Supply chain collaborates on bending project
Planning, cooperation keep project on schedule despite short time frame
Faced with a daunting bending contract and a short timeframe, Rick Williams of Rockford Process Control (RPC) sought cooperation from a tubing supplier, bending machine manufacturer, and a tooling supplier at the beginning of the project. The information exchange enabled RPC and its supply chain partners to develop a process, deliver a bender, and engineer and manufacture tooling without the benefit of tubing samples.
Fabrication projects don't just happen on their own; they require planning, which often involves an OEM, a fabricator, a tooling provider, and a metal supplier. If it's a new project, it might even involve an equipment vendor and a software developer. Likewise, any problem that crops up during any stage of the project likely involves the OEM, the fabricator, the tooling provider, and the metal supplier, and maybe even the equipment vendor and the software developer.
Depending on the nature of the project and the complexity of the problem, and the amount of planning that went into the project, the collaborators might find themselves trying to find and solve a problem that could have been prevented in the first place.
Successfully managing a big fabrication project hinges on developing a strategic plan. It requires sharing all the available information about the processes involved; understanding the capabilities and limitations of the equipment, tooling, and materials; and knowing how the finished part is intended to fit and function. If the players are brought into the project in the conventional way—that is, one at a time, at varying stages of the project's development—they don't get a chance to exchange all the available information, which impedes the project's progress and stymies the troubleshooting of any problems that crop up.
Rockford Process Control (RPC), Rockford, Ill., recently entered into a contract to manufacture a large, complicated tubular component and invited all the members of the supply chain to the table at the same time. This led to an information exchange that allowed the members of the supply chain to work together to identify, discuss, and resolve many potential problems that could have derailed or slowed the project.
Stamper, Welder, Fabricator, Coater
A proprietary product manufacturer and component supplier, RPC is no stranger to the intricacies of dealing with suppliers and OEMs. Founded in 1983 by entrepreneurs Paul Colloton and Richard Gleichman, the company initially provided components to the automotive industry. In 1987 it acquired a cabinet hardware product line from another Rockford-based manufacturer, and suddenly it was a multifaceted company. It provided parts for the auto industry and hinges, knobs (or pulls as they are known), and other hardware for the cabinetry and casework industry. It also developed a line of wire doors, the type of doors often used on school lockers.
The company's main manufacturing activity was stamping. No one-trick pony, it expanded its capabilities to include robotic welding, tube bending, and machining, turning itself into a contract manufacturer of fabricated components and assemblies. It also has powder-coating capabilities.
Although the company is well-positioned for contract manufacturing work, some contracts require a little more care than others. One such project landed in the lap of Rick Williams, a manufacturing engineer with decades of experience. It was a high-visibility project from the start. Not only did it involve one of RPC's largest customers, it also was an existing part that the customer was having trouble incorporating into an assembly. Complicating matters was the material itself, rectangular tubing, which RPC had limited experience in fabricating (see Figure 1).
Managing Variables or Manufacturing Parts?
Some manufacturing processes are straightforward, such as using a saw to cut a tube to length or a drill to put a hole in a length of barstock. Adding more processes multiplies the variables and the number of things that can go wrong. Introducing forming processes, those that force the metal to flow, makes manufacturing all the more tricky.
This project was a frame rail for a golf cart manufacturer. The previous supplier delivered a component that seemed to meet the OEM's specifications, but the OEM had an occasional problem with it. The specific nature of the problem was elusive. It seemed that no matter who Williams asked—manufacturing engineers, quality control personnel, purchasing agents—he got a different answer. Eventually he worked his way through the company and talked to the equipment operator, the person who actually loaded the part into the robotic welding cell. The problem turned out to be a gap between the frame rail and another component, a cross member. The gap varied; on many parts it was small and the welding process went without a hitch. Occasionally it was too large for a good weld. Williams gathered that the OEM had made some changes in the final assembly fixture to accommodate the variations in the frame rail.
Williams' detective work left him with a couple of questions. What was causing the gap to vary? If RPC manufactured parts that conformed to the OEM's specifications, would they fit into the final assembly fixture?
Creating a New Process
In its simplest form, manufacturing a component means taking a series of steps that make changes to a workpiece. Manufacturing a component successfully requires strict control over any variable that can affect the outcome. In the case of the frame rail project, the manufacturers had the luxury of starting from scratch.
"We knew the goal, and those of us in the supply chain got to start with a clean sheet of paper to see how we could most cost-effectively get there,” said Chuck Kuhn, president of GL Precision Tube, the tube supplier for this project.
"Everybody's process has limitations in the factors that can be controlled,” Kuhn said. "If you're hitting the mean objective, things often go fine,” he said, referring to the center of the tolerance band, regardless of the specification—the material's chemistry, the workpiece's cut length, the number of degrees of bend used to form the workpiece, and so on.
"When you hit the boundaries of what the process was intended to do, problems can creep in,” he said. This isn't to say that manufacturers work outside the specified limitations; in fact, problems often crop up when everyone is working within the agreed-upon tolerances. If just one or two factors of the process are at the limit, the process is likely to work smoothly. "When you get into a situation where you have a culmination of [many] factors outside the norm, but still within specifications, pretty soon the process doesn't work,” Kuhn explained.
This is why bringing everyone to the table early is critical.
"Getting everyone together gives all the players an opportunity to understand all the limitations, learn what can be controlled, and maybe some aspects that need to be controlled beyond the norm,” Kuhn said. "Having access to all this information prevents one player from adversely affecting another one downstream."
Tube and pipe complicate matters because they can be more difficult to work with than sheet metal. Sheet metal fabricators start with raw material that has been rolled flat. Welded tube and pipe start flat, but they have been processed further; they have been coiled, uncoiled, formed, welded, sized, and so on.
Relying closely on ASTM specifications can be problematic. Some of the specifications, such as those for hardness and elongation, apply to the original coil, not the tube or pipe. Forming the coil into a tubular shape work-hardens the material, changing its hardness and elongation properties.
Another factor is that tube forming experience in the fabricating industry is spotty. "Many engineers and quality control technicians aren't trained in tube bending and treat a tube bending process like they would a stamping process,” Williams said.
Stamping isn't always easy or predictable, but it has fewer variables than tube bending. Stamping involves two dies that constrain a sheet of metal, controlling its flow; a rectangular tube has four surfaces, each of which reacts differently to the bending forces, and the material isn't fully constrained. Because of this, a small change in one aspect of tube bending can result in a big change elsewhere. Intersect points, where two straight sections meet, are critical.
"A deviation of just a few tenths of a degree in a bend angle can throw an intersect point way off,” noted Williams. On the other hand, good intersect points don't necessarily translate into a good finished component. A change in the material's hardness from one heat to the next affects the material's elasticity, or its ability to stretch. "Your intersect points can be OK, but your end lengths can be way off,” he said.
This is why experience and intuition matter.
"Tube bending is a science, but it's not an exact science,” Williams said. "It's part science, part art. When you have a problem, you have to look at the whole thing, then home in on the problem area."
Process Development and Information Exchange
Although each link in the supply chain is critical in every project, RPC had the central role, which revolved around the process that Williams developed.
Process. Williams used the customer's print as a guide, but he didn't stop there. He also focused some of his attention on the final assembly fixture. His main concern was the fit-up between the frame rail and the cross member. The OEM's assemblers had to join the frame rail to several other components, but all of these areas had looser tolerances than the gap between the frame rail and the cross member. Williams figured that this area of the tube should be of most importance and developed every step of the manufacturing process around it. In other words, he visualized how the component would fit into the final assembly fixture and then designed the process from finish to start.
Furthermore, he put substantial emphasis on maintaining continuity throughout the entire process.
"The frame rail has a datum hole that locates the cross member,” Williams said. "This is the critical area. After bending the tube, we drill the hole using the datum point for the cross member as our stop point in order to maintain consistent fit-up in the final assembly fixture and our in-process gauges."
Because the part is made from bent tubing, the finished part length can vary substantially from lot to lot. If Williams hadn't known this, he might have been tempted to use one of the tube ends as a reference, and the locations of the part's features would have varied as the tube length varied from heat to heat.
Verifying the part's dimensions also raised some questions. Two obvious choices were a coordinate measuring machine (CMM) and a test fixture. Williams felt that an in-process gauge would be the more appropriate of the two.
"Some parts are more conducive to a hard gauge, others to a CMM,” he said. "Rectangular tubes aren't perfectly rectangular, straight tubes aren't perfectly straight, and bending a tube introduces quite a bit of distortion. A CMM picks up all of these discrepancies,” Williams said. "Also, if you use a CMM and you check different points from one tube to the next, you'll get varying results. In some cases a tube fits into a test fixture just fine, but the CMM data says it shouldn't,” he said (see Figure 2 and Figure 3).
At times like this, Williams refers to an evaluation that has little to do with dimensional tolerances and process control, and a lot to do with instincts.
"Sometimes I ask myself, 'Does this pass the look-right test?'" he said. This test relies more on common sense than it does on formal manufacturing principles, but it's handy for many situations, especially bend measurement. And it's simple.
"If it doesn't look right, it doesn't pass the test,” he said.
RPC prefers test fixtures to verify bent tubular parts, especially complex parts with several bends. When RPC does use a CMM, it checks the dimensions of straight sections only, and relies on the software to calculate the bends (see Figure 4).
The most important question still wasn't answered. Would the three critical bending elements—the machine, tooling, and material—interact in such a way that the finished parts would be accurate and consistent?
Material. "Bob Want, the sales engineer for Tools For Bending, was ready to design the tooling around normal ASTM tolerances,” Kuhn said. "That's fine, but tolerances are a worst-case scenario, and people don't want to deal with the worst-case scenario. I told him that GL Precision Tube could provide material at half the standard wall tolerances, gauge-restricted steel, chemistry-restricted steel, and specific weld locations.
"In a common steel such as 1008 or 1010, the carbon content can vary from virtually none to 13 points; that will significantly affect the way the tube forms,” Kuhn said. "We can do better than that, but we have to know upfront if that's what's needed. The mill can do it and it doesn't add cost."
Tooling. "We had the steel guy and the tooling guy at the first meeting,” said Chuck Schooley of KCS Industrial Sales, referring to Chuck Kuhn and Bob Want. KCS is a sales representative for both Tools For Bending and SMT Industries Inc., the bending machine manufacturer for this project.
"Getting them together was crucial because they were able to discuss the details,” Schooley said. "They were able to haggle back and forth to determine the specifications, the tolerances, the amount of elongation, and all that good stuff regarding the steel. There was no time to produce samples of tubing before making the tooling, so we had to agree to it on paper. That's a recipe for disaster because you can spend several thousand dollars on tools and it could turn out that they don't fit."
Want concurred. "We didn't have time to wait for the mill to make some samples, so we worked back and forth to come up with an appropriate compromise on the tolerances and built the tooling without a physical material sample."
Ultimately it worked, because GL Precision Tube was able to hold tolerances that were approximately half of ASTM tolerances. According to Want, ASTM tolerances for nonround tubing are too loose to be practical for modern manufacturing requirements.
"I could manufacture a mandrel for a specific size of nonround tubing, contact three mills, and place orders for three shipments of acceptable product, all of which conforms to ASTM standards, and the mandrel might fit one of them,” Want said.
"The tube supplier stepped up and asked what would be required to meet the quality requirements that this customer demands,” Want said. "It was especially tricky because this tubing required bends on both planes,” he said, referring to easy-way and hard-way bends.
"We had a lot of negotiating about the outside dimensions, the inside dimensions, the corner radius, the weld seam location, the height of the weld flash, and so on, and we hit on a combination that they felt they could hold,” according to Want. "GL delivered—they held the tolerances, and to my knowledge, Rockford Process Control has not had a single problem with the tooling not working because of material variation. And that is a very common problem,” Want said.
The agreement between GL Precision Tube and Tools For Bending was the crucial tooling issue, according to Want. "Otherwise, we would have had to manufacture two different-sized mandrels, just to be on the safe side,” he said.
Bender. Want worked with the bender supplier, SMT Industries, to develop an unusual way to use the mandrel.
"For a part like this, normally we would have used ball-and-socket links between the mandrel balls,” Want said. "These allow the mandrel to flex in any direction, so it would allow bending on two planes. The drawback is that ball-and-socket links aren't as strong as hinged links. They also allow the balls to rotate, and the operator has to manually line them up to load the part,” Want said.
"We told SMT that we thought the E-plane bend should be supported with a mandrel made with hinged links, and that the H-plane bend wouldn't need a mandrel at all,” Want continued. "This posed a problem. Because hinged links flex in just one direction, the first H-plane bend would shatter the mandrel. The solution was to insert the mandrel, make the E-plane bend, then retract the mandrel to the home position to get it out of the way, and then make the H-plane bend. This is unusual because you generally don't make a bend with the mandrel in the home position. Bender software usually has a fail-safe that prevents bending in this situation. SMT changed the software so it would bend with the mandrel retracted."
As everything started to come together—the tooling was finished, the first lots of tube were available, and the bender was ready—SMT's role changed. It did more than supply the machine and software; it was in charge of the product runoff.
"Our role was to bring in the material and the tooling and do some fine-tuning on the setup and the programming,” said SMT President Ron Duval. "We had to get it right because RPC got the contract for the business on a commitment to deliver a part that looked better and was more accurate and more repeatable. They didn't dare submit parts that were about the same and hope to improve them as time went on."
KCS's Schooley summed up the runoff: "The tooling made good parts out of the chute."
Each participant comes to a project with a unique body of knowledge—bending, tooling, materials, and so forth. Getting access to each others' knowledge in the planning stage was the key to this project's success, Williams said.
"To be able to sit down and talk made all the difference,” Kuhn said. "Everyone was exchanging information early on. In today's environment, it's key to engineer smarter, develop processes for efficiency, and eliminate excess costs, and the best way to achieve that is to get all parties involved as early as possible, ideally from the concept stage."
This isn't to say that RPC didn't have to deal with a problem or two along the way. For example, despite keeping the raw material's chemistry restricted to half the normal tolerances, the amount of elongation still varies from heat to heat. RPC determined that the most ductile heats of the material used for this project have elongation properties that vary from 35 percent to 50 percent. To deal with this, it developed four bending programs tailored to handle various elongations in increments of 5 percent. Still, troubleshooting this problem wasn't as difficult as it would have been without the information exchange upfront.
Bending the tube was just one portion of the project, of course. Williams and the staff at RPC had to visit the OEM on a number of occasions; develop the entire fabrication process, including drilling and machining (see Figure 5); engineer the fixtures for drilling, machining, and verifying the dimensions of the tube; purchase the bender and tooling; haul a load of the raw material to SMT's facility in Ohio to run off several hundred parts; ship those parts to the customer for evaluation; then get the workcell set up and running back in Rockford.
Not bad for a project with a 10-week time line.
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