Comparison Metal Water Jet Cutting with Laser and Plasma Cutting
Nowadays, we used many different materials for the production which generally have to be respectively formed by a more or less complicated cutting operations. In this article, I will focus attention on three ways of shaping the workpiece materials, namely: water jet cutting, laser and plasma cutting. These methods will be compared only in terms of cutting metals, which significantly reduces the scope of discussion and at the same time doing their precision. In the literature I have not found a good comparison of these three techniques cutting simultaneously.
Water jet cutting
Water jet is the method consisting of cutting the material (or a water jet can relate to cleaning it) by the use of thin water jets under high pressure with added abrasive slurry used to cut the target material by means of erosion. The technique with using high pressure water for cutting materials was first time patented in 1968 by researcher in USA, but fast development of water jet cut method was starting in early ‘80s. Today is a rapidly developing technology, that is used in industry for processing variety of engineering materials. It is an emerging technology, which has many advantages over the other non-conventional cutting technics. Often, in order to improve the performance of the process additive is used in the form of abrasive grains of garnet, which allows cutting of very hard materials. The correct name of this technology is so cutting hydro-abrasive treatment (abrasive water jet).
Cutting process can be described briefly as follows. The water fed by a pump under pressure, after passing through the water causes the suction nozzle to the abrasive mixing chamber. Then a mixture of water and abrasive is directed to the mixing nozzle in order to form and stabilize. The result is a stream of hydro-abrasive, which has enough power to cut through even the toughest materials.
Water jet we can applied in many areas of modern industry, such as automotive industry, aerospace industry, construction engineering, environmental technology, chemical process engineering, and industrial maintenance. Typical water jet cutting machines have a working space from few square feet to hundreds of square feet. In this moment the high pressure water pumps are available from 276 MPa up to 689 MPa.
Laser cutting
Laser cutting is a technology, which enables a laser to cut of various materials using the high-point of the cutting jet by the introduction of energy and technical gas of high purity. Laser radiation is characteristic, and thus practically unattainable by other methods. Gets plenty of power here in the choice, but a very narrow area of the spectrum. Its characteristics include consistent in time and space radiation, and the most polarized beam with low divergence. Depending on the cutting device is carried out in three ways: by burning, melting or sublimation.
Creation of the laser beam generally involves stimulating a lasing material by electrical discharges or lamps within a closed container. The beam is reflected internally through a partial mirror, until it achieves sufficient energy to escape as monochromatic coherent light. Generally, the narrowest part of the focused is less than 0.32 mm in diameter. Of course, taking into account the thickness of the material, the width of the gap as small as 0.10 mm are possible.
Plasma cutting
Plasma cutting is basically a process that is used to cut generally steel and sometimes other metals of different thicknesses. This process consists of metal melting, and then disposing of the cut metal from the slot. This is done by means of a concentrated plasma arc, having a large kinetic energy. In fact, plasma cutting uses a high temperature that prevails in the core plasma arc and high speed plasma stream. The electric arc is formed between the tungsten electrode and the cut object. Principles of its formation is as follows: by passing the gas stream in the compressed form of the arc for the phenomenon is a result of ionization and high power density is possible to produce a stream of plasma. The most commonly used gas plasma cutting is air and also in high-power devices used argon, nitrogen, hydrogen, carbon dioxide. Plasma arcs are extremely hot and are in the range of 25 000 ° C. Due to the high temperature plasma cutting edge of a destructive influence on the confluence. This method we can usually cut from 50 mm to 150 mm thick. This is a large range of metal cutting.
Comparison of water jet, laser and plasma cutting
Water jet cutting as compared to laser and plasma technology is primarily a more versatility. We cut here virtually any type of material, are not so restricted as in the case of a laser to homogeneous materials which do not reflect light, or as in the case of the plasma to the conductive material. For example, a perfect choice for cutting stone of varying thickness, shape and properties. Due to the small cutting force ceramic does not break, when cutting of complicated shapes. Here, there are quite a lot (in terms of waterjet technology) limitations on cutting speed, but keep in mind that this technology does not release harmful gases, UV radiation and other hazardous substances harmful to the machine operator. Technology same water jet cutting is so safe that we can use it to cutting food. Strength cutting head is relatively large, and the price of water - the basic cutting agent low. Texture of cut material here does not affect the quality of the processing, as is the case with the laser, where it may lead to distortion of the beam. You may also cut highly reflective materials such as aluminium and copper, and the cutting of holes does not make too much trouble here as in the case of plasma cutting.
One of the greatest advantages of hydro-abrasive machining, compared to the technique of laser cutting of materials is the possibility of limited practical geometry of the machine. It is mainly about cutting materials over 30 mm, where the waterjet works very well, and lasers are starting to have the first problem. When we continue this thought and move on to more than 100 mm cut material, it turns out that the hydro-abrasive machines are already unbeatable compared to laser technology, and it is not yet the end of their ability. Please note that, as usual, the thickness of the cut material is associated with a relatively longer duration of cutting, which should also be taken into account because it is not the fastest cutting techniques. Compared to laser plasma seems to be more universal, because it can be used to cut materials in the range from 0.5 mm to about 160 mm. However, as the thickness of the material also increases the wear of the electrode.
Another important advantage of water jet machining is the lack of any thermal deformation of the material being cut, and thus no melted edges. The temperature in the vicinity of the treatment increases in small areas as compared to other methods, and the process further location in the water, will accelerate the removal of heat from the treatment zone. As a result, the amount of heat generated at the cutting does not affect in any way the shaped material. For these reasons we can declare that the structure was preserved, because no significant change in the existing structure.
The structure of the cut surface in waterjet machining has a very high quality. The edges are rounded here, and additionally are not formed burrs. This results in a lack of need for a finishing process, which can significantly shorten the duration of the production of parts, and allows you to reduce the number of machines. Water jet machining this is a way to cut without heat Interactions. The lack of high-temperature cut material is not distorted area at near cut, so you do not need to use excess material that must then delete. It is the preferred method when the materials being cut are sensitive to the high temperatures generated by other methods. Little downside hydroabrasive technique is less precise cuts and relatively large compared to the noise in the laser cutting. However, the worst falls here plasma technology, which is both highly inaccurate and loudest.
Both the water jet machining, laser and plasma cutting can be easily attached the starting material. In water jet cutting the cut object is laid directly on a special grid, which prevent to the object from falling into the tank with water. In the absence of forces which could move the workpiece is used specific weights or fixtures. All compared treatment techniques are easy to program, which practically consists of the appropriate cutting path planning. There is no a problem choice of tools. With the right software it is possible to determine the shape of the cutting time.
Introduction to CNC Milling
In the past, complex machine parts or molds were often broken down into small, simple parts. Then, they are joined together into complete parts by welding and riveting methods. This machining does not guarantee high accuracy and is expensive. Later, thanks to copying technology, it is possible to process more complex details. However, this machining still has many disadvantages such as low productivity and difficulty in ensuring high accuracy. Therefore, the application of CNC to machine tools is a leap forward in machining technology. It ensures high precision, can process complex parts.
In the process of industrialization and modernization of the country, our country's mechanical industry is developing in the direction of automation, design and processing with the help of computers. That requires experiential learning, technology mastery and continuous development of mechanical production facilities. In order to achieve the above goal, domestic mechanical enterprises and training institutions have been investing more and more modern machine tools, completing the problem of exploitation and effective use of technical and economic efficiency. is an urgent requirement, especially the issue of training scientific and technological human resources capable of accessing, mastering and effectively exploiting modern CNC machines.
CNC: stands for Computer Numerical Control and is a type of machine that is controlled automatically with the help of a computer. Parts are automatically programmed to operate according to the sequence of events set by the user to create a product of the required shape and size.
Milling: is a type of machining that uses cutters to shape a workpiece, often on a moveable tabletop, although some milling machines also feature movable cutters. Milling started out as a manual task performed by humans, but most milling these days is done by a CNC mill, which utilizes a computer to oversee the milling process.
CNC milling, or computer numerical control milling, is a machining process which employs computerized controls and rotating multi-point cutting tools to progressively remove material from the workpiece and produce a custom-designed part or product. This process is suitable for machining a wide range of materials, such as metal, plastic, glass, and wood, and producing a variety of custom-designed parts and products.
Overview of CNC Milling Process.
Like most conventional mechanical CNC machining processes, the CNC milling process utilizes computerized controls to operate and manipulate machine tools which cut and shape stock material. In addition, the process follows the same basic production stages which all CNC machining processes do, including:
Designing a CAD model
Converting the CAD model into a CNC program
Setting up the CNC milling machine
Executing the milling operation
The CNC milling process begins with the creation of a 2D or 3D CAD, CAM part design. Then the completed design is exported to a CNC-compatible file format and converted by CAM software into a CNC machine program which dictates the actions of the machine and the movements of the tooling across the workpiece. Before the operator runs the CNC program, they prepare the CNC milling machine by affixing the workpiece to the machine’s work surface (i.e., worktable) or work holding device (e.g., vise), and attaching the milling tools to the machine spindle. The CNC milling process employs horizontal or vertical CNC-enabled milling machines—depending on the specifications and requirements of the milling application—and rotating multi-point (i.e., multi-toothed) cutting tools, such as mills and drills. When the machine is ready, the operator launches the program via the machine interface prompting the machine to execute the milling operation.
Once the CNC milling process is initiated, the machine begins rotating the cutting tool at speeds reaching up to thousands of RPM. Depending on the type of milling machine employed and the requirements of the milling application, as the tool cuts into the workpiece, the machine will perform one of the following actions to produce the necessary cuts on the workpiece:
Slowly feed the workpiece into the stationary, rotating tool
Move the tool across the stationary workpiece
Move both the tool and workpiece in relation to each other
As opposed to manual milling processes, in CNC milling, typically the machine feeds moveable workpieces with the rotation of the cutting tool rather than against it. Milling operations which abide by this convention are known as climb milling processes, while contrary operations are known as conventional milling processes. Once the milling operation is completed, and the part is produced to the custom-designed specifications, the milled part passes to the finishing and post-processing stages of production.
CNC Milling Machine Operations
CNC Milling is a machining process suitable for producing high accuracy, high tolerance parts in prototype, one-off, and small to medium production runs.. The versatility of the milling process allows it to be used in a wide range of industries and for a variety of part features and designs, including slots, chamfers, threads, and pockets. The most common CNC milling operations include:
Face milling refers to milling operations in which the cutting tool’s axis of rotation is perpendicular to the surface of the workpiece. The process employs face milling cutters which have teeth both on the periphery and tool face, with the peripheral teeth primarily being used for cutting and the face teeth being used for finishing applications.
Plain milling, also known as surface or slab milling, refers to milling operations in which the cutting tool’s axis of rotation is parallel to the surface of the workpiece. The process employs plain milling cutters which have teeth on the periphery that perform the cutting operation. Depending on the specifications of the milling application, such as the depth of the cut and the size of the workpiece, both narrow and wide cutters are used. Narrow cutters allow for deeper cuts, while wider cutters are used for cutting larger surface areas.
Angular milling, also known as angle milling, refers to milling operations in which the cutting tool’s axis of rotation is at an angle relative to the surface of the workpiece. The process employs single-angle milling cutters angled based on the particular design being machined to produce angular features, such as chamfers, serrations, and grooves.
Form milling refers to milling operations involving irregular surfaces, contours, and outlines, such as parts with curved and flat surfaces, or completely curved surfaces. The process employs formed milling cutters or fly cutters specialized for the particular application, such as convex, concave, and corner rounding cutters.
The main types of CNC Milling Machines.
Machine interface: The machine interface refers to the machine component the operator uses to the load, initiate, and execute the CNC machine program.
Column: The column refers to the machine component which provides support and structure to all other machine components. This component includes an affixed base and can include additional internal components which aid the milling process, such as oil and coolant reservoirs.
Knee: The knee refers to the adjustable machine component which is affixed to the column and provides support to the saddle and worktable. This component is adjustable along the Z-axis (i.e., able to be raised or lowered) depending on the specifications of the milling operation.
Saddle: The saddle refers to the machine component located on top of the knee, supporting the worktable. This component is capable of moving parallel to the axis of the spindle, which allows the worktable, and by proxy the workpiece, to be horizontally adjusted.
Worktable: The worktable refers to the machine component located on top of the saddle, which the workpiece or work holding device (e.g., chuck or vise) is fastened. Depending on the type of machine employed, this component is adjustable in the horizontal, vertical, both, or neither direction.
Spindle: The spindle refers to the machine component supported by the column which holds and runs the machine tool (or arbor) employed. Within the column, an electric motor drives the rotation of the spindle.Arbor: The arbor refers to the shaft component inserted into the spindle in horizontal milling machines in which multiple machine tools can be mounted. These components are available in various lengths and diameters depending on the specifications of the milling application. The types of arbors available include standard milling machine, screw, slitting saw milling cutter, end milling cutter, and shell end milling cutter arbors.
Ram: The ram refers to the machine component, typically in vertical milling machines, located on top of and affixed to the column which supports the spindle. This component is adjustable to accommodate different positions during the milling operation.
Machine tool: The machine tool represents the machine component held by the spindle which performs the material removal operation. The milling process can employ a wide range of milling machine tools (typically multi-point cutters) depending on the specifications of the milling application e.g., the material being milled, quality of the surface finish required, machine orientation, etc. Machine tools can vary based on the number, arrangement, and spacing of their teeth, as well as their material, length, diameter, and geometry.
Applications of CNC Milling Machines.
With increasingly affordable tooling costs and the ability to create a wide variety of complex parts, CNC milling is a popular solution for projects from prototypes to full fabrication of unique precision parts. CNC milling machines have been widely applied in life and production. CNC milling serves industries such as:
Aircraft and aerospace
Prototypes and custom designs
Commerce
Maintenance
Electronic device
Medical, disposable and non-implantable
Entertainment
Technology and security
Telecommunication
Transport and cars
Industrial and O.E.M.
Laser cutting of Aluminium alloy BS 1100 sheet using CO2 laser
The problem under focus for this research work was gas laser cutting of ―difficult-to-laser-cut‖ material. Non ferrous materials such as aluminium alloys, brass and copper alloys were the difficult-to-laser cut materials. They were named so since they were less absorptive to laser beam by nature and reflected majority of the laser beam that falls upon them. Hence laser cutting of such material became a tedious task subject to various limitations such as sheet thickness, higher kerf width and higher surface roughness.
Abrasive Water Jet Machining process
Abrasive water jet (AWJ) has been finding extensive use in the manufacturing industries for machining wide range of materials such as metals and non-metals. The reason behind the selection of AWJ machining process is that does not generate heat at the cutting zone, but the heat is less while machining hard materials; ability to cut all kinds of materials such as metals, non-metals, composites, ceramics; a higher material removal rate than the Wire EDM process, and production of a better surface integrity than the laser machining process; cutting thick components in the range of 250 mm (depends on materials); absence of thermal distortion to work materials but ability to cut intricate shapes; existence of minimum cutting force on the work materials and yield of better dimensional accuracy due to insignificant deformation; rock drilling and surface cleaning.
Major influence of AWJ machining parameters
a) Water jet pressure: This is an important process parameter in the AWJ machining process. Kinetic energy of the AWJ depends on the pressure level of water. Pressure is less than the threshold pressure range when no material removal takes place. Similarly, the pressure equal to the critical pressure range represents the limit indicated for effective cutting. The machining process becomes ineffective if extended beyond this stage. Water jet pressure is directly proportional to the penetration depth and the material removal rate. It has influence on the distribution of water as well as abrasive particles in the jet. It is frequently denoted by MPa or bar or PSI.
b) Traverse rate: It determines the quality of the cut surfaces produced by the AWJ process. The major influence on the traverse rate on the AWJ machining process is the determination of the exposure time. A Lower traverse rate increases the surface quality by allowing a larger number abrasive particles to impinge on the target material surface. It also impacts on the cutting rate of the process. The traverse rate is indicated by mm3 /min.
c) Abrasives: Various types of natural (garnet) and artificial abrasives (silicon carbide, aluminium oxide) are used in the AWJ machining process. The abrasive particle size, shape and hardness have a significant influence on the AWJ cutting performance. The higher the hardness of the work material, the harder should be the abrasives to be used. An increase in the size of the abrasive particle increases the particle disintegration. A decrease in the depth of penetration and material removal rate is seen following an increase in the limit of the size of the abrasive particles, which happens due to a reduction in the impingement frequency on the target material surface. The range of the abrasive size is universally indicated by mesh size (#).
d) Abrasive mass flow rate: The flow rate of the abrasives along with the water jet pressure has an influence on the AWJ material removal rate. An optimum supply of abrasives yields a higher cutting performance with a better surface finish. The flow rate of the abrasives depends on the diameter of the focusing nozzle used in the AWJ machining process. A transformation of the jet momentum is required for ensuring effective acceleration of the abrasive mass flow rate. The rate of abrasive mass flow is usually expressed as kg/min.
e) Stand-off distance: It is defined as the distance between the target material and the nozzle. It is usually maintained by an optimal level of distance in mm, considering the large influence of SOD on the kerf profile produced by the AWJ.
Jet impingement angle: This angle is associated with the tilting of the cutting head. It is defined as the angle between the initial AWJ flowing direction and the target material surface. Any change in the jet impingement angle eventually changes the jet attack angle on the target material with an effect on the mode of erosion. The use of the changeover jet impingement angle has a great impact on the AWJ cutting performance without the involvement of any additional costs. There are two different types of jet impingement angles used in the machining process, namely, forward and backward directions. The advantages include lower kerf taper formation, less striation formation, less contaminated zone, and machining of very soft and fragile materials.
g) Work material: The work material desired in the AWJ machining process usually has any size and shape. The AWJ process uses hard materials like titanium alloy, tungsten carbide, ceramics, stainless steels, composites, etc. The major phenomena found in the work material after the machining process are abrasive contamination and striation formation. The abrasive contamination for soft materials is higher with the hard materials due to the hardness of the work materials. In contrast, striation formation is significantly present in the hard material cut surface rather than the soft materials. This happens due to the increase in the abrasive attack angle at a larger cutting depth, as the kinetic energy of the abrasive particles is reduced. This cutting energy is greatly reduced by machining the hardened material at a lower cutting region.
Laser cutting explained...
Laser cutting is one of the largest applications of lasers in metal working industry. It is based on the precise sheets cutting by focused laser beam. The laser beam is a new universal cutting tool able to cut almost all known materials.
Laser cutting is the process of vaporizing material in a very small, well defined area. Laser cutting process utilizes coherent light developed within an optical resonator cavity. An electrical discharge, through premixed He, N2, CO2 causes photon emission between reflecting mirrors mounted perpendicular to axis of the resonator.
Beams exit a 50 percent reflecting, 50 percent transmitting mirror at one end of the resonator cavity. The photon energy derived from the excited CO2 molecule has a 10,6 |im wave length (infrared), phase aligned to permit focusing through processing optics. The output beam typically focused to a spot diameter of 0,1 mm using a 63,5 mm focal length lens. Cutting temperature is approximately 18000 F and energy concentration is 10 MW/cm2 (enough to vaporize most materials).
However, 90 percent of the light beam is reflected from the surface of steel, necessitating an oxygen assist. Cutting speed ranges from 2 to 10 m/min for laser power 1,5 kW.
The laser beam' effect upon a workpiece material can be divided into several characteristic phases:
Absorption of the laser radiation in the workpiece surface layer and transformation of the light energy into the heat one,
Heating of the workpiece surface layer at the place subjected to the laser beam,
Melting and evaporation of the workpiece material,
Removal of the break-up products, and,
Workpiece cooling after the completion of the laser beam' effect.
Part of the laser beam power is lost due to its passing through the workpiece, by the molten material and process gas. Still, its greatest part is absorbed and used for inducing melting and evaporation of the material at the cut point. The absorbed energy quantity is mostly depending on thermal and physical properties of the workpiece material; the choice of material also depends upon them.
Absorption is essential only at the first moment of the interaction between the laser beam and the workpiece material. Later on, heat diffusion is of crucial influence. In cutting, the aim is to vaporize the material as quickly as possible and to produce as narrow a heat-affected zone as possible with minimum distortion of the workpiece. Most industrial laser cutters employ a gas stream coaxial with the laser beam.
The gas stream helps to remove molten material from the region of the cut. In the laser cutting operation, in addition to the heat obtained by focusing the laser beam, the process gas is used for removing the molten material from the cutting zone, to protect the lenses from evaporation and to aid the burning process. The useful power can be increased in the case that the process gas is oxygen due to the exothermic reaction. The gas blowing increases the feed rate for as much as 40%.
By combining the laser as the light radiation source and the machine providing motion, in addition to the applied numerically controlled system, it is possible to provide for a continual sheet cutting along the predetermined contour. The laser is a cutting tool has been successfully applied to a large number of materials. Since the laser beam exerts no force on the part and is a very small spot, the technology is well suited to fabricating high occuracy parts, especially flexible materials. The materials which are currently being cut by lasers include: metals, wood, cardboard, fabrics, plastics, composites, ceramics, glasses and quartz. The part keeps its original shape from start to finish Laser cutting systems have enjoyed the reputation of producing parts that are below 0,1 mm tolerance with narrow heat affected zones and cutting speeds to 5 m/min and greater in gauge thickness materials.
The downside of laser technology was it's relatively high capital cost. While current developments in laser technology and motion systems have not reduced the capital cost, capabilities have improved in areas of system reliability, speed, material thickness capacity and even tighter tolerances in finished parts. In industrial application nowadays various types and constructions of laser cutting machines can be met. By contour cutting two dimensional formed thin sheet workpieces the use of machines with X-Y table coordinate is effective and real when CNC control unit is used for control. By contour cutting three dimensional formed thin sheet workpieces the laser robots are often used. Technological problems related to the application of laser machines to continual sheet cutting are in insufficient knowledge of the laser technique application as well as due to absence of sufficiently reliable practical data and knowledge about the parameters influencing the work process itself.
Laser Cutting: Advantages and disadvantages
Laser cutting do offer a number of advantages, however, when compared to more conventional techniques, for example:
Laser radiation is a very "clean" form of energy, in that no contaminating materials need come into contact with the workpiece. In fact, the working atmosphere can often be controlled to suit a particular task;
Laser beams, because of their high spatial coherence, may be focused onto very small areas. This intense local heating can take place without neighboring areas being affected;
It is comparatively easy to control the beam irradiance;
The beam is readily directed into relatively inaccessible places; it can pass through transparent windows and be directed round sharp corners;
Most of the laser energy is deposited very near the surface of the target, thus enabling shallow surface regions to be treated without necessarily affecting the bulk.
There is almost no limit to the cutting path; the point can move in any direction unlike other processes that use knives or saws;
The process is forceless allowing very fragile or flimsy parts to be laser cut with no support. Since the laser beam exerts no force on the part and is a very small spot, the technology is well suited to fabricating high accuracy parts, especially flexible materials. The part keeps its original shape from start to finish;
The laser beam can cut very hard or abrasive materials;
Sticky materials that would otherwise gum up a blade are not an obstacle for a laser;
Lasers cut at high speeds. The speed at which the material can be processed is limited only by the power available from the laser;
Cutting with lasers is a very cost effective process with low operating and maintenance costs and maximum flexibility.
Disadvantages of laser cutting are:
Very large resonator cavity required per cutting head, therefore, not normally used in multiple-head configuration;
High capital equipment cost;
Requires isolation of cutting head for safety;
Mirror alignment critical and power level reduces as mirrors degrade;
Double material thickness is equal to one-half the cutting speed;
Generally not used for steel above 20 mm.
Laser Cutting: Working quality
Working quality obtained by laser cutting is determined by the shape and dimension precision as well as by cut quality. The work piece shape and dimensions' accuracy are determined by the characteristics of the coordinate working table as well as by the control unit quality as in the case of CNC laser cutting machine.
The cut quality refers to the cut geometry, the cut surface quality and physical and chemical characteristics of the material in the surface cut layer. The cut geometry comprises the following: cut width, cut sides' inclination and rounding out of the cut edges. The surface quality includes the accessed roughness, waviness and deviation of the shape - surface error. The physical and chemical properties of the material in the surface cut layer refer to the surface layer formed in the laser cutting process due to the heat effect of the laser beam upon the work piece material.
The cut width is an essential characteristic of the laser cutting process giving it advantage over other sheet cutting procedures. The cut width of metals is small, it ranges 0,1÷0,3 mm with steel sheets’ cutting. The cut sides' inclination also determines the cutting quality. The cutting of material by means of the focused laser beam is characterized by narrowing of the cut. Its size depends on many factors, primarily on the focal distance of the focusing lenses as well as on defocalization, in addition to the properties of the work piece material and the laser beam’s polarization.
In order to determine quantitatively the cut sides' inclination the cut sides' inclination tolerance (u) and the cut sides' inclination angle (β) are used. The cut edges at the laser beam entrance side are rounded out due to the Gauss distribution of radiation intensity over the laser beam cross-section. The edges’ rounding-out is very small. The cut edge rounding radius ranges from 0,5 mm to 0,2 mm with steel sheets cutting. The round increases along with a rise in sheet thickness.
The laser cut surface reveals a specific form of unevenness. Observation of the cut surface can reveal two zones: the upper one in the area of the laser beam entrance side and the lower one, in the area of the laser beam exit side. The former is a finely worked surface with proper grooves whose mutual distance is 0,1÷0,2 mm while the latter has a rougher surface characterized by the deposits of both molten metal and slag. That is why it is determined to measure roughness of the cut surface at the distance of one third of sheet thickness from the upper cut edge.
The laser cutting is a high-temperature process causing a noticeable yet small heat damage of the material surrounding the cut zone, that is an insignificant change of the basic properties of the work piece material. The shape of the changes upon the materials induced by the laser radiation can be of various forms. The changes may involve the crystal structure as micro and macro cracks of the material on its surface or as zones molten together or evaporated. Since the laser cutting is actually the thermal way of cutting then the structure of the material changes in the cut zone. Changes of hardness in the surface cut layer are due to the fact that the work piece material is heated to high temperatures exceeding the critical transformation points with the onrush of the laser beam.
After the passing-through of the laser beam the process of self-cooling occurs causing a rapid cooling of the heated surface layer. In most cases the laser thin sheet cutting is successful in removing material from the cut zone with no dross produced. In sheets of greater thickness and some kinds of materials deposits of the molten metal dross appear along the exit cut edge. The cutting speed change along with the material thickness change for a variety of laser power at the assist gas pressure of 70 kPa.
The cutting speed rapidly decreases when material of greater thickness are cut. The cutting speed can also increase along with the laser power. However, it has to be remembered that the laser almost always works with an optimal radiation power.
Laser Cutting: Surface of Laser Cut
The laser cut surface reveals a specific form of unevenness. As either semicircular grooves or proper grooving are the consequence of the focused laser beam shape, the cutting velocity and formation process, as well as of the removal and hardening of the molten material at the cut place.
Observation of the cut surface can reveal two zones: the upper one in the area of the laser beam entrance side and the lower one, in the area of the laser beam exit side. The former is a finely worked surface with proper grooves whose mutual distance is 0.1…0.2 mm while the latter has a rougher surface characterised by the deposits of both molten metal and slag.
That is why it is determined to measure roughness of the cut surface at the distance of one third of sheet thickness from the upper cut edge. There is a difference between the cut surface roughnesses in the direction of the laser beam fand that in the direction perpendicular to the laser beam axis that is in the cutting direction. The former is of no crucial importance in considering the problem of the cut surface roughness due to the fact that the laser is applied to thin sheet cutting. The latter is a more obvious phenomenon that can be observed and analysed.
In laser cutting, the edges of the workpiece have a characteristic grooved pattern. At low cutting speeds, the grooves run almost parallel to the laser beam. As the cutting speed increases, the grooves bend away from the direction of cutting. Groove lag refers to the greatest distance between two drag lines in the direction of the cut. The groove lag is evaluated visually. The evaluation is carried out on a picture of cut with the aid of a magnifying glass or a microscope.
Parameters that are most often used for accessing the surface roughness are the standard roughness (ten point height of irregularities) Rz. and the mean arithmetic profile deviation Ra. The standard roughness Rz is the arithmetic mean calculated from the roughness (scallop height) of five consecutive, representative, individual measured sections.
The standard roughness Rz is measured e.g. with a brush analyzer corresponding to ISO 3274. The measuring itself is carried out at continuous distances in the cutting direction, in accordance to ISO 4288. The point at which the roughness is measured is dependent on the sheet thickness and the material type.