Snap! Your Tubes are Cracking, But Guess What - Bend Tools Aren't The Culprit!

A couple of days ago, I found myself on location, extending a helping hand to a novice client who was having trouble getting a new component set up on their tube bending machine.

They had a gleaming new set of bending tools but had been trying in vain to churn out satisfactory parts for the past few weeks. Regrettably, after inspecting the application and material closely, it became clear that they weren't likely to achieve their goal unless they altered their current approach.

So, what exactly went wrong, and how could this predicament have been evaded?

Unraveling the Intricacies of a Tricky Application

At first glance, this task may have seemed rather straightforward, but this particular component proved challenging to bend. The tube boasted a rectangular shape, a relatively thin wall, and a tight bending radius. Furthermore, the final product needed to retain its form throughout the bending process and exhibit no tool marks since it was a noticeable component of the final assembly aimed at the retail market.

Scott Mitchell, the president of OMNI-X, points out, “Beyond just conceiving and fabricating a tool set, a project's parameters require using a machine equipped with the right capabilities and selecting a material possessing the qualities that allow it to be formed into the desired shape."

He goes on to share a checklist of seven critical queries that should be addressed before creating a set of bending tools:

1. What is the composition of the material? Most materials come with a specific set of characteristics that aid in defining the requirements of a part shape.
2. What are the exact dimensions? This refers to the outside diameter, wall thickness, and tube shape. Corner radii are also required if square or rectangular tubing is being used.
3. What is the maximum bending degree for the part shape?
4. Are there any quality constraints? These could encompass permissible ovality, radius tolerance, outside wall thinning, or aesthetic and beauty considerations.
5. Are there blueprints for the part available? The final shape of the part helps identify constraints that might influence tool design. Factors like distance between bends determine the maximum length of clamps needed to grip the part.
6. What are the specifics of the machine? It is crucial to acquire comprehensive specifications of your bending machine, as merely knowing the make and model does not provide enough insight into the machine's real abilities.
7. What are the details of the application? Aspects like production volume, pre- or postbending procedures, and other details can have a considerable impact on how the tools are fashioned.

Jeff Jacobs, the owner of Tube Form Solutions, underscores the importance of maintaining a good relationship with your tooling partner throughout the quoting and design phases. He explains, "We often encounter customers requiring budget estimates on the fly. However, as the project progresses, they're frequently astounded by the degree of detail required to ensure successful production.

“We have a comprehensive checklist in place that assists us in gathering all necessary information to craft a design that is not only capable of bending their parts but is also compatible with the machine they plan on using."

He also points out that several tubing benders currently in use have been operational for over four decades. "It is quite uncommon to come across a machine of this age that hasn't undergone some modification or redesign."

Identifying the Problem

So, what was the crux of the issue with the project I was tackling last week? The material was a 1.50- x 0.75-inch rectangle with a 0.056-inch wall thickness being bent the easy way on a 1.25-inch inside radius, equivalent to a 1.625-inch centerline radius.

Due to aesthetic requirements, a mandrel was crafted to support the interior of the part. A sample tube was dispatched to the tool manufacturer to create a mandrel, ensuring a snug 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 in use was a single-axis hydraulic rotary draw bender. While it could clamp with sufficient hydraulic force, the pressure die was designed to only follow, without providing any boosting force. The pressure die's return to its initial position was gravity-aided using a cable directed through a series of pulleys with a weight hanging at the end.

In the production phase, the tube snapped nearly as soon as bending began. If the tube did not break instantaneously, the links for the mandrel balls would snap, leaving the part severely warped.

Quest for a Solution

After my arrival and with the tools laid out on a bench, I attempted to assemble them around a sample tube, only to find that I could not fully engage the clamps manually. Swift measurements showed that the tube was almost 0.010 inches above the nominal 1.50-inch dimension, but the tools had been designed to cater to a slightly smaller size. This meant that, once the tools were mounted on the machine, merely clamping would deform the tube wall before bending could even commence.

Luckily, the bend die was a two-part design, and the top could be elevated using a couple of shims, enabling the tools to merge without distorting the tube.

Regrettably, despite this fix, there was no discernible improvement in the bending process.

Upon querying the staff about the specifications of the material being bent, they managed to produce a copy of the invoice from the material order, but they did not possess the full material specifications or data sheets. After making some quick phone calls and conducting online research, I discovered that the material's dimension specification was quite broad: +/- 0.010 inches in the 1.5-inch dimension. As it turned out, the sample sent for tool design was at the lower end of that tolerance, and the material being bent was towards the upper end.

However, the major problem was elongation – a measurement indicating how much a specific material can stretch before failing, expressed as a percentage. It is handy in determining a piece of tubing's formability.

Elongation necessary for bending a part can be estimated using a simple formula: outside radius/centerline radius – 1. For this part, the centerline radius was 1.625 inches, and the outside radius was 2.00 inches, requiring roughly 23% elongation. But the elongation specification for the material used was only 15%.

Sometimes, a part can be bent into a shape demanding more elongation than the material specifies if the bender has a follower boost (also known as a pressure die assist, or PDA).

A PDA's action pushes more material to the bent part's outside radius, which may help surpass the elongation limitations by a few percentage points. Some benders may also be able to boost from behind the tube with a carriage, allowing the part to be bent into a shape that exceeds the amount of elongation by a few points. The clamp design and gripping length can also slightly reduce how far clamps slide over the tube.

Unfortunately, the machine used for this part did not have any of these features, and even if it did, the gap between the material elongation and the required elongation would likely still be too great to overcome.

At this point, the client's viable options are to revert to the material supplier and source metal capable of being bent around its required radius, or alter the design of the part.

Accessible Resources

There are numerous factors at play when assessing the suitability of bend tooling and machine requirements in determining whether a part can be created using

Source : The Tube & Pipe Journal (Written by Jay Robinson)