Tube & Pipe

According to experts at CML USA Inc. Ercolina, the correct tube bender can save time and improve part quality

By Lauren Duensing

December 2018 - One of the most common methods of bending tube and pipe is rotary draw tube bending. When it’s performed correctly, this process produces satisfactory results at a reasonable cost. However, incorrect equipment, material or technique can result in poor-quality bends and a high volume of material headed to the scrap bin.

“Choosing a reliable machine with the necessary capacity is fundamental; however, the best machine won’t overcome poor tooling or material,” says Kim Stevens, graphic design and marketing at Davenport, Iowa-based CML USA Ercolina, a company that has been supplying bending machines to the American market for over 40 years.

Recently, one of Ercolina’s customers purchased a new CNC mandrel machine for bending handrail and “was not only amazed with the mandrel bend quality but also impressed with the time saved in the automation of positioning the pipe between bends,” Stevens says. “Before purchasing our CNC pipe bender, the operator had to calculate the tube length manually, lay the tube out with paint marks and roughly position the pipe between bends by eye. The new  machine allowed the customer to go from a CAD file to a fully bent part in minutes, accurately producing parts with far less wasted material.”

FFJ 1218 tube image1

The TB80 bends any angle to 180 degrees with independent material springback and speed compensation for each bend.

FFJournal surmises there are a few frequently asked questions of tube bending machinery experts whose answers can help everyone. So here are the main factors to consider before beginning to bend.

Q: Why is material so important?

A: When purchasing tube, remember that the cheapest material isn’t always the best choice. Bending poor-quality tubes and pipes often leads to higher rates of scrap and breakage. Even the best machinery and tooling cannot overcome poor material quality.

“Often the last consideration when bending is the tube quality—when in reality, it should be the first,” says Stevens. “Poor-quality tube results in less-than-desirable bends, a greater chance of breakage during the bend and problems due to inconsistent size of the tube and location of seam.”

When selecting material, consider application strength requirements, appearance and quality, and centerline radius specifications. Also, knowing the material’s true dimensions is key to selecting the proper machine and tooling for your application. Pipe sizes are measured by ID, with wall thickness referred to in terms of schedule. Tube sizes are based on OD, and wall thickness is referred to in gauge sizes.

Q: What is rotary draw bending?

A: A rotary draw bend is formed by drawing the workpiece around a rotating bend former. The leading edge of the material to be bent is clamped by the bend forming die, while material rests between the forming die groove and the opposing force, commonly referred to as the counter bend or pressure die. The forming die rotates to the desired degree of bend, completing the process.

Q: What is centerline radius?

A: Often, centerline radius (CLR) is confused with the degree of bend. CLR is the distance from the center of the forming die to the centerline of the material to be bent. This distance is critical to achieving appropriate bend quality. Bending materials to the CLR less than two times the material diameter requires internal support of a mandrel to prevent the tube from collapsing.

Q: What is the maximum degree of bend?

A: Rotary draw bend tooling usually is designed to accept a maximum bend angle of 180 degrees. When selecting bending machinery for your application, be sure the machine program or system allows for slight overbending to compensate for springback during the bend cycle. Machines with programmable bend angle settings can provide high bend accuracy and ease of use. Degree of bend also affects bend quality. When working with a new material, try making two test bends—one at 45 degrees and one at 180 degrees—and review the results.

Q: What is springback?

A: All materials are prone to springback, which is when material relaxes after the counterbend die pressure is released. Most materials are slightly overbent during the bend cycle to compensate for this effect. The higher the tensile yield on the material, the more springback that must be added to the bend, Stevens says. “Soft materials, like zero-temper aluminum, often require no springback, whereas chrome-moly steel can require several degrees of overbend to compensate for the springback effect.” The style of bending can affect the amount of springback needed, too. “Mandrel bending often requires less springback than non-mandrel or open rotary draw bending.”

FFJ 1218 tube image2

An icon menu provides easy access to auto and manual operating modes and system diagnostics.

Q: Why is minimum distance between bends a factor?

A: Tooling for rotary bending requires a straight section of material for the forming tool to clamp the workpiece securely and prevent slippage during the bend cycle. Applications always should be reviewed for the shortest distance between any two bends, and the tooling should be manufactured to compensate for this distance, if possible.

Less expensive bending machines might not allow the operator to achieve this distance, so the workpiece might need to be cut first and then welded back together after the bend is made.

“Ideally, a minimum of two times the material diameter should be used to grip the tube,” Stevens says. “For example, 2-in. tube should have 4 in. of gripping area between bends. A good manufacturer can generally offer tooling with these requirements or make recommendations based on the application.”

Q: What tooling do I need?

A: Material grade affects the tooling selection and composition. Make sure the tooling is appropriate for the material specifications and doesn’t conflict with the material to be bent. For example, stainless tube can conflict with a carbon steel counter bend die.

Improper or low-quality tooling has a “greater chance of poor bend quality or even breakage during the bending process,” Stevens says.

Rotary draw bend tooling varies greatly in quality, availability and cost from manufacturer to manufacturer. Standard tooling generally is more cost effective, so changing a part’s CLR dimension slightly to accommodate stock tooling could save time and money. If special tooling is required, budget for higher costs and extended delivery times.

 When selecting a bending machine, buyers are advised to consider material type and grade, material length, wall thickness, production requirements, desired centerline radius, maximum degree of bend, and quality of bend desired. “Customers should always consider future bending needs and not limit themselves with a lesser-capacity machine,” Stevens says. “Most customers that purchase a bender quickly recognize the time and cost savings achieved by having a bending machine,” she says they soon realize their ability to consider bending for “any job that was previously fabricated by cutting and welding.” Taking this into consideration can effectively eliminate the labor from sawing, grinding, welding and clean up. “The bonus is a far better looking finished product at a reduced cost.” FFJ



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