Common-line cutting obviously saves material, but it’s not always the best choice
October 12, 2012
All things being equal, it stands to reason that a single separating cut is always preferable where possible. But all things are not equal, and there are times when common-line cutting is not the best option.
Anyone who has ever cut anything—metal, wood, even paper dolls—would agree that a single cut along a shared edge between neighboring parts is fast and efficient. A common-line cut reduces cut time, scrap, gas, and, in the case of waterjet, abrasive consumption. In some instances it can help extend tool life. The tightest nest also makes part breakout a literal snap, which can be especially beneficial when cutting thick stock. Fabricating 0.75-in.-thick plate, workers often literally must hammer parts out of the skeleton.
All things being equal, it stands to reason that a single separating cut is always preferable where possible. But all things are not equal, and there are times when common-line cutting is not the best option. In some instances, it’s a downright bad idea. To understand why, fabricators need to know the physical factors that influence the common-line cut.
Skeletons can lack the strength to fully support parts once the cutting process has begun. This instability can adversely affect part quality. As a common workaround, programmers add microtabs to hold parts during the cutting process, and there must be enough space around the part for those tabs.
Unexpected machine shutdowns and restarts also present challenges for common-line cutting. A successful restart hinges heavily on how sophisticated the machine’s CNC is, and on operator experience. In the best-case scenario, a restart will add time to a job; in the worst-case scenario, the job will have to be scrapped.
If a common-line cut compromises edge quality or tolerance requirements, it probably isn’t a good idea. For instance, some thermal cutting processes generate enough heat to cause an expansion effect that shifts the part, creating opportunity for errors. This warping effect becomes especially prevalent when cutting long, narrow parts.
For working with large, simple parts, such as metal for a ship hull, shared-edge cutting can be a great option. But when jobs require precision edges, common-line cutting may not be the best option. In fact, some sectors have such high part quality standards, common-line cutting is not worth the risk.
Laser cutting can be well-suited for common-line cutting. Still, even narrow, focused laser beams can cause problems as the initial burst of energy required to create a pierce point can easily cut a hole that intrudes beyond the allotted kerf.
While it is true that laser pierce points may be reduced with fewer cuts, this is not always the case and should never be assumed. Programmer experience and toolpath sequence will have a direct bearing on the number of pierce points required for a given job.
Most laser cut parts require space for lead-ins and optional lead-outs. The lead-in allows the laser to pierce and enter the cut line like a car merging onto a freeway. With a lead-out, the beam exits into the skeleton web and moves on to the next part. The programmer must leave ample space around parts so that the laser can perform these operations.
The amount of space around parts depends on factors like material type and thickness, the cutting process, and the shape of the parts. Parts nested so that there is a consistent, straight line is a great opportunity for common-line cutting. The laser moves uninterrupted from one part to the next. Indeed, if the world were made up of parts with nothing but straight lines—squares, rectangles, and triangles—there would be few reasons not to use the common-line approach.
Of course, the world isn’t like this, which means the programmer must ensure he leaves enough space between parts for those lead-ins and lead-outs—or, he may also consider using a soft pierce. Here, the laser makes a series of pulsations rather than a sudden burst, and this sometimes allows the machine to initiate the cutting path on an actual cut line, eliminating the need for a lead-in. Soft piercing does add a little more time to the job, but it is less likely to damage parts sharing an edge.
From a numbers perspective, common-line cutting can be especially beneficial when laser cutting thick metal. Thick metal is expensive, so reducing scrap can lead to significant savings. Even more beneficial, thick plate takes longer to cut than thin material, so reducing the number of cuts can reduce cutting time significantly and increase throughput. Over the course of a day, week, or month, these savings in material and time can be staggering.
In a punched nest, skeleton integrity is absolutely critical, especially when parts unload through the machine’s chute. The sheet moves between punch strokes, and the sheet’s center of mass changes dynamically with every hit. As more parts exit, the remaining skeleton weakens. If a skeleton becomes too flimsy, a punch can jostle it to the point where the operation damages the part, the tool, or both. Tightly nesting parts can leave a weak, narrow-web skeleton at the end of a job, and this can present problems. Using a common-line cut between some parts can make an already weak skeleton even weaker.
In this situation, sequencing is important. Parts punched early in the sequence may be able to share a common line. But as the punching operation reaches the end of the program, parts must have enough space to maintain the skeleton’s integrity, to ensure the machine can punch good parts until the very end of the nest.
In many instances, a programmer may use a common-line cut with microtabs. In general, microtabbing adds process stability. These tabbed parts stay in the nest until the end of the punching operation, at which point they are snapped out and sent to downstream processes.
This strategy also can require fewer tool changeovers. When the operation sends parts down the chute, it must complete one or a group of parts, so it must punch all the parts’ various geometries in sequences, which can require frequent tool changes. This can slow a punching operation considerably. But if parts are nested with microtabs and stay in place throughout the operation, the program need not complete one part after another. Instead, the punch program can run each tool as much as it can before switching to the next tool. Depending on the nest and cut geometries required, this strategy may help speed the punching cycle.
Some programmers may choose another iteration of this tabbing strategy. They may decide to leave small parts tabbed together in so-called “mini-nests” that can make downstream operations easier. For instance, a press brake operator may not be able to hold an extremely small part steady against the backgauge without getting his hands dangerously close to the tooling. So instead, he holds a mini-nest of parts and bends them as a group, allowing him to keep his hands a safe distance from the bend line and form the parts much more efficiently. After he’s finished, he simply snaps the mini-nest apart.
Sometimes a programmer will use a combination of strategies—tabbing some parts, and then sending these mini-nested parts down the chute. These tabbed part groups remain attached to the skeleton a bit longer into the punching operation before they are sent down the chute, and this effectively increases the skeleton’s integrity. In some cases, this also allows more parts on the nest to share a common line. Still, there’s a trade-off: As the punch nears the end of the program, the skeleton web must be wide enough so it doesn’t wobble out of position. With this approach, material utilization may not be as high as with a nest that tabs all parts to the skeleton, but it also doesn’t require people to shake parts out of the sheet.
Also note that tabbing parts to the skeleton sometimes isn’t always practical, especially when the operation involves automated part removal and stacking. In these cases, suction cups lift parts out of nests and stack them on an offload table. The suction cups can lift parts tabbed together, but they obviously will have a problem if they try to lift parts held with microtabs to the skeleton itself.
Part orientations can present another hurdle. In a turret punch, not every tool can go into an auto-indexing station that can rotate, and this historically has made nesting less flexible. Certain parts had to be oriented a certain way for specific tools. But today advanced software can determine the most effective part orientation and identify common-line cutting opportunities, taking punch orientation into account.
Consider two identical parts. To save material, they’re nested so they share a common cut edge—but this also requires that one part be oriented differently. Traditionally, this would have been a serious roadblock for a programmer. Now software can analyze this and determine which combination of tools can cut each part. The two parts may be identical, but because they are oriented differently on the nest, they may be punched with entirely different tools, and software can help the programmer determine the best approach almost instantaneously.
When it comes to common-line cutting, there are no simple answers. With its big paycheck potential, shared-edge cutting should always at least be considered. Every company should conduct in-house studies to determine its own guidelines and best practices, so as not to leave money on the table—or excess scrap in the bin.
For the programmer, advanced cutting machines and software have opened the door to a lot of nesting options. Software now can help programmers choose which parts are good common-line-cutting candidates. The nesting program helps them weigh the options, balancing the need to minimize scrap while also reducing the inherent risks with process instability.
With dynamic nesting, programmers can place a number of dissimilar parts on one nest. This allows them to mix and match orders to take full advantage of the material and thicknesses being cut. Programmers often use common-line cutting with dynamic nesting. They balance maximum material utilization with the need to not nest too far ahead on the schedule, to ensure efficient part flow and minimize work-in-process (WIP) downstream. Maximizing material usage is great, but not at the expense of excess WIP.
All these considerations add complexity to the nesting equation. It’s not just about getting the absolute maximum out of the material; it’s also about reducing overall manufacturing time. The best nesting practices take both of these factors into account.