The manufacturing process of "cold forming" applies force to a metal as it is staged in a die. The technology, used originally in the early 1900s to create artillery shells, supports the creation of a variety of sophisticated, small parts, including fasteners, pins, and screws. But how do the residual stresses caused by the process affect the life of the part?
After a "punch," the force from cold forming exceeds the alloy's elastic limit, causing plastic flow until the metal blank assumes the shape bound by the punch and the die. The manufacturing method is achieved by force alone, forgoing the application of additional heat or cutting and shearing.
In a presentation this week titled Warming Up to Cold Forming: An Alternative to Precision Machining, a Tech Briefs reader asked a maker of metal parts:
How will residual stresses caused by the process affect the life of the part?
Madan Mathevan, Vice President of Engineering, Sussex Wire (Easton, PA): Most of our customers actually like the residual stresses. That's the reason they come to us. The reason being: The stress work-hardens the material and gives it more strength.
For example, take an undercut just beneath the head of the screw. If that particular screw is made by machining, material is removed. But cold forming that particular shape helps with the strength of the material.
There are also other customers who come to us and say they don't want any residual stress; for those customers, we anneal the product after it gets cold-headed so we can reduce any kind of residual stresses on these bolts, screws, or other products we are cold heading.
It mostly depends on what the customer wants. Some customers like the fact that we can work-harden the material. Some customers don't want it. What we usually do is perform a stress analysis and show our customers what the residual stresses are going to be in different locations. That gives them an idea of whether they want a product annealed or not.
What percent of increase in tensile strength is seen due to work-hardening or cold forming?
Mathevan: Most of the time, depending on the reduction or increase in diameter, the strength goes anywhere between 10 pounds per square inch to 15 pounds per square inch tensile.
What do you think? How have you used cold forming? Share your thoughts below.