It's all about tool selection -- or is it?


May 10, 2005


Figure 1
The radius gauge fits squarely into the bend.

Air forming, bottom bending, and coining are different forming methods that can be used to create various bends—sharp, radius, and profound-radius. Throw in a mix of operators and engineers with different ideas of what each method is and how each is applied, and confusion about the best way to accomplish a task might arise.

In forming operations, no matter how operators and engineers with different ideas make the project work, who is right and who is wrong is no mystery. They all are correct to some extent.

Say what? Yes, they are all correct to some extent. Take for example the fact that a 90-degree punch mated with a 90-degree V die can be used to air-form, bottom-bend, and coin. Each method will produce a different inside radius and bend deduction; however, all three methods can use a single tool set.

Making the right tooling selection requires understanding what tools you have on the shop floor. It also means understanding what methods your press brake operators are using. Are they air forming? Are they bottom bending? Are they coining? Or are they using some combination of all three? Most likely, it's the latter.

So how do you make competent tool selections at the design stage before the job gets to the shop floor? By asking your experts—those who work the machines daily. It's important that you know your facility's capabilities and understand the methods used.

This leads us then to question how to best deal with that information.

A Is A

First let's look at the inside bend radius, the basis for laying out any sheet metal part. By getting incorrect information at this point, if not fatal, it will at the very least be an expensive lesson. In a speech from Atlas Shrugged, John Galt states that "A is A," and nothing else but A can be A. Isn't it time to look at your shop situation from the same objectivistic viewpoint? In doing so you will find that not only is A, A, but also that a 1/32-inch inside bend radius is a 1/32-inch inside bend radius.

All too often the inside radius specified on a blueprint is never achieved. Just because a punch with the desired radius is used to produce the bend doesn't mean that the required radius is achieved. Shop floor forming method variations and tool selection errors can produce poor, unusable parts.

Many times the inside bend radius isn't verified with a radius gauge (Figure 1). When some operators use bottom bending and others use air forming—occasionally to produce the same part—wide deviations in inside bend radii can occur. Because the bend deduction (BD) or K factor is derived from the inside bend radius, not achieving "A" (the required inside bend radius) manifests itself as inconsistent parts over time.

When methods and tool selection differ from operator to operator, so do the results. When your efforts to get the press brake operators to use a common system fail, that's when you need to remember John Galt's "A is A." A 1/32-inch inside bend radius is a 1/32-inch inside bend radius, regardless of how it's achieved. When the required 1/32 inside radius is a 1/32 inside radius, A equals A, and the part is good.

Understanding Your Shop's Abilities

Does your shop have a variety of tools? Does it seem that no two operators use the same tooling in the same situation? Almost every bend other than a 90-degree bend is by default air-formed, and with air forming, every variation in the V-die opening increases or decreases the inside radius. When a part is bottom-bent or coined, the radius is stamped into the material.

The only deviation from the air forming default for angles other than 90 degrees is when sharp bend radii are required—a sharp bend less than 63 percent of the material thickness. Air forming cannot produce a sharp bend. Instead, it creates a ditch in the bottom of the bend, amplifying all variables within the material—thickness, hardness, and grain direction—while producing a radius derived from the V-die opening.

For bottom bending, the minimum inside radius is no less than 63 percent of the material thickness. Coining is not recommended for sharp bending. Whether the machine is new or old, it is too tonnage-intensive.

Your engineering or design department simply needs to specify an inside radius that you already have confirmed is within your shop's abilities. Insist that quality control verify that the calculated inside radii have been met with the radius gauge sitting squarely in the radius. When this is the case, the bend's mathematics have been achieved.

Assuming you have the tooling that can produce the desired inside bend radii, selecting the forming method and tooling on the shop floor cease to be issues. Air form, bottom bend, or coin as long as A is A. You'll make high-quality parts.


Steve Benson

2952 Doaks Ferry Road N.W.
Salem, OR 97301-4468
Phone: 503-399-7514
Fax: 503-399-7514
Steve Benson, a member and former chair of FMA's Precision Sheet Metal Technology Council, is the president of Asma LLC.

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