Choosing the right CNC sheet goods cutting process depends on many factors.

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Sheet goods—including metals, plastics or even wood, cloth and paper products—are ubiquitous materials in manufacturing, and are used to produce a wide variety of parts. In mass production, these parts form the inputs for more complex processes that shape blanks into three-dimensional forms. In the automotive industry, sheet and plate steel are stamped into blanks using presses and custom-made tooling. Creating the tool steel dies for these processes requires a large initial investment, as well as costly downtime for tool changes.

However, there are other processes for cutting sheet and plate materials. CNC processes offer a significant advantage over machines which require tooling, because they can cut any part described by the program—instead of only the part described by the shape of the die. For cutting sheet and plate materials, CNC laser, plasma and waterjet cutting are popular options. Wire EDM and gas cutting are also available, though these two options cover opposite ends of the speed-accuracy spectrum. Wire EDM is a very slow but accurate process, while oxy-fuel gas cutting is used to cut thick metals with relatively low accuracy.

Laser and plasma can operate at either end of the range. That’s why it is a common question to compare both processes based on cost, accuracy and speed, to determine the best choice for a given project.

CNC Laser and Plasma Cutting—Comparison Highlights

The first difference to consider when comparing plasma and laser cutting is which materials can be cut. Plasma cutting can only be used on conductive materials, while laser processes can cut a wider range of materials, including wood, plastics and textiles. However, because laser cutting machines use a focused beam of light to burn through the material, it is less effective than alternatives—and sometimes is not effective at all on highly reflective materials such as copper. In addition, most laser machines are not powerful enough to cut materials thicker than three-quarters of an inch, though this is changing as the technology develops. Plasma machines can cut metal plate up to an inch and a half in thickness.

Secondly, the accuracy of the cut differs between the two processes. Laser cutting results in a small kerf with high accuracy. Parts are dimensionally very close to drawing specifications. By comparison, the kerf left by a plasma torch is wider, and leaves more spatter, dross and slag on the part which frequently requires post-processing for cleanup. Another accuracy consideration is the heat deformation that can be caused by the processes. Laser cutting causes relatively little heat deformation, mainly in thin materials. Plasma can cause significant deformation due to the intense heat, similar to welding processes.

How Does Laser Cutting Work?

In the process of laser cutting, an intense beam of aligned photons at a specific wavelength is fired at the workpiece, along with a jet of high-pressure gas such as nitrogen. The intense beam melts, vaporizes and ablates the material, while the jet shields and blows the swarf out of the cut. The laser is directed by G-code guiding the cutting head along the x- and y-axes of the workpiece, which is positioned horizontally on the machine bed.

CO₂ vs Fiber Laser Comparison

There are two primary laser cutting technologies. The first is CO₂ laser, with over 30 years in service. CO₂ laser machines are a well-understood and well-characterized technology, and early adopters of this historically expensive technology enjoyed a cost advantage over competitors, especially in high-mix production. While powerful, CO₂ lasers use more electricity to run than fiber lasers—as much as 70 percent more. They also require more consumables during operation in the form of beam path gases, and require more maintenance.

Fiber laser systems are solid-state and run essentially maintenance-free. Cut performance is a significant advantage. Plasma cutters have an average kerf width of 0.150” and work within a margin of error of +/-0.020”, while laser CNC tables have an average kerf width of 0.025” and operate within an error margin of +/-0.005”.

CO₂ Laser Advantages

• CO₂ lasers provide better edge finish on plate aluminum and steel than fiber (less dross and better perpendicularity).
• CO₂ lasers are more flexible, able to effectively cut more and thicker materials than fiber lasers.
• Lower upfront machine cost than fiber lasers.

CO₂ Laser Disadvantages

• Slower cutting time than fiber lasers.
• Complex beam path system requires shielding gas and maintenance.
• Higher operating costs, higher cost per part (in comparable materials).

Fiber Laser Advantages

• Newer technology.
• Machine affordability, support and functionality are still growing.
• Lower maintenance.
• Faster cutting speed, especially in thin materials. For example, a 4KW CO₂ in 16 GA mild steel using N₂ as a cutting gas has a recommended cutting speed of just 260 IPM, whereas an equally equipped fiber laser has a cutting speed of approximately 1,417 IPM.
• Lower operating costs, considering consumables, electricity, maintenance and throughput.
• Smaller focal diameter results in higher accuracy and intensity.

Fiber Laser Disadvantages

• Compared to CO₂, the edge finish in plate steel and aluminum is of lower quality.
• Fiber lasers are not suited to as wide a range of materials as CO₂, especially in non-metals.
• Higher upfront machine cost compared to CO₂.

There are many variables involved in planning and running a cost-effective laser cutting strategy with either technology. Capital costs are also considerable. For manufacturing professionals who need parts quickly, contract manufacturing services such as Xometry are available to manufacture parts on order, using nearly any manufacturing process, including waterjet, plasma and laser cutting services. Xometry will also consult with customers to recommend the best process based on each drawing.

How Does Plasma Cutting Work?

Plasma cutting can be thought of as a cousin to welding technology. In plasma cutting, an electrical arc is struck between a cathode and anode, between the nozzle and the workpiece. Superheated ionized gas (plasma) flows in a directed jet, melting the material to make the cut. The temperature of the plasma jet can reach 20,000°C. Plasma cutting machines clear the cut using compressed air or an inert shielding gas such as argon or nitrogen.

Advantages of Plasma Cutting vs Laser

• Lower cost per part vs. laser systems.
• Able to cut thicker plate stock, up to 1.5” (in general).
• Able to cut highly reflective metals such as copper, which reflect a laser thus rendering it less effective.
• Fastest cutting speed, compared to laser and other options such as waterjet.

Disadvantages of Plasma Cutting vs Laser

• Limited to electrically conductive materials (metals).
• Larger kerf, meaning less accuracy and less geometry capability.
• Does not offer the engraving or part marking functionality that laser can accommodate.
• Dross, spatter and heat distortion can reduce part quality or necessitate additional steps.
• Perpendicularity of the cut edge is not as good as laser, with a beveled edge on plate materials.
• More consumables (shielding gases) may be required.

CNC Table Cutting—Do it In-House or Job it Out?

In any manufacturing operation, whether to subcontract a process or invest in the specialized equipment is a key consideration. Specialized processes may not be sensible to bring in-house. For example, heat treating is a very commonly subcontracted job in machine shops that focus on milling, grinding and turning. Electronics or harness assembly is often subcontracted to shops that specialize in that area. CNC cutting, including laser and plasma, is also a process that makes a good candidate for subcontracting or ordering from a digital manufacturing marketplace such as Xometry.

A critical consideration for manufacturers is the relative value added to work in progress at each stage of the manufacturing process. It is common that the majority of the added value happens later in the manufacturing process, during machining, forming, assembling and packaging. Capital cost allocation into those areas often delivers a higher overall process return on investment. And from a process standpoint, the capability of laser or plasma cutting equipment scales with the surface area of the machine bed. Production floor space may be a premium, and in-house cutting capability may come at the cost of valuable production floor space. Ordering parts pre-cut can speed, simplify and standardize manufacturing of a product.

Perhaps the most important component of the decision whether to bring front-end processes in-house or contract for them is the investment in knowledge. The first steps in many manufacturing processes are size reduction of sheet stock, which may be as simple as cutting material from a reel, or as complex as fineblanking. To cost-optimize these processes requires expertise which must be developed with experience. With increasing pressure to compress time-to-market, many manufacturers can better allocate resources downstream. Frequently, significant cost savings can be realized with minor changes to part design to ease the cutting process.

Service bureaus such as Xometry are equipped with manufacturing experts that consult with customers to improve design for manufacturability and lower cost, such as replacing an inside corner with a radius, adding a draft angle or adjusting a hole size. These tweaks, in addition to an expert recommendation on the best process for a part—including laser vs. plasma—can make a big difference in the quality and performance of the parts, and the manufacturing process as a whole.

Manufacturing is about strategy and tactics. Saving financial and human resources so that they can be deployed for maximum effect is always a route to successful business. Taking the first steps of the part-making process off the shop floor and ordering from a digital manufacturing marketplace such as Xometry can add more value and profitability to many industries, including high-mix, low-volume part-making.

To learn more about Xometry, visit their website.