Laser cutting is a hot cutting manufacturing and fabricating process

The Laser Cutting Machine

Laser light may be used to cut and score a wide variety of materials. With a laser cutting machine like this, paper and plastic can easily be scored and cut very precisely. Even plywood up to 1/4" thick can be cut on this small machine. Larger industrial machines can cut metal as well. Typically the plotting rate and power of the laser are modulated to select various levels of scoring and cutting.

This particular machine is used primarily to create architectural models out of plastic. It has a cutting area of 3 feet by 2 feet, but larger machines have cutting areas of 8 feet by 4 feet. The major components are the gas tanks, the laser plotter itself, and the controller.

These tanks provide nitrogen gas that is used to limit the burn rate when flamable materials like paper are being cut. A program on this PC reads a user's AutoCad DXF file from a floppy disk and controls the laser cutter. Profiles for the current material being cut are set by the operator here. The laser plotter is a mechanical 2D positioning mechanism driven by motors. The laser is mounted in the back. Small mirrors are used to guide light from the laser to a particular location on the material. The base of the cutting area is made from a honeycomb material to let the laser light pass through. This has to be replaced periodically.

Scoring and multiple cutting layers are indicated by labeling the layers 1S, 2C and 3C in the input DXF file. The lines to be scored are given by the layer labeled 1S. All scoring is done before cutting is started. The width of the cuts and scores is approximately 0.009" on this machine.

Any number of cutting layers are given by layers 2C, 3C, etc. These are cut in numerical order. Multiple cut passes are needed for parts that have holes in them. First the holes are cut, then the shape of the piece is cut. This avoids problems that may occur due to parts shifting after being cut. A sample DXF file is given here for your perusal.

I've completed a three projects so far:

• A simple folded shape - laser scoring mylar and paper
• A 3D surface function - cutting and scoring cardboard
• A short production run - making a kit for distribution

This laser cutting work was performed at Terziev Studios in San Francisco, California. Many thanks to Sergé, Ian, and Zdravko for their help.

Continue to learn more about laser cutting, please visit SGI.

Laser cutting services

As part of our continuing inward investment program, we added a Trumpf CNC laser cutting machine to our inventory. The machine is capable of handling our own production with ease, and in early 2005 the decision was taken to offer a sub-contract laser cutting service.

The machine is one of the new generation of solid-state lasers from Trumpf. Unlike the older generation of CO2 gas lasers, which required high voltages, complex gas sytems and high-speed pumps, the solid-state Nd:YAG laser in our machine has no moving parts. It requires no complex gas-mixes and delivers its beam via a fibre-optic cable. The nett result of this modern approach is increased reliability, reduced downtime and better cuts. The benefits are significant, we get from lower running costs, you benefit from assured delivery.

The laser is capable of processing mild steel sheet from 0.5 to 10mm, Stainless from 0.5 to 5mm and a variety of other thin sheet specialist steels. Efficient and modern nesting software allows us to rapidly quote from either Autocad .dwg or dxf files. If required we can also work from paper drawings.

Process Capability - Materials:We have available, mostly same day, a wide range of quality steels. We have taken considerable time and care in establishing our supply chain and working closely with our suppliers to ensure prompt and efficient delivery of first class raw material. Efficiency in our supply chain helps us maintain minimum lead times and efficiently process orders.

Hot Rolled, dry (1.6mm to 6.0mm): BS 1449: Section 1.2:1991, HR1, HR2, HR3, HR4, HR14, HR15, BS EN 10111:1998, DD11, DD13, BS, EN 10025: 1993, S275, S355J2 G4.

Hot Rolled, Pickled & Oiled (1.6mm to 6.0mm): BS 1449: Section 1.2:1991, HRP15, HRP14, HRP4, HR3, HRP2, HRP1.

Cold Reduced (0.7mm to 3.0mm): BS 1EN 10130: 1998, DC01, DC03, DC04, BS EN 10130: 1991, Fe P01, Fe P03, Fe PO4, BS 1449: PART 1: 1983, CR4, CR1.

Plasma Cutting machines. We now have a machine that is faster, more accurate and better built than our previous range. Using the industry-standard 'Burny' control systems and state-of-the-art mass production methods, we now have a product that exceeds our expectations, is better finished and with much shorter delivery cycles than we could previously offer. You get a top quality product and we can now supply the demand without working 24x7 to fulfill the orders. We've taken the step from making 'one-offs' to 'production', and we're certain you'll appreciate the build quality and the performance, yet still at a very competetive price.

Continue to learn more about laser cutting, please visit Pololu.

Estimating your cutting costs

The recent recession has created a new standard for many fabricators: When capacity exceeds work, the reality is that cost does matter—more than ever before.

Many fabricators bid on jobs simply to fulfill short-term needs: to keep personnel employed and machinery running so they can pay the fixed costs associated with owning the equipment. Often such a job is taken at a loss.

Fabricators generally are still in an overcapacity situation today, working in a price-sensitive market. After months of performing low-margin work, they realize that it's time to analyze what type of equipment offers the lowest cost of ownership and highest potential for profitability in today's competitive environment.

While laser cutting technology grabs the headlines, plasma cutting technology has progressed, making its own mark. Every year approximately the same numbers of laser and plasma cutting machines are sold in North America. Also, many are purchased for the same or similar applications—creating flat parts out of mild steel sheet metal and light plate in low to medium operations.

A model was created to compare the total cost of ownership for laser cutting and plasma cutting equipment. A variant of plasma cutting also was evaluated—the punch-plasma combination. Punch-plasma machines commonly are used for the same applications as traditional plasma or laser cutting machine tables. Punch-plasma machines use plasma cutting to contour the external geometry of the part and a punching cylinder with tooling to create internal features.

To compare the economics of these machines, costs were divided into three categories:

1. Labor: costs associated with running the machine, including the time to handle raw material, finished parts and remnants, and attending the machinery while it's running (when required). To place a value on these costs, you must know the hourly cost of an operator, the amount of time it takes to run a part on each machine, the percentage of time allotted for machine setup, and the percentage of time an operator actually attends the machine. These factors all may be unique for each application or facility.
2. Operating: costs associated with operating the process, including gas and power consumption, consumable items, maintenance and repair, and tooling. These costs occur only when the machine is operating.
3. Depreciation: costs associated with the equipment purchase. It may be a monthly lease or loan payment or the initial price of the equipment amortized over a specific amount of time. These costs also include the estimated value of the machine at the end of the payment schedule. Since depreciation costs are fixed, they occur whether the machine is working or idle.

Remember that these machines don't produce parts at the same speed. Because many of the costs just discussed are time-dependent, it's necessary to express the comparative data as a cost per equivalent amount of work, not a cost per hour, which can be misleading.

Also note that these costs all are application-dependent. They may vary depending on part features, the amount of work, and the number of shifts running.

While a job shop may have the ability to solicit additional work to fill a machine’s time, a manufacturer likely will be limited to the amount of product shipped. For this reason, the model was made flexible enough to accommodate a number of different scenarios, using a database of different parameters associated with the processes, the user, and the machinery, each being independently variable.

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About Laser Cutting

Laser cutting is a hot cutting manufacturing and fabricating process using an industrial laser for the cutting of material, usually metal. "Laser" is an acronym for Light Amplification by Stimulated Emission of Radiation. The beam of the laser is an extremely coherent radiation of a wavelength, meaning the beam will not dissipate like conventional light beams. The focused beam of the laser makes it best suited for the energy transfer necessary to cut metals by melting or burning the material along a cut line. Assist gas sweeps the cut area clean. The cutting process is precision controlled through a combination of CNC and CAD computer systems.

There are distinct advantages to laser cutting over other cutting methods. Excellent control of the laser beam with a stable motion system achieves an extreme edge quality. Laser-cut parts have a condition of nearly zero edge deformation, roll-off or edge factor. Laser cutting has higher accuracy rates over other methods using heat generation, as well as water jet cutting. It is also faster than conventional tool-making techniques. There is quicker turnaround for parts regardless of the complexity, because changes of the design of parts can be easily accommodated. Laser cutting also reduces wastage.

There are a few disadvantages to laser cutting. The material being cut gets very hot, so in narrow areas, thermal expansion may be a problem. Distortion can be caused by oxygen, which is sometimes used as an assist gas, because it puts stress into the cut edge of some materials; this is typically a problem in dense patterns of holes. Lasers also require high energy, making them costly to run. Laser cutting produces a recast layer in the kerf that may be undesirable in some applications. Lasers are not very effective on metals such as aluminum and copper alloys due to their ability to reflect light as well as absorb and conduct heat. Neither are lasers appropriate to use on crystal, glass and other transparent materials.

Lasers, even low-powered ones, are potentially hazardous to a person's eyesight. The laser beam can focus on an extremely small spot on the retina, causing permanent burn damage in seconds. Infrared and ultraviolet lasers are even more dangerous because the "blink reflex" protects the eyes only if the light can be seen. Lasers are divided into five safety classes based on wavelength and maximum output power. Lasers in Class I are inherently safe because of a low output power or an enclosure that cannot be opened in normal operation without the laser automatically switching off. In Class II, the blinking reflex will prevent eye damage; most laser pointers are in this class. The lasers in Class IIIa have large beam diameters and are mostly dangerous in combination with optical instruments, which change beam diameter. If the beam of a laser in Class IIIb enters the eye directly or is reflected into the eye, damage can result. Class IV lasers are highly dangerous. Damage to the eyes and skin can be caused even by indirect scattering of light from the beam.

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Avoiding Material Damage with Cold Laser Cutting

The medical device industry’s use of sensitive metal materials has created an important demand for precision micromachining. The technologies chosen to work with such materials must be selected based on precise criteria. When working with the materials, it is critical that heat effects, mechanical damage, particle deposition, and other potential fabrication distortions are avoided. High precision is also generally required for machining delicate metals.

Stent manufacturing is a good example of a demanding application. The delicate operation requires cutting sheets or tubes of metal to make mesh tubes with an intricate structure. A water-jet-guided laser, or laser microjet, technology provides an efficient way to cut stents. The material sustains neither heat nor mechanical damage while being cut, and the microjet prevents any temperature increase in the stent, which is essential. A hair-thin water jet provides a cooling effect as it guides the laser beam. After cutting, even stents made from shape-memory materials retain clean structures to a degree difficult to achieve with conventional lasers. Shape-memory materials tend to melt more than other materials (such as stainless steel), and more burrs are produced during cutting. Moreover, laser microjets significantly reduce the need for postprocessing steps because they generate no burrs or dross on the material surface. Moreover, particle contamination is reduced.

Stents are mesh tubes that improve and ensure blood flow in blood vessels narrowed as a result of atherosclerosis or other vascular conditions. After being tightened over a balloon catheter, the stent is moved into the blocked vessel. When the balloon is inflated, the stent expands into the vessel. There, it forms a scaffold that holds the vessel open (see Figure 1). Stents are designed to remain in the artery permanently.

Various materials can be used in making stents. Stainless steel and nitinol are the most common materials. But other shape-memory alloys and polymers are also viable options. The stent tube is typically about 2 mm in diameter and has a wall thickness between 100 and 200 µm.

Stent cutting is a demanding procedure. Because the stents will be placed inside the human body, certain requirements must be met. First, there can be no cracks. Second, the edges must be clean, without dross or burrs attached. Also, precision and consistency are critical because the cut curvature is fine and highly complicated. Finally, thermal damage must be minimal. Some materials, like the shape-memory metal alloy nitinol, are heat sensitive; thermal loading could damage their shape-memory ability. Stainless steel expands by 16 µm per meter of length per degree Celsius as the material is heated. For example, a variation of 15ÞC of a 2-cm-long stainless steel stent will imply a variation of about 5 µm in length. Biocompatibility is also essential because of the risk of rejection. A good surface finish is required when applying an additional coating that contains antirejection drugs.

Certain mechanical and chemical methods may remove dross and moderate heat-affected zones after cutting. However, it is more productive to obtain the requisite quality and geometry—or as close to such as possible—during the initial cutting step. Obtaining the necessary quality early in the process helps prevent the need for sandblasting at a later stage. It also minimizes the need for electropolishing to round off the edges and achieve a smooth surface finish.

The laser-microjet principle couples a high-powered, pulsed laser beam with a hair-thin water jet. A fiber carries the laser beam to the center of the system. There, it passes through a transparent window and enters a water-filled chamber. Once past the window, the laser beam is focused into a nozzle where it is coupled with the water jet exiting the chamber. From this point, the laser beam is guided along the cylindrical jet by total reflection at the interface between air and water. The reflection is the result of the difference between the refractive indexes. When the beam reaches the workpiece, the laser cuts the material by heating.

The pure, deionized, filtered water is pressurized between 50 and 500 bar. However, its consumption remains low (about 1 L/hr) because of the jet’s small diameter. The nozzles, which can be either sapphire or diamond, may be as small as 25 µm in diameter. The laser sources are diverse, but they are usually pulsed Nd:YAG lasers whose wavelength may be 1064 nm (infrared), 532 nm (green), or 355 nm (ultraviolet).

Because the water jet guides the laser, it ensures a consistent spot diameter, which enables a single, centimeters-long focus. The length for which the water jet is stable is about 1000 times the water-jet diameter. For example, a 50-µm jet will be stable for 5 cm. In addition to guiding the beam, the water jet has other functions. These functions prove significant for precision cutting, especially when requirements are as stringent as those associated with medical device manufacturing.

Between laser pulses, the water jet cools the edges of the cut and its immediate surrounding area, preventing heat damage within the metal. It is efficient for avoiding heat load resulting from laser ablation on both sides of the processed pieces. Because of this cooling function, the water-jet-guided laser is referred to as a cold laser. No oxidation is visible.

When the laser microjet was first used for stent cutting, only planar sheets were processed. Using planar sheets enabled the complex structure of stents to be tested. However, the objective was to process tubes directly. For that reason, a rotary axis was added to machines dedicated to stent cutting.

The challenge was to avoid damaging the internal backside of the cylinder while processing the external front side. With nothing to block it, the laser beam, still guided by the water jet, might have damaged the second wall of the tube after cutting the first. Several strands of copper placed inside the tube during the process solved this problem. The copper efficiently protects the inside of the tube, and the strands do not melt under the heat of the infrared laser because of copper’s high reflection coefficient. The molten material generated by laser ablation and ejected during cutting is not problematic because the water jet cools the particles, avoiding damage inside the tube.

Manufacturing medical stents, which involves cutting metal to obtain mesh tubes with intricate structures, is a highly demanding process. Lasers are commonly used for this application, but they can create burrs and produce substantial material damage as a result of heating. Postprocessing steps are time-consuming and can decrease yield. The water-jet-guided technology’s advantages, which include low particle deposition, small heat-affected zones, and negligible mechanical constraints, can significantly reduce these additional steps. This tool, which combines the strengths of laser beam and water-jet processing, is well adapted to precision cutting of heat-sensitive metal materials.

Continue to learn more about laser cutting, please visit Device Link.