When it comes to framing members and other structural elements for cars and trucks, hydroforming is where it’s at. Hydroforming starts with a tube — generally carbon steel — that has been pre-bent into a three-dimensional shape that will fit into a closed die. Fluid is then pumped into the tube, high pressure is applied to the fluid, and the pressure applies uniform stress to the inner surface of the tube. The tube undergoes plastic deformation, expands, and assumes the shape of the die’s interior surface. This is no light-duty task, considering that tubing typically has an OD of ½ to 6 in. with a wall thickness of 0.020 to 0.120 in. Pressure is then relieved, the fluid drained, the formed part ejected, and preparations are made for the next workpiece.

Bending machineTo pre-bend tubing so it will fit into a die, many OEMs rely on tube-bending machines designed and built by Eagle Precision Technologies Inc., Brantford, Ontario. Eagle Precision also has a line of end forming and muffler-assembly machines. Their end-forming machines perform such tasks as forming a flared or tapered tube end or expanding the ID at the end of one tube so it will mate with another tube of the same size. The muffler assembly machines combine bending, end forming, roll forming and joining, pressing, and cutting (not necessarily in that order) to form mufflers from steel tubing sheet. All three types of machines are used extensively for making exhaust system parts for cars — both at the OEM level and for the huge exhaust aftermarket.

But Eagle’s machines certainly are not limited to just automotive applications or to working with tubing that has a round cross section. At the low end of the scale are machines for bending, end forming, and cutting of ½-in. diameter tubing that may become hydraulic tubing assemblies for aircraft, marine vessels, off-highway equipment, or similar end uses. The finished assemblies may instead use square tubing to serve as a single-piece frame for a kitchen chair or fabricated into a heavy-duty assembly for office chairs. Or high-grade steel tubing may be used for precise heat exchanger tubes in refrigeration, furnaces, chemical processing, and other demanding applications. In the ever-growing health and fitness industry, manufacturers of treadmills and other exercise equipment use Eagle Precision’s machines for producing strong but lightweight tubular frames made from a variety of materials and cross-sectional shapes, even extruded shapes.

Machine anatomy

Jim Sabine, engineering manager at Eagle Precision, reveals, “We use our own CNC control based on a VME bus computer system and write our own software. Bending is a high-level program: you enter the physical parameters of the tube you’re starting with, the basic shape of the tube you want, and the controller takes over from there. The software then factors in the tubing material’s properties, size, and thickness, and executes the bend at optimum speed. “For some materials, such as Inconnel, bending must occur slowly to avoid ripping them. However, because metals work harden as they bend, you can’t go too slow. Plain carbon steel, on the other hand, can be bent at much higher rates. That’s why we have to use servovalves: we must generate extremely high forces at high speed, yet maintain positional accuracy within thousandths of an inch.”

tube-bending machinesEagle Precision’s benders operate at approximately 2,250 psi, relief valves are set at 2,700 psi, and all components are rated for at least 3000 psi. Sabine continues, “We generally specify tandem pumps, because servovalve circuits demand higher pressure than is needed for other functions on the machine — typically clamping functions, which operate at 1,500 to 1,800 psi. Therefore, one pump is dedicated to the servo system. Sabine explains that using a dedicated pump to isolate the servovalve circuit eliminates wide pressure fluctuations in the rest of hydraulic system. Pressure can be set manually, but usually is set automatically by the computer control.

Joint technologies

For a typical part, Eagle Precision’s machines may produce as many as ten bends on a single length of tubing. Jim Overbeeke, vice president, sales and marketing, explains that “Electromechanical functions are more cost effective at lower power levels, but a wide range of power requirements exists where cost differences between hydraulics and electrics is not important. Although in high power ranges where precise control is required, hydraulics certainly has a cost advantage.”

“Based on their awareness of potential leakage, some customers don’t want hydraulics on a machine,” offers Sabine. “However, a lot of clamping occurs to hold a length of tubing in place during bending. Hydraulics is by far the most practical method for performing these clamping operations, so most machines have at least some hydraulics on them. Once a machine contains some hydraulics, you can add more hydraulic functions fairly cost effectively simply by increasing the size of the existing power unit and providing the necessary actuators, valves, and plumbing.

“With electromechanical functions, you have to provide a motor, starter, gearbox, actuator, and associated components for each individual function. It’s almost like starting from scratch every time you add functionality. It’s not only more expensive, but the components are so much larger that it can be difficult to find enough space to mount everything.”

“Many of our customers would prefer an all-electric machine rather than one with electrics and hydraulics,” admits Overbeeke. “In fact, on our 1½-in. machines, many customers are willing to pay $5,000 Canadian ($3,700) more for a machine that is all electric over one that is electric and hydraulic.” Sabine explains that “Much of this additional cost is tied up in a 12-hp servomotor and large, zero-backlash speed reducer. Compare this to a hydraulic servomotor or cylinder driven by a servovalve. Customers are willing to pay the premium, though, because a total machine costs over $100,000 ($74,000). So their justification is that it adds maybe 5% to the total cost. Once you get beyond 12 to 15 hp, though, the cost of an all-electric machine skyrockets, which makes it almost impossible to pay 10% or more for the all-electric machine.”