Common line cutting

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by Sylvain Robidoux

Implement common line cutting/nesting to improve fabricating efficiencies

Manufacturers are scrutinizing all aspects of the business to cut waste and maximize productivity–and the shop floor is taking centre stage. Implementing common line nesting/cutting accelerates cut time, while reducing scrap, gas, and, in the case of waterjet, abrasive consumption. In some instances it can even help extend tool life by reducing the number of pierce points.

Parts with a consistent straight line present a great opportunity for common line cutting. If one cut nothing but squares, rectangles, and triangles, there would be few reasons not to use this approach. Since we must often cut irregularly-shaped parts here are some ways to determine if and when to use common line cutting.

Thinner skeletons mean less scrap–a clear cost-saving benefit of common line cutting. But thin skeletons may lack the strength to fully support parts once cutting has begun. This instability can adversely affect part quality. Microtabs may be added to stabilize parts during cutting and there must be enough space around the parts for such tabs.

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. 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 considerations
Most laser cut parts require space for lead-ins and optional lead-outs. The lead-in allows the laser to pierce and merge onto the cut line. 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.

Common-line cutting can be especially beneficial when laser cutting thick metal. Thick metal is expensive so reducing scrap delivers significant savings. And because thick plate takes longer to cut, reducing the number of cuts can speed cutting time significantly. Over time these savings in material and time can be significant. On the flip side, in instances where tight tolerances and quality are absolutely critical (such as medical devices or aerospace for example), common line cutting might not be the best option.

Common line punching
In punching, skeleton integrity is absolutely critical, especially when parts unload through the machine’s chute. The sheet moves between punch strokes, and the sheet’s centre of mass changes dynamically with every hit. As more parts exit, the remaining skeleton weakens. If a skeleton becomes unstable, the part, tool, or both can be damaged. 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.

Software is key
Today’s advanced nesting software assists programmers in choosing which parts are best for common line cutting, weighing the options, balancing the need to minimize scrap while also reducing the inherent risks with process instability. Dynamic nesting allows programmers to place a number of dissimilar parts in one nest. This allows manufacturers to take full advantage of the material type and thicknesses being cut by combining work orders, again saving time and money.

How to begin?
Fabricators should investigate common line cutting so as not to leave money on the table or in the scrap bin. There are a number of factors to consider and part quality must always be weighed against savings. A reputable fabrication automation solutions provider can help you to determine if common line cutting–and other advanced cutting techniques–are the best option and demonstrate how today’s sophisticated nesting software can improve the process. SMT

Sylvain Robidoux is an engineer with SigmaTEK Canada, LLC.


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