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Perfecting kit-based part flow in metal fabrication

How a panel bender and press brake work together in kit-based part flow

Figure 1
In this kit-based part flow arrangement, parts with etched QR codes are offloaded from the laser and conveyed automatically to forming, passing under a scanner that reads the label and calls up the bending program, either on the press brake or on the panel bender.

Managing a modern sheet metal fabrication operation isn’t easy. You manage thousands of part numbers. If you work at an OEM, it’s highly likely that you need to manage kit-based part flow, cutting and bending one unique piece after another. If you’re at a job shop or contract fabricator, you may also want to do the same, simply because it helps all parts arrive simultaneously (or close to it) at the final manufacturing step, no massive work-in-process inventory required.

Laser cutting or punching one part after another isn’t an arduous affair. Modern software can dynamically nest quite efficiently, with little or no human intervention. Indeed, the soft and flexible tools of modern cutting processes (that is, the laser or punch tools) are tailor-made for a high product mix.

But what about bending? Forming remains one of the most complex processes in fabrication, and new approaches to flow (see Figure 1) are changing how people think about efficiency and productivity through the entire plant.

Forming Kits

If you’re manually setting up punches and dies, it’s impractical to run one different part after another. Even if it took, say, only 10 minutes to switch between one part and another, that’s still too long to facilitate true kit-based part flow.

Here the panel bender has filled a need (see Figure 2). With universal tooling, panel benders have no setup time. A part manipulator slides the flat piece in place, the blank holders clamp the material, and the bending blades fold the flange up or down, even bump-bend complex shapes. The blank holder tool segments move automatically to accommodate different lengths of bend in a sequence.

Panel benders have advanced to the point where they can take on a greater variety of part geometries. A panel bender with a deeper throat (that is, the space behind the blank holder tools) allows for taller flanges. Some of the latest panel benders have auxiliary bending blades designed to handle hidden bends and tabs.

Most significant, panel benders with automated workpiece manipulation and bending blades that move up and down can usually form a part much faster (and more safely) than an operator can manipulating a part through a bend sequence on a press brake.

But panel benders have their inherent limitations. For instance, its blank holder tools can be only so narrow, and a manipulator needs to be able to hold the part. This limits how small parts can be.

A decade ago most panel benders could handle parts down to 8 by 20 in. Now they can handle parts as small as 5 by 10 in. (though they often can produce even narrower parts if the bender has an integrated shear; in this case, after the bending blades form the flanges, the shear blade rises to cut off the narrow part from the larger sheet). Moreover, to position workpieces for bending, the panel bender also needs the part to have a flat surface.

So it’s usually unavoidable: Some parts need to go to a press brake. Even large automated fabrication centers—those that shear, cut blanks, then convey those blanks to the panel bending station—have an offload table after cutting where operators can remove blanks that need to be formed on a press brake.

Figure 2
A panel bender’s blank holders secure the workpiece in place, and bending blades fold in the positive (top) and negative direction.

A press brake is a universal bending tool. It can make nearly any bend in any material at any time. That’s its beauty. But the brake’s downside is, of course, its setup time. Setup times on conventional press brakes have plummeted in recent years, thanks to advances in software, backgauges, and quick-change tooling. Regardless, you still need to change out tools between jobs.

But now we have press brakes that can change tools automatically, which has reduced changeover times to minutes and, in some cases, seconds. These brakes rearrange and replace tools using various mechanized manipulators. One version has manipulators that quickly move top punches while the bottom V-die opening widens or narrows as needed, changing over from one job to the next in less than 10 seconds (see Figure 3).

Moreover, both press brakes and panel benders now have automatic angle correction, using lasers or other means to account for changes in material thickness and hardness between heats. The correction systems also account for differences in grain direction, a real benefit if you want to take advantage of dynamic nesting in laser cutting and punching.

So now the changeover times on certain press brakes are nearly as fast as the changeover times on a panel bender. Both panel benders and certain press brakes with automatic tool change can perform changeovers faster than parts can be moved in and out of the forming station. In effect, both now have a setup time of zero, and this opens the door for efficient kit-based part flow of almost any fabricated product. The operator can form one different part after another and have no noticeable setup time in between (see Figure 4).

Thinking Anew

This kind of arrangement is a reality now. All the same, if you were to look at such an arrangement in action, you might need to fight the urge to think about old notions of manufacturing efficiency.

Consider an operator who mans a cell that has both a press brake with automatic tool changing and a panel bender. He accepts blanks delivered via a conveyor or other means. Each blank has a bar code or QR label on it, which the operator scans in to call up the program. This tells him which machine to form the part on, the panel bender or press brake, or a combination of both machines. In this Industry 4.0 scenario, the machines are in constant communication with systemwide software. And before the job was released to the floor, engineers have already determined how parts should be cut and bent—again, either using a brake, a panel bender, or both.

By the time the operator delivers the part to the bending machine, the program has been downloaded and the tooling arranged. The machine forms the part, uses automatic angle correction as needed, then completes the job as the operator sends the formed part downstream and retrieves the next, entirely different part.

In one scenario, a formed part could be put on a conveyor to be offloaded by a material handler, which places the formed parts on one of a series of carts, each designed to carry a kit destined for joining and assembly. As soon as a cart has every piece in the kit, it’s immediately moved to welding.

Thinking traditionally about manufacturing efficiency, you might see several red flags. For one thing, parts to the forming cell aren’t carefully sequenced but appear to arrive randomly. Several parts may be formed by the panel bender while the press brake sits idle, or vice versa.

Under traditional thinking, managers might view this as poor utilization of equipment. To really make use of both machines, why not alternate between brake and panel bending parts? The panel bender manipulates a part automatically while the operator manipulates the next part through the bending sequence on the adjacent press brake. Voila, you’ve doubled your throughput, right?

Figure 3
This press brake with automatic tool change has an adjustable-width V die and manipulators that rearrange punches between jobs.

Yes, if you looked at the forming cell as an island. But what about the time and resources required to arrange the parts so that they arrive in the forming cell in the right sequence? To avoid starving the forming cell, you may have to hire more people to break parts out of a nest and arrange them just so. And besides, the act of carefully sequencing parts that flow into the cell is rife with human error. You’d be spending more time and resources just to achieve a little more throughput in bending. Put another way, you’d move the constraint, and increase the chaos, to just before the forming station.

This explains why part labeling, achieved automatically (laser etching a QR code, for instance) or manually, is so important. It allows all the parts from a nest of multiple jobs to be sent in any order to the forming cell.

Still, if you’re thinking traditionally, you might see part labeling as another red flag. Automatic labeling isn’t too time-consuming; it takes only about 3 seconds for a laser to apply a QR code to each part, though that time can add up, depending on how many pieces are on a nest. But if you have no choice but to apply labels manually (if, say, a QR code wouldn’t work for cosmetic reasons), that act still slows throughput downstream, right?

Yes, but again, only if you look at the cutting operation in isolation. The cutting cycle time is longer if you label every part. But the overall cycle time—from loading the raw sheet through cutting to when the first bend is made at the forming station—is much, much shorter.

How? Three factors come into play. First, having parts already labeled as they are offloaded from the cutting machine (or applying labels manually immediately thereafter) eliminates the chaos in part sorting. Second, the downstream forming station can read that code and start forming immediately. Third, and most critical, the forming operation can bend different parts in the order they’re received, and because setup time is essentially zero, it doesn’t matter in what order those parts arrive.

People operating the cutting machine need not concern themselves with the sequence of parts that are offloaded. They simply can nest for maximum material utilization. Parts arrive in random order, and the operator scans them into the forming system and starts forming immediately.

Two Key Components: Dynamic Nesting and Labeling

In a traditional batch-style flow system, both dynamic nesting and comprehensive part labeling (that is, every piece is labeled) can in some cases cause more trouble than they’re worth. If a fabricator dynamically nests a sheet for maximum material utilization, the arrangement sometimes creates chaos for those sorting parts, particularly if parts emerge without labels. A material handler may mistake one part for another and put the blank in the wrong job routing.

If that person needs to apply a bar code to parts, he may stick it on the wrong part. And if the part is made of pickled and oiled material, those labels are bound to fall off sooner or later.

Depending on your situation when batching, it may well be easier to limit the variety of parts on any one nest. You’d lose a little in material utilization, but you’d gain efficiency from simpler sorting. And if you’re batching like parts together, it may not be worth labeling every blank.

The very reason you batch is to minimize the number of changeovers you have on downstream equipment. But now that downstream forming equipment has virtually no changeover time, why run batches at all?

Figure 4
In this setup, one operator operates both a press brake and a panel bender. Scanning in the part label automatically loads the part program, either on the brake or panel bender. All the disparate parts that go into a kit are then placed on a pallet, ready to be moved downstream.

If you’re running kits instead of batches, you need to nest dynamically so that you can get as many parts in one kit (assembly) cut as quickly as possible. The only reason that parts in one kit would appear on multiple nests is if some pieces required different material grades or thicknesses. And in this case, the laser (itself with automated nozzle changeout and other automation) would run the different materials sequentially.

From here parts are lifted out of the sheet and the skeleton is disposed of. Here, some cutting systems can remove parts automatically from the nest and stack them. It does prolong the cycle time, but it eliminates the need for a person to shake out parts manually. Whether this makes sense depends on the resources you have and how long it takes to cut a nest. You may not want your extraordinarily fast fiber laser just sitting there waiting for the automation to remove parts and deliver the next sheet. In fact, many choose to separate parts manually simply because it often keeps the laser running a higher percentage of the time.

Once offloaded, parts are then sorted, and this brings us back to part labeling. It can be a challenge, especially if it’s not practical to etch a part identifier automatically during the cutting process. But in a world in which both cutting and forming equipment effectively have no changeover time, part labeling has become a challenge worth overcoming. In fact, in the chaotic world of high-product-mix metal fabrication, the entire methodology behind kit-based part flow really falls apart without accurate and reliable part identification.

Design-for-manufacturability efforts may allow the laser to etch a QR code in a noncosmetic location. If you have P&O material that for whatever reason can’t have a label etched onto its surface, you may be able to use a removable label attached mechanically to a part feature. Whatever the solution, if you get part labeling right, you eliminate the chaos downstream.

Couldn’t a shop use simple move tickets with QR codes, perhaps attached to a kit of parts coming off the skeleton? This is theoretically possible, but it adds to the havoc in part sorting and more than likely would starve the forming cell of parts. And between two critical fabrication processes, cutting and forming, is the last place you want your chaos to be, particularly when your forming cell has zero setup time.

Single-piece Fabrication

Put dynamic nesting and smart part labeling together, then add equipment with no changeover time, and you make kit-based, single-piece part flow possible in the fab shop. Such flow cuts down overall manufacturing time dramatically—and that overall manufacturing time is what governs your ability to respond quickly to customer demand. It also allows you to ship more products in less time, which, if you’re not spending significant money on additional resources, increases profits.

It’s not just about a machine’s uptime or strokes or inches per minute, though those are important pieces of the puzzle. When you get right down to it, shipping more in less time is what really makes the difference.