DfFB Corner Radii Rules: Minimizing Stress in Fine Blanking Parts

In fine blanking, corner geometry is not merely an aesthetic detail—it directly governs die life, part quality, and manufacturing cost. Sharp internal corners concentrate stress at levels 3 to 5 times the nominal applied force, leading to premature die chipping and unplanned maintenance. This guide establishes engineering rules for internal and external corner radii in fine blanking parts, drawn from four decades of die design and production experience at HS-FINEB.

Stress Concentration

Why Corner Radii Matter in Fine Blanking

In fine blanking, the V-ring exerts high compressive stress on the material surrounding the cutting edge. This stress field interacts directly with part geometry, and at any sharp internal corner, the local stress magnifies dramatically. The stress concentration factor (Kt) at a zero-radius internal corner reaches 3 to 5, meaning the die material experiences three to five times the nominal blanking force at that point. Under cyclic loading of 200 to 1,200 tons, this amplified stress initiates microcracks that propagate until chipping occurs—the number one failure mode for fine blanking dies.

External corners present a different but equally important challenge. Sharp external corners restrict material flow during shearing, causing incomplete shear zones, increased burr formation, and dimensional variation at the cutting edge. Parts with zero-radius external corners consistently show 30 to 40 percent more die roll than parts with properly radiused corners.

The fundamental principle is straightforward: in fine blanking, where the V-ring forces material into a compressive state to enable clean shear, every geometric discontinuity becomes a stress amplifier. Proper corner radii are the single most cost-effective design change a product engineer can make to extend die life. Unlike material upgrades or surface treatments, which add per-part cost, corner radii modifications cost nothing once the die is built correctly—they simply require specifying the right radius on the part drawing.

Fine blanking die showing corner geometry and V-ring configuration

Internal Corner Radii

Rules for Internal Corner Radii

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Minimum Radius

The minimum internal corner radius should be 0.5 times the material thickness (t), and never below 0.3mm regardless of material thickness. For a 2mm part, this means R ≥ 1.0mm. For thin materials below 0.5mm thickness, use 0.3mm as the absolute minimum. Specifying radii below this threshold creates die corners that cannot be machined reliably and will chip within the first few thousand strokes. For high-strength steels above 600 MPa tensile strength, consider increasing to 0.7t to account for higher blanking forces.

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Stress Concentration Factor

At zero radius, the stress concentration factor Kt reaches 3 to 5. At R = 0.5t, Kt drops to approximately 1.5. At R = 1.0t, Kt approaches 1.2. This non-linear improvement means that even a small radius provides dramatic stress reduction. The most cost-effective improvement comes from moving from R = 0 to R = 0.3t, which cuts Kt by more than half. The relationship is non-linear: doubling the radius from 0.25t to 0.5t reduces Kt by 40 percent, but doubling again from 0.5t to 1.0t only reduces Kt by an additional 20 percent.

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Die Manufacturing Impact

EDM wire cutting leaves a corner radius equal to the wire radius plus the spark gap, typically 0.25 to 0.35mm total. Specifying radii below this limit forces the die maker to use slower, more expensive grinding processes or accept an undersized radius that will fail prematurely. For radii below 0.25mm, the die maker must use a combination of wire EDM and precision grinding, increasing die cost by 15 to 25 percent. Always consult the die maker about achievable radii before finalizing the part drawing.

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Material Flow Effect

Larger internal radii improve material flow during shearing, reducing die roll (the tear zone at the cutting edge). Parts with R = 0.5t show 30 to 40 percent less die roll than parts with zero radius, improving dimensional accuracy and surface finish at the shear zone. This is especially critical for parts with tight flatness requirements, where die roll variation across the part edge can cause assembly interference. The material flow benefit compounds with die life: better flow means less uneven wear on the cutting edge, extending the interval between resharpening.

External Corner Radii

Rules for External Corner Radii

Fine blanking die detail showing external corner geometry and punch configuration

External Corner Design Principles

External corners are less critical than internal corners because they are in compression during the blanking stroke—the material pushes inward against the die, not outward. However, the punch corner at the external radius is still the weakest point on the punch itself. A sharp punch corner will chip under the same cyclic compressive stress that breaks internal die corners.

The minimum external corner radius should be 0.3 times the material thickness. For a 2mm part, this means R ≥ 0.6mm. While this is smaller than the internal minimum, it should never be zero. The recommended practice is to use the largest radius the part design allows. Even a small external radius of 0.2mm dramatically improves punch life compared to a zero-radius corner.

External corners also affect material flow. A properly radiused external corner allows smooth material flow around the corner during shearing, reducing burr height and improving edge finish. Parts with sharp external corners show visible burr growth and edge roughness at the corner within the first 10,000 strokes of die life, requiring premature maintenance that proper radii would have prevented.

Alternatives & Common Mistakes

Sharp Corner Alternatives and Common Mistakes

Fine blanked part detail showing corner geometry and chamfer alternatives

When Sharp Corners Are Unavoidable

When a zero-radius corner is absolutely required by the part function, several alternatives exist. A stepped punch design can move the sharp corner to a non-cutting surface, preserving die life while meeting the dimensional requirement. A secondary chamfering operation can be added after blanking to create the sharp corner, accepting the additional processing cost. In all cases, the engineer should accept that die life will be shorter—typically 5,000 to 8,000 strokes before chipping.

A chamfer is the most practical alternative to a radius. A 0.2mm × 45° chamfer reduces the stress concentration factor from approximately 5 to 2, cutting peak stress by more than half while maintaining a functionally sharp corner. This is particularly useful for mating surfaces where a radius would interfere with assembly.

Three common mistakes account for most corner-related die failures. First, designing zero-radius internal corners without consulting the die maker—the die maker can often suggest minor geometry modifications that preserve function while improving manufacturability. Second, not specifying the burr side on the drawing—the burr side directly affects corner quality, and leaving it unspecified means the die maker guesses, with a 50 percent chance of getting it wrong. Third, ignoring the material thickness effect on minimum radius—a radius that works for a 3mm part may be too small for a 1mm part.

Die Life & Cost

Effect on Die Life and Cost

The financial impact of corner radii design is direct and measurable. An internal corner with zero radius typically lasts 5,000 to 8,000 strokes before chipping occurs. With a proper radius of R = 0.5t, die life extends to 15,000 to 25,000 strokes—a 200 percent improvement. Each chipping event requires die resharpening at a cost of $200 to $500 per occurrence, plus production downtime of 2 to 4 hours.

Over a typical production year of 100,000 strokes, a die with zero-radius corners will require 12 to 20 resharpening events, costing $2,400 to $10,000 in maintenance alone. A die with properly designed radii requires only 4 to 7 resharpening events, costing $800 to $3,500. The difference—$1,600 to $6,500 per year per die—represents the direct cost saving from proper corner design. When multiplied across a production line with 5 to 10 active dies, the annual saving reaches $8,000 to $65,000.

HS-FINEB provides DfFB (Design for Fine Blanking) review as part of every die design service. Our engineers examine customer part drawings for corner radii, hole spacing, web thickness, and other manufacturability factors before die construction begins. This review typically identifies 3 to 7 design modifications that improve die life by 100 to 300 percent without compromising part function. The review process takes 2 to 3 business days and is included at no additional cost with every die design order. Contact our engineering team to schedule a DfFB review for your part drawings.