Tube bending with no straights ?
By Tim Heston, Senior Editor
Wisconsin tube shop invests infreeform bending technology
Picture a prototype of a medical patient lift with an interior skeleton consisting of a single, contoured tube designed to fit the human body perfectly, making life easier for both patient and caretaker. To most the lift itself might be interesting, but not jawdropping. But for the staff at Sharpe Products, and anyone else familiar with tube bending, for that matter, the lift represents something extremely unusual. It breaks the rules of tube design.
The lift consists of a single tube with only one 2-inch straight section and a multitude of radii. Years ago a salesperson at the New Berlin, Wis., tube bending job shop would have taken one look at the drawing and turned it down. The job just couldn’t be done, at least not cost-effectively. Without straight sections, the shop’s all-electric rotary tube bending machines would have nowhere to grasp the tube during the bend. They perhaps could have split the design into sections, bent each contour using a straight section that would be cut off afterward, welded the tube sections together, and (finally) hand-ground all the joints to a smooth finish. And all this didn’t even include the most costly component, at least for the initial setup: the tooling. Rotary benders would have required a rotating die for each unique bend radius. The tooling alone would have added up to $15,000 or more, said Robbie Krickeberg, Sharpe Products’ estimator.
Sharpe didn’t turn down the job, though, because of an unusual bending technology—a machine that works by starkly different bending principles (see Figure 1).
History and CapabilitySince launching in 1990, Sharpe Products has transformed itself from primarily an architectural handrail fabricator into a diverse tube bending shop. Some revenue still comes from railing, but the lion’s share is from custom pipe and tube bending. The company sells stock handrail components (see Figure 2) as well as offers custom pipe and tube bending and fabrication services such as high-speed cutting, end forming, and welding.
Its 52,000-square-foot facility has a dozen bending machines. Eight are CNC, four are NC, and together they can handle tube and pipe from 0.25 to 6 inches in diameter. The shop also uses an Aicon TubeInspect system, a kind of noncontact 3-D inspection machine using a series of CCD cameras. Technicians place a tube inside a work envelope, compare the actual workpiece with a 3-D drawing, and send corrections back to the bending machines on the floor if needed. Operators use a Bluco fixturing table for quick go/no-go inspection during production. For precision work, technicians use a Romer coordinate measurement machine designed for tubes.
The shop’s three all-electric rotary draw benders can stack tooling to accommodate multiple radii in a part. The machines can roll and punch tubing too. Another machine, a twin-head compression bender, can form two bends simultaneously in tube up to 1.5 in. OD.
Freeform BendingBut no machine at Sharpe is anything like the one that bent the contoured tube for the patient lift, the tube with one—and only one—2-in. straight section in it. It’s a part that couldn’t be made with traditional methods. For rotary draw bending, “you usually need a straight section that’s at least 1X diameter,” Krickeberg said.
Several years ago the shop invested in a bending system from Tokyo-based Nissin Precision Machines Co. Ltd. The machine uses MOS, a term dubbed by Japanese researchers for a unique freeform bending method. Tubes are inserted into a guide cylinder and pushed through a mobile bending die. Bent workpieces emerge from it almost in Play-Doh® fashion, and the die position dictates the resulting bend angle and radius. At the machine’s front, the mobile die assembly consists of a bearing into which is placed bending dies made for specific tube diameters. The arrangement allows for sequential and even continuous bends of various degrees and directions.
A Sheet Metal AnalogyIn a rough sense, such freeform tube bending is to panel bending as rotary draw bending is to press brake bottoming. Bottoming uses a matched punch and die set to form a specific bend; rotary draw bending uses, in concert with other tooling, a rotating die designed to bend a tube to a specific radius. In panel bending, the sheet is clamped with hold-down tools, and a bending blade is moved a precise amount to form the desired angle in a sheet. This is somewhat analogous to Sharpe’s freeform tube bending machine.
A technical paper published on tubenet.org, authored by the machine’s inventors —Makoto Murata of the University of Electro-Communications in Tokyo, and Tadao Kato of Nissin—describes the details. The tube is fed from a guide cylinder through the center of the mobile die. Bend geometries are determined by the position of the mobile die. An AC servomotor can move that die continuously throughout the bend, in any direction on a vertical plane: side to side, up and down, diagonally, and every direction in between.
A CNC monitors the entire operation. A rotary encoder positioned at the guide-cylinder exit measures tube length, feeds that information back to the CNC, which in turn instructs the servomotor to move the mobile die to the correct position to form the desired bend angle, bend direction, and bend radius at the correct location on the tube (see Figures 3 and 4).
Low Tooling CostsOperators change the bending die only when changing tube diameter. If a 1-in.-dia. stainless tube is followed by a 1.25-in.-OD aluminum tube, the operator switches out the bending die from 1 to 1.25 in., and then calls up a new program. Sharpe’s system can bend tube from 0.25 in. to 1.25 in. OD.
The technique can bend tube in any direction as long as the workpiece doesn’t interfere with the machine. According to Krickeberg, the machine generally can bend any radius 3X the tube diameter or greater, depending on the material.
It can 3-D bend beyond 360 degrees too. The tube feeds into the mobile die, which can be set to bend a tube into a continuous spiral. If bending on a flat plane (the X or Y), the system can bend tube to about 270 degrees. Determining the bend radius is what the machine inventors call the offset, or the vertical distance between the centerline of the tube in the guide cylinder and the center of the bending die. The greater the offset, the sharper the radius.
The machine doesn’t have a mandrel to plug the inside of the tube, so the process can’t handle workpieces with extremely thin walls. According to Krickeberg, the wall thickness, depending on the material, must be between 5 and 10 percent of the tube diameter. So 1-in.-OD tube would require a wall that’s between 0.050 and 0.100 in. thick.
“We’ve bent tube a little thinner than that, but not much,” he said. “You need to worry about wrinkling, because there’s no mandrel used in the process.”
Material consistency is paramount. The resulting degree of bend (DOB) and radius can change slightly with varying material characteristics, usually from excess springback (an issue press brake technicians who air-bend high-strength steel are all too familiar with). For instance, if a bad batch of tube has a tensile strength beyond the upper limits of the material specification, the resulting bends may change slightly.
Machine ProgrammingThe CNC machine works off a library of material parameters that calibrates the machine. The more jobs the machine processes, the bigger the library becomes. Once parameters are set for a specific material and bend, they need not be set again.
To calibrate the machine for a 1-in.-OD stainless steel tube with 0.065-in. wall thickness, an operator inserts the material and bends it to various DOBs and radii. “Then he measures those specific radii and plugs in those numbers into the machine, so it knows where those bends actually came out,” Krickeberg said. If, say, a measurement shows a bend that sprang back 5 degrees, the programmer enters this characteristic into the machine. After this initial calibration, the system will know to overbend such geometries in identical material for future jobs.
Working with a calibrated machine, an operator programs a part by entering a few go-to commands into the CNC. In their tubenet.org paper, the machine’s inventors gave the following example program for a tube requiring six bends:
G01 is the starting point. L200 is a 200-mm straight length; R40 is a 40-mm bending radius; T90 is a 90-degree bending angle; F and E show moving speeds of the die; P90 is a bending direction; and M02 ends the bending operation.
This complex, six-bend part requires six lines of CNC code. Changing radii doesn’t require different tools, only a different number in the code. For instance, if a technician needed this program to produce a 50-mm-radius bend, he would change “R40” to “R50.”
Relearning Tube DesignSince installing the system several years ago, the machine has bent plenty of prototype parts, mainly for jobs that couldn’t be done any other way. “It’s a new process,” Krickeberg said. “People are still discovering what this technology can do. It’s like learning the basics of tube bending all over again.”
The technology frees tube design from previous manufacturing constraints. To really take advantage of the system, though, product designers may need to unlearn some basic bending principles, Krickeberg explained. Adding more bends of different radii does not add tooling costs, and a part need not have straight sections.
“We explain to people that, yes, we can actually bend a tube without a single straight [section] in it,” he said. “We just offer a manufacturing option that not many are familiar with—yet.”