Bending tube with an all-electric machine

Characteristics and capabilities

By Jeff Tapper

December 13, 2001

This article discusses the ins and outs of all-electric tube bending machines, discussing the challenges, performance characteristics, and typical options of all-electric benders.

In the tube bending industry, all-electric, CNC benders are a fairly new development but are gaining ground. They have been commercially available in Europe since 1995 and in the U.S. since 1998. Most manufacturers now either offer all-electric bending machines or are developing them.

On all-electric tube benders, the motion and force needed for positioning the tube or the tooling during production are controlled by a servo motor that drives a ball or roller screw. A basic machine has five axes of CNC motion — distance between bends, plane of bend, degree of bend, clamp die, and pressure die — as well as a control system. Most manufacturers are not limited by the number of axes their servo motors can control.

Currently two- to seven-axis twin-head compression benders with up to 50-millimeter capacity and five- to 14-axis rotary draw benders with up to 150-mm capacity are available in all-electric versions from several manufacturers.

Watch for Challenges

Although the prospects for all-electric tube benders look good, this is new technology for many manufacturers, and they still are working out the kinks. Integrating new software and new mechanical bits with existing machine designs requires a lot of study and testing on the part of manufacturers. If they rush the project through the research and development process, they will create headaches for buyers. Once the kinks are worked out, all-electric machines could produce as much as 30 percent more parts than comparable hydraulic machines.

As with any new technology, buyers should look for vendors who have several years of experience with it. If no experienced vendors are available, it might be wise to stick with proven technology.

Performance Characteristics

Power Consumption. An all-electric tube bender uses power only when it is moving, so it typically consumes less power than standard hydraulic machines, which use power to create pressure, noise, and heat whether they are making parts or idling. Typical sound levels for all-electric machines are 55 to 60 decibels, and the heat generated is negligible.

Speed. As with hydraulic benders, all-electric bending machines are capable of moving tube too fast, which can whip it out of shape. The specifications for revolutions per minute or feet per minute of the different motions for all-electric benders typically are about the same as for hydraulic benders.

Accuracy. Because it uses no fluids, an electronic motion control system is not subject to positioning variations created as hydraulic fluid warms up during the first part of the production day. Additionally, spikes and other random noise, normal in hydraulic systems, increase the hysteresis and erode the latency, or timing, of a closed-loop, CNC hydraulic servo circuit. This randomly affects positioning and the forces applied, regardless of temperature, forcing the operator to waste time and material tweaking the machine. Without the hydraulic pump and circuits, an all-electric machine is not subject to these errors.

Setup and Programming. Assuming no tooling problems, new-part setup on an all-electric bender takes 30 minutes or less. Setup for an already-programmed part with mandrel and wiper dies of different outside diameters (ODs) can take as little as 5 minutes to the first good part.

An operator can program and save both the position and force used for the clamp die, pressure die, pressure die assist, and mandrel with the part number. This is important for two reasons:

1. Because all adjustments are entered and saved with a part number in the computer, they come up the same way the next time that part number is loaded, reducing setup time on future runs.
2. Because these adjustments are made on a computer program, not the bender, no hand tools are used, reducing setup time and the skill levels needed for quick setups.

Helpful setup features include setup notes by part number; tooling database; percentage speed control for each motion; and percentage force control for the clamp die, pressure die, and pressure die assist.

Operation and Productivity. No hand tools are used with an all-electric bender, minimizing tweaking time to accommodate variations in material hardness. Some benders also include bend arm torque monitoring and logging. For instance, if a mandrel breaks during a bend, the internal drag is reduced, which also reduces the bend arm torque. The bend arm torque monitor notes this drop and stops the machine, flashing a message on the operator's screen. If the material hardness increases and the tubes start breaking, the operator can show this "before-and-after" log to the material vendor.

Helpful operating features can include a part counter, cycle time display, springback compensation, collet recapture software, bend-on-bend software, and mandrel anticipation software.

Scrap. An operator can program close-to-optimal tool positions and forces from the beginning rather than using trial and error with hand tools. As a result, initial part setup generates minimal scrap. Tool positions and forces can be saved and loaded exactly the same for the next run. During a production run, an all-electric bender generates no part variations that wind up in the scrap bin because of oil warm-up or machine error. The issue of scrap becomes even more important for companies that bend expensive materials in repeated short runs.

Service and Diagnostics. Service and diagnostics depend on the manufacturer. Some bender manufacturers provide on-site factory support using a modem and video camera shipped with the machine, greatly reducing the need for on-site service calls. On-screen diagnostics also are common. They provide information on the state of the machine, instructions, and alarm messages when there is trouble.

Design. Good basic machine design practices minimize the number of components used and the amount of friction to be overcome. The fewer components used, the fewer there are to break. The less friction to overcome, the less wear and tear, and the less power consumed. Machines designed to these standards tend to run longer and more reliably.

Environmental Impact. All-electric benders have no fluid systems that can leak, make heat, or make noise, providing a friendly working environment.

Safety. Many manufacturers produce machines that are compliant to the CE 392/4 standard. Safety is a prime concern with a machine that moves as quickly and quietly as an all-electric bender, because people generally do not expect a machine to move unless it makes noise. Hard perimeter fencing; emergency-stop buttons; light barriers; and an antitiedown, two-handed start cycle control are recommended.

Control Features. Today many benders are shipped with a personal computer operating Windows® with Visual Basic software to coordinate the various motions. The PC simplifies the integration of a bender into a workcell or system.

Reverse bend order, XYZ to or from LRA, mirror part from right hand to left hand or vice versa, three-dimensional part display, and line editor help simplify programming. Operation can be made easier with a choice of fast or slow speed, manual or automatic loading and unloading, and straight or hitch feed, as well as sequencing through 10 different part numbers.

Typical Machine Options

Following are some machine options available from many manufacturers:

• Servo-driven pressure die assist, mandrels, and wiper dies help improve the quality of tight-radius, thin-wall tube bends. A servo-controlled mandrel can be programmed to extract partially near the end of the bend to eliminate mandrel hump. Pressure die assists, servo locked to the motion of the swing arm, help prevent slippage.
• Calendering allows the generation of large-radius bends by substituting a set of rollers for the normal bend set. It usually requires a heavy-duty carriage and drive.
• Servo-driven vertical and horizontal carriage and mandrel motion allows the stacking of bend dies for multiple-radius bend dies or clamp types with no fixed centerline heights between grooves.
• Under CNC, a punch and actuator can be mounted so that the tube can be clamped and then punched during the bending cycle. Similar to the punching option, a two-bladed cutoff can be mounted in the clamp section. This is appropriate for small-OD, short parts with one bend.
• The bend head can be extended away from the body of the machine to reduce part-machine interference under the head. Another way to reduce interference is to mount a second die set for right- and left-hand bending on the same machine.
• Many options are common for loading and unloading. An automatic bundle descrambler loader holds 200 to 400 round parts. A rack loader holds 20 to 30 round parts. A chain loader holds 15 to 20 nonround parts or round parts with nonround fittings or features. These loaders usually are available with optical or electromagnetic weld seam detectors. Unloaders range from pick-and-place to a robot.

Bending for Tomorrow

Like robotics and plastic injection molding machines, servo motor-driven machines are taking a foothold in the tube bending arena, and equipment manufacturers are getting onboard. Some already offer all-electric benders. Many offer hybrids with all-electric bend heads that still have hydraulic clamping, pressure die, and mandrel. And more are developing this all-electric technology.