MINIMAL CONTACT
  Because both exert minimal contact, tooling can be simple, thus less expensive. In most cases, tooling is used to hold material flat and/or off a referenced edge.

KERF SIZE
  The small kerf produced by laser and AWJs permit very fine cutting and drilling. The smaller kerf also allows close nesting of parts that can be a significant cost saving for expense materials. The typical kerf width for AWJ is 0.875 mm (0.035 inches) and 0.250 mm (0.010 inches) for laser. The difference in kerf width is insignificant, but there may be times when a small radius is necessary, in which case laser cutting is more suitable. .

MATERIAL HARDNESS EFFECTS
  Conventional machining requires material to be soft state. Material is cut and in some cases heat-treated. This often creates problems in distortion, shrinkage, elongation, and/or warping. By heat-treating materials prior to laser or waterjet processing , grinding and/or re-forming due to distortion during the heat-treating process can be avoided. The cutting and drilling speeds of metals are independent of material hardness, in most cases. Additionally, the minimal heat input from both processes reduces distortion and/or stresses. Abrasive waterjet cutting provides no distortion because it does not depend on the exothermal reaction to aid in the cutting process.

MATERIAL THICKNESS
  Laser cutting becomes very limited when material thickness increases. Cutting carbon steel over 10 mm thick may not be effective. The thickness limitation is further reduced by the composition of the material. Laser cutting thickness limitations are dependent on heat conductivity, surface reflection at 10.6 microns, vaporization point of alloys, the types of alloys, surface tension of molten materials and geometry of the part. With the increase in thickness, the likelihood of a "blow-out" (thermal runaway) will occur. AWJ can process much thicker material. One-hundred-fifty-millimeter-thick carbon steels can be cut with the AWJ. AWJ seems to be most effective at 25 mm thick steel. Generally, it is desirable to stack thin parts on top of each other to obtain a thicker section. In cutting steels, the only limiting factor other than thickness is abrasive resistance.

Differences

ABRASIVE RESISTANCE
  The more abrasive resistance, the more difficult it is to cut with AWJ. The process is slowed down to compensate for the abrasive resistance of the material. It is sometimes practical to change the abrasive from garnet to silicon carbide or nitride to improve the cutting speeds, however, these materials are more costly and they create more wear on the abrasive feed system. Accuracy and speed decreases in proportion to abrasive resistance qualities of the material. Taper and AWJ stream tailing are more pronounced in abrasive resistance materials. Generally, the abrasive resistance qualities do not affect laser cutting.

MATERIAL COMPOSITION
  The composition of the material will affect laser processing dramatically over that of AWJ cutting. Laser processing is influenced by the melting point of the material, conductivity, viscosity of metal in liquid state, surface tension, absorbability of material at 10.6 microns, and carbon content. For example, copper is very difficult to cut with a laser, but not for AWJ. Set-up time for laser can be quite lengthy and costly when cutting unfamiliar materials. Nozzle size, power, optical focal length , assist gas, gas pressure, speed, and focal point can all affect the process. These parameters are so important to the process, if not correct, the material will not cut. Once the combinations are correct, however, the speed of cutting can be two to five times faster than AWJ. Material composition other than abrasion resistance has little effect on AWJ cutting, so set-up time can be much shorter. The parameters set for one material can generally be used for other materials. This allows AWJ to cut a combination of material in layers very easily.

MATERIAL QUALITY
  Laser processing is more sensitive to material quality than AWJ cutting. The surface finish can dramatically affect the quality of cutting. In most cases, steel must be clean, pickled, and oil free to effectively cut. Hot rolled steel presents serious quality problems in cutting because of the surface scale. The surface scale tends to melt in with the metal creating an undesirable surface finish. Surface texture must be smooth, otherwise the assist gas and laser focus can be altered, affecting quality of the cut. AWJ is oblivious to these problems.

RECAST LAYER
  Laser processing will leave a recast layer on the surface. Laser cutting melts and burns some of the metal. It will deposit some re-melted materials on the side of the cut edges and on the bottom of the cut. This layer of deposited materials is highly stressed and may crack, especially if it is oxide. Although these cracks are small, they can propagate into the material creating larger cracks. This is especially true for inside corners with small radius where stresses are higher. This can be eliminated in certain cases with high pressure nitrogen cutting. However, this method of cutting is more expensive and more difficult to control. The motion system for high pressure cutting must be very precisely controlled. The results of high pressure nitrogen cutting are impressive. Cutting speeds over AWJ is at least three times faster. Presently, high pressure cutting is typically used in 300 series stainless steels. AWJ cutting does not melt the material so there is no problem with recast.

HEAT-AFFECTED ZONE (HAZ)
  Laser cutting does produce a heat affected zone (HAZ). HAZ occurs in metals when the temperature rises above the critical transformation point. In laser cutting this is localized near the cutting zone. In carbon steel, the higher the harden-ability the greater HAZ. For example, on 4140 steel 7 mm thick will produce a HAZ zone of about 0.18 mm. Since HAZ is brittle, this area has a low tolerance for cracking during bending or stress. In most cases the HAZ can be eliminated by post heat treating the part, but there is a risk of distortion. In AWJ there is virtually no HAZ.

STRESSES
  Laser cutting creates more stresses in material than does AWJ cutting. In most cases, laser processing will produce little distortion in material, but this is dependent on the laser parameters, material thickness, and composition of material. Distortion from laser processing comes from the sudden rise in temperature of the material near the cutting zone. Distortion will also come from the rapid solidification of the material remaining on the sides of the cut. The heat induced stresses can be reduced by adding a water quenching system to the laser cutting nozzle. AWJ does not create any stress.

TAPER--WANTED AND UNWANTED
  AWJ can produce more taper than laser cutting. One of the biggest drawbacks with AWJ cutting is the taper created during the process. Typical taper in AWJ is twice that of laser. Although taper can be controlled with speed, it is usually about 25 percent of the nozzle diameter (typically 0.23 mm or 0.008 inches) at the most cost-effective speed. Slowing down the process speed can correct this problem, but will increase part price. In practice, the process is slowed down only in those areas where the taper needs to be controlled. Taper becomes greater by the increased abrasive resistance properties of the material. The more resistant the material the more taper. This taper can actually be a benefit in some cases. Taper can be programmed into a part by altering the angle and/or speed of the jet.

COMPLICATED GEOMETRY
  AWJ is more capable of cutting more geometric shapes. Laser has limitations to side wall thickness. This thickness limitation is dependent upon the material and thickness. For example, to cut a 0.250 mm thick sliver in 6 mm is next too impossible with a laser. AWJ has no problem.

START HOLE DRILLING
  Drilling starting holes is faster with laser but not as safe. Laser can drill holes very quickly. Often during the drilling process with laser, there will be a "blow-out", especially if the hole is small in relation to the thickness. This can scrap parts. AWJ is much slower, however, the drilling with AWJ is more controllable.

ADDRESSING SECONDARY OPERATIONS AND PROCESSING PROBLEMS
  Both processes have some clean-up operations. In some cases the laser recast and slag from side and underside will need to be removed. Slag from the cutting process must be ground off. This can be a difficult task if the part has a complicated geometry. Also, some serious quality problems can result due to over grinding. Some materials often oxidize very quickly so oil protection or coating may be required. Most parts must go through a secondary operation to remove the residual garnet. In some cases there may be a slight burr that may need to be removed.

  Fumes are a big concern with laser cutting. Certain metals such a stainless steel can give off toxic vapors which if inhaled can be very hazardous. With AWJ cutting, the toxicity of fumes or mist is not fully understood. If the material is toxic in its natural state, the fumes will also be hazardous. AWJ cutting does not alter the chemical composition, whereas Laser processing can change a harmless material into a toxic gas.

  Both processes have their safety problems. Some concerns with using Lasers include: radiation burns, eye problems, potential for fires from the process and equipment, and most important, the high voltage hazards that have claimed several lives. AWJ is hazardous due to the possibility of water and garnet penetration under the skin. Water jet can also be extremely noisy, therefore hearing protection and monitoring is required. Both processes require their own restricted area. In the case of the laser it is required by law. In the case of the AWJ a separate room for authorized personnel is required.

  Environmental control is very important for both. The AWJ process produces waste disposal problems. The garnet itself is not a problem, the cutting material removed during the process can be a problem to dispose. In certain areas of the country it is not permitted to dispose of used garnet at local land fill. It is best to obtain information from the local county for land fill restrictions prior to disposing. As AWJ cutting becomes more popular the problem could become more pervasive.

 The Motion system used to cut materials in both processes is important. Laser motion systems can be much more complicated because of beam and acceleration requirements. Beam alignment is significant if accuracy is to be maintained. 

  Acceleration is very important in laser processing. Lasers needs constant speed. Typically a good laser system will need at least 0.5 G of acceleration. With AWJ 0.25 G is adequate. Most AWJ cutting systems move the head for positioning. Moving the work piece is impractical in most cases because there needs to be a tank to catch the garnet stream. AWJ does create other motion problems. There is some backward force from the AWJ stream (16 lb.). If the head is not well secured, it can create tolerance problems. AWJ need not maintain constant speed. The acceleration and de-acceleration must be controlled for effective AWJ cutting. By reducing the need for rapid acceleration, a system can be built lighter and less expensive than a laser system. The protection of the unit from abrasives may however, off-set any cost savings of the system.

Original article by Robert Ulrich (1949-2002), Founder, LAI International, Inc.
Copyright 1999, 2007

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