Draw bending for tubing with special needs
November 7, 2002
Changes taking place in tube and pipe producing and fabricating may require you to review your lubrication strategies. One of these changes is the increased use of aluminum, aluminized steel, and stainless steel tubing.
The draw bending of these materials requires special lubrication expertise. As a manager, you should understand what changes in production and process control are necessary to achieve productive and profitable fabrication.
Properly chosen lubricants can profit tubemakers who perform draw bending. They can contribute to high production rates, good part quality, and improved secondary operations. Some key questions to ask about lubricant properties include:
After a lubricant is selected that meets these criteria, operators sometimes find they still have production problems. Do not panic. Too many operators blame the lubricant first, without checking the process requirements. Too often what they perceive as lubrication failures actually can be traced to variables in the process.
A draw bending lubricant requires unique properties. The key ingredient is the barrier film, which is the heart of the lubricant. The physical and chemical properties of this film should minimize frictional forces in order to protect the tooling without wrinkling or breaking the part.
Another job of the barrier film is to surround the metal oxides present, especially on aluminum, aluminized steel, and stainless steel materials.
This presents a paradox: Draw bending lubricants must stay static; that is, not squeeze out or flow away from the pressures of the draw bending operation. So the purpose is not hydrodynamic lubrication but to be a static film that enhances the material flow during bending.
The surfaces of aluminum and aluminized steel have one common characteristic: They generate metal particles during the forming operation.
During the forming operation, the pressures exerted on the material and tooling peen off metal oxides present on metal surfaces. These metal particles are small, a micron or submicron in size, and are quite abrasive. They have a tendency to pack up at the pressure points on the tooling. This buildup is referred to as black smut sometimes.
These are the same aluminum oxides used to manufacture grinding wheels and sandpaper. Their continued presence in the tooling area and on the part can result in acute tool wear and poor part quality. The lubricant must encapsulate and remove these particles continually.
Several methods can minimize the generation of metal oxides during the forming operation. To begin, let’ s look at the inside of the tube. Debris remaining on the inside after cutoff and end finishing can consist of dirt and metal fines. Some companies prewash their tubes with dip tanks, power washes, or cleaning wands.
Particles inside the tube restrict clearances between the mandrel body and the mandrel balls during draw bending. When metal particles are trapped in this area, they increase friction, which generates more metal oxides and results in metal pickup, galling, and tool wear.
To combat this elevated friction, use a draw bending compound with sufficient body and wetting to encapsulate the debris in the tooling area. Heavy-duty barrier films inhibited with solid lubricants have been found to provide optimum wetting and the physical strength to reduce the overall frictional forces present.
This 3-in.-diameter aluminized steel tube was bent for use in a truck exhaust system. A lubricious compound with anti-wipe additives and a solid lubricant blended into an invert emulsion prevented both the tooling and material from picking up metal fines and oxides.
Another consideration is lubricant compatibility with the tooling material. The mandrel tooling material should be hardened and polished tool steel, hard chrome-flashed to a thickness of 0.002 inch. A coating that can be helpful on aluminum and aluminized steel forming is Kro-lon®. This coating is 0.004 to 0.005 in. thick generally.
Stainless steel also generates metal fines and oxides during forming. Stainless steel has two other properties that make it harder to form: work hardening and the higher energy inputs needed to form the tube.
The amount of heat produced during deformation of stainless is greater generally than that produced with cold-rolled steel or aluminum. Forty percent more energy is required to form stainless, which produces more heat and fiction. However, the forming temperature must be low enough to prevent work hardening, which increases friction and metal pickup and shortens tool life.
The increased pressures needed to form stainless require draw bending lubricants that have high film strength and superior extreme-pressure protection and can provide a physical barrier that will not squeeze out under the increased forming pressure.
The following examples show how the combination of proper lubricant properties, tooling materials, and tool coating can help improve productivity.
A company bent 3-in.-diameter aluminized steel tubes for use in a truck exhaust system (see Figure 1).
When the company used a conventional water-soluble draw bending lubricant, both the tooling and material picked up metal almost immediately. The company changed the bending lubricant to a lubricious compound with anti-wipe additives and a solid lubricant blended into an invert emulsion.
The combination of additives in the new bending compound helped to encapsulate the metal fines and eliminated the metal pickup. Chrome-flashing the tooling further improved performance.
In another situation, 409 stainless steel heat exchanger tubing required a 1-1/4 D bend on a bronze mandrel (see Figure 2).
This 409 stainless steel heat exchanger tubing required a 1-1/4 D bend on a bronze mandrel. The producer selected a chlorine- and sulfur-free lubricant paste to prevent wrinkling, wall thinning, scratching, and metal pickup.
In this case, the tube bending lubricant had to fulfill the following requirements:
To meet all of these requirements, the tube producer selected a chlorine- and sulfur-free lubricant paste that could be applied by a mandrel lubricator. The paste preserved tool life and was readily removed.
By paying close attention to a draw bending lubricant’ s compatibility with the workpiece, tool material, and tool coating—especially when working with aluminum, aluminized steel, or stainless steel—you can minimize friction and increase tool life.
Joseph Ivaska Jr. is vice president of engineering, Tower Oil & Technology Co., 205 W. Randolph St., Chicago IL 60606, phone 773-927-6161, fax 773-927-3105, Web site www.tower oil.com. Tower Oil manufactures metalworking lubricants for pressworking and tube and pipe manufacturing.
This article is taken from the book Tube Forming Processes: A Comprehensive Guide© by Greg Miller, published by Society of Manufacturing Engineers, Dearborn, Mich.