Springs are used in a myriad of applications and are as essential to machines as bearings. Springs come in a wide variety of constructions but few are as tightly engineered as the style commonly referred to as compression or die springs. Die springs are engineered to give predetermined pressure at a given compression reliably and consistently.

This article will look at the basics of selecting die springs for punch tools using the following variables:
- Pressure required to perforate the material (shear off, punch hole)
- Stripping or Hold Down pressure
- Travel (compression distance)
- Available space in the tool

There are 3 main types of springs. Each type of spring has its best application depending on the pressure required.
- Engineered metal die springs ­ good for any general application where performance is critical; pressure increases with the amount of compression
- Urethane springs ­ provides alternatives for punch tool design where traditional design is cumbersome; very good for forming curves; reduces marring stock surface

Gas springs ­ can provide high pressure in small areas beyond that of die springs or urethane; gas springs provide a flatter pressure curve through the compression

The die springs primary task is to provide pressure to hold the target material in place while the punch perforates and then retracts ­ or strips off (hence 'stripping pressure'). There are standard formulas for calculating perforating pressure. Stripper pressure is a function of perforating pressure ­ usually 10 ­ 25% of perforating pressure. These are usually based on mild steel. Mild steel has a shear pressure of 50,000 lbs psi or 25 tons. The perforating pressure calculations are usually done using 25 tons and then adjusted for other metals with a ratio reflecting the hardness of the other material to mild steel.(see chart 1 below)

Perforating pressure is calculated as follows:
(Length of shear in inches) x (Material Thickness in inches) x 25 tons (for mild steel)

- the perimeter of a hole is the same as the length of the shear

Example

Let's look at punching a ¼" hole in 20 gauge mild steel
- (Perimeter of hole) x (Material thickness) x 25
- Perimeter of a circle = 3.14 x Diameter
- (3.14 x .25" x .036" x 25 tons)

Perforating         Stripping 15%
(Pressure in tons)
.71 tons             .11 tons (220 lbs)

Assume 4 springs moving ½" to attain minimum 220 lbs
- This could be accomplished with a 2" x ¾" Blue spring giving 72 lbs @ ½" x 4 = 288 lbs. (see chart 2 for other options)
- A single 2" urethane stripper ¾" OD and ¼" ID would deliver 430 lbs and may simplify the tool design especially if there is limited space available.
- 4 Gas springs 2.24" x ¾" compressing approx.600" with a force of 65 lbs deliver 260 lbs

Excess pressure for perforating and stripping is usually not a problem ­ only too little pressure. Total pressure is only limited by the power in the punch press to be used.

The amount of travel (or compression) in a spring is based on the punch tool design. Obviously different travel distances will require different spring lengths. Die springs generally do not compress more than 50% with most no more than 30%. The spring above traveled 25%.

The permutations caused by the number of different punch presses and applications require a large selection of springs. Die springs come in 4 incremental strength grades by color ­ blue, red, gold, and green. There are over 120 spring designs in each color grade.

Punch Tools Inc. carries over 500 types of springs including urethane and gas springs to meet your needs. Call our experts to help you with your next project. We¹ve got our calculators ready!

Chart 1   Material Hardness Tonnage Ratios

Chart 2   Metal Die Spring Pressure Comparison