This paper shows the impact of mixing water and abrasives in water jet cutting process on the quality of the machined surface. The tests were done with polymer material SIPAS, where the influence of cutting parameters was researched (cutting pressure, cutting feed and abrasive mass flow). The surface roughness was measured on several zones, regarding the depth of materials, because the roughness is increased with the material thickness

A. Stoić

B. Kosec

I. Samardžić

M. Duspara

M. Stoić

Hrčak - Portal of scientific journals of Croatia

239METALURGIJA 53 (2014) 2, 239-242A. STOIĆ, M. DUSPARA, B. KOSEC, M. STOIĆ, I. SAMARDŽIĆTHE INFLUENCE OF MIXING WATER AND ABRASIVES ON THE QUALITY OF MACHINED SURFACE Received – Prispjelo: 2013-05-03Accepted – Prihvaćeno: 2013-09-25Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 53(2) 239-242 (2014)UDC – UDK 621.929:621.937.004.6:621.6=111A. Stoić, M. Duspara, I. Samardžić, Faculty of Mechanical Engineering in Slavonski Brod, Slavonski Brod, Croatia, B. Kosec, Faculty of Natural Sciences and Engineering, University of Ljubljana, Ljubljana, Slovenia, M. Stoić, University of Applied Sciences, Slavonski Brod, CroatiaThis paper shows the impac t of mixing wat er and abrasives in water jet cutting pr ocess on the qualit y of the ma-chined surface. The tests were done with polymer mat erial SIPAS, where the infl uence of cutting paramet ers was researched (cutting pressure, cutting feed and abrasive mass fl ow). The surface roughness was measured on several zones, regarding the depth of materials, because the roughness is increased with the material thickness.Key words: mixing, abrasives, nozzle, water jetINTRODUCTIONAbrasive water jet (AWJ) cutting process differs from the pure water jet cutting because abrasive particles erode the cutted material in the kerf zone. AWJ is much faster  than pure water jet and hard materials such as steel, met-al, glass, stone, etc. can be cut more efﬁ ciently.The abrasives are added to the jet after the creation of pure water jet. The most commonly used abrasive in abrasive water jet is sand, which is of dif ferent grain size. Garnet is the most widely used abrasive. Characteristics of AWJ process are:- extremely versatile process- no heat affected zone- no mechanical stresses- easy to program- thin stream- detailed geometry- less material loss- simply to ﬁ xture- low cutting forces- reduced secondary operations- little or no burrABRASIVESTherefore, different types of abrasives can be used; some of them cheapen the machines value. For exam-ple, if it is cut lot of aluminum it can be used softer abrasives, thus slowing down wear in nozzle hose of mixing tube. The most commonly used abrasive in the water jet industry is red garnet. Garnet is pretty tough and when it breaks, it creates sharp edges; these characteristics are advantageous in water jet cutting. They are chemically inactive and will not react with the material that one needs to cut.However, one must never use abrasives that contain silicon, such as beach sand because the dust created by silicon abrasives may cause silicosis, a very painful and deadly disease.The abrasive characteristics that are required are:  twice sifted, sharpness (sharper abrasives cut better), pu-rity (impure abrasives inﬂ uence cutting), and the price.CUTTING HEAD The cutting head in water jet cutting mainly consists of two parts, Venturous chamber and mixing tube. The compressed water goes from tube into the opening (ori-ﬁ ce). This specially designed and made, opening is used to turn water into a highly compressed accelerated wa-ter stream. Therefore, depending on dif ferent applica-tions of diameters it can be from 0,2 mm to 0,35 mm.Three most commonly used types of materials for making opening are sapphire, ruby and diamond.Abrasive particles are fed into the cutting head where they come into contact with the water spray inside the Venturi chamber (Figure 1). Like a bullet in the riﬂ  e, abrasive particles are accelerated down the mixing tube and they go to the cutting point.MIXING OF WATER AND ABRASIVEIn the water jet cutting, water jet accelerates abra-sive particles that erode the materials [2]. Abrasive wa-ter-jet is formed in the mixing chamber . Water, at high pressure, is injected through the jet head. The diameter of the mixing chamber is usually from 0,2 to 0,4 mm.The water jet, at high speeds, creates a vacuum that pulls abrasive particles into the mixing chamber along with air. Jet accelerates abrasive particles and air in the 240  METALURGIJA 53 (2014) 2, 239-242A. STOIĆ et al.: THE INFLUENCE OF MIXING WATER AND ABRASIVES ON THE QUALITY OF MACHINED SURFACETherefore, when exiting the mixing tube both phases have the same velocity (Figure 2).Mixing tube is mainly made of tungsten carbide with an internal diameter of 0,8 mm to 1,6 mm and with a length of 50 to 80 mm. The tungsten carbide is used because of its resistance to abrasives.The mixing process can be presented by mathemati-cal models:  2 wwjwpv =  (1)  Where is: vwy = velocity of water jet,  ψ = velocity coef-ﬁ cient of the opening (oriﬁ ce). For example, if oriﬁ ce diameter is 0,65 mm velocity coefﬁ cient is 1,01 [5], pw = water pressure, ρw = density of waterVolume ﬂ ow:  w wj orificeq v Aφ= ∗ ∗  (2)  2 *4ow wjdq vπφ= ∗ ∗  (3)  2 * 24o wwwd pqπφ ψρ= ∗ ∗  (4)  2 * 24o ww dwd pq cπρ= ∗ ∗  (5)where is:Φ =  “vena-contracta” coefﬁ cient. The typical value may be taken as 0,64 for a sharp oriﬁ ce. [6]Aoriﬁ ce = surface size of oriﬁ cedo = diameter of oriﬁ cecd = the oriﬁ ce coefﬁ cient discharge. [7]The total power of the water jet can be given as:  wj w wP p q= ∗  (6) 2 * 24o wwj w dwd pP p cπρ= ∗ ∗ ∗  (7) 2 3* 24o wwj dwd pP cπρ= ∗ ∗  (8)Energy and momentum do not remain constant due to losses during the mixing process. But, in the begin-ning, it can be assumed that there is no loss of jet mo-mentum before and after the mixing [8].Table 1 shows us achievable high dif ferences at maximum cutting of depth. Generally , hard materials mixing tube. The cutting width depends on the diameter of the mixing tube and the distance from the work piece. If the mixing tube of 1 mm in diameter and the distance from the work piece is 3 to 5 mm, the cutting width is about 1,2 mm [3].The water jet that is coming out of the mixing tube consists of three elements: the abrasive particles, air and water. The ability of material removal for each jet formed in a given nozzle depends on the pressure pump, type and abrasive mass ﬂ ow. Generally, with the higher pump pressure the material removal ability increases.When the ﬂ ow of abrasive jet is of relatively small capacity the removing material increases with the abra-sive mass ﬂ ow. Because, when the abrasive mass ﬂ ow rate is increased there are more abrasive particles in-volved in the cutting process. Also, if the cutting head has a lot of abrasive particles, kinetic energy of one par-ticle is degraded due to limited kinetic ener gy of the water jet cutting. Therefore, there is an optimum mass ﬂ ow of abrasive particles. If the mass ﬂ ow is uneven it can cause pulsing jet, which is causing damage to the work piece.The mixing process is a gradual mixing of abrasive particles and water jet.During the mixing process, abrasive particles are gradually accelerated because of the gradual torque transfer from the aqueous phase to abrasive phase. Figure 1  Cuting head with all parts. 1 - Pressurized Water, 2 - The water is forced through an oriĀ ce, 3 - The high velocity of the jet creates a Venturi eff ect, 4 - The abrasive is fully mixed in nozzle, 5 - The abrasive waterjet stream [1] Figure 2 Schematic of mixing process [4]Table 1 Thickness of the cutting materialCutting material Material thickness / mm1 Stainless steel 702 Wood 1503 Glass 504 Ceramics 305 Polymer 1306 Stone 907 Copper 808 Aluminium 1009 Steel 100241METALURGIJA 53 (2014) 2, 239-242A. STOIĆ et al.: THE INFLUENCE OF MIXING WATER AND ABRASIVES ON THE QUALITY OF MACHINED SURFACEcan be cut with smaller thicknesses, while cutting of softer materials can be done with larger thicknesses. For instance, wood can be cut with a thickness of 150 mm and the stone with a thickness of 90 mm.EXPERIMENTThe tests in the experiment were made on polymer material SIPAS 60 with properties shown in Table 2.Table 2 Mechanical properties of the cutted material [9]Typical values at 23 oC ValuesPropertiesAbbreviated term PA6Density 1,14 / g/cm3Viscosity number 140 / ml/gProcessingMelting temperature 220 / oCMechanical propertiesTensile modulus 3,9 / MPaYield stress 90 / MPaFlexural modulus 2,9 / MPaFlexural strength 95 / MPaBall indentation hardness 190 / MPaSIPAS 60 is a material with excellent resistance to abrasion and impact load. It is used for making gears, coupling elements for eccentricity, sealing rings, screw elements, sliding elements and other elements exposed to impact loads.The quality of the machined surface is determined by its roughness. Theoretical roughness depend primary on the geometry of the abrasive water -jet tools and ap-plied process. One has to take into account that the real roughness exclusively depends on theoretical rough-ness, although higher or lower roughness depends on several factors. These factors make the surface as a tra-jectory of action in abrasive water-jet cutting. [10]Limit values for the parameters were chosen as the marginal cases of the selected machine. The parameters were kept between minimal and maximal mass ﬂ ow and water pressure that can be obtained in the machine (marginal cases). The cutting head velocity was chosen as a calculated value obtained from software “velocity calculator”.To measure the surface roughness, it was used Mitu-toyo Surf test SJ301.The worst result of surface roughness in Table 3, we obtained on the sample I3, because the sample has the smallest abrasive mass ﬂ  ow (0,4 kg/min), the lowest pressure (250 MPa) and a top speed of cutting head (160 mm/min). As for the materials with thickness of 19 mm, because of poor quality surface, roughness could not be measured (Figure 3). The cutting material has a thick-ness of 20 mm. Table 3 The Cutting ParametersMass fl owkg/minPressure/ MPaVelocity of cutting head / mm/min0,4350 80 G1120 G2160 G3300 80 H1120 H2160 H3250 80 I1120 I2160 I32,8 350 80 J1120 J2160 J3300 80 K1120 K2160 K3250 80 L1120 L2160 L3Table 4 The measurement results DepthNum.Roughness / µm1 / mm 11 / mm 19 / mmRa Rz Ra Rz Ra RzG1 8,1 47,7 8,9 52,8 10,5 58,4G2 7,6 44,5 9,8 55,1 11 51,9G3 7,8 41,7 8,2 49 11,5 62,8H1 7,2 44,5 7,9 55,8 10,2 55,6H2 7,9 51,6 10,7 52,3 16,1 83H3 9,7 60 10,5 55,6 15,4 87,9I1 8,2 47,9 11,7 67,9 9,4 57,1I2 7,5 44,1 12 66,1 13,4 73,6I3 7,2 45,5 12,6 68,3 / /J1 6,9 36,8 6,9 48,4 8,3 48,2J2 6,7 43,7 9 47,3 8,6 49,3J3 8 45,1 8,1 49,2 9,2 54,2K1 6,1 40 7,5 42,9 7 44,3K2 7,9 41,5 6,8 42 8,7 49,6K3 8,1 47 8,6 47,3 9,4 58,6L1 5,7 35,6 6,9 46,5 7,9 45,8L2 6,2 36,1 6,9 45,1 8,8 48,3L3 6,8 35,8 7,5 45,8 8,2 50,1Figure 3 Surface of I3 specimen 242  METALURGIJA 53 (2014) 2, 239-242A. STOIĆ et al.: THE INFLUENCE OF MIXING WATER AND ABRASIVES ON THE QUALITY OF MACHINED SURFACECONCLUSION Measurements of surface roughness were conducted on the cutted surface created by AWJ cutting while in-ﬂ uence of cutting parameters were examined (increas-ing pressure and abrasive mass ﬂ ow rate). The increase of the abrasives mass ﬂ  ow has a greater impact than increase the pressure of water.If the results of roughness measurements in the low-er reaches of the sample I3 and L3 are compared with the same parameters (water pressure and cutting head velocity) with a different mass ﬂ ow it can be concluded that with the mass ﬂ ow rate increased 7 times there is incomparably better quality of surface roughness. In sample I3 water and abrasives do not have a good inter-action because there is not enough abrasives in the sys-tem. Therefore, all results of measurements are approx-imate. The cutting tool is only water with a small per -cent of abrasives. With increased water pressure from 250 MPa to 350 MPa with the same mass ﬂ ow rate and the cutting head velocity (for samples L3 and J3), in the cutting lower zones there is 10 % better surface rough-ness. In the upper and middle zones the impact of chang-ing parameters is 10 %. Also, with the increased cutting head velocity from 80 mm/min to 160 mm/min (for samples K1 and K3) in the cutting lower zones there is 25 - 30 % better surface roughness. In the upper and middle zones the impact of changing parameters is 15 - 20 %. If there are materials with low density they can be cut with a low-speed cut-ting heads and low pressure, thus saving machine and increasing proﬁ ts.AcknowledgementAuthors would like to thank Company SIGMA T from Slavonski Brod and its director Mato Sigurnjak, BSc. for their unweaving support in the experimental part of the work.REFERENCES[1] http://www .wardjet.com/01-waterjet-cutting-characteristi-cs.html (17.04.2013)[2] P. Hreha, S. Hloch, J. Valíček, K. Monková, P. Monka, M. Harničárová, M. Fusek, P . Konjatić; Impact of abrasive mass ﬂ ow rate when penetrating into a material on its vi-bration, Technical Gazette, 17(2010)4, 475-480[3]  http://cdn.intechopen.com/pdfs/15606/In Techtribology_in_water_jet_processes.pdf (13.12.2012.)[4]  http://nptel.iitm.ac.in/courses/Webcourse- contents IIT%20Kharagpur/Manuf%20Proc%20II/ pdf/LM-37.pdf (17.04.2013)[5]  J. H. Lienhard; Velocity Coefﬁ cients For Free Jets From Sharp-Edged Oriﬁ  ces; Journal of Fluids Engineering, 106(1984), 13-17[6]  http://content.alterra.wur.nl/Internet/webdocs/ilri-publica-ties/publicaties/Pub20/pub20-h1.2.pdf (29.03.2013)[7]  http://www.engineeringtoolbox.com/ oriﬁ ce-nozzle-ven-turi-d_590.html (29.03.2013)[8] h ttp://nptel.iitm.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Manuf%20Proc%20II/pdf/LM-37.pdf (29.03.2013)[9]  http:/www.sipas-galdovo.hr/materijali.html (12.03.2012)[10] A. Stoić, M. Duspara, B. Kosec, M. Stoić, I. Samardžić. The application of water jet in cutting polymer materials. Metalurgija 52(2012)2, 255-258Note:  Responsible person for english translation: J. Vujčić, Lecturer University of Applied Sciences, Slavonski Brod, Croatia

The influence of mixing water and abrasives on the quality of machined surface

Abstract

Similar works

Full text

Available Versions

Hrčak - Portal of scientific journals of Croatia

Directory of Open Access Journals