How to Choose a Die Spring for Your Stamping Die

Die springs play a crucial role in the performance and longevity of stamping dies, ensuring optimal pressure for restraining sheet materials during various operations such as piercing, forming, flanging, or trimming. Choosing the right die spring involves considering factors such as pressure requirements, spring size, and travel needs. Here’s a guide to help you make an informed decision.

Pressure Requirements

Determining the spring pressure for a die spring involves calculating the piercing pressure for the entire die. The formula for piercing pressure is:

PiercingPressure=S×T×L

Where:

• (S) is the shear strength of the part material.
• (T) is the thickness of the part material.
• (L) is the total length of the shear or cut distance.

Typically, die springs are driven with a pressure equal to 10% to 15% of the total piercing pressure. This range allows for safe alignment of the downward punching speed, ensuring secure engagement of die components before material contact. The formula for stripping pressure is:

StrippingPressure=PiercingPressure×(0.10 to 0.15)

In forming dies, the formula for bending pressure is different but still includes the same three elements: pressure, the quantity and size of springs, and travel. The forming pressure should be at least 1.5 times greater than the force required for bending the part.

Where:

• (S) is the tensile strength.
• (T) is the material thickness.
• (R_1) is the radius on the forming punch.
• (R_2) is the radius on the forming die.
• (L) is the length of the part.

Spring Size and Quantity

The available space in the die will help determine the number and size of springs to use. Whenever possible, opt for multiple low-pressure springs rather than a few high-pressure ones. This reduces pressure on individual springs.

Consider placing springs as close as possible to the perforated, formed, flanged, or trimmed areas for enhanced control over sheet material while positioning maximum pressure in the working region. Additionally, using features like spring cams, fixed bolts, or spring caps can protect springs from debris and safeguard tool details in case of spring fatigue or breakage.

Travel Requirements

When springs are installed in the die, they are compressed by approximately 0.125 inches, known as preload force, to prevent springs from detaching. Preloading also reduces the impact associated with rapid loading and unloading of the die, extending spring life.

The travel of the die spring depends on preload force, material thickness, the distance the punch enters the die, and the distance the stripper hangs below the bottom of the punch. In summary, preload plus stripper travel equals the total spring travel.

Spring Selection

Now that you have the pressure, approximate size, and travel requirements, you can begin the actual spring selection process. Match the chosen springs with the three aspects of die design. Ensure that the selected springs, under preload, provide the required pressure. Springs must offer sufficient travel within their normal operating range.

Remember that springs should not compress beyond their maximum working range, as this significantly affects spring life and may damage the die. Choosing springs with additional travel is advisable to prevent overtravel when tools are re-sharpened.