Bending Basics: Strategies for forming offsets
With the right tooling used in the right way, you can form offsets easily and efficiently
If you pick the right offset tool for the job, you'll find you will be able to produce many different offset geometries quickly and safely.
Many offset bends are too close together for standard press brake tooling, but they can be produced easily using the correct type and style of tooling. If you pick the right one for the application, you’ll find that you can produce many different types and styles of offset bends accurately and safely.
Offset tooling comes in two varieties: upspring (or upsweep) and horizontal. The upspring tool is used to form two bends that are too close together for conventional forming methods. The horizontal offset tool is designed to offset the material by one material thickness (see Figure 1).
To determine the appropriate offset tool for a given job, subtract the material thickness from the outside dimension of the specified offset. The resulting number is the offset depth as measured from the material surface.
Say you need to form an offset, specified as 0.156 inch, in 0.059-in.-thick material. Here you would subtract the material thickness from the specified offset: 0.156 in. -0.059 = 0.097 in. The required offset of 0.097 in. is the A dimension shown in Figure 2 and is the depth of the offset tool, as measured between the two faces of the die (see Figure 3).
The Upspring Tool
The upspring style is a bottoming tool designed to stamp the angle, radius, and dimension of the offset into the material. As when bottoming or coining with conventional tools, the offset tool radius equals the bend radius. A 90-degree upspring offset tool can cause the part to momentarily take on a Z shape; that’s the overbending that occurs just before being forced back to 90 degrees by the bottoming process.
The upspring tool isn’t always used in a bottoming operation, though. Before the tool reaches the bottom, there are plenty of air-formed angles available. For many years these tools came in only 90 degrees, unless they were custom-made. But over time tooling vendors started to make the transition to air forming requirements, and so today offset tools are available with die angles appropriate for air forming. The variations in tool angle allow for springback when forming.
The Horizontal Offset Tool
The horizontal tool is not a bottoming tool but instead used primarily for “stepping” material one material thickness. The angle and radius normally are unimportant in such operations; the main overriding factor is the clearance between the tool faces (see Figure 4).
The maximum bend angle that can be achieved safely with the horizontal offset tool is approximately 70 degrees complementary. Attempt any angle tighter than this and the tool will begin to act like a shear, cutting the material rather than bending it.
Dealing With Side Thrust
You may need to consider side thrust when working with either type of offset tool. In a standard die set, the thrust is applied equally to all surfaces, which cancels out any thrusting effect during the forming process. But with an offset tool, the side thrust “pushes out” in both directions, and at times this can get out of hand (see Figure 5).
Left unaccounted for, the side thrust can damage the tooling or workpiece and, even worse, cause serious injury. Should this become a problem, you can attach a thrust plate to both ends of the tooling to reduce the effect of side thrust (see Figure 6).
Test Pieces and Tonnage Considerations
Although you can certainly calculate your way through an offset bend, it’s really not practical because the material is being constrained between the two bends. As the material is forced to the offset dimension, the material between the two bends cannot elongate normally. That elongation needs to go elsewhere, and that elsewhere is the two flanges exiting the tool set. You also lose some material thickness from the formed dimension. Considering all this, it quickly becomes clear that a test bend is the best and quickest option for determining the flat dimension of the blank.
Moreover, required tonnages can vary greatly when using offset tools, mainly because they can involve either air forming or bottoming. Bottoming will produce the best results but does require a vast amount of tonnage. Air forming requires much less tonnage, but the final bend will take on more of a Z shape rather than a true 90-degree offset.
If you look closely at the tool set, you’ll notice it has no front or back. You can install it to face either direction, and you might find it natural to install the tooling so that the workpiece swings upward during forming in the same way it does with a standard V-die set—but this isn’t the best practice. When using a standard V die, material swings upward both in front and behind the die. This doesn’t happen with an offset tool set, however. When the material swings up in front, it swings down in the back (see Figure 7).
The backstop entering the die space may not be a problem for most of your bends, but it only takes one unintended collision to ruin the part, the tool, and the backstops. For this reason, it is best practice to install this type of tooling so that it moves the workpiece downward in the front. This keeps the backstops out of the die space.
Various Offset Angles
Many times a blueprint will specify an offset geometry with bend angles other than 90 degrees: say, a 45-degree bend angle and a 0.250-in. inside dimension, as shown in Figure 8. This is large enough so you could form it using a standard punch and V die, bending the workpiece to the desired angle and dimension using two separate hits (see Figure 9).
Alternatively, you could use an upspring offset tool to form it in one hit, in which the bend angle is controlled through the depth of penetration and the size of the die set. Like so much else in precision bending, a little calculation can save you a lot of time.
Offset Bending Formulas at Work
The formulas here include both ideal and actual tool dimensions. If the calculated ideal die width is unavailable, you may have to give a little on the bend angle or the dimension to produce a good part. Though it depends on the application, it’s often easier to give a bit on the bend angle rather than on the dimension.
Nevertheless, the following variables and formulas do work down to a bend angle of about 30 degrees complementary. With bend angles less than 30 degrees, the relationship among the tool width, offset dimension, and the bend angle becomes realistically unworkable.
These should give you a valid dimensional value for the ram setting or depth of penetration, measured up from dead bottom of the stroke, where the die faces are touching without the material being present.
A = Required inside dimension
B = Actual measured tool dimension
Rp = Radius of the punch or inside radius
Dp = Developed penetration
Di = Half of the difference between optimum and actual measured tool dimension
Mt = Material thickness
Od = Optimal tool dimension
Od = (90 /complementary bend angle ) × A
Dp = [(B × sine of bend angle) / 2] + Mt
Di = (B - Od ) / 2
Actual machine input depth = Dp + Di + Rp - 0.03
Continuing with the example introduced in Figure 9, our required inside dimension is 0.250 in. (A), our angle is 45 degrees, and our material thickness (Mt) is 0.250 in. So the optimal tool dimension would be as follows:Od = (90/45) × 0.250
Od = 2 × 0.250
Od = 0.500 in.
In this case, we’ll assume the optimal tool dimension (Od) is the same as the actual measured tool dimension (B). From here, we calculate the developed penetration (Dp).Dp = [(B × sine bend angle)/2] + Mt
Dp = [(0.500 × sine 45)/2] + 0.250
Dp = [(0.500 × 0.7071)/2] + 0.250
Dp = [0.35355/2] + 0.250
Dp = 0.176775 + 0.250
Dp = 0.427 in.
At this point you would add this developed penetration to the difference between the ideal and measured tool (Di, in the current example, is zero), the inside radius (Rp), and then subtract 0.03. In this example, the inside radius is 0.157 in., which, at 63 percent material thickness, is the minimum inside radius that can be achieved, where the bend turns sharp. (Editor’s note: For more on sharp bends, see How the inside bend radius forms.)
Actual machine input depth = Dp + Di + Rp -0.03
Actual machine input depth = 0.427 + 0 + 0.157 - 0.03 = 0.554 in.
This gives you an approximate ram depth setting that you can use when setting up a job at the press brake, and it is generally valid up to 18 in. of bend length. Note that 0.590 in. is the dimension up from dead bottom, where the dies meet but are not under a load. The actual input for your machine may vary based on where the controller’s origin or “zero point” is.
Also note that if you apply right-angle trigonometry, you will find that the tool is 0.353 in. from top to bottom (click the slideshow images to get to the two "sidebar" images). That 0.590 in. value, as calculated here, may seem to not even enter the die space, but there is penetration once you consider the 0.250-in. material thickness (see Figure 10 and Figure 11).
These formulas show that if you penetrate a 0.500-in. offset die 50 percent into the offset die’s opening, you can create a 45-degree bend at 0.250 in., per the drawing in Figure 9. Moreover, if you work the optimal tool dimension (Od) formula, you will discover that all of the offset dimensions in Figure 12 could be produced in the same 0.500-in. upspring offset tool.
This shows how you can produce different inside dimensions, angles, and radii with the same offset tooling set. Ultimately, if you consider all the application variables, including available tonnage, you will discover how valuable and time-saving offset tooling can be.
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