Cold-rolled coil is cut by shearing or slitting at almost every processing stage, from the mill side trimmer to the service-center slitter. The quality of the cut edge is judged by four features: the bright band, the fracture zone, the burr and the collapse of the top edge. This article explains how each feature is formed, what values are considered acceptable for cold-rolled material, and how the shear gap, blade condition, clamping force, cutting speed and material properties can be adjusted to obtain a clean and consistent cross-section.
What a Good Sheared Cross-Section Looks Like
An ideal sheared section combines a high proportion of bright band, a fine fracture zone and small, uniform burrs. The bright band is the smooth burnished strip left by the cutting edge and is the primary indicator of section quality; a minimum of one-third to one-half of the material thickness is normally expected. The fracture zone should be fine and slightly inclined, without secondary tearing or coarse grain spalling. Burr height on cold-rolled steel should generally be held within 5 to 10 percent of the material thickness, for example 0.05 to 0.10 mm on a 1.0 mm sheet, while precision slitting for exposed automotive parts often agrees limits below 0.02 mm. The collapse angle at the top edge must also be controlled, because excessive collapse reduces dimensional accuracy and creates stress concentration at the edge.
| Evaluation indicator | Typical acceptable value |
|---|---|
| Bright band proportion | At least 1/3 to 1/2 of material thickness |
| Burr height | 5-10% of thickness (0.05-0.10 mm at 1.0 mm); precision lines often agree below 0.02 mm |
| Fracture zone | Fine and oblique, no secondary tearing or coarse spalling |
| Collapse angle | Small and uniform, no severe edge roll-over |
Setting the Shear Gap
The gap between the upper and lower blades is the most influential parameter of the shearing process. If the gap is too small, the material is sheared a second time, producing a second bright band, accelerating edge wear and even chipping the blade. If the gap is too large, the material is overstretched before fracture, increasing the collapse angle, roughening the fracture zone and enlarging burrs. For cold-rolled coils with a tensile strength up to about 450 MPa, the recommended bilateral gap is 5 to 10 percent of the material thickness. Smaller values in the range of 5 to 7 percent are preferred when a high bright-band proportion is required and the machine is rigid enough to avoid edge chipping; larger values of 8 to 10 percent suit thicker plates and rough cutting where jamming must be avoided. Trial shearing remains the final check: the gap is optimal when the bright band occupies about one-third to one-half of the thickness and the fracture zone is smooth and free of steps.
| Material condition | Bilateral gap guidance |
|---|---|
| Cold-rolled coil, tensile strength up to 450 MPa, precision work | 5-7% of thickness |
| Thick plate or rough cutting | 8-10% of thickness |
| Higher-strength or harder material | Use a smaller gap and ensure sufficient machine rigidity |
| Soft, high-elongation material | Enlarge the gap slightly so cracks propagate cleanly |
Blade Condition and Maintenance
The cutting edge is where the shearing force is applied, and its condition decides the section. A sharp edge produces early indentation cracks and a clear bright band but wears quickly; an overly dull edge squeezes material into the gap instead of cutting it, causing abnormally large collapse angles and heavy burrs. Practice shows that a very small, uniform edge radius of about R0.02 to 0.05 mm, created by a light dulling or micro-passivation treatment with an oilstone or special equipment, inhibits premature crack propagation, increases the bright-band proportion and improves edge durability. Maintenance follows three rules: establish a blade wear curve and regrind when the burr height exceeds 1.5 times the allowable value; micro-passivate newly ground edges to remove microscopic serrations; and for high-strength cold-rolled steel, consider hard coatings such as TiN or CrN to reduce friction and stabilize section quality.
Clamping Force and Cutting Speed
Insufficient clamping lets the sheet lift during cutting, which reduces the indentation depth, steepens the cut slope and can tear the edge; excessive clamping damages the surface and increases feeding resistance. As a rule of thumb, the clamping force should be 5 to 10 percent of the shearing force, and the clamp foot should grip the material 0.1 to 0.2 seconds before the blades meet, as on mechanical shears. High-speed lines such as oscillating shears and flying shears must be treated differently because the strain rate changes the fracture behaviour of the steel: moderately increasing speed helps produce a smoother cut, but excessive speed creates impact vibration that degrades the surface. The recommended strategies are strong pressure with slow shearing for thick material and steady pressure with faster shearing for thin material, with a deliberate slow-down near the end of the stroke for precision blanking.
Compensating for Material Variation
Real coils are never perfectly uniform. When tensile strength and hardness rise, fracture becomes more sudden and the fracture zone rougher; when elongation is high, the material resists breaking and produces large burrs. Test the incoming material online or from the certificate and compensate in the process: for high-strength steel, reduce the gap, increase the blank-holder force and use a coated cutting edge so the material cannot slip; for soft, high-elongation steel, enlarge the gap so cracks propagate cleanly instead of the material being torn, and spray a small amount of volatile lubricating oil on the blade side to reduce adhesion and frictional heat.
FAQ
What bright band proportion is considered good?
A bright band of at least one-third to one-half of the material thickness is the normal acceptance criterion; higher proportions indicate better indentation control but require a smaller gap and a sharp, well-supported blade.
What shear gap should be used for 1.0 mm cold-rolled coil?
For a 1.0 mm coil with a tensile strength up to about 450 MPa, the bilateral gap is typically 0.05 to 0.10 mm, that is 5 to 10 percent of the thickness, adjusted toward the lower end for bright-band requirements.
Why do burrs increase when the blade is dull?
A dull edge cannot indent the surface early enough, so the material is squeezed into the gap and torn rather than sheared, which produces large collapse angles and burrs. Regrinding is required when burr height exceeds about 1.5 times the allowable value.
How much clamping force is needed on a shear?
Clamping force of 5 to 10 percent of the shearing force is the usual guidance, with the clamp foot engaging 0.1 to 0.2 seconds before cutting on mechanical shears to prevent sheet lifting.
How should the gap be changed for high-strength cold-rolled steel?
Higher-strength material fractures more suddenly, so the gap should be reduced, the blank-holder force increased and the blade edge coated or passivated to prevent slippage and chipping.
Can soft steel be sheared with the same settings as normal steel?
No. Soft, high-elongation steel tends to tear and form large burrs; enlarge the gap slightly and use edge lubrication so the cracks propagate cleanly instead of the material being stretched into the gap.