All About Die Springs
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 provide 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 lb 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.
Material Hardness Tonnage Ratios
Material Description 
Tons/
sq. inches 
Shear Strength/
sq. inches 
Multiplier to
Nominal
Mild Steel 
Aluminum
≠ Soft Sheet 
7.5 
15,000 psi 
.3 
Aluminum
≠ Half Hard 
9.5 
19,000 psi 
.38 
Aluminum
≠ Hard 
12 
25,000 psi 
.5 
Brass
≠ Soft Sheet 
15 
30,000 psi 
.6 
Brass
≠ Half Hard 
17.5 
35,000 psi 
.7 
Copper
≠ Rolled 
14 
28,000 psi 
.57 
Steel
≠ Mild 
25 
50,000 psi 
1.00 
Steel
≠ ASTM A36 
30 
60,000 psi 
1.20 
Steel
≠ 50 Carbon 
35 
70,000 psi 
1.40 
Steel
≠ Cold Drawn 
30 
60,000 psi 
1.20 
Steel
≠ Stainless (188) 
35 
70,000 psi 
1.40 
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 (Pressure in tons): .71 tons
Stripping 15%: .11 tons (220 lbs)
Assume 4 springs moving ½" to attain minimum 220 lb.

This could be accomplished with a 2" x ¾" blue die spring giving 72 lb at ½" x 4 = 288 lb (see Metal Die Spring Pressure Comparison chart below). 

A single 2" urethane stripper ¾" O.D. and ¼" ID would deliver 430 lb 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 lb deliver 260 lb. 
Metal Die Spring Pressure Comparison
Die Spring
Colour 
Free Length (inches) 
O.D.
(inches) 
Maximum Compression
% 
Maximum Compression Distance
(inches) 
Maximum Compression
(lb) 
Blue 
2.0 
3/4 
50 
1.0 
144 
Red 
2.0 
3/4 
37 
.74 
183 
Gold 
2.0 
3/4 
30 
.60 
297 
Green 
2.0 
3/4 
25 
.50 
340 
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 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. We've got our calculators ready!
Tel: 604.521.6444
Toll Free: 1.800.668.4996
Email: sales@punchtools.com 
