Variables in Bump Bending

• Arc Length
• as measured on the inside surface of the radius
• Radius Pitch
• This is the distance between bumps
• The greater the # of steps, the smoother the radius will be
• Radius Pitch = # of steps / Arc Length
• A narrow pitch multiplies any error that might occur from the machine, the material, or the tooling
• If an already bent face rests inside the die, it aggravates an easy bending calculation
• Such a condition also develops offset forces to tools that the machine must manage
• Die Width
• An optimal die opening is double the radius pitch
• If the die width were wider than double the radius pitch, previously formed sections would sink slightly inside the die opening.
• If the right tools are available, the die width governs the radius pitch
• Punch Tip Radius
• It's best practice to use a punch tip radius large enough so it doesn't leave a deep bend line with each bump, which will create a rougher outside surface
• The punch radius should be no more than 63 percent of the thickness of a mild steel plate; the punch-nose radius can be larger for high-strength plate, where you might use a punch-nose radius several times the material thickness
• Depth of Penetration
• For a smooth bend, the depth of penetration shouldn't be much deeper than the pinch point, where the punch nose firmly holds the material
• As a starting point for test bends, the Depth of Penetration = (Die width / 2) + Material thickness minus .02.
• When it comes to die with and punch penetration, be careful about tonnage. Despite only slight punch penetration, forming tonnages add up quickly, especially with thick or hard material.
• Accounting for Springback
• Springback is accounted for by changing the depth of penetration
• Hard materials have significant springback, and make everything more complicated
• If the die width is narrow, a tiny change in penetration changes the bend angle dramatically. This can be problematic when bending only a few degrees at a time. Tiny errors stack up over dozens of bumps.
• Limitation of angle measurment in progress
• Some modern press brakes have angle measurment tools, but these generally only start to work when the angle exceeds 9 - 25 degrees, and they need a flat surface to measure against, which is a problem during bump bending.
• Using Templates
• In the face of all these limitations and challenges, templates can be a great help
• Bump a little, compare to template, bump a little more; don't overbend
• You may need to turn the plate around to form a flange or bumped radius on the other side.
• Draw bend lines on the metal and use these
• Backgauging
• One bump can cause the material to shift against the backgauge, so that the punch doesn't hit the workpiece correctly on the next bump.
• A conventional backgauge has a vertical backstop and a horizontal part that supports the material. There are special 6-axis backgauges that have special fingers that clamp and position the material.
• Material support and backgauging off the most complexities and challenges
• Variable Dies
• With a variable die, you can change the die opening between hits
• Variable dies can also be used in situations that require, for example, a wider radius at one end and a shorter radius at the other end.
• Because radius pitch is related to die opening, you can change the radius pitch continuously by adjusting the die opening as you go
• Crowning
• Deflection due to heavy tonnage can become a big issue
• A constant error of a fraction of a degree can result in finished workpiece that has a bow or a kink in it
• Some modern press brakes have automatic crowning compensation systems
• If not, shimming is the way to do it