Tubing requires precise control of material flow
Anyone looking to meet the challenge of processing tubing for today’s motorized vehicles ought to talk with BLM Group USA (Wixom, MI), a global leader in the field. BLM Group’s North American product manager, Lindley Searles, said there are generally around 20 tube parts on a car, “depending on what the maker chooses to use tube for.”
Besides obvious examples such as exhaust systems, fuel systems, and the aluminum tubing in air conditioning systems, an automotive manufacturer might also use tubing for the trunk hinge. Searles said, “There are other parts you might not think of as including tubing, such as the rail that runs along the edges of a truck’s roof. Some trucks even have a piece of tubing that creates the frame underneath.”
Whatever the application, Searles said, “Tubing is often the last thing that’s designed in the automotive world. They get everything laid out and then they have to figure out how they’re going to get the exhaust from the engine manifold to the back of the vehicle, and the fuel from the tank to the engine, and they’ve only got a tiny space to do it in. So this is driving the requirement to create tighter radii than we’ve done before.
“Everything used to be bigger and there used to be fewer components to maneuver around. The industry is challenging both us, the equipment supplier, and the raw material manufacturers to make a product that can be formed within these constraints.”
Tight Bends Require Precise Control
Bending tube in a tight radius requires precise control of the material flow. That requires specialized tooling and multiaxis CNC, as opposed to the old technology of hydraulically controlled bending machines. Searles explains that the material on the outside of a bend “has to come from someplace. It either has to stretch, break, or pull from the end of the tube. Historically, we’d allow the material to stretch, pulling material from the end of the part, and we’d often use heat to get the material to bend properly. We’d make single-bend parts, cut it off, and weld these segments together to create an assembly. As recently as five years ago any finished tube with multiple bends would have included much more cutting and welding than needed today.
“That’s because our electric machines now actually push material into the outside of the turn as the tube is bending. The servo-controlled axis also maintains enough control to keep material from pushing on the inside of the bend, where it thickens and can cause problems like wrinkles. We can control the material flow so well that radii as tight as one times the diameter are doable.”
But it’s a balancing act. BLM Group’s engineers have to determine the right amount of pushing required and the perfect timing for applying the force. And they have to create a repeatable process that takes just a few seconds. With cycle times like that and full automation, these machines are pushing out 20,000 or more finished tubes per day.
Cut to Length in Seconds
In the vast majority of high production situations, cutting the tube to length precedes the bending operation. Searles said. “Typical cutting times are a second and a half to two seconds, depending on the material and the thickness of the wall. A three-inch tube with a quarter-inch wall might take three or four seconds.
“In the automotive world, walls are generally getting thinner to help make cars lighter. We can process these lighter tubes faster. On the other hand, the material selection plays a big role, and of course it depends on the application. For example, a structural component like tubing in the door for collision protection would likely be a thin wall tube, but a much harder material that we’d cut slower. If it’s an exhaust system it’s generally pretty soft steel, which we can cut pretty fast.”
BLM Group uses either saws or lasers for the actual cutting, typically holding about 0.0050″ (0.13 mm) for sawing and 0.0040″ (0.1 mm) for laser cuts, both of which are perfectly acceptable. The goal is to produce parts in very high volume that meet the accuracy requirement without expending unnecessary resources. BLM Group’s CM602 automatic CNC sawing machine handles tube and bar up to 4″ (102 mm) in diameter, and depending on the material, runs at up to 5000 rpm cutting speed. The cutting head meets the tube surface at 45° and its linear movement across the tube provides a uniform load on the blade resulting in less noise, less scrap and blade life for 2.5 m³ of material.
Laser has the advantage of being able to cut additional features such as holes and slots either before or after bending the tube. Searles said that while it’s more cumbersome to make cuts after bending the part, there are applications in which you can’t cut the material before bending it without the likelihood that the material would tear or deform in the process of bending it. “We look at each application and determine the best approach to accomplishing the goal, and we have equipment that can process tubes in whatever sequence of operations makes sense.”
Laser is also better at cutting the stainless steel used in some exhaust applications. In one such case, a European Tier 2 auto supplier achieved roughly 30% higher productivity using the BLM Group LT5 laser cutting machine versus the fastest sawing machine they had considered.
Handling the ‘Spaghetti’
Like a lot of high-volume production tasks, just handling the raw material in the tubing business is a major challenge. Searles said that, “Quarter-inch tube tends to become like spaghetti when you drop it into an automated system, so we need to take special steps to handle such material properly.”
Product specialist Tom Worley added, “The biggest problem isn’t machining or cutting the tube, it’s getting one tube at a time to feed in properly at production rates. You’re talking about tubes that can be smaller in diameter than my little finger and 20 to 40′ (6–12-m) long, so it doesn’t take much of an error to create a mess. It takes a very good loader to make a high-production sawing machine perform at its best.”
For straight tubes, BLM Group uses bundle loaders. A crane or forklift operator delivers a load of tube to the hopper and the loader automatically separates the tubes and feeds them individually into the machine. Smaller-diameter material like air conditioning tubing may come from the supplier in a coil, requiring a different approach. Worley said, “It’s typically ductile material, so you wouldn’t want to straighten it into long segments and then process it. Our machine unwraps and straightens the coil as it processes it.”
End Machining in a Flash
Some applications also call for machining the ends of tubes (e.g. for use as bushings) and BLM Group delivers with machines like the BC80, a highly productive and completely automated CNC end-machining center. The BC80 can cut bushings up to 3.15″ (80 mm) in diameter, in lengths from 0.4 to 13.75″ (10–349 mm) and then chamfer, ID, OD, and face both ends, flush the chips, and measure the length relative to center—all automatically. The machine rejects any part outside spec, and can adjust the next cut based on any deviation from the normal. Because the machine has a rotating head with four stations (each of which simultaneously machines both ends of the part), what might have been, say, a 40-second total cycle time is effectively only about 12 seconds.
Making the Job Easier
Searles said, “The automotive world wants a complete, turnkey solution. And that’s what we do. We take the process from the bundle of raw tubing to the finished product, with a complete product line of automated tube loading systems, cutting systems, and bending and machining systems.
“Our biggest challenge is getting customers to design for manufacturing. We can provide a solution to deal with whatever they want, but it often becomes a question of how cost effectively you can do this. For example, if someone designs a piece of tubing with five different radii, that’s potentially five different sets of tools. It’s more complicated to create a machine with all this tooling. If they can compromise on two or three radii they can lower their manufacturing costs.”
The malleability of the material is also still a critical factor, so the end user should take care to specify the right material at the outset, and establish quality control procedures for the supply.
“And you don’t use the same machine to make exhaust systems that you’d use to make air conditioning components,” Searles said. “If you’re in contract manufacturing and you service the off-road and passenger market, you’d need to handle a broader range of applications so the equipment needs to be more flexible.
“These are the kinds of conversations we have with our customers. That’s why we like to get involved in the projects early on, rather than at the end, when the customer already has the contract and flexibility is limited. Customers in all industries are leaner and leaner and look to suppliers to do this type of evaluation for them. Our job is to make the manufacturing engineer’s job easier and help them make money.”