Tube breakage? Bend tooling may not be the problem

It's not uncommon for metal fabrication shops to struggle with producing precise parts on tube bending machines. The solution? Often times it comes down to simple math and communicating with the machine manufacturer.

Last week I went on-site to help a new customer that was trying to set up a new part on its tube bender.

The company had a brand-new set of bend tooling but had struggled for weeks to make any good parts. Unfortunately, after looking at the application and material, we determined that the customer was not likely to be successful without changing its plan.

So, what happened, and how could the problem have been prevented?

Questions for a Difficult Application

Although it seemed like a simple enough project, this part was difficult to bend. The tube is rectangular, the wall is fairly thin, and the bending radius is very tight. In addition, the final part needs to maintain its shape throughout the bend and have no tool marks because it is a visible part of a final assembly going into a retail market.

“Given the parameters of a project, more is involved than just designing and building a tool set,” said Scott Mitchell, president of OMNI-X. “A machine with the correct capabilities must be used, and a material with properties that allow it to be formed to the desired shape must be selected.”

Mitchell provided a list of seven questions that need to be answered before manufacturing a set of bend tools:

1. What is the material? Most materials have a set of specifications that can help define the requirements of a part shape.
2. What are the nominal dimensions? These include outside diameter, wall thickness, and tube shape. If square or rectangular tubing is used, the corner radii also are needed.
3. What is the maximum degree of bend for the part shape?
4. Are there quality restrictions? These may include allowable ovality, radius tolerance, outside wall thinning, or aesthetic and cosmetic concerns.
5. Are part prints available? The final shape of the part helps to determine restrictions that may affect tool design. Things like distance between bends will determine the maximum length of a set of clamps to grip the part.
6. What are the machine details? Get thorough specifications on your bender. Just the make and model number do not always provide enough detail into the machine’s true capabilities.
7. What are the application details? Information such as production volume, pre- or postbending processes, and other details may play a significant role in how the tools are designed.

“Staying engaged with your tooling partner is incredibly important to the quoting and design process,” Tube Form Solutions Owner Jeff Jacobs said. “We often have customers that need budgetary pricing right away. However, when the project moves forward, they are often surprised at the level of detail needed to be assured they will be successful in production.

“We provide a very detailed checklist that helps us make sure we have all the information needed to create a design that is not just capable of bending their parts but will fit the machine they intend to use.”

Jacobs pointed out that some tubing benders being used for regular production today have been in service for 40 years or longer. “It is pretty rare to find one of those machines that has not been modified or redesigned in some way.”

Defining the Problem

So, what was the problem with the project I was on last week? The material is a 1.50- x 0.75-in. rectangle with a 0.056-in. wall thickness being bent the easy way on a 1.25-in. inside radius. This equates to a 1.625-in. centerline radius.

Because of aesthetics needs, a mandrel was designed to support the inside of the part. A sample tube was sent to the tool manufacturer to design a mandrel, making sure it fit inside the tube, and at the inside radii, with a close tolerance. The mandrel was designed with three balls to support a 90-degree bend.

The machine being used was a single-axis hydraulic rotary draw bender. Although it clamped with plenty of hydraulic force, the pressure die was designed to follow only, with no boosting force. The return of the pressure die to its original position was accomplished by gravity using a cable directed through a series of pulleys with a weight suspended at the end.

In production, the tube broke almost immediately after bending started. If the tube did not immediately break, the links for the mandrel balls would break, leaving the part badly deformed.

Searching for a Solution

After I arrived, with the tools on a bench, I tried to assemble them around a sample tube and found that I could not fully engage the clamps by hand. Some quick measurements determined that the tube was right at 0.010 in. over the nominal 1.50-in. dimension, but the tools were designed for this to be slightly under nominal. Because of this, with the tools on the machine, just the act of clamping deformed the tube wall before bending ever started.

Fortunately, the bend die was a two-piece design, and the top could be raised with a couple of shims, allowing the tools to come together without deforming the tube.

Unfortunately, even with this accomplished, there was still no real improvement in bending.

I asked the staff if they had specifications on the material being bent. They were able to provide a copy of the invoice from the material order, but they did not have the full material specifications or data sheets. With a few quick phone calls and internet research, I found that the dimension specification for the material is pretty wide: +/- 0.010 in. in the 1.5-in. dimension. As it turned out, the sample sent for tool design was toward the bottom end of that tolerance, and the material being bent was toward the top end of the tolerance.

The big problem, though, was elongation—the measurement used to describe how much any given material can be stretched before failure. It is expressed as a percentage and is a useful value in determining the formability of a piece of tubing (see “Understanding steel tube and pipe metallurgy”).

The elongation required to bend a part can be estimated through a simple formula: outside radius/centerline radius – 1. In this case, the centerline radius of the part being bent was 1.625 in. and the outside radius was 2.00 in.:

2.0/1.625 = 1.23

1.23 – 1 = 0.23

Therefore, this part required roughly 23% elongation. However, the specification for elongation for the material being used was 15%.

Sometimes a part can be bent to a shape that requires more elongation than the material specifies if the bender has follower boost (often referred to as pressure die assist, or PDA).

The action of a PDA forces more material to the outside radius of the bent part, and this may help overcome the elongation limitations by a couple of percentage points. Some benders also may have the ability to boost from behind the tube with a carriage, also allowing a part to be bent to a shape that exceeds the amount of elongation by a few percentage points. Gripping length and clamp design also can help a little by reducing how far clamps slip over the tube.

The machine being used for this part did not have either of these options, however, and even if it did, the difference between the material elongation and the required elongation would probably still be too much to overcome.

Presently, the customer’s only real options are either to go back to the material supplier and find a metal that can be bent around its required radius or change the part’s design.

Readily Available Resources

There are dozens of factors that go into the applicability of bend tooling and machine requirements in determining if a part can be made with a specific machine and specific material. We have gathered a list of formulas we regularly use for bend tooling design and manufacturing and are happy to share it.

In addition, most bend tooling manufacturers are eager to share bending information with anyone who wants it, offering free mandrel calculators and other information through their websites.