theartofpressbrake.com http://www.theartofpressbrake.com/wordpress/?page_id=1023 Forming Methods Bend Allowance, Outside Setback, Bend Deduction If you calculate these with precision, you have a better chance of bending a good part on the first try. But, to make this happen, you need to make sure every factor in the equation is what it should be, and this includes the inside bend radius . How exactly is this inside bend radius achieved? To uncover this, we must first look at the different methods of bending on a press brake: air forming, bottom bending, and coining. The Methods of bending There are three different types of bending methods used in the forming of sheet metal: “ air forming“, “Bottom Bending and “Coining. Each of these methods has a specific purpose and application. In this chapter the benefits and the inefficiencies of each will be discussed. Coining Note that there are three bending methods, not two. Bottom bending and coining often are confused for the same process, but they are not. Unlike bottoming, coining actually penetrates and thins the material. Coining is the oldest method and for the most part, no longer practiced. Why? Because of the extreme tonnages this method requires. An amount of tonnage so great that the material “flows” on a molecular level while under this extreme pressure. Coining forces the punch nose into the material, penetrating the neutral axis, figure 1. Technically, any radius may be coined, but traditionally, coining has been used to establish a dead-sharp bend. This method not only requires excessive tonnages, it also destroys the material’s integrity. Coining forces the entire tool profile to less than the material thickness, and thins the material at the point of bend. This also requires dedicated or special tool sets for each bend angle and radius. The punch nose produces the inside radius, which is then used to establish the bend deduction. Figure 1 With this penetration of the neutral axis by the punch tip, the springback will almost completely disappear. The high pressure removes springback during the forming process. The material will remain permanently deformed at the point where the pressure was released. In the coining process it would not be uncommon to find the tonnage exceeding 50 tons per foot. Because of this excessive tonnage requirement, the use of this method of forming is impractical for the heavier gauges of material or material with high tensile strength. Bottom bending Bottom bending forces the punch nose into material, using various punch angles in conjunction with a 90 ° V die. In bottom bending, only the punch nose radius is “stamped” into the material. 000159 In air forming (described more fully later), the punch penetrates into the die space producing the required bend angle, plus a small amount to account for springback. The punch backs out of the die, and the material “springs” back to the desired angle. Figure 2 Like air forming, bottom bending requires the punch to descend to a point that produces the bend angle plus a small amount. But unlike air forming, the ram continues past this point and descends farther into the die space, forcing the workpiece back to the set angle of the bend; this process is called Springforward. On average, the bend bottoms at 90°, at a point in the die Figure 2 space that is about 20% above the material thickness. For instance, 0.062-in. thick cold-rolled steel bottoms somewhere between 0.074 and 0.078 inches; where the zero point is measured from the bottom of the V-die under a light load; your machine my reference differently. As shown in figure 2, there is enough angular tooling clearance between the punch and die to allow the springback in the material to be compensated for. This angular tooling clearance should be equal to the springback. As pressure is applied to the radius, the material passes by the required bend angle, touching off the punch faces and then Springing Forward to the set angle at 90°. Bottom Bending and Springback For example, an 88° complementary punch angle is capable of bottoming soft aluminum and stainless steel, but would over-bend a piece of cold rolled steel. Aluminum and stainless steel both have springback factors of one and one half to two degrees of springback. On the other hand, cold rolled steel has approximately one half of a degree. You can also take advantage of a sharp bend. The “ sharp bend” pinching action at the bottom of the ditch compensates for the half a degree of Figure 3 springback. This punch and die relationship will vary with different material types, but the idea still works the same. The bottoming die should be the set angle for the bend, usually 90° and the punch that is angularly equal to or less then the 90° die angle. Examples: 90° – 90°, 90° – 88°. Bottom bending offers the consistency in bend angle that coining does, with far less tonnage. This type of forming is still best done by highly skilled operators. Severe damage could still be done to the workpiece, tooling, or the press brake due to the tonnages that are required. Remember, bottom bending is stamping the radius instead of the entire working surfaces of the punch, material and V-die. One last bottoming note, what do you do when the clearance between the punch and die does not match the springback? The answer is a small piece of tape. It has been a common practice in the past to use masking tape high on the punch face to Figure 4 “change” the angle of the punch from one-half to a full degree, figure 3. While a common practice, it is time consuming because the tape constantly needs to be removed and replaced. A good solution is vinyl pin stripping tape! It serves the purpose and will last the entire job without replacement. A point 000160 of note, do not cover the entire face of the punch, that will only shift the center of the tool and will not change the angle, figure 4. Special dies like Rolla-V’s and urethane pads also force the material around the punch nose. This again is the radius used in the calculations for bend deduction. Bottom Bending Bottom Bending Courtesy of Asma LLC Air forming So far it all seems pretty straightforward. In air forming, the radius is produced as a percentage of the die opening, regardless of the die style, be that a V, channel, or acute die. The die opening determines the inside bend radius in the material. To determine the inside radius that is going to be developed over a given die opening and for various material types and thicknesses, technicians use what’s known as the 20-percent rule. The 20% rule states that in a free form, the resulting inside radius is a percentage of Figure 5 the die opening. The die opening is multiplied by the material type percentage. With the many alloys available today, including both new and recycled metals, it is impossible to determine an absolute percentage multiplier. Nevertheless, the rule gives you a good starting point. The rule percentages are: 304 stainless steel: 20-22 percent of the die opening. AISI 1060 cold-rolled steel, 60,000-PSI tensile: 15-17 percent of the die opening. H series soft aluminum: 13-15 percent of the die opening. Hot-rolled pickled and oiled (HRPO): 14-16 percent of the die opening When you begin to work with these percentages, start with the median value until you find the value that best matches the material characteristics you receive from your metal supplier. Multiply the opening by the median percentage to determine the developed inside radius of the die/material combination. If you have a 0.472-in. die opening, and you’re bending 60,000-PSI cold-rolled steel, start with the median percentage of 16 percent of the die opening: 0.472 × 0.16 = 0.0755. In this case, an 0.472-in. die opening will produce an 0.0755-in. floated inside bend radius on the part. When your die opening changes, so does your inside radius. If the die opening is 0.551 in. (0.551 × 0.16), the inside bend radius changes to 0.088 If the die opening is 0.972 in. (0.972 × 0.16), the inside bend radius changes to 0.155. On the other hand, if you’re working with 304 stainless steel and its median value is 21%; multiply that median value, 000161 21% by the die opening and you have the radius. So, for that same 0.472-in. die opening we a much different inside radius: 0.472 × 0.21 = 0.099 inch As before, when you change the die opening, you change the inside bend radius. A 0.551-in. die opening (0.551 × 0.21) calculates out to a 0.115-in. inside radius. A 0.972-in. die opening (0.972 × 0.21) gives you a 0.204-in. inside bend radius. If you change the material, you change the percentage. If you work with material not listed here, you can look up the material on the Internet and compare tensile strengths to the baseline value of 60,000 PSI for AISI 1060 cold-rolled steel. If the tensile value is 120,000 PSI, then your estimated percentage value will be two times that of cold-rolled steel, or 30 to 32%. Air forming and Bend Angle Remember, air forming is a “three point” bending operation. These three points are the punch tip and both top corners of the die, as shown in figure 5.