Why should you care about inside bend radii?

Photo courtesy of TRUMPF Inc.

A minimum bend radius is a function of the material and has little or nothing to do with the press brake punch tip. A minimum bend radius for one material thickness is not the same for another material thickness. In cold-rolled mild steel, the minimum bend radius is 63 percent of the material thickness. For example, a piece of 12-gauge (0.104-in., 2.64-mm) material turns sharp at 0.065 in.(1.66 mm), making any punch radius less than that value, by definition, a sharp bend, such as 0.062 in. (1/16 in., 1.58 mm), 0.032 in. (1/32 in., 0.82 mm), or 0.015 in. (1/64 in., 0.38 mm). Radius bends occur between the 63 percent threshold and a bend radius no greater than 10 times that of the material thickness.

Can a press brake operator achieve the desired bend radius on-the-fly? Well, maybe, maybe not. I suppose that depends on the operator's skill level.

Bending Basics

Before we continue, a discussion of bending principles is in order. First, we must establish a common language about forming methods, bend types, and their subsequent mathematical effects.

During any forming process, material elongation occurs. Elongation is a bit of a misnomer, because the material actually changes shape, which may or may not include elongation. The part of the material that normally would square into the corner must go somewhere else as a result of the radius effect. This elongation commonly is called a bend deduction (BD) or K factor—mathematically equivalentterms.

How many times have you referred to a bend deduction or K factor chart to find a bend deduction value only to discover later that it wasn't correct for the finished part? What happened? Was the chart wrong? Was the problem operator error? Was engineering asking for something that couldn't be done in practice? Any or all of these factors could have caused the error.

Chart Confusion

Let's look at examples from five BD charts. Sixteen-gauge cold-rolled steel (0.059 in., 1.50 mm) with a 1/32-in. (0.032-in., 0.83- mm) inside bend radius has the following values:

• 1/32-in. radius in 0.060-in. material

• Chart #1 N/A

  Chart #2 0.063

  Chart #3 0.083

  Chart #4 0.097

  Chart #5 0.102

• 1/16-in. radius in 0.060-in. material

• Chart #1 0.106

  Chart #2 0.108

  Chart #3 0.110

  Chart #4 0.132

  Chart #5 0.136

The difference between the high and the low values for one 1/32-in. single bend radius is 0.039 in. Multiplying 1/32 times three bends gives 0.117 in.

The difference between the high and low values for one 1/16-in. bend radius is 0.030—0.090 for three bends.

Are the charts wrong? No. Every value from every chart is valid for an achieved radius under a given set of circumstances. The problem lies in the fact that the radius called for rarely, if ever, matches the achieved radius. Most test samples confirm the descrepancy.

Correct Bend Deduction

Empirical-formula charts do not take into account all the variables. They just assume a 1/32-in. (0.83-mm) radius in 14-ga. material, rather than the natural minimum radius of the material of 0.046 in. (1.18 mm), 63 percent of the material thickness. This changes the value of the calculations. If the 0.032-in. (0.83-mm) value is used in the calculations, the resulting BD will be incorrect,because 0.046 in. (1.18 mm)—the achievable radius—produces an incorrect bend deduction value, which, in turn, makes the corresponding flat blank layout incorrect.

BD or K factor is developed in the following manner: