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Guest Editorial

Eliminate guesswork

By Steve Parrow

FFJ 1217 guest lead

Above: The mandrel and wiper die serve as bearing surfaces while containing the cross section during bending.

December 2017 - The perception that aluminum is typically tougher to weld or paint than most other metals used for fabrication is pervasive, so it’s not surprising to learn that bending aluminum tubing requires a bit more consideration than most other tubular materials.

Material selection

Those familiar with rotary draw tube bending, or mandrel bending, can attest to the fact that aluminum-bronze mandrel and wiper tooling is a tried-and-true material for bending mild steel, stainless steel, titanium, Inconel and many other alloys.

Aluminum-bronze has a relatively low friction coefficient and is commonly used in bearing applications for a variety of industries. Because the mandrel and wiper die essentially serve as bearing surfaces against the tube while containing the tube’s cross section during bending, there’s no question that aluminum-bronze would be the ideal choice for most bending applications. However, if an aluminum tube is loaded over an aluminum-bronze mandrel and further sandwiched together with an aluminum-bronze wiper die, the bending process will most likely come to a screeching halt.

One contributing factor to this is that aluminum is prone to galling in certain situations. Galling is the microscopic transfer of material between two sliding surfaces. Although the initial transfer of material begins at the microscopic level, it’s not uncommon for quite visible and substantially large deposits to form on either contact surface.

Aluminum-bronze contains a significant amount of aluminum, roughly 10 percent in most alloys. In the bending scenario described above, we are compressing an aluminum surface against an aluminum-bronze surface, and because both materials contain aluminum molecules that naturally attract to each other, the sliding motion creates an ideal condition for severe galling. In this case, it will occur between the tube interior and the mandrel and between the tube exterior and the wiper die. As the aluminum deposits increase in size, so too will the amount of friction caused by them. Eventually, the material will not move as it should, and tube breakage often follows.

Because hard surfaces resist galling, the solution is to use a polished and coated material for the mandrel and wiper tooling’s working surfaces, such as hard chrome, PVD or similar coatings. Reduction in the friction associated from the galling allows material to flow more freely during the bending process. Further, a proper bending or drawing lubricant formulated for bending aluminum should be applied directly to the working surfaces of the mandrel and wiper die and reapplied, as needed, throughout the production run.

Tempered for bending

Unlike most other tubular materials, aluminum is available in a variety of tempers or heat treatment conditions. It is important to select a temper that will allow the tube to be formed without breaking. The key property to look for when determining a material’s formability is its elongation properties. Most tubing may be easily bent on a relatively tight bend radius if it has an elongation rating of 20 percent or higher.

For example, 6061-T4 is a 6061 aluminum alloy with a T4 temper grade. A lower temper grade indicates greater elongation characteristics, so a T0 temper (25 to 30 percent elongation) is ideal for forming, whereas T6 temper grade (10 percent elongation) is better suited for machining. The example mentioned above, T4, typically has an elongation rating of 16 percent. So, although it is formable, it falls below the 20 percent guideline. Therefore, it may not be possible to achieve a tight radius bend.

Recognizing how specific materials react differently to otherwise routine elements of an application and understanding why they react differently is the first step to finding a repeatable solution—no matter what size tube or which bending machine is being used. Once a solution is adopted, it can easily be incorporated into a standard routine and no longer perceived as a tough or intimidating process. FFJ

Steve Parrow is sales manager at Bend Tooling Inc., which offers tooling sets for most CNC rotary-draw tube benders and specializes in inserted and solid-body mandrel and wiper tooling. He has 15 years of industry experience.

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