Chapter 4 Notes: Design of Blanking Dies

4. Design of Blanking Dies (Chapter 4) – Key Concepts and Design Guidelines

• Scope and focus
  • This chapter centers on blanking and punching with dies and presses (metal sheet cutting and forming).
  • Emphasis on how to design and select tooling (punches, dies, stripping, guides) for efficient blanking and high-precision parts, including small electronic components.
  • Differentiates blanking (making a separate blank) from piercing (creating holes) and notes that many presses are used for both blanking and piercing in industry (Japan example: 70–80% of presses used for blanking/piercing).

4.2 Edge characteristics and burrs of punched parts

• Edge types and their significance

  • The edge of a blanked part changes with cutting conditions and can indicate process suitability.
  • Four parts of the cut edge are discussed in relation to trimming and burr formation:
  • Curved edge (rollover)
  • Smooth sheared edge (burnished surface)
  • Fractured surface (rough, irregular)
  • Burr (thin, sharp metal jutting from the edge)
  • The total edge profile height should sum to the workpiece thickness; burr height is the excess over the original sheet thickness.

• 4.2.1 Mechanism for rollover (curved edge)

  • When the punch presses material into the die, the material between the punch and die experiences high compressive stress; sides experience tensile stress and flow according to punch movement, causing a domed (cupped) deformation called rollover.
  • Rollovers can also be explained by zero clearance (no clearance) versus positive clearance; when clearance is zero, material fills cavity more fully, promoting rollover in both the edge and the pierced part.
  • Rollover is shown to occur at the edge near the punch/die edge, especially with insufficient clearance control.
  • Practical implication: rollover reduces the straight edge length and local material around holes used for fasteners, reducing clamp area and edge strength.

• 4.2.2 Mechanism for burr formation

  • Bur is formed when fracture/plastic flow initiates slightly above the cutting edge due to high tensile stress around the tool edge.
  • Burr grows as the material at the sides of the punch/die edge is pulled away first and remains attached along the edge, forming a thin, sharp projection.
  • With very small clearance, rollover and burr can be minimized, but if clearance is too small, secondary shearing (Secondary Shear) can occur as the blank is displaced, increasing the chance of burrs along the edge.
  • If clearance is too large, the part may separate more by being pulled away (not fully cut), producing large burrs and taller burr heights, making deburring costly or impractical.

• Burr production and edge quality controls

  • Burrs should be minimized to avoid post-processing costs and safety concerns.
  • Deburring (boring out burrs) increases manufacturing cost; acceptable burr size becomes a key design parameter for die life and cycle time.
  • Burr height is a function of clearance, material properties, and tool wear; designers balance edge quality with tool life.

4.3 Principles of blanking die design

• 4.3.1 Clearance between punch and die (Clearance)

  • Definition: clearance is the gap between the cutting edges of the punch and die.
  • Normal design rule: clearance should be less than the workpiece thickness to ensure the part breaks away, not pulled through.
  • For circular blanking and piercing on round parts, one-sided clearance can be used (as half the difference of punch and die diameters):
  • CL = 0.5 (Dp − DD)
  • Dp: punch diameter; DD: die diameter
  • For irregular shapes, clearance can be treated analogously.
  • Importance: clearance strongly affects edge quality, cutting force, part dimensional accuracy, die/punch wear, and blanking stiffness.

• Optimum clearance and material dependence

  • Optimum clearance is material dependent and is obtained from Table 4.1 (brief summary below).
  • Table 4.1 provides recommended clearance as a percent of sheet thickness (%t) for various materials. Typical ranges (per table) include:
  • Iron and mild steel family: ~6–9% of thickness
  • Hard steels: ~8–12% of thickness
  • Silicon steel: ~7–11% of thickness
  • Stainless steel: ~7–11% of thickness
  • Copper/brass/brass alloys: variable within ~6–10% depending on hardness
  • For burr-free/low-burr processes or multi-stage cutting, the clearance may be reduced (e.g., down to <= 0.5% of thickness for very fine blanking or burr-free steps).
  • Important notes:
  • Clearance % is provided for t < 3 mm in Table 4.1; special processes may require adjustments.
  • If parts slide off the die (debris), use smaller clearance than the recommended table in some cases.

• 4.3.2 Force and work in blanking

  • The maximum cutting force (P) can be estimated from the shear strength and the sheared area:
  • Sheared area is the length of cut around the blank (perimeter) times the material thickness:
  • Let K be the shear strength of the material (kgf/mm^2) and S1 be the length of the cut around the part (mm); t is the material thickness (mm).
  • Then the maximum cutting force is:
  • $P = K \, S_1 \, t$
  • K can be taken from Table 4.2 or approximated as K ≈ 0.8 × σt (where σt is the tensile strength of the material, in kgf/mm^2).
  • S1 is the total length of the sheared perimeter (for a circular blank S1 = πD, where D is the blank diameter).
  • Example calculation (from the text): Cutting stainless steel with 2 mm thickness to a blank diameter of 30 mm
  • K ≈ 36 kgf/mm^2 (from Table 4.2; for this material/condition)
  • S1 = πD ≈ π × 30 ≈ 94.25 mm
  • P ≈ 36 × 94.25 × 3 ≈ 10,174 kgf ≈ 99.7 kN
  • Practical interpretation: This demonstrates why blanking dies are sized to deliver near-peak force, but with margins and safety factors for punch/die wear and press curve limitations.

• 4.3.2b Shearing work (energy) in blanking

  • The work required to shear (W) is the area under the force vs. stroke curve.
  • The approximate formula used in the text is:
  • $W = m \, t^2 \, K / 100$
  • Where:
  • W is the work (J)
  • m is a material-dependent factor (Table 4.3)
  • t is material thickness (mm)
  • K is the shear strength (kgf/mm^2)
  • Table 4.3 provides the factors m for different material groups; typical values include:
  • Group 1: Aluminum (soft), Brass (soft), Mild steel (low carbon) → m ≈ 0.76
  • Group 2: Aluminum (hard), Copper (hard), Mild steel (0.2–0.3% C) → m ≈ 0.64
  • Group 3: Spring steel, Brass (hard), Steel (0.3–0.6% C) → m ≈ 0.50
  • Group 4: Steel (0.6% C and higher) → m ≈ 0.45
  • Group 5: Cold-rolled steel (not annealed) → m ≈ 0.40
  • Group 6: (Other) → m ≈ 0.30
  • Practical takeaway: W increases with material thickness and the chosen K; the factor m captures material behavior, hardening, and workpiece/forming characteristics.

• 4.3.3 Stripper plate and stripping force

  • A stripper plate holds the sheet blank in place as the punch moves down, preventing ejection of the blank through the die opening.
  • Stripping helps ensure a clean blank edge and reduces burr formation but requires additional force and spring/cushion capacity.
  • Stripping force is typically a portion of the maximum cutting force (often 5–20% of P, depending on material, clearance, lubrication, and stripper design).
  • Types:
  • Fixed stripper (non-movable): simpler but limited in high-accuracy or continuous-feed lines.
  • Movable stripper: external actuation to press the sheet against the stripper during cut; more effective for light or precise cuts but adds machine complexity.
  • The stripping system also supports blank ejection and sheet alignment, and affects feed reliability and wear on punches.

• 4.3.4 Counter punch

  • Counter punches are used when higher blanking accuracy is required, particularly for thin or high-accuracy parts.
  • They apply external pressure to press the blank against the punch-die interface from below, stabilizing the sheet under cutting and improving edge straightness.
  • Counter punches typically require external force (e.g., springs or gas) and add to the press capacity needs.
  • In some designs, the counter punch sits beneath the die and helps hold the sheet, reducing burrs and improving edge quality; in other cases the punch remains at the bottom and the counter-punch helps hold the sheet under the punch.

• 4.3.5 Shapes of punches and dies

  • Punch shapes
  • Generally divided into 3 parts: the cutting tip, the body, and the head for mounting.
  • Two main punch types:
    • Straight punch (no shoulder): common for high-precision, burr-minimized blanks; less prone to buckling, but if used with a stripper or EDM/wire-cut parts, issues may arise with fasteners.
    • Shoulder punch: used for smaller shapes and when blanking with stripper support; helps prevent punch fracture (buckling risk).
  • For mounting, the punch head often includes a flange; the punch body is cylindrical, and for non-round shapes, anti-rotation dowel pins are used.
  • Die shapes
  • Four main types (as per Fig. 4.2 and 4.21):
    • (a) Straight Die: no bevel/relief, not typically used for general cutting due to clogging by scrap; used for high-precision or special operations (e.g., push-back blanking).
    • (b) Die with clearance angle (beveled die): mitigates die clogging and keeps the edge from folding; increases clearance requirement.
    • (c) Die with “land” and clearance: a land segment exists between cutting edge and body; helps rigidity and edge consistency.
    • (d) Die with land and open: larger land area with open edges to facilitate shedding; helps with burr control but raises clearance.
  • 4.3.5.1 Design notes for punch/die surfaces
  • The bevel angle a is typically small to moderate (often ~0.25–1 degree for the die edge) to avoid edge collapse and ensure consistent clearance.
  • The straight die often gives the most consistent edge if properly aligned and deburred, but its tendency to clog requires careful scrap management (shape choice and cut strategy).
  • The land height (h) design is crucial for preventing edge collapse and to control burr, particularly for complex shapes; Table 4.5 provides recommended h values for specific die types (shape 4.21 (c) and (d)).
  • 4.3.6 Strip layout (Strip Layout / Blank Layout)
  • The layout of parts on a strip heavily affects material cost (50–70% of total cost is the blank material).
  • For simple shapes (circles, squares, etc.), parallel rows and tight spacing minimize scrap width and material waste; for hollow or complex shapes, alternate layouts reduce scrap and avoid blanking in central holes.
  • Considerations for layout:
    • Alignment with the sheet rolling direction to minimize springback and bending resistance in downstream forming.
    • Ensure that the scrap width (gap between blanks and between parts) is minimized to reduce waste but not so tight as to risk edge damage or tearing.
    • When producing hollow parts or isolated spaces, plan to place other parts in the hollow region’s open space to minimize waste.
    • For shapes like “L” or “U”, stagger/offset layouts help minimize scrap.

4.4 Die life and wear (sustainability and maintenance)

• 4.4.1 Factors affecting die life

  • Die life is determined by 5 main groups of factors:
    1) Press and equipment rigidity, speed, and drive system reliability.
    2) Die-set and guide posts; precision and wear of guides affect edge quality.
    3) Die condition (shape, clearance control, build quality) and tool geometry.
    4) Workpiece material properties (hardness, brittleness, thickness, and the presence of coatings or surface films).
    5) Lubrication and material handling (lubricant type, application method, surface finish, and feedability).
  • The practical result is that wear is influenced by hardness, toughness, and surface interactions; high-carbon and carbide tools extend life but may suffer from accelerated crack growth if misapplied.

• 4.4.2 Wear modes on punches and dies

  • Wear typically initiates near the cutting edge and on the punch surface near the edge; four primary wear modes are identified:
    1) Flank wear (side wear) along the long axis of the punch and die; affects the effective clearance and ultimately the edge dimension.
    2) Edge wear (at the cutting edge itself) leading to burr growth; progressive edge dulling increases burr height.
    3) Face wear (front face wear) near the cutting edge due to repeated impacts; results in fatigue of the surface.
    4) Crater wear (crater wear) on the punch face away from the edge center, caused by repeated elastic deformation and sliding between the punch and workpiece.
  • Figures 4.24 conceptually illustrate these wear modes.

• 4.4.3 Wear control to extend die life

  • Manage wear through a combination of tooling material, coating, and lubrication strategies:
  • Tool materials: hard, wear-resistant materials (e.g., carbide) for long life, particularly for high-volume or hard-material blanks.
  • Surface coatings: hard chromium, nitriding, or thin hard coatings (TiC/TiCN by CVD/PVD) can dramatically extend die life and improve surface finish of finished parts, often by 2–4x or more depending on conditions.
  • Coatings and microstructure: micro-hardness and carbon content influence wear resistance; micro-hardness often governs abrasive wear resistance better than macro hardness.
  • Lubrication strategies: fluids with EP additives reduce adhesive wear; chlorinated lubricants with sulfur-containing compounds often help in stainless steels and high-hardness materials; MoS2 and PTFE can also reduce wear in certain cases.
  • Choice of coatings also depends on workpiece material (e.g., TiN, TiC/TiCN show different effectiveness depending on Austenitic vs Ferritic stainless steels).
  • Deburring and finishing strategies to maintain edge geometry without excessive tool wear.

• 4.4.4 Counter-punch considerations for wear and accuracy

  • Counter punches add stiffness and improve edge straightness for parts requiring high positional accuracy.
  • They require external actuation (springs, gas) to hold the blank under the punch and prevent misalignment and slip; however, their use increases machine size and complexity.
  • Counter-punch implementation must balance the required holding force with available press capacity, often around 10% of the cutting force, and may require stronger drive systems or alternative actuation (e.g., nitrogen springs).

• 4.4.5 Die life expectancy and life estimation

  • Life expectancy is influenced by press rigidity, die set quality, lubrication, workpiece properties, and the geometry of the punch/die combination.
  • Estimation frameworks often use empirical data from prior runs and wear models to determine how quickly edge wear will progress under given conditions and what maintenance intervals are appropriate.

4.5 Practical design summary and guidelines

• Best practices for stable blanking operations

  • Start with optimum clearance values from Table 4.1 for the specific material; adjust for burr-free or multi-step blanking, or for delicate fine-blanking operations which may require reduced clearance (< ~0.5% of thickness, or ~10–20 μm) to preserve edge integrity.
  • Use a counter-punch or a robust stripper system to minimize burrs and maintain edge straightness, especially in high-precision applications or high-speed lines.
  • Design strip layout to minimize scrap while also aligning blanks with rolling direction to manage springback in downstream forming.
  • When long or complex strips are required, consider feed direction and alignment to minimize burr height and ensure consistent edge quality across the strip.

• Quick reference to key equations

  • Clearance for circular blanks (one-sided):
  • $CL = 0.5 \, (D<em>p - D</em>D)$
  • Maximum cutting force (approximate):
  • $P = K \, S_1 \, t$
  • Where K ≈ 0.8 × σ_t (material tensile strength) or K read from Table 4.2 for the material; S1 is the length of the cut (for a circular blank, S1 = π D).
  • Stripping or blanking energy (work):
  • $W = m \, t^2 \, K / 100$
  • m is a material-dependent factor (Table 4.3).
  • Note: All material constants (K, σ_t, m) are material dependent and taken from Tables 4.2 and 4.3.

• Real-world considerations and design trade-offs

  • A tighter clearance reduces burr but increases stamping force and wear; larger clearance reduces punch wear but increases burr and edge rounding.
  • The press curve (especially for mechanical presses) means actual peak force occurs near BDC; engineers should design with 20–30% margin between press capability and required blanking force to accommodate dynamic effects.
  • For heavy or multiple-stage blanking (burr-free or push-back blanks), clearance and tool geometry may require special sequence design (e.g., first stage with larger clearance to separate, then secondary stage to finish with burr-free edges).
  • Store and maintain tool life data to optimize replacement schedules; coatings and microstructure management via heat treatment and surface modification dramatically improve die life in high-volume environments.

References (as cited in the transcript):

• Saxonia, Online Catalogue. 2008.
• Koga & Aoki, Press Working – Blank ing Operation. 2002.
• Eary & Reed, Technique of Sheet Metal Working, 2nd Edition. 1974.