In order to understand what makes a laser suitable for cutting, one must distinguish its uniquefeatures in comparison to ordinary light.Conventional light produces waves, which radiate out in all directions to fill up and illuminate a wide area. The energy intensity rapidly decreases as waves moves away from the source,just as the sun's intensity is diminished when it finally reaches the earth.The laser on the other hand provides a stream of collimated, coherent light waves which giveit exceptional intensity and direction ability. Lacking the dispersion of conventional light, alaser can be easily projected as a beam over relatively long distances while maintainingnearly all of its useful power output.The use of lasers for cutting can be thought of in the same way as that of focusing sunlightwith a magnifying glass to produce a concentrated source of heat energy. While this methodonly results in a few burned holes in paper, it gives us an illustration that light is indeed asource of energy with potential material processing capabilities.A laser can be used for cutting by exposing material to the intense heat energy developed byits beam. If that heat input to the material is greater than that material's ability to reflect,conduct, or disperse the added energy, it will cause a sudden rise in temperature of thematerial at that point. If the temperature rise is substantial enough, the input heat is capableof initializing a hole by vaporizing the material. The linear movement of this intense heatenergy with respect to the material provides cutting action.In most cases the "raw" (unfocused) beam of even high power (multi-kilowatt) industrial lasershas inadequate energy to do much more than slowly heat a surface. Therefore, the beam isdirected through a focusing lens. This allows the energy to be concentrated into a spot of lessthan 0.25 mm thus producing power densities of over a million watts per centimeter squared,capable of vaporizing many materials.While intense heat is capable of vaporizing material, the control of that heat is essential indetermining quality. The key performance features of a laser are those beam characteristicsthat affect the resultant power density as it is directed onto the work piece.
ModeA cross-section of a laser's beam profile is commonly referred to as mode. Described in termsof TEM (Transverse Electromagnetic Mode) mode relates to the beam's ability to be focused.It is also comparable to the degree of sharpness of a cutting tool. The lowest order orreference mode is TEM00, of which the beam's profile simulates a Gaussian distributioncurve. Modes that approach this energy distribution can be focused down to the laser'stheoretical minimum spot size and give the sharpest energy density.Higher order or multi-mode beam profiles are characterized by a tendency to spread out theenergy distribution away from the centre of the beam. The resultant spot is large with thismode causing lower energy concentration. Therefore, higher order mode lasers areconsidered to be duller cutting tools than low order mode lasers of equivalent power output.
Power OutputLasers are rated by their power output in terms of watts. Since laser cutting is a thermalprocess, the amount of heat produced relates to its capabilities. Whereas a 300 watt laserwith a high quality output is more than adequate for the cutting of paper products, it lacks theheat producing capabilities to effectively couple into aluminum. Given all other considerationsbeing equal (eg power distribution, spot size, etc), increased power allows for fasterprocessing speeds and the ability to cut thicker sections of materials.
StabilitySince quality results are obtained by the application of consistent energy, the stability of thelaser's output is a key feature in cutting. This includes maintaining unwavering output energy(power stability), consistent beam quality (mode stability), and fixed energy concentration(pointing stability). Should the power increase or decrease by more than a few percent overthe short term operation, the beam quality oscillate between a Gaussian and multi-modeprofile, or the location of the beams direction shift more than a few tenths of a milliradian dueto the outputs instability, there will result a noticeable change in the available power densityfor cutting.
PolarisationParticularly evident in metal cutting and ceramic processing, studies have shown that randomoccurrences of inconsistent edge quality, namely variations in kerf, edge smoothness, andperpendicularly, are attributable to the effects of polarisation. Uncontrolled or randompolarisation is characteristic of most standard material processing lasers. It can unpredictablyaffect the relative degree of absorption of the beam's energy that is coupled into the materialat a given moment. To correct this inconsistency, lasers can be equipped with opticalpackages that either fix the polarisation to be aligned in the same direction of the cuttingaction or circularly polarise the output to give equivalent coupling regardless of the directiontravel.An important asset of laser cutting is the high level of control, which is available over thevariables affecting the process. The cut can be tailored to meet the exact requirements of thejob and the results can be readily duplicated. The principle parameters are:
SpeedLaser cutting feedrates have been found to fit empirical formulas based on the available laserpower density and the properties of the material to be cut. Above a threshold amount, thefeedrates are directly proportional to available power density, which takes into account thelaser's performance features (eg power, mode) in addition to the focusing system'scharacteristics (eg spot size). Cutting rates are likewise inversely proportional to the materialsdensity and thickness. Therefore, given all other parameters are constant, feedrates willincrease with:· Additional power (1700 watts vs 3500 watts)· Improved mode (TEMoo, vs multimode)· Smaller focused spot size (2.5 vs 5" F.L lens)· Lower required energy to initiate vaporisation (plastic vs steel)· Lower material density (white pine vs hickory)· Decreased thickness (1.25" vs.250")Feedrates can be varied for a particular set of parameters in order to obtain different edgequality results, particularly for metals, the plot of cutting speed versus thickness for a materialhas two curves. The upper curve reflects the top speed at which through cuts are achievedwhile the lower curve shows the limit below which the material is self-burning. The resultantwindow of acceptable cut speeds is usually wider at the thinner range of a material.
Focusing LensSince speed is a function of available power density, the choice of the focusing lens has agreat impact on the resulting cut quality. Imaging of lasers beams is usually accomplishedwith transmissive lenses of focal lengths ranging from 2.5 to 10 inches. Because the focusedspot size is proportional to the focal length, the power density that is produced is proportionalto the square of that length. Short focal length lenses give very high energy densities, but arelimited in their application due to a shallow working depth. They are appropriate for use withthin materials and in high-speed operations where the material can be held within the limiteddepth of field. Longer focal length lenses have lower power densities but are able to maintainthose densities over a much broader range and therefore can be used for thicker crosssections of materials given that they have enough energy initially.
Focal Point PositionDuring the laser cutting process, the focal point of the lens should be consistently positionedin order to provide the best cutting results. In most cases, the focal point is positioned at orslightly below the surface of the material. Above or below this point the power density willtaper off until it is insufficient to produce an effective cut. Cutting systems that employ shortfocal length lenses must ensure constant monitoring of the lens-to-work piece distance.
Assist GasRecall that assist gas is supplied coaxial with the focused beam to protect the lens and aid inthe material removal process. Generally, compressed air or inert gas is used to purge meltedand evaporated material from the cut zone while minimising any excess burning. For mostmetal cutting applications, a reactive gas assist can be employed to promote an exothermicreaction. The enhanced energy intensity from the use of oxygen can improve cutting speedsby 25% - 40% over the results obtained with use of air.In addition to gas type, delivery pressure is an important consideration. Typically, pressures of45-60 psi (3-4 bar) developed in the gas jet nozzle are used in cutting thin material at highspeeds to help prevent the clinging of slag or dross to the back edge of the cut. The pressureis reduced as the material thickness increases or process speeds slow.
LASER CUTTING PROCESSLaser cutting systems combine the heat of the focused beam with assist gas, which isintroduced through a nozzle coaxial to the focused beam. The high velocity gas jet serves to:· Aid in material removal by blowing out excess material through the backside of the work piece· Protect the lens from spatter ejected from the cut zone· Assist in the burning process.The best example of the chemical effect of the assist gas is the use of oxygen for the cuttingof steels where performances are increased by the exothermic reaction of combustion of ironin oxygen. Another example is clean cutting stainless steel with high-pressure nitrogen. Asthe laser beam cuts the stainless steel, the high-pressure nitrogen blows the melted materialaway.While carbon dioxide lasers are capable of generating tremendous heat intensity, it is anincorrect assumption that they are capable of vaporising and cutting all known materials.Rather, each material has its own unique response, some of which are not suitable, to theeffects of CO 2 lasers. Therefore, the question of suitability of using a laser for cutting thatmaterial hinges on how well it handles the added energy input. That interaction is dependentupon three key factors of the material.• Surface condition - how well it initially absorbs the energy• Heat flow properties - its coefficients of thermal diffusivity and conductivity• Heat phase-change requirements - the amount of excess heat required to induce achange as a function of the materials density, specific heat, and latent heat of vaporisation.The following information is intended to provide general inputs on the major categories ofmaterials, keeping in mind these factors.
NON-METALSIn general, non-metallic materials are good absorbers of infrared energy as produced by a CO2 laser. Likewise, they are generally poor conductors of heat and have relatively low boilingtemperatures. As such, the energy intensity of a focused beam is almost totally transmittedinto the material at the spot and will instantly vaporise a hole.
Plastics (Polymers)Lasers have found their way into many plastic machining operations because of their ability tocut complex geometrise, at high feedrates without contacting the work piece. Since the laseris an intense heat source, it uses its energy to vaporise the binder and quickly breaks downthe material's polymer chains.Thermoplastics with relatively low melting temperatures typically display clean cuts with firepolished edges as a result of resolidified melting. Process control can be exercised tominimise or eliminate bubbling or the presence of small burrs on the backside of the cut.As the tensile strength of the polymer increases, there is a correlation to a marked increase ofcharring present along the cut edge. Greater energy intensity per unit time is required tobreak the stronger chains and therefore leads to a burning action. Reasonable results havebeen obtained with polyester and polycarbonate while there is generally a substantial layer ofdecomposed material along the edge of phenolic, polyamides, and PVC.As a caution, in the cutting of some polymers, specifically lucite, and PVC, careful attentionmust be directed at the containment and appropriate filtering of potentially hazardous and/orcorrosive fumes that are generated as the result of burning.
CompositesNew lightweight, fibre reinforced polymers are difficult to machine with conventional, cuttingtools. This has led many users to the non-contact cutting capabilities of a laser. Prior to thecuring of laminates stacks, thin prepreg sheets in thicknesses up to 0.5mm can be trimmed orsized at speeds up to 40 metres per min without gumming up a cutting tool. The heat from thelasers cutting action fuses the edges, thus preventing fraying of the fibres.For thicker sections and fully cured composites, particularly boron and carbon fibre material,there is a higher probability of charring, and thermal damage along the cut edge, thusreducing the acceptability of laser cutting for structural members. As with the cutting ofpolymers, care should be exercised in the removal of fumes.
RubberBoth natural gum and synthetic rubber materials in thicknesses up to l9mm readily vaporisefrom the heat of a focused laser beam. This allows precision sizing of items such as gaskets.Material with fibre or steel cord reinforcement can be cut with a laser at considerably slowerspeeds due to the higher energy intensity per unit time necessary to sever the cords.The advantage of laser cutting is the simplicity of handling without having to worry aboutstretching or distorting of the material due to the impact of a cutting tool. Fresh cut samplestend to exhibit slight stickiness along the edge so they require care in post-process handling.Additionally, some rubber, particularly those containing carbon black, may require a clean-upoperation to wipe clean any edge charring.
WoodThe laser offers a number of attractive advantages for the cutting of timber, plywood, andparticleboard. In particular, it provides narrow kerfs of 0.3-0.8mm, the absence of sawdust,the ability to contour cut in any direction and no tool wear and noise. While the use of a laserlikewise eliminates rough, torn-out, and fuzzy edges as evident with conventional sawingtechniques, it is characterised by "burned" edges produced by the laser's heat. Greateramounts of charring will result when the material thickness is increased, thereby slowing thecutting feed-rates.While lasers are routinely cutting slots in die boards for mounting of steel rule dies theiracceptance for other industrial applications has been hampered by process limitations andrelatively high initial cost. Since practical power outputs are limited to a few kilowatts, lasersare limited in their ability to cut up to 75mm thick for timber and 25mm for particleboard andplywood.
Other OrganicsPaper products and leather, as well as natural and synthetic textiles, can easily be cut with alaser. The lack of thickness; coupled with their high combustibility minimises the power outputrequirements of a laser to no more than a few hundred watts. The resultant edges are cleanand free from fraying.
QuartzSince it has a relatively low co-efficient of thermal expansion, quartz responds well to thecutting action of a laser. Though there is the presence of a shallow heat affected zoneadjacent to a cut, the resultant edges are crack-free and have a smooth appearance therebyeliminating clean-up operations required by saw cutting. Thicknesses up to10mm can be cutat speeds that are a couple orders of magnitude greater than sawing and without impartingforce to the work piece.
GlassAs opposed to quartz, most types of glass are prone to thermal shock and are thereforegenerally not suitable candidates for laser cutting. The instantaneous heat of the laser's beamprovides cutting action by both vaporisation and the blowing away of molten glass from thecut zone.Some materials such as boro silicates have a low co-efficient of expansion and, withadequate head cycling, can tolerate the heat input from a laser. However, most other forms ofglass including soda lime experience thermal shock that results in crack propagation alongthe cut edge. Also, based on the reflow characteristics of the particular glass, there will bevarying degrees of resolidified material that will adhere to the edges and underside of the cut.
Stone & RockWhile they tend to absorb the heat energy from a laser, granite, concrete, rock, stone andvarious minerals are not suited for laser cutting. The explosiveness from heating moisturewithin the materials can lead to undesirable cracking. Aside from the lack of uniformity in theirstructures, stone and rock are typically found in thicknesses greater than 25mm, far in excessof the practical depth. of field of useable focussed laser energy.
MetalsAlthough at room temperature, almost all metals are highly reflective of infrared energy, theCO 2 laser with its 10.6-micron wavelength (far infrared) is successfully employed on manymetal cutting applications. The initial absorptivity can range from only 10% to as little as 0.5%of the incident energy. However, the focusing of a beam to provide power densities in excessof 1 million watts per square cm can quickly (in a matter of microseconds) initiate surfacemelting. The absorption characteristics of most metals in their molten states increasedramatically, raising the absorptivity of energy to as much as 60% - 80%.
Carbon SteelConventional steels of up to 16 mm lend themselves reasonably well to oxygen assisted lasermating. The kerfs are narrow (as little as 0. 1 mm for thin material) and the resultant heataffected zones are negligible, particularly for mild and low carbon steel. At the same time, thecut edges are smooth, clean, and square.It has been found that the presence of pockets of phosphorus and sulphur within mild steelcan cause burnout along the cut edge, as such, the use of low impurity steels (eg cold rolled)will result in improved edge quality over results obtained with hot-rolled material. A highercarbon content within the steel does yield a slight improvement in edge quality yet will makethe material subject to an increased HAZ.
Stainless SteelLasers have been shown to be viable cutting tools for the fabrication of sheet metalcomponents made from stainless. The controlled heat input of the laser beam serves tominimise the HAZ along the cut edge, thereby helping the material to maintain its corrosionresistance. Since stainless does not react with an oxygen assist as efficiently as does mildsteel, cutting speeds for stainless are slightly slower than those for comparable thicknesses ofplain steel. At the expense of up to 50% of the speed for oxygen-assisted cutting, an inertassist gas can be employed to obtain a "weld ready", oxide-free cut edge.As for the resultant cut quality, martensitic and ferritic (400 series) stainless provide cleansmooth edges. The presence of nickel within austenitic (300 series and precipitationhardened) stainless steels affects the energy coupling and transfer within the material.Specifically, the viscosity of molten nickel generated during the cutting action causes it tomigrate and adhere to the backside of the cut. While the use of high velocity gas jets caneffectively eliminate slag for material up to 1.0 mm thick, slag deposits up to 0. 5mm aregenerally present on thicker cross sections.
Alloy SteelSince care is taken to control the amount and distribution of additives to the base iron, mostalloy steels are considered ideal candidates for the laser cutting process. High strengthmaterials such as AISI-SAE 4130 (chrome moly steel) and 4340 (chrome nickel moly steel)display exceptional laser cut edges that are square and clean.
Tool SteelSimilar in many ways to allow steels, most tool steels respond reasonably well to the cuttingaction of a laser. The most notable exceptions are the tungsten high speed (Group T) andtungsten hot work (part of Group H) materials that retain heat in a molten state, therebyresulting in burned out and slaggish cuts.
Aluminium AlloysDue to its high thermal conductivity and high reflectivity to a CO 2 laser's wavelength,aluminium requires considerably higher laser energy intensity in order to initiate cuttingcompared to steel. This means the need for a laser possessing exceptional beam quality andcapable of outputting at least 500 watts, in addition to precise focus control. Due to thereduced coupling efficiency, even 1-2 kilowatt lasers are limited to cutting of thicknessesunder 3.8mm.During the cutting process, the assist gas serves primarily to blow the molten material fromthe cut zone. This helps to produce edge quality that is generally superior to that produced bya bandsaw. However, the melted material tends to flow along the edge and cling to thebackside of the cut. While this slag is easily removable, there are intergranular cracksemanating from the cut surface on some alloys. Concern over the presence of this microcracking has prevented the use of lasers for manufacturing structural components such asaircraft.
Copper AlloysCopper has less ability than aluminium to absorb energy from a CO 2 laser. Due to its highreflectance, copper generally cannot be cut. Brass on the other hand can absorb someenergy. It essentially behaves like aluminium with slag adhering to the backside of the cut.
TitaniumPure titanium responds well to the concentrated heat energy of a focused laser beam. Theuse of an oxygen assist enhances the cutting speeds but tends to promote a larger oxidelayer along the cut edge. Aircraft alloys such 6AL-4V tend to exhibit some slag that adheres tothe bottom side of the cut but is relatively easy to remove.
Primary ConsiderationsThis section discusses the criteria that are important to successful cutting. It is intended as aguide only, since there is no substitute for operator experience.These are the principal considerations with which the operator must concern himself at alltimes. Note that it is the combined effect of these adjustments that determines the result. Thevarious items cannot be considered independently.
Laser Power SettingThe most important point regarding laser power is that maximum power is not necessarilybeneficial. Firstly, there is some trade-off between power and mode - the mode (or quality ofthe beam, which determines the fineness of the focus) is of significantly greater importance tocutting than the power level. Secondly, limiting the power is frequently beneficial in terms ofreducing thermal input into the material - especially when cutting thin material, or materialswhich can be adversely affected by excess heat. It is simply wasteful to use more power thannecessary.
Cutting SpeedThe actual feedrate in use for a job will directly affect the cutting results; the feedrate isdecidedly a function of the type of material and material thickness to be used. In anyparticular case, there will be some feedrate that is too high and the cut will simply fail topenetrate the material fully; at the other extreme, excessive heat input is likely to damage thematerial adjacent to the cut. In general, some feedrate closer to the maximum limit will beoptimum, but always the choice is made experimentally on the basis of cutting results; theoperator, with a little experience, can make this determination quite readily, making use of thefeedrate override control.
Focal HeightFocus assemblies provide support for the lens in order to image the beam. These assembliesgenerally provide means to adjust the focal point in or at the part. Height sensing devices canbe incorporated to automatically maintain the proper focal point position regardless ofundulations in the work piece surface. These devices measure the lens-to-work piece spacingeither through contact probes riding on the work piece surface or via a comparison of noncontactoptical, acoustic, or electrical (inductance or capacitance measuring) signals bouncedoff the material. The feedback can trigger compensation of the vertical axis position.For best results, the focal point of the beam must impinge on the surface of a work piece. Thisfactor is of greater or lesser importance, depending on the material; in general, materials thathave a high intrinsic reflectivity to the laser beam will be most critical of the focal heightsetting (eg. mild steel 45% reflective; stainless steel 66%; aluminium 99%). The focal point onaluminium and stainless steel should be approximately 4/5 buried into the material. Thickercarbon steel will cut better when the focal point is 1-2 mm above the material.The operator may find that the focal point needs to be "tweaked" occasionally during a job;the precise focal point can change slightly owing to thermal effects in the lens.Nozzle Lateral Adjustment (Spot)Gas jet nozzle assemblies are usually integrated with the focusing assembly below the lens inorder to develop the desired gas assist. A properly designed nozzle tip is very important to thecutting process. It can promote higher feedrates, and better quality with minimum gasconsumption.Nozzle adjustment is an important factor, ensuring that the beam emanates centrally throughthe orifice. Misalignment of the nozzle normally causes noticeable variations in cut quality withrespect to the direction of the cutting traverse. Severe misalignment results in the laser beamhitting the inner walls of the nozzle, with consequent poor cutting performance, and heating ofthe nozzle and surrounding assembly.The nozzle adjustment must be made whenever a lens is changed, or even if a lens isremoved temporarily for cleaning. During a working shift, the operator might "tweak" thisadjustment a few times; the slight changes in pointing angle of the beam (through the externaloptical system) account for this requirement.
Secondary ConsiderationsThese are considerations with which an operator must become concerned when cuttingresults are below expectations, and all primary considerations (listed above) have beenchecked.
Choice of lensAs a general rule, the shortest focal length lens (5") produces the most sharply defined focalpoint. Thus, the 5" lens is used when maximum intensity is important - that is, cuttingmaterials with high intrinsic reflectivity (metals). In practice, there is only a slight (but usuallynoticeable) difference between a 5" lens and a 7.5 lens in this respect.The longer focal length is required, however, to achieve parallel-sided cuts in some materialswhen the material is reasonably thick. For example, to cut 1" thick acrylic, it is found virtuallyimpossible to keep the sides of the cut parallel with the 5" lens, whereas the 7.5" lens makesthis quite easy. Note that the choice of laser power, assist gas pressure, and feedrate allcombine to influence the cut quality in this respect, apart from the lens itself.
Condition of the lensCleanliness of the lens is of major importance, since any contaminants on its surfaces willcause it to absorb energy and become warm. Thermal distortion in the lens inevitablyproduces fuzziness in the focal point of the beam, and consequent reduction in cuttingperformance. Eventually, if a lens becomes excessively heated, thermal stress and gaspressure will cause it to shatter.The operator should inspect the lens regularly (and clean as necessary). In fact, commonsense is the rule here; the source of contamination is virtually always airborne particlesproduced by the cutting. Therefore, if material being cut produces contaminants (eg. sheetmetal often has oil on the surface; rubber produces black smog when cut; etc.), the lensshould be inspected as often as convenient. The assist gas greatly helps in keepingcontaminants away from the lens, but the operator must be aware that this is by no meanstotal protection. Lifting the focal height while piercing will also help protect the lens.
Condition of the nozzleThe copper nozzle may become damaged or blocked in time, usually as a result of hot metalspatter thrown up from the work surface. The orifice can be cleared by a conventional oxy tipcleaner.
After some usage, the orifice may become "out of round"; this causes swirling or vortex actionin the assist gas jet, which usually produces highly directional effects in the cutting. Often, thiscan be rectified by carefully drilling the orifice; if the orifice is of large diameter then excessiveassist gas will be consumed. Eventually, the nozzle will need replacement.
External Optical AlignmentThe optical system (set of mirrors) external to the laser cavity (including the mirror mounted atthe top of the Beam Control Unit) should normally be checked and adjusted on a routine basis(say, once per month). However, if the integrity of the alignment is under suspicion in themeantime, it can be checked by using the cross-wire method of allowing the raw beam topass through the system onto a target. The image produced by the cross-wire (its shadow)will indicate whether the beam passes through the position centrally.Misalignment of the external beam will generally cause haphazard cutting results, with highlynoticeable directionality.
Assist Gas PressureGenerally oxygen is used for metal cutting, and air is used for non-metal cutting. Highpressurenitrogen can be used to cut mild steel, stainless steel and aluminium. Using nitrogenas an assist gas leaves the cut edges clean and free of dross but is expensive because up to25 bar is needed. The general rule with assist gas is: there must be sufficient flow (pressure)in each case, but an excessive amount is wasteful. Normally, high-pressure cutting requiresincreased pressure with increased material thickness and cutting with oxygen requiresdecreased pressure with increased thickness. Of course, the type of material is also aninfluence; very low carbon steel, for example, will be adversely affected by excessive oxygenflow since it is highly reactive. In any particular case, the pressure used will be experimentallydetermined, and is usually not highly critical. Note that no material can be cut without anyassist gas.