1.What are the ideal shear section characteristics? What are the core evaluation indicators for improving section quality?
An ideal sheared cross-section should possess the characteristics of "high proportion of bright band, fine fracture zone, and small and uniform burrs."
Core evaluation indicators include:
Bright band proportion: Typically required to be ≥ 1/3 to 1/2 of the material thickness. A smooth and flat bright band is a primary indicator of cross-section quality.
Fracture zone roughness: The fracture zone should be fine and oblique to avoid secondary tearing or large grain spalling.
Burr height: Burrs from cold-rolled steel shearing should generally be controlled within 5% to 10% of the material thickness (e.g., for a 1.0mm thick material, burrs ≤ 0.05 to 0.1mm).
Cross-section collapse angle: Excessive collapse angle not only affects dimensional accuracy but also leads to edge stress concentration.
2.How can the cross-sectional quality be significantly improved by adjusting the shear gap? What is the optimal gap range?
Insufficient gap: This easily leads to secondary shearing, creating a second bright band, accelerating edge wear, and even chipping.
Insufficient gap: The material is overstretched before fracture, resulting in increased collapse angle, a rough fracture zone, and severe burrs.
Optimization methods:
Theoretical benchmark: For cold-rolled coils (tensile strength ≤ 450 MPa), the optimal bilateral shearing gap is typically 5%~10% of the material thickness.
Dynamic adjustment: For high-brightness bands: Use a smaller value (e.g., 5%~7%), but ensure sufficient equipment rigidity to avoid edge chipping.
For thick plates or rough machining: Use a larger value (e.g., 8%~10%) to prevent jamming.
Practical tips: Through trial shearing, the optimal gap is achieved when the bright band on the cross-section occupies approximately 1/3~1/2 of the material thickness, and the fracture zone is smooth and without steps.
3.How does the condition of the cutting edge (sharpness and radius) affect the sheared section? How should it be maintained?
The cutting edge is the direct point of application of shearing force, and its state plays a decisive role in the cross-section:
Sharp cutting edge: Produces early indentation cracks and a clear bright band, but wears quickly. An overly dull cutting edge causes material to be squeezed into the gap instead of being sheared, resulting in abnormally large collapse angles and a significant increase in burrs.
Small rounded cutting edge: Practice has shown that a cutting edge with extremely small, uniform rounded corners (R0.02~0.05mm, commonly known as "reverse taper" or "dulling") can actually inhibit premature crack propagation, appropriately increase the proportion of bright bands, and improve cutting edge durability.
Maintenance strategies:
Regular regrinding: Establish a cutting edge wear curve. When the burr height exceeds 1.5 times the allowable value, regrinding is necessary.
Micro-passivation treatment: For newly sharpened tools, it is recommended to perform micro-passivation treatment with an oilstone or specialized equipment to eliminate microscopic serrations and form a stable small rounded corner.
Coating application: For high-strength cold-rolled steel sheets, coatings such as TiN and CrN can be used to reduce the coefficient of friction and improve the cross-sectional quality.
4.How can the clamping force and shearing speed be coordinated to obtain a smooth cross-section?
Insufficient clamping force: During shearing, the sheet metal warps upwards, resulting in insufficient indentation depth of the cutting edge, excessive slope of the cut surface, and even tearing.
Excessive clamping force: May damage the sheet surface or increase feeding resistance.
Speed matching:
Low-speed shearing (e.g., mechanical shears): The clamping force needs to be sufficiently large (usually 5%~10% of the shearing force), and the clamping foot must clamp the material 0.1~0.2 seconds before shearing.
High-speed shearing (e.g., oscillating shears, flying shears): Dynamic response needs to be considered. The faster the shearing speed, the higher the material strain rate, and the different fracture toughness. Generally, appropriately increasing the shearing speed (within a certain range) helps to obtain a smoother cut surface, but too high a speed will lead to impact vibration, deteriorating the cut surface quality.
Recommended strategy: Use "strong pressure, slow shearing" or "steady pressure, fast shearing," adjusted according to the material thickness. For precision blanking, it is recommended to appropriately slow down before the end of the shearing to reduce the impact of vibration on the cut surface.
5.When material properties (such as tensile strength and elongation) fluctuate, how can the shearing process be dynamically adjusted to stabilize the cross-sectional quality?
Material Testing: Test the actual tensile strength and hardness of the material online or offline. The harder and stronger the material, the more sudden the fracture and the rougher the fracture zone.
Process Compensation:
High-strength Steel: Reduce the shear gap (to a smaller value), increase the blank holder force, and apply a reinforcing coating to the cutting edge to hold the material in place and prevent slippage, resulting in a relatively clean cross-section.
Soft Steel/High Elongation Materials: These materials are not easily broken and tend to produce large burrs. In this case, the gap should be appropriately enlarged to allow cracks to propagate smoothly, avoiding the material being "torn" rather than "shorn" due to an insufficient gap.
Lubrication Adjustment: For highly viscous soft steel, a small amount of volatile lubricating oil can be sprayed onto the side of the cutting edge to reduce material adhesion to the cutting edge, decrease frictional heat, and improve the cross-section.