Solutions to Bending Marks Problems
Solutions to Sheet Metal Bending Marks Problems
When a customer demands that pesky press-brake-tool marks on bent metal be totally eliminated, or if the shop personnel are faced with bending a radius on a part that just can’t be done with regular tooling, how do they approach these problems, especially if they don’t know how to do either?
Sometimes a part can be finished to remove tool marks, but bending a unique radius is more of a challenge. Both require a different way of bending parts, but not with traditional press-brake tooling.
The way it’s done is with urethane-press-brake tooling that can eliminate tool marks along with producing bends that no other types of tooling can do.
This tooling can make perfectly finished parts like the leading edge of a wing for a light airplane where tool marks would disrupt the flow of air and the plane’s lift. It can even perform non-kinked bending of small-diameter tubing that needs to wrap around another component.
Urethane tooling can be as strong as plastic and as elastic as rubber. Urethanes are used in many areas of manufacturing as their physical properties include excellent abrasion resistance, good impact resistance along with excellent resilience and toughness. Plus it offers high load bearing capacity.
As with any press brake tooling, the application will always dictate what type of urethane should be used. Urethane can be used for either the punch or the die, but about 99 percent of the time as the die, depending on the application.
But what does this matter to the person who needs to bend a component? Really, not much, as it’s the company building the tooling that will select the proper urethane and tooling to do the work. The tooling style and type would include the hardness and the design to control springback in the material that’s being bent.
Carl Michelsen, V.P. of Polyurethane Products Corp., is a long-time supplier of urethane-press-brake tooling and components. He mentions that polyurethane tooling can be used to keep the carbon in the steel of standard-press-brake tooling from embedding in stainless steel parts. As this carbon can later rust in the parts, marring them.
“One job that our company builds is a U-shaped trough from stainless steel,” he says. “The problem with this is that the sides have to be straight and in the center it has to have a perfect U-shape. These troughs are used for sewage movement. And they are made from 0.125-in. to 0.250-in.-thick stainless steel. They’re usually 10 in. to 24 in. in diameter. They use a shaftless-augur in the U-shape. Usually if this is done using conventional press brake tooling, it takes 40 to 50 hits to get the proper configuration. And it entails two operators handling a large piece of stainless that probably weighs 200 to 300 pounds. By using urethane-press-brake tooling, we can do the same thing in four to five hits.”
No tool marks
Michelsen also mentions that their primary customers for press-brake tooling are sheet-metal fabricating shops that need parts that don’t show tool marks.
“Although a lot of our tooling is sold to eliminate press-brake-die marks, large radius bending is also a key element to using this tooling. For instance, I do all types of business with companies that make fire trucks and emergency vehicles, because they use polished plate that they don’t want marks from the press brake tooling showing up on it, and they often have large radii to bend.
“We also do a lot of lighting fixtures. Because these fixtures use perforated screens over the lights as diffusers and when using normal press-brake tooling, they look horrible from the tool marks. Many bends can be done with this tooling to materials as thin as 0.005 in. and as thick as 0.250 in. Primarily it’s all geometry related,” he says.
Michelsen also notes that urethane tooling has a natural crowning ability. He adds, “There is another odd thing that happens with urethane. A lot of press brakes have a way to control their crowning to give a straight part. By using urethane-press-brake tooling, even if you have an old, inaccurate press brake, if your punch is straight, the urethane will act as a poor man’s crowning device. You can make dead straight parts with urethane, where you would struggle with it if you’re using steel dies.
“For instance, if you start making lighting fixtures and your press brake is not dead straight, you’ll be producing junk parts. Because the part will have a bow in the center. It won’t be true. But if you use urethane-press-brake tooling, it will be true every time. If you have a straight punch on the press brake you’ll have a straight part.”
Michelsen notes that urethane can only be used in bottom-bending press-brake applications. It primarily does not work with air bending.
Urethane comes in four main grades for metalforming, and deflection for the die components will dictate what should be used. Hardness and deflection are measured on the Durometer A or D scale.
Very hard urethane is a 75D Durometer and has a five-percent deflection. Hard urethane is a 95A Durometer with a 15-percent deflection. Medium urethane is a 90A Durometer with a 25-percent deflection and soft urethane is an 80A Durometer with a 35-percent deflection. Urethane is always offering a constant volume, and when it’s properly contained, it can generate an infinite amount of pressure, Michelsen says.
Asked about how the different Durometers of the urethane come into play, Michelsen says, “It’s the amount of force that’s needed to do a specific job. If you’re bending 0.250 in. material, it will need more pressure. You need stiffer urethane to do stiffer materials basically. Also the other factors are how much elongation the urethane will see within the forming operation and hardness is directly related to elongation. The harder the urethane the less it will elongate.
“I characterize this as more of a shape change factor in the urethane. The softer the urethane the more it can be changed. A soft urethane you can change by 50 percent and it will remain fully elastic, and you want to work within the elastic limits.”
“Then there’s springback to contend with, which is strictly a function of the material itself,” Michelsen says. “Typically springback is factored into the form-punch tool. I’ve done materials that have as much as 70-percent material springback.”
But even though all of this must be considered in the press-brake tooling, it’s really the tooling manufacturer that will build it to meet the part’s challenges. Michelsen mentions though, that “there are cases that use polyurethane-press-brake tooling where productivity has improved by a factor of 1000 percent.”
When you’re confronted with a part that has a unique radius or requires a perfect, unmarked finish, urethane tooling could be the way to go.