Page 13 - MetalForming January 2010

Crowning Achievements Deliver Press Brake Accuracy
and Process Efficiency
State-of-the-art press brakes all offer high accuracy and repeatability; what separates high-end machines from the rest of the pack is the ability to adjust to the changing properties of the work- piece material. Even with the most accu- rate and repeatable machine in the world, if its crowning system cannot adapt as material properties change from part to part, there is no way to guarantee accu- rate, repeatable forming results.
Crowning—the method by which press brakes compensate for deflections in the upper beam and lower bed when pressure is applied—occurs by different
John Kemp is product manager, bending, Bystronic Inc., Hauppauge, NY: 631/231- 1212; www.bystronic.com.
methods depending on the make and model of press brake. And while any type of crowning system is better than none at all, fabricators must carefully consider the effectiveness of the crowning system when comparing and contrasting the press-brake models on the market.
Any time pressure is applied to the machine, deflection occurs and is direct- ly related to the amount of pressure applied, where it is applied and over what area. For a crowning system to be effective, it not only must account for all of these variables, but also must be able to adjust for them on the fly, without operator input.
Springback Problem?
While you may believe that you
have a springback problem, it’s more likely you have a crowning problem being attributed to springback. Accord- ing to Hooke’s Law, springback is pro- portional to force, which is propor- tional to material tensile strength. The reason a fabricator may experience a large variation in bend angle from one workpiece to another—when the mate- rial’s tensile strength changes—has more to do with the pressure required to complete the bend than the actual amount of springback.
Consider, for example, bending a section of 1⁄4-in. plate to a 90-deg. angle in a press brake. Assuming 3 deg. of springback (typical for steel) and a ten- sile-strength variation of 10 percent; the amount of springback, therefore, also will change by 10 percent. This results in an angle variation of only ±0.3 deg. What causes fabricators to experience angle variations much larger than this? The answer: insufficient crowning.
When tensile strength increases by 10 percent, the press brake must apply 10 percent more force to bend the part.
While any type of crowning system is better than none at all, fabricators must carefully consider the effectiveness of the crowning system when comparing and contrasting the press-brake models on the market.
BY JOHN KEMP
www.metalformingmagazine.com
METALFORMING / JANUARY 2010 11