Unleash the potential of automated tube end forming

Jan 07, 2024

A multistation end forming machine completes its cycle, forming a closed bead at the end of a copper tube.

Imagine a value stream in which pipes or tubes are cut and bent. In another area of the plant, rings and other machined parts are processed, then sent to assembly to be brazed or otherwise fitted onto the tube ends. Now imagine the same value stream, this time with end forming. In this case, end forming doesn't just expand or reduce the tube-end diameters but also creates various other forms, from intricate grooves to beading that replicates the rings that were previously brazed into place.

Within the tube and pipe fabrication arena, end forming has quietly evolved, with manufacturing technology introducing two levels of automation to the process. First, operations can combine multiple precision end forming steps within one work envelope—in effect, a done-in-one setup. Second, such complex end forming has been integrated with other tube and pipe fabrication processes, like cutting and bending.

Most applications that involve such automated end forming have been in high-end precision tube fabrication—usually of copper, aluminum, or stainless steel—in industries like automotive and HVAC. Here, end forming eliminates the machined connections designed to secure a leak-free connection for the flow of air or fluid. Such tubes usually have outside diameters of 1.5 in. or less.

Some of the most cutting-edge automated cells start with small-diameter tube delivered in a coil. It first travels through a straightening machine, after which it's cut to length. After that, a robot or mechanical device transports the work to end forming and bending. The sequence that occurs depends on the application requirements, including the distance between the bend and the end form itself. Sometimes the robot can carry a single workpiece from end forming to bending and back to end forming, should the application require a tube with end forms on both ends.

Making such cells even more productive is the number of manufacturing steps certain high-end tube end forming systems can incorporate. Some systems carry tubes through as many as eight end forming stations. To develop such a setup starts with knowing what modern end forming can accomplish.

Precision end forming tools come in several varieties. Ram punches, the "hard tooling" of tube end forming, reduce or expand a tube end to a desired diameter. Rotary tools can chamfer or face a tube to ensure a burr-free surface and consistent process. Other rotary tools perform a rolling process for creating grooves, barbs, and other geometries (see Figure 1).

An end forming sequence might start with chamfering, which provides a clean surface and ensures a consistent hang-over length between the clamp and the end of the tube. Next, a ram punch performs a beading process (see Figure 2), expanding and compressing the tube, forcing the excess material to form a ring around the outside diameter (OD). Depending on the geometry, other ram punches might insert barbs on the tube OD (which can help secure hoses to the tube). A rotary tool might groove a portion of the OD, followed by a tool that cuts threads on the surface.

The exact sequence of tools and processes used depends on the application. And with up to eight stations in an end forming machine's work envelope, the sequence can be quite extensive. For instance, a series of punches progressively form a bead on the tube end, with one ram expanding the tube end followed by two more that compress the end to create the bead. Performing the operation in three steps can, in many cases, create a better-quality bead, and multistation end forming systems make such progressive operations possible.

An end forming program sequences the operation for optimal precision and repeatability. The latest all-electric end forming machines can precisely control the position of its tooling. But aside from chamfering and tapping, most end forming processing steps are just that—forming. How metal forms can vary depending on the material type and quality.

Consider again the beading process (see Figure 3). Like a closed hem in sheet metal, a closed bead in end forming has no gap. This allows the punch to form that bead to a precise location. The punch is, in effect, "stamping" the bead to its specific shape. But what about open beads, analogous to open hems in sheet metal? A gap in the middle of the bead might, in some applications, create some repeatability issues—at least if it were formed the same way as the closed bead. A ram punch could form the open bead, but because nothing is supporting the bead from the tube inside diameter (ID), one bead geometry might be slightly different from the next, a tolerance difference that might or might not be acceptable.

FIGURE 1. A rotary tool forms grooves around a tube OD, creating barbs for a secure hose connection.

In most cases, the multistation end forming machine can take a different approach. A ram punch first expands the tube ID, creating a wave-like preform in the material. Next, a three-roll end form tool, designed with the negative shape of the required bead, clamps around the tube OD and rolls the bead.

Precision end forming machines can create a variety of shapes, including asymmetrical ones. That said, end forming does have its limitations, and most of them have to do with how the material forms. Materials can withstand only a certain percentage of deformation.

Punch surface heat treatments vary with the kind of material they’re designed to form. Their design and surface treatments account for the differing levels of friction and other end forming variables that change with the material. A ram punch designed to end-form stainless steel tube will have characteristics different from a punch designed to end-form aluminum tube.

Different materials also require different kinds of lubrication. Harder material like stainless might use a thicker mineral oil, while aluminum or copper might use a nontoxic oil. Lubricant application methods vary as well. Rotary cutting and rolling processes often use a mist while ram forming can utilize a flood or mist lubricant. Some punches have oil emerging directly from the punch into the tube ID.

Multistation end forming machines have different levels of ram and clamping power. All other variables being equal, higher-strength stainless steel will require more clamping and ram power than a soft aluminum.

View a close-up of tube end forming in action, and you can see the machine advance the tube before the clamp secures the tube in place. Maintaining a consistent hang-over—that is, the length of the metal beyond the clamp—is critical. For straight tube, which can be moved to a defined stop, maintaining that hang-over is straightforward.

The situation changes when end forming previously bent tube (see Figure 4). The bending process can elongate the tube ever so slightly, which adds another dimensional variable. In these setups, orbital cutoff and facing tools can cut and finish the tube end to ensure it's exactly where it should be, per the program.

This begs the question, why end form a tube after bending? It has to do with tooling and workholding. In many cases, the end form is placed very close to the bend itself, leaving no straight section for the bend machine tooling to clamp onto during the bend cycle. In these cases, it's much easier to bend the tube and transfer it to end forming, where it's secured in a clamp that matches the bend radius. From there, the end forming machine cuts off excess material, then creates the required end-formed geometries (which, again, ends up being very close to the bend).

In other cases, end forming before bending can cause complications in the rotary draw process, especially when the end form interferes with the bend tooling. For instance, clamping a tube for bending can end up distorting that previously made end-form. Creating a bending setup that wouldn't damage the end-form geometry ends up being more trouble than it's worth. In these cases, it's easier and more cost-effective to end form the tube after bending.

Cells that incorporate end forming can involve a host of other tube fabrication processes (see Figure 5). Some systems have both bending and end forming, a common combination considering how linked the two processes are. Some applications start with end forming a straight tube, which then heads to rotary-draw bending to form a radius, then heads back to the end forming machine to work on the opposite end of the tube.

FIGURE 2. A multistation end forming machine produced these end-formed beads, with a ram punch expanding the ID and another compressing the material to form the bead.

In this case, the sequence governs the process variables. For instance, since the second end forming operation occurs after bending, an orbital cutoff and facing operation in the end forming machine ensures a consistent hang-over length and better-quality end form. The more consistent the material is, the better and more repeatable the end forming process will be.

Whatever combination of processes an automated cell has—be it just bending and end forming or a cell that starts with the tube in a coil—the way a tube progresses through the stages depends on the application requirements. In some systems, tube is fed directly from a coil, through a leveling system into the clamps of a rotary draw bender. Those clamps secure the tube as the end forming system moves into position. Once the end forming system completes its cycle, the rotary draw bender commences. After bending, a tool cuts the completed workpiece. The system can be designed to run different diameters, with dedicated ram punches in the end former and stacked tooling in the left- and right-hand rotary draw bender.

However, if the bending application requires a ball mandrel in the tube ID, the setup wouldn't work since the tube being fed into the bending process comes directly from coil. The arrangement also wouldn't work for tubes that require forms on both ends.

In these cases, a cell involving some combination of mechanical transfer and robotics might suffice. For instance, a tube could be decoiled, leveled, cut, then staged for a robot to place the cut piece into a rotary draw bender, where a ball mandrel could be inserted to prevent distortion of the tube wall during bending. From there, the robot could transfer the bent tube to the end former. The order of operations could vary, of course, depending on the job requirements.

Such systems can be set up for high-quantity production or small lot sizes, processing, say, five of one shape, 10 of another, and 200 parts of yet another. Machine designs also can vary depending on the sequence of operations, especially when it comes to the location clamps and providing the needed clearance for various workpieces (see Figure 6). For instance, location clamps in end forming that accept bent tubes must have sufficient clearance for the bent tubes to be located consistently.

The right sequencing can allow for concurrent operation. For instance, a robot can place a tube in an end forming machine; then, as the end former cycles, the robot can transport another tube to the rotary draw bender.

For a newly installed system, programmers will set up templates for the job mix. For end forming, this might involve details like punch stroke feed speed, the center between the punch and the clamp, or the number of revolutions for a rolling operation. Once these templates are set up, though, programming is quick and straightforward, with programmers adjusting the sequence and originally set parameters to suit the application at hand.

Such systems are also set up to be connected in an Industry 4.0 environment, with predictive maintenance tools measuring motor temperatures and other details, as well as machine monitoring (like how many parts are produced in a particular period).

On the horizon, end forming will get only more flexible. Again, the process has limitations when it comes to percentage of deformation. Still, there's nothing stopping a creative engineer from designing a unique end forming setup. In some operations, the ram punch inserts into tube ID and expands the tube against the cavity within the clamp itself. Certain tools can create end forms that extend 45 degrees, resulting in an asymmetrical shape.

The foundation for all of this comes from the capabilities of the multistation end forming machine. When the operation can be "done in one," all sorts of end forming opportunities arise.