Air-bending sharp on the press brake: A fight not worth fighting

Question: We recently replaced our 15-year-old punches and dies with new tooling. Our old tooling still dictates our 3D modeling, and I have been tasked with changing the 3D modeling parameters, along with adjusting our press brakes to produce more accurate parts. Most are bent to 90 degrees, and each part has between two and five bends. Several are box-style, with four flanges being bent inward, but we do have some stacking parts that need to be precise to fit within one another.

We work with 60,000-PSI cold-rolled steel, galvanized material, and numerous stainless steel grades. Typical material thicknesses are between 10 and 16 ga. A few months ago, we purchased new tools to air-bend these materials. We have bottom dies that are 0.3937, 0.62992, and 0.98425 in. We use an 86-degree top punch with a tip radius of 0.024 in. Based on our tooling and material, we are forming over die openings that are about 6x the material thickness, and our top punch is producing sharp bends to the point of putting a crease into every part we manufacture, based on the 63% rule of air bending (that is, bends turn sharp when the inside radius is less than 63% of the material thickness).

Under the old tooling standard, we set our K-factor to a varying amount based on material type. Based on your suggestions from previous columns, we should be keeping our inside radius as close to the material thickness in order to get accurate angles and dimensions.

We could get punches with nose radii of 0.060 in., 0.090 in., and 0.118 in. Considering we make sharp bends more often than not, I believe the tooling options available to us should work sufficiently for our bending needs. With the right tooling lined up, the blanket 0.031-in. inside radius we set in our 3D modeling would be adjusted for the material type once we get the new punches.

Using calculations from your website, the K-factor would then be calculated correctly, and therefore the parts would have the correct bend deduction. Would this be the best course of action to get our parts to be more accurate?

Answer: Let’s start with the dies and die openings. The chart in Figure 1 shows the angle variation that can be expected based on the material thickness and die opening. The smaller the die opening gets relative to the same material thickness, the greater the amount of angle variation you will need to deal with during forming.

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