Stamping Die | Blanking Clearance

Blanking Clearance
The quality of blanked parts refers to the cross-sectional condition, dimensional accuracy, and shape error. The cross-sectional condition should be as perpendicular, smooth, and with minimal burrs as possible. Dimensional accuracy should be within the tolerance range specified in the drawing. The part's shape should meet the drawing requirements: the surface should be as flat as possible, i.e., minimal camber.

1. Cross-sectional Characteristics of Blanked Parts
Based on the characteristics of blanking deformation, the cross-section of a blanked part can be clearly divided into four characteristic areas: rounded corner zone, bright zone, fracture zone, and burr zone.

Rounded Corner Zone: This area is formed when the punch cutting edge presses into the material, causing bending and tensile deformation of the material near the cutting edge, pulling the material into the clearance.

Bright Zone: This area occurs during the plastic deformation stage. After the cutting edge cuts into the material, the material is squeezed against the side surfaces of the punch and die cutting edges, forming a bright, perpendicular cross-section, typically occupying 1/2 to 1/3 of the entire cross-section.

Fracture Zone: This area forms during the fracture stage. It is a tear surface formed by the continuous expansion of micro-cracks near the cutting edge under tensile stress. Its cross-section is rough and has a metallic appearance. And slightly angled.

Burst Zone: Burrs form during the later stages of plastic deformation when the cutting edges of the punch and die penetrate a certain depth into the sheet metal. The material on the cutting edge face is compressed, and the tip is under high hydrostatic stress. This prevents crack initiation at the cutting edge, instead causing it to occur on the side of the die not far from the tip. Under tensile stress, the crack lengthens, the material fractures, and a burr is produced. The distance between the crack initiation point and the cutting edge tip is called the burr height.

Among the four characteristic zones, the wider the bright band, the better the cross-sectional quality. However, the size of the four characteristic zones and their proportion on the cross-section are not constant but vary depending on material properties, blanking clearance, cutting edge condition, and other conditions.

2 Factors Affecting Cross-Section Quality

Influence of Material Properties: Materials with good plasticity develop cracks later during punching, resulting in a greater shearing depth and a larger proportion of bright bands and larger fillets on the cross-section. Materials with poor plasticity are easily broken, cracking soon after shearing, resulting in a smaller proportion of bright bands, smaller fillets, and mostly rough fracture surfaces.

Influence of Die Clearance: Whether cracks overlap on the fracture surface during punching depends on the size of the punch-die clearance: 1) When the punch-die clearance is appropriate, cracks near the die cutting edge... 1) Cracks generated in the direction of maximum shear stress can converge during punching. Although the cross-section is not perpendicular to the material surface, it is still relatively straight and smooth with small burrs, resulting in a better cross-sectional quality of the part.

2) When the clearance increases, the tensile stress within the material increases, causing tensile fracture to occur earlier. This widens the fracture zone, narrows the bright zone, and increases bending deformation, thus increasing the collapse angle and camber.

3) When the clearance decreases, the bending moment in the deformation zone is small, and the compressive stress component is high. Cracks generated near the die cutting edge enter the compressive stress zone below the punch and cease development. Cracks originating near the punch cutting edge cease propagating in the compressive stress zone on the upper surface of the die. The upper and lower cracks do not coincide. The material between the two cracks undergoes a second shearing process. When the upper crack is pressed into the die, it is squeezed by the die wall, producing a second bright band. Simultaneously, some material is extruded, forming thin, high burrs on the surface.

4) When the clearance is too small, although the collapse angle and camber are small, the cross-sectional quality is still defective. For example, delamination appears in the middle of the cross-section, bright bands appear at both ends, and burrs are extruded at the end faces.

5) When the clearance is too large, because of the bending moment... Large tensile stress components and non-coincident cracks near the die edges result in increased slope of the fracture layer after separation, leading to two beveled angles on the part's cross-section and suboptimal cross-sectional quality. Furthermore, large collapse angles, large arches, small bright bands, and high, thick burrs further degrade the quality of the stamped part.

Therefore, the die clearance should be maintained within a reasonable range. Additionally, uneven die assembly clearance will result in products exhibiting localized issues of excessively large or small clearances. Therefore, die design, manufacturing, and installation must ensure uniform clearance.

3. Principles for Determining Blanking Clearance

1) During blanking (final shearing of the product), the die dimension is the required workpiece dimension, and the punch dimension is the required workpiece dimension minus the blanking clearance (the blanking clearance is subtracted from the punch);

2) During punching (flash), the punch dimension is the required workpiece dimension, and the die dimension is the required workpiece dimension plus the blanking clearance (the blanking clearance is added together on the die).

4 Blanking Clearance Selection Reference Table

1. Single-Operation Die Blanking Clearance

1. For electrolytic iron and single-sided blank materials, the blanking die clearance is 0.10t (both sides), and the punching clearance is 0.12t (both sides);

2. For aluminum alloy materials, the blanking die clearance is 0.05t (both sides), and the punching clearance is 0.12t (both sides);

3. For various stainless steel and stainless iron materials, the blanking die clearance is 0.14t (both sides), and the punching clearance is 0.16t (both sides);

4. For copper materials, the blanking die clearance is 0.06t (both sides), and the punching clearance is 0.12t (clearance) on both sides;

5. For materials with a thickness of 0.3mm or less, or equal to 0.3mm, the blanking clearance for both blanking and punching is 0.10t (both sides);

6. For materials with a thickness of 2.1mm or more, the above specifications do not apply, and the following empirical values ​​for blanking clearance should be selected: The blanking clearance for aluminum material A1050 is 0.02t~0.05t (double-sided); the blanking clearance for aluminum material A5052 is 0.05t~0.06t (double-sided); and the blanking clearance for iron materials is 0.06t~0.10t (double-sided).