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Don't be afraid of skeleton handling

Technology advancements are helping to eliminate this burdensome material handling task

Figure 1
Maximizing the number of similar parts you can nest on a sheet not only improves sheet utilization, it reduces the size of the skeleton that has to be destroyed by the punch. Photo courtesy of TRUMPF

Skeletons can be scary—whether hung in a corner in a dark biology lab or clinging to a tree in a suburban yard at Halloween.

Skeletons can be scary in a metal fabrication facility as well. Make no bones about it, the remnant left over after a punching or laser cutting job is a nightmare to deal with.

Skeleton handling simply doesn’t make for the most efficient production. Where automated material handling upgrades are not part of the machine tool design, the operator has to remove the parts and the skeleton manually. Separating the parts from the skeleton takes some time, but more than likely, the parts are of a size that one operator can handle them easily. Removing and disposing of the skeleton, on the other hand, is an awkward task. It’s much larger in size than any one part that came out of the nest, and the operator is charged with pulling it off the table, bending it into a more manageable size, and moving it to a scrap bin. If the skeleton is plate, the operator might be using a jib crane or require someone else to assist with the handling. No matter how you look at it, the additional time associated with wrestling skeletons can be scary, particularly when the activity is taking time away from the production of parts.

From a safety perspective, handling skeletons can be even more frightening. The edge of a thin-gauge remnant has the potential to cut an operator even if he’s wearing gloves. When an injury such as that occurs, getting a machine tool back up and running is the last thing on the operator’s mind.

Machine tool vendors have recognized this metal fabricating challenge and have been trying to assist fabricators in addressing this troublesome material handling task.

Sending It Down the Chute

Fabricators might wish they could simply close their eyes and wish skeleton handling away. It’s never going to be that simple, but TRUMPF has devised a way to make skeletons disappear in rapid fashion on its punching machines.

The machine tool development group calls it skeleton-free processing. The skeleton is destroyed as similar parts are processed, starting from the top of the sheet and working downward toward the clamps—similar to writing a letter on a sheet of paper.

For a fabricator to make the most of this arrangement, said Mike Kroll, TruPunch and TruMatic product manager, the shop has to meet a couple of parts requirements. This type of part processing is best suited to fabricating the same part a majority of the time or one job that occurs regularly. Also, the parts need to have a simple geometry, such as a rectangle, and cannot contain any forms.

This entire exercise, Kroll said, is about boosting sheet utilization (see Figure 1) and eliminating the task of skeleton handling. As a result, dynamic nesting—where the software automates the placement of parts on a nest to ensure blanks are cut as they are needed—is not a priority. Parts must be arranged so that contour machining is overlapped and in such a fashion to allow all scrap pieces or skeletons to be destroyed.

“We go in, look at the customer’s parts, and nest them, trying to get them to mate up to each other,” Kroll said. “What is going to happen is when you are nesting the same part on the sheet, it is going to give you the best sheet utilization. You are going to make them back to back, and you don’t need any web width.”

Figure 2
With a quick twist of a long, narrow part, it too can disappear down a part chute on the punch table. Photo courtesy of TRUMPF.

As the punch cuts the parts, the sheet is positioned so that the parts drop down a part chute or feed onto a conveyor. Any scrap that remains during this process also is sent away in a similar manner.Software directs a deflector flap on the parts chute to move one way to send good parts into one bin and scrap into another. When the single-station punch finishes the job, the operator doesn’t have to remove a skeleton because there isn’t one to remove.

Narrow parts that fit within the machine part chute size also can be processed the same way. In this instance, the punching head moves down on the newly cut part and uses a bilevel stripper to rotate the part 90 degrees, which positions it to fall directly into the parts chute (see Figure 2).

Kroll said this nesting approach can lead to a sheet utilization improvement between 10 and 30 percent. That’s just part of the story.

“Customers see the benefit,” he said. “They can see this increasing their productivity and having less scrap, while at the same time needing less area to store these skeletons.”

Fabricators that sell their scrap know that recyclers are more interested in receiving a bin packed tightly with small metal parts rather than a container of bent skeletons that actually contains more air than metal. In some instances, Kroll said, scrap buyers might pay 2 to 3 cents more per pound for a more densely packed container of metal.

Cutting up the Skeleton

If a fab shop has a laser cutting machine, it wants the equipment to be cutting parts, not skeletons. The cutting prowess of modern lasers helps the devices to churn out parts fast, which is why some companies invest in automated material loaders and unloaders that keep parts moving, even in the middle of the night with no one around. If the fabricator has not invested in advanced part separation capability, it has to hire someone to remove parts—either during the morning shift or right as they are being cut—from nests. It’s a necessary evil.

That doesn’t mean the operation can’t be made more efficient. Bystronic Inc. has included skeleton destruction as part of its CAM software for more than a decade.

When a programmer is setting up a job for the laser cutting machine, he or she uses a “sheet separation” option, according to Frank Arteaga, Bystronic’s head of product marketing. The laser then incorporates cutting action that not only frees the parts during cutting, but also sections of the overall skeleton.

“The program automatically puts all of the slice cuts in between the parts first in the skeleton, so when you cut into the channel to release the part, you also are breaking into the previously cut separation line,” Arteaga said. “That way there is no witness mark from the skeletal cuts being put onto the final parts.”

These skeletal cuts can occur both in the X and Y axes. Thinner materials might require cuts only in the shorter (Y) axis, while thicker materials might require cuts in both axes to accommodate ease of handling. The programmer can set the distance between cuts according to the size of the skeletal sections that the operator is most comfortable handling.

After the cutting is done, the operator can unload the parts and the skeleton quickly. In many instances, a material hopper can be found right next to the machine. The cut-up skeleton fits snugly in the hopper, and the operator can get back to loading another sheet for the next job.

The same principles of skeletal cutting apply to fiber lasers, but because of the high speed at which these machine tools cut, the fabricator almost always chooses to use some sort of automation for material handling, Arteaga said. Such an approach lends itself to uninterrupted operation, even when skeletons need to be removed.

In this type of laser cutting machine setup, vacuum suction cups and multiple trays are used. Vacuum cups are used to load the raw material from a top tray, and the same cups are used to place cut parts on a middle tray. With the parts gone, the shuttle table actually moves to a position where a double-fork system lifts the skeleton off the table, the table then moves back to the shuttle position and the skeleton is lowered onto a skid located at the floor level.

This type of processing efficiency has been the goal for many fabricators investing in new laser cutting capacity, Arteaga said.

“It’s all part of automation: the consistency factor,” he said. “If you can maintain the consistency [in all areas, such as material loading and unloading, parts separation, and skeleton handling], the process becomes more consistent. Your quoting becomes more consistent. That’s all part of it.”

See. There’s nothing to be afraid of.

About the Author
The Fabricator

Dan Davis

Editor-in-Chief

2135 Point Blvd.

Elgin, IL 60123

815-227-8281

Dan Davis is editor-in-chief of The Fabricator, the industry's most widely circulated metal fabricating magazine, and its sister publications, The Tube & Pipe Journal and The Welder. He has been with the publications since April 2002.