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28.10.2021

Tracing the evolution of bending knowledge, bending machines, bender tooling

If you were to read a collection of tube bending articles written at various times over the last three decades, you might be surprised by how much bending technology has changed, especially within the last 20 years. Benders have advanced, new features have been developed, and bending knowledge has improved and spread.


To get a sense of how much the industry has advanced, consider two people doing two different things in the early 1990s. One is a tube bender technician working for an aircraft parts manufacturer. He's well-versed in bending technology and bending machine and knows the combinations of tubing alloys, tooling sets, lubricants, and bender settings that result in tight-radius, repeatable bends. The other owns a pickup truck, and he's changing the spark plugs. The truck is so big and has so much room to spare under the hood that he sits on the fender with his legs inside the engine compartment. He rests his feet on the wheel well as he works.


What do these two activities have to do with each other? Everything.


A Tale of 1D Bends in Two Industries


The aircraft industry has always been one of innovation. From the first flight in 1903, the technology it used to build aircraft was almost unrecognizable a mere 50 years later. Wooden frames covered in cloth and internal combustion engines had been replaced by aluminum airframes, aluminum skins, and jet-powered engines. The first flights were measured in seconds for time and feet for distance; five decades later, jets were flying for hours and hundreds of miles. Always space- and weight-conscious, aircraft companies led the way in reducing component size and weight. In tube bending or pipe bending machine, this means tighter bending radii and thinner walls. By the 1990s, it wasn't uncommon for a tubular aircraft part to have a bending radius equal to the tube's diameter, known as a 1D bend.


Back then, the automobile industry wasn't as weight-conscious as it is now, and most vehicles were bigger than they really needed to be. The corporate average fuel economy (CAFE) legislation, which mandated the fuel efficiency for passenger cars and light trucks, didn't change much in the1990s. After the initial target, 17.2 miles per gallon (MPG) for two-wheel-drive trucks and 15.8 MPG for four-wheel-drive trucks in 1979, they climbed to 20.7 and 19.1 by 1991. After that they rose slowly, just 1.4 MPG over the next 15 years. Gasoline was little more than $1 per gallon throughout much of the 1990s; consumers were hardly clamoring for fuel efficiency, so automobile manufactures didn't have much incentive to reduce vehicle weight or size. In the early '90s most automotive applications used centerline radii larger than 1.5D and straight lengths between bends long enough for a standard 2D clamp and insert. Exhaust systems were big and cumbersome, so engine compartments were large to accommodate them.


The automotive industry at the time hadn't started the big push to lighten cars and make them more fuel-efficient. Exhaust tubing was 0.060 to 0.080 inch thick, and in the '90s, some considered 0.060 in. to be thin wall.


Reducing Straight Lengths, Eliminating Welds. At that time, 1D bending was limited to elbows, or single bends, which were welded to straight sections to produce complete components—a time-consuming way to make a part.


In the early to mid-1990s the straights between bends shortened to less than 2D. Stack benders become the coveted machines of the time. The early stack machines were limited to either two or three tools: The so-called two-stack and three-stack machines with shearing machine. The latest generation of machines is more advanced still. They have a specific stack height but can accommodate as many tools as can fit onto the stack.


In that same time period a three-stack machine held a single straight clamp and two bend dies, so part designers still had limitations: either use the same straight clamp for every bend on a given part, or design the part so it used several straight clamps, which required additional tooling setups. The tradeoff was design creativity versus production efficiency.


The auto industry followed the aircraft industry by reducing the bend radii, which ultimately led to a new machine equipment feature: Boost. This process, a pushing force that advances the tube into the bend die, gave manufacturers the ability to make several 1D bends in succession with no straight length between the bends. In other words, make a 1D bend, rotate the tube a bit, and make another 1D bend right after the previous one. Voila! A compound 1D bend.


Bend and Cut. In the late 1990s another innovation came along to improve production efficiency and conserve material: Cutting the tube immediately after bending. Bender manufacturers developed various types of bender cutoffs, but combining bending and trimming on one machine wasn't viable for every application. One disadvantage was that the time needed to trim the tube on the bender uses more bender time, so less tubing was being bent in a specified time. Fabricators soon learned that cutting and trimming in a second operation allowed the bender to bend more parts.


Tooling Evolves Too


Wiper dies also have changed. The wiper prevents the terminal hump or wrinkle on the inside radius of the bend. Many companies have switched to inserted wiper dies, which cost much less than conventional wipers and are considered disposable. The older style, which is a squareback, is more expensive but can be recut and reused several times


The squareback type is much more rigid than the disposable type and therefore works better for 1D applications. The key setting for wiper dies is the rake, which is the maximum angle relative to the tube's centerline at which the wiper can be set before the terminal hump forms. More rather than less rake is normally desirable because increasing the rake angle increases wiper life.


As materials have gotten thinner and bending radii smaller, more pressure has been applied to make good bends without wrinkles, and the wiper tower needs to be rigid enough to maintain position when this additional force is applied. While machine builders, including slip roll machine, incorporated one innovation after another, the one area that seemed to be neglected was the wiper towers, the posts to which the wipers were mounted. Bends have increased in severity, but among many bending machine manufacturers, wiper die towers have stayed the same.


Any experienced bend technician knows that a wiper tower that deflects during bending creates several problems, especially wrinkles. The setup person has a difficult time setting the wiper because all of the rules he or she knows no longer apply and uncertainty sets in. How to set the wiper on a tower that moves during bending? It's impossible to say. A wiper tower that moves introduces a variable that nobody wants to deal with.


Replacing the bender's original wiper stack with a heavy-duty wiper tower might be the way to go. A heavy-duty, multistack wiper tower might seem expensive in the short term, but this investment has long-term benefits. The two offsets are less scrap (a waste of material and time) and less downtime needed to resolve setup problems. In shops that have a handful of troublesome applications, the cost can be justified without too much effort, and the return on investment comes quickly.

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