Stamping Process Parameters (MFG): Forming Force and Die Design in Disc Spring Manufacturing

1. Process Overview

The manufacturing of disc springs typically includes blanking/piercing (producing annular blanks) and conical forming (pressing the cone angle). Stamping process parameters determine the dimensional accuracy, residual stress distribution of the spring, and ultimately affect its force‑deflection characteristics and fatigue life.

Based on sheet metal forming theory, the calculation formulas for the most critical forming force, die clearance, and press tonnage in disc spring stamping are provided below.

2. Core Calculation Formulas

2.1 Blanking/Piercing Force ($P_c$)

In compound or progressive dies, the force required to shear the annular blank and center hole is:

• $P_c$ — Blanking force (N)
• $L$ — Shear contour length (mm), for blanking $L = \pi D_e$, for piercing $L = \pi D_i$
• $t$ — Material thickness (mm)
• $\tau_b$ — Material shear strength (MPa), can be approximated as $\tau_b \approx 0.8\,R_m$, where $R_m$ is the tensile strength

When selecting a press in practice, a safety factor of $1.2\sim1.5$ should be applied to account for edge wear and other factors.

2.2 Conical Forming Force ($P_f$)

Pressing a flat annular blank into a conical disc spring involves combined bending and flanging deformation. The forming force can be estimated by:

Or using a more precise empirical formula:

• $\sigma_b$ — Material tensile strength (MPa). For spring steel, typically in the annealed state before quenching, $\sigma_b \approx 500\sim700$ MPa; if in the quenched and tempered state, a higher force is required
• $C$ — Shape coefficient, depending on $h_0/t$ and lubrication conditions, generally taken as $0.5\sim1.5$. The larger the cone angle, the larger $C$
• Other symbols are the same as above

Note: Disc springs are usually formed in the annealed state and then heat-treated (quenching + tempering) to achieve final properties. Therefore, forming force calculations should use mechanical properties of the annealed material.

2.3 Blank Holder Force ($Q$)

When the sheet metal has a tendency to wrinkle during forming (especially for thin materials and large cone angles), a blank holder is required. The blank holder force is:

• $A_p$ — Blank holder area (mm²), typically the annular area
• $q$ — Unit blank holder pressure (MPa), for steel can be taken as $1.5\sim3.0$ MPa

Excessive blank holder force can cause tearing, while insufficient force can lead to wrinkling; adjustment through die tryout is necessary.

2.4 Stripping and Ejection Forces

After blanking, parts may stick to the die or punch, requiring friction to be overcome for removal.

• Stripping force $P_{strip} \approx K_{strip} \cdot P_c$, $K_{strip}=0.02\sim0.06$
• Ejection force $P_{eject} \approx n \cdot K_{eject} \cdot P_c$, $K_{eject}=0.03\sim0.07$, $n$ is the number of parts stuck in the die

2.5 Die Clearance ($c$)

Blanking clearance directly affects the quality of the shear edge and die life. For spring steel plates used in disc springs, the recommended total clearance is:

The clearance for forming dies is slightly larger than the material thickness, considering springback. Typically, the punch-die clearance is taken as $t + (0.05 \sim 0.1)$ mm.

3. Springback Compensation

Disc springs exhibit springback after forming, causing the free cone height $h_0$ and cone angle to deviate from the die design values. Process-wise, over-bending compensation is applied to the die cone angle, meaning the die cone height $h_{0,die}$ is greater than the target cone height $h_0$. The compensation amount is determined by the material yield strength and thickness:

Where $k$ is an empirical coefficient, typically determined through die tryout or finite element simulation.

4. Press Tonnage Selection

The required press nominal force $P_{press}$ should be greater than the total working force:

Considering energy consumption during the blanking stroke and overload protection, select a press with a nominal force $1.3\sim1.5$ times the calculated value.

5. Calculation Example

Disc spring: $D_e=40$ mm, $D_i=20.4$ mm, $t=2.0$ mm, annealed spring steel $\tau_b = 400$ MPa, $\sigma_b=600$ MPa.

Blanking force: $L = \pi \times 40 \approx 125.66$ mm

Forming force: Take $C=0.8$

Note: The forming force is much smaller than the blanking force, so blanking is the main process determining press tonnage.

Adding stripping force, etc., a press with a nominal force of 160 kN is sufficient.

6. Relationship Between Process Parameters and DIN 2093 Performance

• Insufficient forming force leads to inadequate cone height and low force values, failing to meet standard load requirements.
• Improper die clearance produces burrs or micro-cracks, reducing fatigue life (OM point, I point are prone to early failure).
• Poor springback control causes $h_0$ to exceed tolerances, deviating stiffness and energy storage from design values.
• Post-stamping stress-relief annealing and shot peening can improve residual stress distribution and enhance fatigue limits; these subsequent processes directly affect DIN 2093 fatigue group performance.

Therefore, accurate calculation and strict control of stamping process parameters are fundamental to manufacturing qualified disc springs.

Summary: The stamping process parameters for disc springs center on blanking force and conical forming force, combined with reasonable die clearance, blank holding, and springback compensation, to ensure part geometric accuracy and mechanical properties meet DIN 2093 requirements. Design must use annealed material data and optimize final parameters through die tryout.