Fabricate and Repair Dies (Stamping, Forming, and Progressive Dies)

Manufacturing and Repairing Dies (Stamping, Forming, and Progressive Dies)

Introduction to the Die Trade

The die is the cold-forming tool that uses plastic deformation to transform a flat sheet of metal into a three-dimensional part. For the Red Seal exam, you must master not only the geometry of the tool, but also the physical phenomena of deformation, functional clearances, tooling materials, and fitting and repair procedures.

The punch is the male component, and the die (in the strict sense) is the female component. Together, the punch and die form the tooling. The press supplies the deformation energy. The distinction between a stamping die (deformation by drawing), a forming die (bending, rolling, curving), and a progressive die (multiple operations in a single pass) is fundamental.

Principles of Metal Deformation

Elastic and Plastic Deformation

Any metal subjected to stress first deforms elastically (Hooke's Law: σ = E × ε, where σ is stress in MPa, E is the modulus of elasticity, and ε is strain). Beyond the yield strength (Re), deformation becomes plastic: permanent. For stamping, you exploit plastic deformation without fracture.

The tensile strength (Rm) is the maximum stress before fracture. The useful zone for stamping lies between Re and Rm. The ratio Rm/Re is called the strain hardening coefficient (n). The higher the value of n, the more the material work-hardens and the more it can be drawn without fracturing.

Work Hardening and Recrystallization

Work hardening is the hardening of metal through cold plastic deformation. Dislocations accumulate, hardness increases, and ductility decreases. For mild steels, work hardening is significant; for brasses, it is moderate.

Recrystallization is the heat treatment that eliminates work hardening. For mild steel, the recrystallization temperature is approximately 550 °C. In practice, you do not recrystallize stamped parts — instead, you select a material with a good strain hardening coefficient.

Springback

Springback is the elastic deformation that occurs after the punch is withdrawn. The part tends to return to its original shape. The springback angle Δα depends on:

Practical formula: Δα = (Re × r) / (E × e) × 180/π (in degrees)

To compensate, you overbend the tool: the die angle is closed more than the final desired angle. You can also use a coining bead that plasticizes the bend zone and eliminates springback.

Minimum Bend Radius

The minimum bend radius is the smallest inside radius that can be formed without cracking. It is expressed as multiples of the thickness:

Exam rule: if the required radius is smaller than the minimum radius, you must either change the material or perform a two-pass bending operation (pre-bend followed by final bend).

Stamping Dies

Stamping Principle

Stamping consists of deforming a flat sheet into a hollow shape (cup, automotive body panel, etc.) through the action of a punch that pushes the sheet into a die. The sheet is held around its perimeter by a blank holder that applies controlled pressure.

The primary deformation is drawing: the metal is stretched radially toward the inside of the cup. The flange (outer edge) moves inward while compressing circumferentially. If the blank holder pressure is too high, the flange cannot slide and fracture occurs at the bottom of the cup. If it is too low, wrinkling of the flange occurs.

Blank Calculation (Developed Blank)

For a cylindrical cup with diameter d, height h, and thickness e, the blank diameter D is:

D = √(d² + 4 × d × h)

This formula assumes constant thickness and neglects the bottom radius. For a bottom radius r, use:

D = √(d² + 4 × d × h − 0.5 × r × d)

Exam trap: height h is measured on the outside of the cup, not the inside. Always check the dimensions on the drawing.

Draw Ratio

The draw ratio β = D/d (blank diameter / punch diameter). For a first draw without annealing:

If β exceeds β_max, you must perform multiple drawing passes with intermediate annealing (for steels) or progressive drawing (for non-ferrous metals).

Punch-to-Die Clearance

Clearance is the difference between the die diameter and the punch diameter, divided by 2 (radial clearance). For stamping:

Clearance = e + 0.1 × e (i.e., 10% of thickness added)

For blanking/piercing:

Clearance = 5 to 10% of thickness (depending on material)

Clearance that is too small produces excessive burnish and rapid wear. Clearance that is too large produces a rounded edge (roll-over) and significant burrs.

Lubrication

Lubrication reduces friction between the sheet and the tool. For deep drawing, use rolling oils or emulsions. For stainless steel, use chlorinated oils (note: chlorinated oils are banned in some shops for environmental reasons — use synthetic oils instead).

Forming Dies (Bending, Curving)

Air Bending

Air bending: the sheet is placed on two supports (the die V), the punch descends and bends the sheet without it touching the bottom of the V. The final angle depends on the depth of penetration. Springback is significant and must be compensated for.

Bottoming / Coining

Bottoming / coining: the sheet is crushed between the punch and the die. The bend zone is plasticized in compression, which almost completely eliminates springback. The force required is 3 to 5 times higher than for air bending.

Bending Force

The air bending force (in kN):

F = (k × Rm × L × e²) / (8 × V)

Where:

Rule of thumb: V = 8 × e (for sheet ≤ 3 mm), V = 10 × e (for sheet > 3 mm).

Developed Length in Bending

The developed length L_d is the length of the sheet before bending. For a 90° bend with inside radius r and thickness e:

L_d = L1 + L2 + (π/2) × (r + k × e)

Where k is the neutral axis position factor:

Exam trap: the neutral axis is the zone where the stress changes sign (compression on the inside, tension on the outside). It is not at the center of the thickness for small radii.

Progressive Dies

Principle

A progressive die performs multiple operations (blanking, piercing, bending, forming, stamping) in a single press stroke. The strip advances by one pitch with each press stroke. Each station performs a partial operation.

Advantages and Disadvantages

Pitch Design

Pitch is the distance between two successive positions of the strip. It equals the part width plus the bridge width between parts.

Minimum bridge: 1.5 × e (for e ≤ 1 mm), 1.0 × e (for e > 1 mm). Side bridge (strip edge): 1.5 × e.

Pitch calculation: P = part length + bridge

Strip width: B = part width + 2 × side bridge

Order of Operations

The typical order in a progressive die:

Exam rule: pilot holes are always pierced first. Bending is done before final trimming. Final trimming is the last operation.

Pilots

Pilots are conical pins that enter the pilot holes in the strip to position it precisely before each operation. The pilot diameter is 0.02 to 0.05 mm smaller than the hole diameter. The pilot must penetrate at least 2 × e into the strip.

Reliefs

Reliefs are cutouts in the strip that allow bending without tearing. For example, for a U-bend, you cut slots at the ends of the bend line.

Tooling Materials

Tool Steels

Exam rule: for production over 100,000 parts, use carbide. For medium production (10,000 – 100,000), use an alloy steel (O1, D2). For low production (< 10,000), use a carbon steel.

Heat Treatments

The heat treatment of tool steels includes:

Hardness is measured in HRC (Rockwell C). For a blanking punch: 58 – 62 HRC. For a stamping die: 55 – 58 HRC (more ductile to avoid cracking).

Coatings

Coatings (PVD, CVD) improve wear resistance:

Manufacturing and Fitting

Machining of Components

Punches and dies are machined by milling, grinding, wire EDM, or sinker EDM. Wire EDM is preferred for complex contours with a precision of ± 0.005 mm.

Grinding is used for flat surfaces and external profiles. Final surface finish must be Ra 0.4 µm or better for functional surfaces.

Fitting

Fitting consists of assembling the die components and verifying clearances. Procedure:

Exam trap: clearance must be checked around the entire perimeter, not just at one location. Uneven clearance produces deformed parts and premature wear.

Tryout

Tryout is the commissioning of the die on the press. You verify:

You adjust the blank holder pressure, depth stop, and lubrication.

Die Repair

Wear and Damage

Common damage:

Repair Procedures

Exam rule: before welding tool steel, you must preheat to 200 – 300 °C to avoid cracking. After welding, cool slowly (in sand or lime) and temper at 150 – 200 °C.

Punch Sharpening

Sharpening of blanking punches is done by surface grinding. Remove 0.05 to 0.10 mm per pass. The relief angle is 1 to 2° for punches and 2 to 3° for dies.

Safety and Standards

Press Safety

Presses must be equipped with guards (light curtains, safety barriers, two-hand controls). Maintenance must be performed with the machine stopped and locked out (lockout/tagout procedure).

Canadian Standards

The Canadian Electrical Code, Part I (C22.1-21) applies to the electrical installations of presses. Rule 8-200 concerns grounding conductors: minimum size of 6 AWG for power circuits.

Standard CSA Z142 (Code for power press protection) defines the safety requirements for mechanical and hydraulic presses. It requires:

Standard CSA B149.1 (Natural gas and propane code) applies if the die is used with a gas-fired heat treatment furnace.

Exam rule: CSA Z142 is the reference standard for press safety in Canada. It replaces older provincial standards.

Practical Calculations for the Exam

Blanking Force Calculation

F = P × e × Rm

Where:

Example: blank a circle of Ø 50 mm from 2 mm mild steel sheet (Rm = 350 MPa).

P = π × 50 = 157 mm

F = 157 × 2 × 350 = 109,900 N = 110 kN

Stamping Force Calculation

F = π × d × e × Rm × (D/d − 0.7)

Where d = punch diameter, D = blank diameter.

Press Power Calculation

Power (kW) = (F × v) / 1000

Where F in kN, v in m/s. For a mechanical press, the speed at the point of impact is approximately 0.3 to 0.5 m/s.

Tolerances and Fits

Fits are defined by ISO 286. For tooling:

Exam trap: do not confuse H7/p6 (interference) with H7/g6 (clearance). H7/p6 is a fit with interference (the punch is larger than the hole). H7/g6 is a fit with clearance (the pilot can slide).

Testing and Inspection Procedures

Dimensional Inspection

Inspection of stamped parts is done with:

Springback Inspection

Springback is measured with an angle protractor or goniometer. If springback exceeds tolerance, adjust the penetration depth (air bending) or modify the die angle (bottoming).

Hardness Inspection

Tool hardness is verified with a Rockwell hardness tester (C scale for hardened steels, B scale for mild steels). The hardness of the stamped part is verified with a Vickers (HV) or Brinell (HB) hardness tester.

Pitfalls to Avoid

Summary

To succeed on the exam, practice calculating forces, developed lengths, and clearances. Memorize typical values (β_max, clearances, temperatures). Review progressive die diagrams and orders of operations. Good luck with your preparation.