Software helps to scrap bad material handling habits
March 13, 2007
What's the point in purchasing an expensive laser cutting machine if you are going to waste money-making opportunities by unloading parts manually? Good nesting software, proper maintenance, and the latest unloading technology can help to make automated laser cutting and unloading a reality.
Automated parts removal helps to eliminate the non-value-added aspects of laser cutting, but a closer look at nesting is necessary to get the most efficiency out of the automated operation.
In the world of sheet metal fabrication, intelligent fabricating with a laser cutting machine can lead to increased productivity and flexibility and reduced setup and tooling costs—as well as faster design and prototype turnaround.
The laser's versatility, whether you run one or 1,000 pieces, makes it a powerful weapon in the highly competitive battle for manufacturing market share, especially when many companies are in search of labor savings and are sourcing fabricated goods from offshore facilities. Running the laser in a lean manufacturing environment where just-in-time principles and single-piece part flow help to minimize waste makes a fabricating operation a formidable competitor in the world market.
A fabricator must reduce non-value-added phases in manufacturing, limit production costs, and maximize production efficiency if it is to survive. This can be achieved by not starving the laser and by considering automation of parts removal, but the latter requires a more in-depth discussion.
Don't starve the laser. Lasers employ impressive acceleration rates and sophisticated control of laser energy to deliver quality parts quickly. The operational challenge is to balance laser processing time with loading and unloading of the system.
Heat generated during the laser cutting process can create "ribbons" that may cause head crashes or inaccurate height sensing. Nesting software can assure these thin parts rest on the grid and do not tip up.
A far too familiar sight is seeing a laser sitting idle—starving—while an employee removes parts from the unload area and stages the next sheet for cutting to resume. Every moment the laser is waiting is a missed opportunity. The cost associated with the pursuit of competitive manufacturing keeps going up.
Whenever the laser is idle, high-speed part processing is not possible. The principal advantage of the laser cutting machine is nullified. That's why the goal of modern laser cutting should be to limit production stops to spark-to-spark time—the time production is stopped after the final spark of sheet current to the first spark of sheet next.
Consider automated parts removal, but keep an eye on unloading parts. Unloading laser-cut parts poses an interesting set of challenges:
Laser cutting is a thermal operation, and as material is being processed, stress relieving can occur and long, thin strips of scrap can bow above the sheet or curl under the parts. If the scrap region between parts bows up, it can cause the laser head to "track"; an incorrect surface height from the previously processed item, instead of the true dimension of the current part.
Internal pieces of scrap can tip or rest between grids and interfere with speedy removal of the material once laser cutting is finished. Tip-ups and ribbons of scrap are commonplace in laser cutting (see Figure 1).
Perimeter notches and contours can cause binding points that may interfere with the efficient removal of parts.
Unloading one sheet of cut parts on top of another can lead to scratched parts.
Automation can help reduce starvation time by delivering raw sheets and removing the processed nests. However, unless the previously listed challenges can be solved or mitigated, the shop floor will continue to experience problems in achieving consistent, uniform stacking. If any of these problems occur, employees must unload and stack processed sheets in a timely manner, or the bottom sheets could be damaged as successive nests are placed on top of one another (see Figure 2). Of course, if manual part unloading is necessary, part identification, part sorting, and scrap disposal also must be manual operations.
How can manufacturers spend hundreds of thousands of dollars on equipment to eliminate manual processes and then saddle the equipment with manual delays? Sorting parts automatically must occur if laser cutting is to be incorporated into a lean manufacturing environment.
The laser has the advantage of being able to cut virtually any geometry. However, after laser cutting is completed, interior and exterior scrap remains on the worktable along with the finished parts. In essence, what you see is what you get.
A lead-in cut is sized to the width of nearby slots so that it can aid in the destruction of interior scrap.
Compare laser manufacturing to punching and notice how interior scrap is managed. By definition, the punching process eliminates a portion or all of the interior scrap. Once a punching machine finishes with the interior holes on a nest of parts, the equipment operator will be left with a sheet that is manually sheared or tabbed portions that require a manual "shake and break"; sequence. Punch/shear combination machines were developed to eliminate many, if not all, manual operations in the punching process.
The greatest hurdle in eliminating manual part separation in laser processing is managing both the interior (see Figure 3) and the exterior scrap. No other single concern in automating laser cutting comes close to the challenge posed by developing an efficient means to separate scrap from finished parts.
Software developments to assist the sorting devices can lead to a dramatic advantage and may result in more consistent automated part sorting.
Just as in a carefully orchestrated chess match, each calculated maneuver must have a detailed and accurate reply—not only to anticipate subsequent movements, but also to plan for the final stages of the match. Fabrication is no different.
It is imperative to keep in mind that many of the challenges associated with material handling after laser cutting may not occur on each nest. Software thus needs to be able to anticipate certain problems, respond effectively to each occurrence, and be flexible enough to address different scenarios.
Take, for instance, free part nesting. Free part nesting refers to the addition of perimeter cuts that either create a final product that may look like a "box"; or help to reduce points or notches within the bordering scrap regions. Sorting software should not impede the nesting ability to rotate parts and optimize sheet utilization.
If an unloading device is capable of rotating the part liberated from the sheet and stacking the piece in routine fashion, it helps to create optimal nesting with optimal unloading. Couple that feature with dual-head unloaders, and fabricators can have two pickup cycles with one pass, effectively optimizing unloading cycles and increasing the odds of balancing laser cutting with unloading throughput.
Dual-head unloading devices also lend an advantage with large parts. The two heads work together to pick the large pieces and remove them consistently.
A cluster cut nests similar parts together in one area of the sheet.
In the case of very small parts being cut, a group or cluster of parts can be produced. The small parts are tabbed into a blank and unloaded in one cycle, to speed up manual separation later. Small-part clusters also may be paired with a larger part or placed in a larger scrap window to help optimize sheet utilization (see Figure 4).
Nesting pairs are often parts with numerous holes or parts just too narrow to unload by themselves. The parts may have "partner"; pieces in the material and thickness specs that allow the parts to be tabbed together and unloaded as one. Software can index the part count correctly for real-time invoice update quantities.
Software can help determine where to place parts to avoid tipped parts or parts that fall between the grids.
Common-line cutting is an obvious advantage when sorting parts off a laser cutting machine. Minimal scrap is inherent in this process. Critical factors when dealing with common-line cutting are the sequencing of the cuts in the common line shared by parts and the software's ability to implement or abandon the common-line approach as the application requires.
Finally, when permissible, total skeletal destruction is possible, which leaves the unload table bare of scrap and full of laser-cut parts. In these cases, the software intelligence to destroy the interior scrap and exterior skeleton truly positions automated part unloading as a must-have tool in any lean laser cutting operation.
In the end, what is the best scenario for laser cutting and automated sorting? A fabricator cuts blanked-to-size parts and relies nesting software that promotes common-line cutting. This is not reality, however. It is for that reason that such sophisticated software must evolve to make laser-cut part sorting automatic and a practical reality.