
- 3Thin sheetDown to about 0.5 mm, with distortion risk.
Key considerations for a clean cut
Match blade clearance to material and thickness. Mild steel at 3 mm runs well around 0.15–0.3 mm clearance. Stainless needs a slightly larger gap because it work-hardens. Aluminium tolerates a wider range but tends to smear if the gap is too tight. The controller usually holds a table of these values, but the operator still has to confirm the blade is sharp and seated correctly.
Check blade condition before a critical run. A dull blade raises cutting force, increases burr height, and can pull the sheet into the gap. Rotate or regrind when the burnished band on the cut face widens or the burr grows. On a four-edge blade, rotating takes minutes and restores edge quality without a regrind.
Control the material handling. Long sheets sag without front supports, and the back gauge can be pushed out of position. Hold-downs must clamp firmly but not mark soft material. For thin or soft sheet, a protective film or a softer hold-down pad prevents dents. After cutting, deburr if the next process is welding or if the part is a visible panel.
- 1Clearance tableSet by material and thickness, not one fixed value.
- 2Blade sharpnessWide burnished band or tall burr means rotate or grind.
- 3Support and clampingFront supports and correct hold-down pressure.
Common questions about CNC shearing
Can a CNC shear cut curves or holes?
No. A shear makes one straight cut per stroke. The blade is a straight edge, so the cut line is always linear.
Curves, holes, and notches need laser cutting, plasma cutting, waterjet, or a turret punch. Many shops shear the straight outline first and then cut features on a second machine.
How thick can a shear cut?
Industrial hydraulic shears commonly handle up to about 25 mm of mild steel. Heavy-duty models go thicker, but they are specialized machines.
The real limit depends on material. Stainless steel and high-strength alloys need more force than mild steel at the same thickness, so always check the capacity rating against the actual tensile strength.
What blade clearance should I use?
Start at 5% to 10% of sheet thickness for mild steel. A 3 mm sheet runs around 0.15–0.3 mm clearance.
Stainless steel needs a slightly larger gap because it work-hardens. Aluminium tolerates a wider range but can smear if the gap is too tight. Confirm with a test cut and inspect the fracture zone.
Why is my sheared edge curved?
A high rake angle pushes the sheet sideways during the cut, and long thin parts bow as a result. Reducing rake or using a machine with a lower rake setting helps.
Other causes include uneven blade clearance across the width, a loose back gauge, or the sheet not being held flat. Check the hold-downs and the blade gap at both ends.
When should I choose shearing over laser cutting?
Choose shearing for straight-edged rectangular blanks in volume. It is faster per edge and costs less per part than laser cutting the same outline.
Choose laser when the part has curves, holes, or tight corner radii. Laser also wins on one-off profiles where a shear would need multiple setups.
Does shearing change the material properties?
The cut is cold, so there is no heat-affected zone and no change to the bulk material. The sheared edge does work-harden locally, which matters if the edge is later bent sharply.
For parts that will be formed close to the cut line, leave enough material or anneal the edge if the alloy is sensitive. For most blanks that go on to machining, the cold edge is not an issue.
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