Technical Guide for CNC Hydraulic Shearing Machine

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  What a CNC hydraulic shearing machine controls
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  How the hydraulic circuit and controller work together
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  What controls sheared edge quality and burr height
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  What dimensional accuracy to expect from shearing
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  Matching machine size and tonnage to the job
• 6
  When shearing is the right process

A shear cuts by forcing a moving upper blade past a fixed lower blade. Hydraulics supply the force, and on a CNC hydraulic shearing machine the controller sets three things that decide the result: blade gap, rake angle, and back gauge position. Everything else on the machine exists to hold those three stable while the ram travels.

Blade gap is the horizontal clearance between upper and lower blades. It is set as a percentage of sheet thickness, typically 6–10% for mild steel and lower for stainless. Too small and the blades rub, raising tonnage and dulling edges. Too large and the material tears instead of shearing, leaving a rough fracture face and a rolled burr.

Rake angle is the tilt of the upper blade. A larger rake concentrates force into a short contact length, so a lighter machine can cut thicker plate. The trade-off is angular distortion: the cut edge curls, and long thin blanks bow. A small rake cuts flatter but needs far more tonnage.

The back gauge positions the material for each cut. On a CNC machine it is driven by a servo axis, so repeat position is typically within ±0.05 mm on good machines. That number, not the blade, is usually what limits blank length consistency on a production run.

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  Blade gapSet as a percentage of thickness; wrong gap is the most common cause of bad edges.
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  Rake angleTrades flatness for tonnage. Short rake for flat parts, long rake for thick plate.
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  Back gaugeServo positioned. Repeatability drives blank length consistency.

The hydraulic system on a shear is not a single pump pushing a ram. Most machines use two or three cylinders fed through proportional valves, with pressure held by an accumulator. The controller ramps flow at the start of the stroke, holds speed through the cut, and decelerates before the bottom. That ramp matters: a sudden start shocks the blades and the frame.

Hold-down cylinders clamp the plate just behind the cut line before the ram moves. If the hold-down pressure is too low, thin plate slides and the blank comes out short. Too high, and soft aluminium or pre-painted sheet gets marked. Pressure is normally adjustable per material family.

The controller also runs the blade gap adjustment on machines that have a motorized gap. Older shears need a hand wheel and a feeler gauge; a CNC machine moves the lower blade a programmed distance. That removes a setup step and, more importantly, removes operator judgment from a variable that has a big effect on edge quality.

A shear is a single-purpose machine. It produces straight cuts only. If a part needs a notch, a slot, or a curved contour, shearing gives you a rectangular blank that goes to a second process such as CNC milling, laser cutting, or punching.

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  Proportional valvesRamp flow at stroke start and end to avoid shock loads.
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  Hold-down pressureEnough to stop sliding, not enough to mark the surface.
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  Motorized gapRemoves a manual setup step on mixed-thickness jobs.

A sheared edge has four zones if you look at it under magnification: a small rollover at the top, a smooth burnished band, a rougher fracture band, and a burr at the bottom. The ratio between burnished and fracture band tells you how well the gap was set. A clean cut shows a burnished band that runs most of the thickness.

Burr height is the number most shops actually measure. On mild steel cut at the correct gap, burr is usually small enough to ignore for welded assemblies. On stainless and high-strength steels the burr is harder and taller, and it will show up as a fit problem or a handling injury if it is not removed.

Blade sharpness changes the picture over time. A dull blade raises the required force, pushes the material rather than shearing it, and grows the burr. Rotation intervals depend on material: cutting stainless wears an edge far faster than cutting mild steel, and coated or abrasive plate wears it faster still.

If the burr matters, plan a deburring step. Tumbling, brushing, or a light face mill on a CNC machine all work. For parts that will be anodized, the burr must come off first, because anodizing locks it in place and it will chip later.

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  Burnished bandWide band means gap and blade condition are in range.
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  Burr heightGrows with dull blades and with harder, stronger materials.
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  Deburr before finishAnodizing over a burr locks in a future chip.

Shearing is a blanking process, not a finishing process. Blank length is set by the back gauge and holds well, but the cut edge itself is not a machined surface. Squareness depends on the machine being set up correctly and the plate sitting flat against the gauge.

Typical shop-floor numbers: blank length repeat within ±0.1 mm on a well-maintained machine with a good gauge, squareness within about 0.1 mm per 1,000 mm of cut, and edge roughness that no one quotes in Ra because the fracture band makes it meaningless. If a drawing calls out Ra 0.8–1.6 μm on a cut face, shearing will not meet it.

Thickness tolerance of the incoming plate often dominates. Hot-rolled mild steel plate can vary in thickness across a single sheet, and the shear cuts what it is given. If the plate is 0.3 mm heavy in one area, the part is 0.3 mm heavy there, and no blade gap setting changes that.

For parts that need real dimensional control, shear produces the blank and then a CNC operation produces the critical features. At GreatLight we hold ±0.005 mm on machined features and Ra 0.8–1.6 μm on machined faces, which is a different class of operation from shearing.

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  Blank lengthBack gauge repeat, roughly ±0.1 mm on a good machine.
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  SquarenessAround 0.1 mm per 1,000 mm when the setup is right.
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  Plate thicknessIncoming variation often dominates the final part tolerance.

Tonnage for shearing is roughly proportional to thickness squared, times the cut length, times the material's shear strength. Doubling thickness roughly quadruples the force. That is why a machine rated for 6 mm mild steel will not cut 12 mm, and why stainless needs noticeably more force than mild steel at the same thickness.

Cut length sets the machine width. A 3,000 mm machine handles a 2,500 mm blank with room for the hold-downs. Cutting right at the rated width with no margin is a bad habit; the ends of the cut are the least supported region and the most likely to bow.

Rake angle is the lever you pull when tonnage is short. Increasing rake reduces peak force and lets a smaller machine handle the same thickness, at the cost of edge flatness. If flatness matters more than machine size, you need more tonnage and a shorter rake.

For thin sheet under about 2 mm, shearing competes directly with laser and punch. Shear is faster per cut and cheaper per part on straight blanks. Laser wins as soon as the part has any contour, hole, or tight corner radius.

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  Thickness squaredForce scales with t²; 2× thickness is roughly 4× tonnage.
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  Cut lengthLeave margin inside the rated width, especially at the ends.
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  Rake vs flatnessMore rake means less force and more part distortion.