leave two empty, 16 pt single lines

Gpszet 2010

Budapest, 25-26.May 2010.

G-2010-Section-No

MODERN HIGH EFFICIENCY CUTTING PROCESSESAntun Stoi

J. J. Strossmayer University of Osijek, Mechanical Engineering Faculty in Slavonski BrodTrg Ivane Brli-Maurani 2, HR-35000 Slavonski Brod, CroatiaIvan Samardi

J. J. Strossmayer University of Osijek, Mechanical Engineering Faculty in Slavonski BrodTrg Ivane Brli-Maurani 2, HR-35000 Slavonski Brod, CroatiaDraan KozakJ. J. Strossmayer University of Osijek, Mechanical Engineering Faculty in Slavonski BrodTrg Ivane Brli-Maurani 2, HR-35000 Slavonski Brod, CroatiaSergej Hloch

Faculty of Manufacturing Technologies of Technical University of Koice with a seat in Preov, Bayerova 1, 080 01 Preov, Slovak Republic

Miroslav Duspara

J. J. Strossmayer University of Osijek, Mechanical Engineering Faculty in Slavonski BrodTrg Ivane Brli-Maurani 2, HR-35000 Slavonski Brod, CroatiaAbstract: Paper presents modern cutting processes with emphasis on efficiency and surface cut quality. Beside that, an observation of suitability for application from the standpoint of costs is given. Authors give some practical experience and examples of cutter specimens by modern cutting processes, as well as comparison of different cutting process such as water jet, wire EDM, plasma and laser from the accuracy, thickness, cutting speed, heat affected zone and material distortion point of view.

Keywords: Laser cutting, plasma cutting, water jet cutting, comparison these cutting processes

1. INTRODUCTIONWater jet Cutting: Water jet machines use cold supersonic abrasive erosion to cut almost any material, both metals and nonmetals. The highly pressurized water stream carries abrasive garnet to etch away the material. Plasma Cutting: Plasma is a thermal process that cuts electrically conductive materials. Plasma cutting involves a near-supersonic jet of ionized gas that leaves a negatively charged electrode inside a torch tip. The ionized gas then cuts the positively charged metal. In other words, the plasma stream cuts by heat about 11000 to 25000 degrees C, essentially melting the material being cut. [1]Laser Cutting: Laser cutting is a thermal process. Special lens are used to focus energy (3 to 6 kW) in a small area to melt away material. EDM Cutting: Wire electrical discharge machining (EDM) uses spark erosion to remove material from electrically conductive materials. The wire is negative and the work piece is positive. Direct-current electric pulses are generated between the wire electrode and the work piece. During cutting, material is melted away by the lightning bolt and flushed out of the kerfs area by the dielectric solution.2. GENERAL OF THE CUTTING PROCESSES2.1 Water jet Cutting

While water jets are perceived as competing with laser, plasma, and EDM, many shops have more than one of these types of machines and consider them complementary processes.

Water jets use cold supersonic erosion to cut almost any material, both metals and nonmetals. It can cut metals ranging from thin shim stock to more than 250 mm thick with accuracies of 0.007 to 0.4 mm. Seventy-five percent of water jets are used to cut material less than 100 mm thick. Repeatability is 0.025 mm. Thick cutting (more than 100 mm) with a water jet loosens tolerances by at least two times. [2]The same parameter set (water pressure, abrasive flow rate, cutting nozzle, etc.) is used for nearly all cutting; only the cut speed varies from material to material. The fact that the parameters usually do not change from one material to another means that setup is fast, and the opportunity for operators to select improper parameters for a particular job is greatly reduced.

With EDM, plasma, and laser, various parameters, gases, or wires must be used to process different materials. Operator expertise and proper setup are essential with these other processes, while they are of less consequence with a water jet. The productivity of this machine can be improved by cutting stacked material and running with multiple water jet heads when cutting sheet metal. For materials 25 mm or more thick, users typically put all the power through one head and do not stack layers.

Materials most often cut with water jet are aluminum, stainless steel, and high-strength metals such as titanium, Hastelloy, INCONEL alloys, nickel alloys, composites, and metal laminates.Water jets often are used to cut short-run prototypes that require minimum tooling or fixturing. They also are used for net-shape cutting, in which the final part is produced without the need for secondary operations to achieve required tolerance or surface finish; or for hogging out, in which the abrasive water jet rough-cuts a part to within about 0.4 mm tolerance and it is finished on a milling machine to achieve a tighter tolerance and smoother surface.

2.2 Plasma Cutting

Plasma is a thermal process that cuts electrically conductive materials. Plasma cutting involves a near-supersonic jet of ionized gas that leaves a negatively charged electrode inside a torch tip. The ionized gas then cuts the positively charged metal. In other words, the plasma stream cuts by heat about 11000 to 25000 degrees C, essentially melting the material being cut.

Assist gases are used to prevent the superheated surface from reacting with air (which can cause oxides or nitrides to form on the surface). They also help blow away the molten material, cool the part, and minimize double arcing.

For cutting mild steel, the assist gas usually is oxygen or air. Stainless steel up to a thickness of 130 mm often is cut with oxygen, air, or argon-hydrogen. Aluminum is cut using air as the primary gas, and methane sometimes is used as the shield gas. Methane is more expensive but produces a better edge, and aluminum up to 150 mm thick can be cut with this process.

Plasma cutting is a good choice for cutting mild steel between 7 and 30 mm thick when the HAZ does not need to be removed before the part is used in a final application. Most of the time, hand grinding is required to remove HAZ before the part can be used in the final application. Secondary operations such as this often add unexpected costs to part production. A majority of the time, plasma is used on mild or carbon steel up to 30 mm thick. Beyond this thickness, oxygen acetylene torch cutting also can be used.

Plasma cuts at high speeds and generates heat that leaves rough edges. Dross sometimes builds up on the bottom side of the cut. Accuracies are in the range of 0.7 to 1.5 mm, depending on the thickness of the material. Kerfs width typically is 3 to 6 mm and can be much smaller with precision plasma. (Figure 1.)

Fig. 1. Comparison cutting processes from the HAZ and size of kerfs point of view [3]2.3 Laser Cutting

Lasers cut materials that bear the right thermal properties to be melted away. In general, lasers are designed for cutting mild steel from 0.5 to 25 mm, stainless to 20 mm, and aluminum to 15 mm. Most of the time, a laser is used on 1 mm mild steel. (Figure 2)Lasers perform best when cutting high-quality, homogeneous materials. Impurities and inclusions in a target material may have adverse effects on cut quality and might cause a reaction that sprays molten product in the direction of the lens or produces defects on the cut surface.

On single layers of thin sheet metal, lasers cut at fast speeds with tolerances of 0.025 to 0.15 mm. On this material, smaller, lower-kilowatt lasers have a fine wave pattern to allow for a precise cut.

While lasers have been used primarily for sheet metal cutting, their use has increased for cutting increasing thicknesses of stainless steel and aluminum. However, cutting thick materials requires powerful lasers and can require secondary operations to remove the heat-affected zone (HAZ), which can cause micro cracking if not removed. New high-power lasers (3 to 6 kW) can be tuned down to cut thin material with a precision equal to that of a lower-powered laser.Initial setup time for a laser is fairly quick. Different materials often require different gases, so material changes also require system setup changes.

If a shop is cutting only mild steel sheet metal parts in medium to large batch sizes, a laser may be the best choice. However, a gray area occurs with materials from about 6 to 12 mm thick. The water jet becomes more viable for thicker materials.

Also, shops using both laser and water jet find a recurring need to process materials other than typical mild steel. Although the laser can cut a variety of metals, the water jet is usually found to be a more productive method for many materials.For example, water jets are usually the most productive method of cutting aluminum. Titanium, nickel alloys, copper, brass, glass, and stone also can be cut productively with a water jet. The quick setup time with a water jet makes it suitable for small to medium batch runs. The part geometry, batch size, and material type must be considered before the cutting method is chosen. [2]2. COMPARISON CUTTING PROCESSES FROM THE STANDPOINTS THICKNESS THE MATERIAL

Fig. 2. Usage cutting processes for different materials and different thickness of material [3]

Fig. 3. Cutting speed for mentioned cutting processes [3]On Figure 3 you can see cutting speed for water jet, conventional plasma and laser cutting. Laser cutting has a maximum speed of cutting (more than 7.5 m/min), but if you look at Figure 2 we see that the laser used for a minimum material thickness. For highest thickness of material (more than 250 mm) we used water jet cutting, but with smallest cutting speed (less than 0.025 m/min).

Fig. 4. Comparison processes with thickness of the material and precision of cutting [3]If you see Figure 4 and Figure 5, we can conclude that initial investment depend on precision or accuracy. Laser cutting has highest precision of these cutting processes, but water jet has a similar precision, water jet cutting can be used for thicker materials than laser. Price for laser cutting machining is 650000 $ and for water jet cutting machining is 100000 $.

Fig. 5. Initial investment [3]3. CONCLUSION

Each cutting process has its advantages and disadvantages, when buying these machines for cutting must know exactly the material we use it, for which the thickness of the material and how much accuracy we need to cut.Water jetWire EDMPlasmaLaser

AccuracyWaterjet cutting accuracy average of 0.003" (0.1 mm). Better result can be achieve with more advanced software.Wire EDM is Extremely precise parts at 0.0001" (0.025mm)

Plasma cutting accuracy is in the range of 0.030 to 0.060"

Accuracy to 0.001"(0.025mm) or better in thin material.

ThicknessWaterjet Machines mostly cut under 3" (75mm) Thicker parts can be cut with reduced accuracy and slower speed.Very thick parts can be cut with wire EDM. Over 12" (30 cm) reported.Plasma cutting usually cuts less than 1.25"Usually cuts thin mild steel less than 0.25" (6.35mm)

Cutting speedWaterjet machine cuts five to ten times faster than EDM when thickness is under 1"Wire EDM cuts at very Slow speed compare to waterjet (5-10 times slower)Fast with thin sheets

Very fast cutting in thin, non-reflective materials.

Quality of edgeWaterjet cutter yields Good edge qualityExcellent

Fair

Excellent

Heat affected zone (HAZ)

Waterjet produces No HAZ

Some HAZ

Some HAZ

Cuts by melting the material, resulting HAZ, often need additional process to avoid micro cracking.

Material DistortionWaterjet machine generates No Distortion but may have hazing near the cut. No internal stress built up.Wire EDM Cuts by melting, resulting material heat distortion.Cuts by melting, resulting material heat distortion.Cuts by melting, resulting material heat distortion.

Other characteristicsWaterjet cutter works well in non-conductive and conductive materials. Can pierce material directly without other drill equipment.Fast setup and rapid programming.Minimal requirement of fixture.EDM is limited to cutting only conductive materials.

Relatively low capital costs. Special gas need for laser cutting.Limited to non-reflective materials.May need different gas for cutting different materials.

Table 1. Comparison matrix of different cuting process such as waterjet, wire EDM, laser, plasma [1]4. REFERENCES

[1] http://www.iwmwaterjet.com/waterjet_EDM_laser_plasma.html (15.02.2010)

[2] http://www.thefabricator.com/article/waterjetcutting/exploring-complementary-cutting-methods (17.02.2010)

[3] http://www.fandmmag.com/print/Fabricating-and-Metalworking/Comparative-Cutting--Waterjets--Lasers--and-Plasma/1$3141 (18.02.2010)3 / 6