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This article contains comprehensive information about laser cutting and laser drilling. Read further to learn more about:
What is Laser Cutting and Laser Drilling?
Theory and Working Principle
Methods of Cutting
Laser Drilling Techniques
And much more…
Chapter 1: What is Laser Cutting?
Laser cutting is a material-cutting method that uses an intensely focused, coherent stream of light to cut through metals, paper, wood, and acrylics. It is a subtractive process that removes material during the cutting process using vaporization, melting, chemical ablation, or controlled crack propagation. Laser optics controlled by Computer Numerical Control (CNC) can drill holes as small as 5 microns (µ). The process does not produce residual stresses on materials, making it possible to cut fragile and brittle materials.
Laser drilling uses several methods, including single-shot, percussion, trepanning, and helical. Single-shot and percussion laser drilling produce holes at a higher rate than the other processes. Trepanning and helical drilling, produce more accurate, higher-quality holes.
Laser cutting is a non-contact process where cutting is completed without making contact with the cut material. It can shape high-strength, brittle materials such as diamond tools and refractory ceramics. The first production laser cutting was introduced in 1965 and was used to drill holes in diamond dies. It was later used for cutting high strength alloys and metals such as titanium for aerospace applications. Its range of applications covers the cutting of polymers, semiconductors, gems, and metallic alloys.
Chapter 2: Laser Cutting Theory and Working Principle
Laser stands for "light amplification by stimulated emission of radiation". Aside from the cutting applications of lasers, they are used for joining, heat treating, inspection, and free form manufacturing. Laser cutting differs from other laser machining processes since it requires higher power densities but shorter interaction times.
Lasers are generated by a high-intensity light source inside a reflective laser cavity, that contains a laser rod that generates the radiation. The light source stimulates the laser rod's atoms as they absorb wavelengths of light from the light source. Light is composed of small bundles of photons that strike the lasting media atoms energizes them. The photon's energized atoms, give off two more photons with the same wavelength, direction, and phase, called stimulated emission. The new photons stimulate other energized atoms producing more photons, causing a cascade of excitations.
The photons move perpendicular to parallel mirrors located on the ends of the laser rod but stay within the laser rod. One mirror is transmissive, enabling the partial escape of light from the cavity. This escaping stream of coherent, monochromatic light is the laser beam used to cut the material. Another set of mirrors or fiber-optics direct light into a lens that focuses the light into the material.
The three main types of lasers used for cutting, are CO2, Nd-YAG (Neodymium Yttrium-Aluminum-Garnet) lasers, and fiber-optic lasers. They differ in the materials used to generate the laser beam.
Fiber-optic lasers are the newest and most popular types of lasers because that they can generate different wavelengths for more precise cutting. They use an optical fiber cable made of silica glass to guide the light. The laser beam produced by fiber-optic lasers is more precise because it is straighter and smaller.
Fiber lasers vary according to their laser source mixture, including ytterbium-doped, thulium-doped, and erbium-doped. The choice of mixture is dependent on the application where they will be used and their wavelengths. For example, erbium generates light in the 1528 nm to 1620 nm range. Ytterbium produces light with wavelengths of 1030 nm, 1064 nm, and 1080 nm.
The two modes of fiber optic lasers are single and multiple with the core diameter of single-mode lasers being between 8 µ to 9 µ while multiple mode lasers have diameters of 50 µ up to 100 µ. Of the two modes, single-mode lasers are more efficient and produce a better quality beam of light.
Fiber-optic lasers are classified as solid-state since their power source is silica glass mixed with rare earth elements. This is contrary to CO2 lasers that use gas to create their power. An additional difference between the two forms of power is their wavelengths, with fiber-optic lasers producing wavelengths of 780 nm up to 2200 nm while CO2 lasers have wavelengths of 9600 nm up to 10,600 nm.
This type has a gas discharge lasing medium filled with 10 – 20% carbon dioxide, 10 – 20% nitrogen, traces of hydrogen and xenon, and helium for balance. Instead of light, laser pumping is done by discharging an electrical current. When the electrical discharge passes through the lasing medium, nitrogen molecules become excited, bringing it to a higher energy level. Unlike what was described before, these excited nitrogen molecules do not lose their energy by photon emission. Rather, it transfers its vibrational mode energy to CO2 molecules. This process continues until most of the CO2 molecules are in a metastable state. The CO2 molecules then emit infrared light at either 10.6 µm or 9.6 µm, which brings them to lower energy levels. The resonating mirrors are designed to reflect the emitted photons on those wavelengths. One mirror is a partially reflecting mirror allowing the release of the infrared beam that is used for cutting the material. After releasing infrared light, the CO2 molecules return to the ground state by transferring their remaining energy to the doped helium atoms. The cold helium atoms then become hot which are cooled by the cooling system of the laser. The efficiency of a CO2 laser is around 30% which is higher than other lasers.
Crystal (Ruby, Nd, and Nd-YAG) Lasers
Unlike the CO2 laser, this type is a solid-state laser that uses a synthetic crystal as a lasing medium. The most popular is the YAG (Y3Al5O12) crystal doped with 1% ionized neodymium (Nd3+). The Nd ions replace the Y ions in the crystal structure in this crystal. The length of the rod is about 4 inches (10 cm) with a diameter of 2.4 to 3.5 inches (6 to 9 cm). The ends of the YAG rod are polished and coated by highly reflective materials acting as the resonator system.
Laser pumping is achieved by krypton flashlamps or laser diodes. This laser pumping excites the Nd ions into higher energy levels. After a short while, the excited Nd ions move into a lower, more stable state, without emitting photons. This process goes on until the medium is populated with excited Nd ions. From its metastable state, the Nd ions release infrared light with a wavelength of 1064 nm.
It is known that as light travels through a fiber-optic, it remains inside with minimal energy losses. This makes fiber-optics more stable than other types that require them to be aligned accurately.
Laser cutting uses assist gasses, such as compressed air, nitrogen, or argon, injected at the nozzle to supplement the cutting process. Assist gasses help start the cutting by using an exothermic reaction, a chemical that releases energy through the use of light or heat. The use of assist gasses helps in a more effective transference of heat than can be created by the beam alone. In cutting metals, assist gasses help remove the molten metal.
The previous chapter discussed the different types of lasers according to how the laser beam is formed using different types of lasing pumps and lasing media. Next will be the methods of laser cutting—how the small bits of materials are removed to produce a cut. There are four main methods of laser cutting: sublimating, melting, reacting, and thermal stress fracturing.
Sublimating or Vaporizing
Sublimation is a type of phase change from a solid state to a gaseous state, with no intermediate liquid phase. This is the same process of how dry ice turns into a vapor without becoming a liquid. The material quickly absorbs energy in which there is no chance for melting to occur. The same principle is applied to laser cutting, wherein a high amount of energy is imparted into the material in a relatively short time that causes direct phase change of the material from solid to gaseous states, with as little melting as possible.
The cut begins by creating an initial keyhole or kerf. In the kerf, there is more absorptivity which causes the material to vaporize more quickly. This sudden vaporization creates a material vapor with high pressure that further erodes the walls of the kerf while ejecting materials from the cut. This deepens and enlarges the hole or cut made.
This process is suitable for cutting plastics, textiles, wood, paper, and foam, which requires only small amounts of energy to be vaporized.
In comparison with sublimation, melting requires less energy to achieve. The energy required is about a tenth of the sublimating laser cuts. In this process, the laser beam heats the material, which causes it to melt. As the material melts, a jet of gas from the coaxial nozzle with the laser beam expels the material from the cut. The assist gasses used are inert or non-reacting (e.g., helium, argon, and nitrogen), which only aids the cutting through mechanical means.
Because of its low energy requirement is used for cutting non-oxidizing or active metals such as stainless steel, titanium, and aluminum alloys.
Reactive Laser Cutting
In this process, a reactive gas is used to generate more heat by reacting with the material. The process begins by melting the material with a laser beam. As the material melts, a stream of oxygen gas comes out of the coaxial nozzle, reacting with the molten metal. The reaction between the metal and oxygen is an exothermic process which means heat is released. This heat assists in the melting of the material, which is about 60% of the total energy required to cut the material. The molten metal oxides are expelled by the pressure of the oxygen jet.
Aside from the lower energy required from the laser beam, cutting speeds using reactive gasses are faster than laser cutting with inert gasses. However, since this process relies on a chemical reaction, the molten metal oxide that is not expelled by the oxygen jet forms along the edge of the cut. This produces low-quality cuts than using inert gasses.
This process is used to cut thick carbon steels, titanium steels, and other easily oxidized metals.
Thermal Stress Fracture
This process involves introducing a small kerf at depths of about one-third the thickness of the material using a laser. The laser is then used to induce localized stresses. This is achieved by heating a small spot which creates compressive forces around it. After passing the laser beam, the area slightly cools, creating thermal stresses. In some designs, coolants are used to assist in the generation of thermal stress. When these induced stresses reach failure levels, a crack is propagated that causes separation.
The movement of the laser beam directs this separation in a controlled manner. This method usually requires less power than laser vaporization with better cutting speeds. Localized heating is normally carried out below the glass transition temperature.
CO2 lasers are widely used for this application since infrared light with a wavelength of 10.6 µm is ideal for cutting most nonmetals. However, not all materials can be cut by one type of laser since different materials absorb light at different wavelengths. Thermal stress fractures are widely used to cut brittle materials such as ceramics and glass.
Another newer method that utilizes principles of thermal stress fracture is Stealth Dicing. This is a laser cutting technology originally developed by Hamamatsu Photonics which is used in cutting semiconductor wafers and parts of microelectromechanical systems or MEMS. In this type of cutting, the initial kerf is created at an internal point within the material. Stealth dicing is a dry cutting process where the cut produced is clean with no molten deposits.
This is a laser cutting technology originally developed by Hamamatsu Photonics which is used in cutting semiconductor wafers and parts of microelectromechanical systems or MEMS. In this type of cutting, the initial kerf is created at an internal point within the material. Stealth dicing is a dry cutting process where the cut produced is clean with no molten deposits.
Chapter 4: Laser Drilling Techniques
There are different ways to create a hole using a laser. These are classified according to the movement of the laser beam relative to the workpiece. Each technique has its advantages and disadvantages.
Single-shot Laser Drilling
In this type of laser drilling, a single laser pulse with high energy is used to create a hole. This single beam laser focuses on a single location until the material melts layer by layer. The melting process is done efficiently and in a short amount of time, which makes this process desirable to produce multiple holes quickly.
Percussion Laser Drilling
In percussion drilling, the laser beam diameter is the same as the hole diameter. To compare it to single-shot drilling, successive low-energy pulses are used to remove material instead of using a single laser pulse. These repeating pulses eventually penetrate the material, which takes about 4 to 20 pulses depending on the depth of the material and laser beam properties. This process is also completed quickly, which makes it effective in working with thick materials and producing multiple holes in a short amount of time.
Trepan Laser Drilling
In trepan laser drilling, the laser beam spot size is significantly smaller than the hole size. When an initial hole is made, the laser beam then traverses around the hole, expanding the drilled hole size into the desired diameter. This is done to drill large holes more efficiently than single-shot and percussion drilling. Trepan drilling is slower but can produce holes with better metallurgy and geometry.
Helical Laser Drilling
Like trepan drilling, this type uses a moving laser beam to drill through a material. However, it does not require an initial hole. In this method, the laser beam is rotated relative to the workpiece. The laser beam‘s rotation is similar to that of a conventional drill bit. Rotation is achieved by a spinning dove prism or other optic systems rotated by a high-speed motor. The quality of the hole produced is comparable to holes made by trepan drilling.
Chapter 5: Laser Cutting Machine Configurations
At first, the method for using a laser cutter was to manipulate the workpiece by hand. It was positioned, the cut was made, the laser removed, and the next cut was made. At the time, CNC programming and other technological advances did not exist. Modern laser cutting has removed the need for manual positioning of the workpiece and uses computer-controlled equipment to quickly and efficiently make the proper cuts.
Gantry Laser Cutting Machines
The main types of gantry laser cutting machines are made of aluminum, They have a long horizontal bed and a gantry positioned over the bed. They can be programmed with multiple cuts that are performed with one pass of the laser, which can be a fiber optic or CO2 laser. Gantry machines use CNC-controlled programming to produce efficient and accurate cuts quickly and easily. Unlike hand manipulation machines with 8 foot to 16 foot (2.4 to 4.9 m) footprints, gantry machines have a footprint of 4 feet to 8 feet. (1.2 m to 2.4 m).
Moving Material Configuration
In this setup, the laser cutter is stationary while the material surface moves. Since no movement from the laser is required, the optics system is simpler than other configurations. However, this is slower than other methods and is usually limited to cutting flat materials.
Flying Optics System
This setup is the opposite of the moving material configuration. The flying optics system involves a stationary material and a movable laser cutter. Since the laser is constantly moving, the laser beam length must be adjusted constantly as well because of the divergence of the laser beam. Greater divergence produces a poorer quality of cut. To mitigate this, re-collimating optics and adaptive mirror control are used. This setup is the fastest of the three since the movement of the mirrors is easier to control.
In the hybrid system, the material moves on one axis while the optics move on the other axis. This setup combines both advantages and disadvantages of the previous two setups. One advantage of this system from flying optics is that hybrid systems provide a more constant beam path, which reduces power losses.
Chapter 6: Laser Marking
Laser marking is the process of creating marks using lasers by cutting the surface of the workpiece at a shallow depth or by inducing chemical changes through burning, melting, ablation, polymerization, and so forth. Like laser cutting and laser drilling, laser marking can be a non-contact process. Issues of tool wear and unwanted work hardening on the surface of the workpiece are eliminated. Moreover, laser marking does not use inks, an advantage over traditional printing. Different types of laser marking processes are summarized below.
This process involves removing specific regions of the coating layer previously applied on the surface of the workpiece. The workpiece has a different contrast from the coating which makes the regions removed significantly visible. Materials for this type of laser marking are special films and coated metals.
This is a laser marking where the surface is cut at the desired depth. The cut is usually made by the laser vaporization process. The main advantage of this method is that it can be done at high speeds.
This is done by fusing additional pigmented materials such as glass powders or crushed metal oxides on the surface of the workpiece. The heat applied by the laser fuses the materials.
This process involves heating specific regions using a laser. The heat applied by the laser causes the metal to oxidize producing different colors such as black, yellow, red, and green.
In this process, plastic bonds between polymers are broken, releasing hydrogen and oxygen and producing a darker color. This process is done on plastics and organic materials.
This is usually done on plastics where the color pigments and carbon are destroyed and vaporized resulting in foaming. The foaming process is done on dark-colored materials that need to have lighter colored markings.
This process induces chemical reactions on the surface of the workpiece where the products of the reaction have different colors.
Chapter 7: Advantages and Disadvantages of Laser Cutting
Laser drilling is widely used in aerospace, automotive, electronics, and tool machining. Below are the main advantages of using lasers for drilling.
As mentioned earlier, since the laser drilling process has no cutting tools involved, there is no issue of tool wear or damage. In conventional drilling, drill bits can become dull, making the cutting slower and producing more heat. This can distort the material and change its mechanical properties due to heating.
Precision and Accuracy
Since laser beams can be focused, this allows precise drilling of small holes that conventional drilling cannot achieve. The hole depth can be controlled even for micro-scale holes. Moreover, the process is digitally controlled by CNC methods. All parameters can be automatically controlled, producing consistent and repeatable results.
Minimal Burrs Produced
Secondary processes such as deburring are required in manufacturing precision parts to remove surface irregularities, metal spurs, raised edges, slags, and dross. Even the most accurate fabrication techniques such as laser cutting technology, tend to develop dross or thermal burrs. However, in comparison with conventional cutting, laser-cut parts still have superior edge quality. This effectively lessens the cost of secondary processes, particularly deburring, which can be as high as 30% of the operating costs.
High Aspect Ratio
This means very deep holes with small diameters can be drilled without issues. Drilling these holes using conventional drills causes the tool to heat up, wobble, and break due to torsional stress. Using a laser creates no frictional resistance and is only limited by the laser generator and the optical systems used.
Suitability for Difficult Materials
Lasers can cut and drill different materials that are difficult for conventional machining. Lasers can cut high-strength metals such as titanium and steel superalloys. Aside from these high-strength metals, laser cutting is used for cutting crystals, ceramics, and even diamonds because of its ability to do controlled fracture.
Fast Drilling Speeds
Since there is no required tool positioning against the workpiece, drilling speeds only depend on the configuration of the optical system and the movement of the cutting head. Moreover, the complexity of the profile to be cut has minimal effect on the incremental cost of operating the machine.
No Residual Stress
Since most of the molten material is blown off by the assist gas, no residual stresses are present along the drilled edges. This results in a clean, mechanically stable cut.
Despite these advantages, current laser drilling technology cannot completely replace conventional methods. Below are the main reasons.
High Investment Cost
Laser cutting machines can reach prices twice as much as waterjet and plasma cutters. The investment‘s rate of return may not be sufficient to produce any economic advantage.
High Expertise Required for Operation and Maintenance
Operating a laser cutting machine requires a specialist with a good technical background because of the range of operating parameters involved. Also, for CO2 and crystal lasers, an expert is needed to bring them back to their operating condition once it becomes misaligned.
Highly Precise Robotic Systems Required
Highly precise movements are required in laser cutting, especially in applications in the order of microns. Two factors can affect the movement of the laser beam. One is the accuracy of the control system and drivers. The control system must be able to process and send precise signals to the high-resolution driver to finely position the laser beam. The other factor is the dimensional accuracy of the laser cutting parts. Linear guides, lead screws, and other parts of the transmission system must accurately mate together. This can be achieved by deburring the laser cutting parts.
Metal Thickness Limitations
The depth of cut depends on many parameters, but the most significant is power. For the same power rating, plasma cutters can cut deeper than lasers. Common industrial laser systems of greater than 1kW can cut carbon steel up to 13 mm in thickness.
Chapter 8: Plasma Cutting vs. Laser Cutting
The terms plasma and laser cutting are sometimes used interchangeably since both are cutting processes. However, regardless of their basic similarities, they are different in how they are applied and their principles. Both methods were developed in the middle of the twentieth century and have been perfected and modernized to fit the needs of present-day manufacturing techniques.
Laser cutting is a process that cuts materials by amplifying of a laser light. It has exceptional precision due to being controlled by a CNC controller. Laser cutting involves focusing a laser light using optics. As the light becomes smaller and hotter, it melts and cuts through a workpiece as a computer directs the process. The workpiece is burned during the melting process, and an assist gas or vaporization blows off waste material.
Plasma cutting is a method for cutting electrically conductive materials using oxygen or nitrogen gas and a jet of hot plasma to melt the surface of a workpiece, regardless of how rugged or tough it may be. The unique characteristic of plasma cutting is the limitation of the materials it can cut, which are electrically conductive and include aluminum, stainless steel, steel, brass, and copper. The cutting plasma is a conductive ionized gas that is extremely hot during the cutting process. Although all plasma cutting tools are the same, the type of tool is determined by its temperature.
All plasma cutting tools burn very hot at temperatures exceeding 40,000 degrees Fahrenheit (22,200 ° C). When the process is combined with CNC machining, it produces parts that do not require further finishing or machining. Unlike laser cutters, plasma cutters discharge radiation, which necessitates the use of protective clothing and glasses or goggles for workers.
Principle Differences Between Laser Cutting and Plasma Cutting
The main difference between the two processes is the fuel used to power the cutting process with plasma cutting using a plasma gas, while laser cutting involves a beam of light. Additionally, there is some danger involved with plasma cutting due to the radiation it emits. Both processes are efficient and precise cutting methods that diverge in accordance with how they complete the process.
Laser cutting is a non-traditional machining method that uses an intensely focused, coherent stream of light called a laser to cut through the material. On the other hand, laser drilling is another type of laser machining process that produces a hole through the workpiece achieved by different techniques.
A laser beam is generated by using a high-intensity light source or electrical discharge device to excite atoms or molecules inside a lensing medium. This lensing medium produces cascading excitations, which result in the production of photons. The photons are then resonated and partially released. The released photons become the laser cutting beam.
Lensing media used for laser cutting are CO2, crystals, and fiber-optics.
There are four main methods to produce a cut or hole. These are sublimating, melting, reacting, and thermal stress fracturing. Each of these methods has its application.
Laser drilling can be done by single-shot, percussion, trepanning, and helical drilling. Single-shot and percussion laser drilling produce holes at a higher rate than the other two processes. Trepanning and helical drilling, on the other hand, produce more accurate and higher-quality holes.
Laser cutting machines can be classified according to the movement of the laser relative to the workpiece. These are moving material, flying optics, and hybrid systems.
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