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机械毕业设计全套

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JCYY01-009@单工位双面卧式车方组合机床的整体设计,机械毕业设计全套

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Study on the diamond tool drilling of engineering ceramicsQ.H. Zhanga,*, J.H. Zhanga, D.M. Sunb, G.D. WangcaCollege of Mechanical Engineering, Shandong University, No. 73, Jingshi Road, Jinan 250061, PR ChinabCollege of Material Engineering, Shandong University, Jinan, PR ChinacShandong Machine Design and Research Institute, Jinan, PR ChinaReceived 27 December 2000AbstractA method for drilling holes in engineering ceramics by using a diamond tool has been developed. In this method, a drilling tool rotates withfixed abrasives. The machining mechanism of drilling based on the fracture mechanics concept is analyzed, and a new theoretical model of thematerial removal rate is proposed. According to this model, the material removal rate increases in accordance with the increase of the staticload applied, the rotational speed of the drilling tool, and the grain size of the abrasive. Selecting 99.5% Al2O3ceramics as the workpiecematerial, experiments have been carried out. The results show that diamond drilling is an effective method for machining engineeringceramics. # 2002 Published by Elsevier Science B.V.Keywords: Engineering ceramic; Diamond tool; Drilling; Machining mechanism1. IntroductionEngineering ceramics have numerous excellent physicaland mechanical properties: high hardness, high thermal resi-stance, chemical stability, and low thermal and electricalconductivity, to name but a few. Because of these specialqualities, engineering ceramics are expected to be usedincreasingly in a number of high-performance applicationsranging from electronic and optical devices to heat- andwear-resistant parts 13. Until today, their applicationshave mostly been limited to electronic and optical devices.One reason is to be found in the limitation on the formingprocess prior to sintering, which restricts the generation ofcomplex geometry and makes it difficult to ensure adequateaccuracy and surface finish. There is also a considerabledeficit in terms of the production or machining of morecomplex geometries in the hardened post-sintering state,with limitations on either the performance or the formingcapacity of the majority of the processes in current use.Machining engineering ceramics to final dimensions byconventional methods is extremely laborious and time con-suming. Tight tolerances and dimensions with acceptablesurface and sub-surface damage are something only attain-able at great cost. Thus research into the areas of moreefficient material removal processes have been beginning togather momentum in recent years, especially in the ways andmeans of reducing the occurrence of faults or cracks in thesub-surface of the machined ceramics 46.A kind of machining method for drilling holes in engi-neering ceramics by using a rotary diamond tool is proposedin this paper. It can increase the material removal rate, andimprove the surface finish.This paper intends to further the understanding of thebasic mechanisms the diamond tool drilling of ceramics andthus to enable the prediction of the material removal rate interms of the static load applied, the grain size of the abrasive,and the rotational speed of the drilling tool.2. The mechanism of diamond tool drillingThe process of diamond tool drilling is shown schemati-cally in Fig. 1. The tool is rotating in the drilling process, theworkpiece is stationary. The material removal mechanismhas been investigated generally by microscopic observationof the abraded surface. The process of material removal isthought to be similar to that of a single tool cutting in that inall cases material is removed by an individual tool or particledisplacing or fracture the work surface. The workpiecematerial is found to be stabbed off in the form of manyminute particles by the abrasive grains grinding.It is concluded that the removal of engineering ceramicoccurs primarily by brittle fracture in diamond tool drilling.To understandthis process, it ishelpful to studythe indentationJournal of Materials Processing Technology 122 (2002) 232236*Corresponding author.E-mail address: zhangqh (Q.H. Zhang).0924-0136/02/$ see front matter # 2002 Published by Elsevier Science B.V.PII: S 0924-0136(02)00016-Xntsof brittle materials, because the abrasive grains acting on theworkpiece surface are just like indenters.2.1. Investigation of indentation in ceramicsThe deformation and fracture pattern observed under thenormal contact of ceramics by a Vickers indenter is illu-strated in Fig. 2. Directly under the indenter is a zone ofplastic deformation. Two principal crack systems have beenidentified, which emanate from the plastic zone: median/radial cracks and lateral cracks. The behavior of both typesof cracks is effected by residual stresses from the non-uniform plastic deformation in the elastic/plastic material.Radial cracks are initiated by a wedge-like action duringloading, and they may continue to propagate during unload-ing due to residual tensile stresses acting on the crack-tip. Lateral cracks are observed to initiate and propagateby residual stresses only as the indenting load is removed.The initiation and propagation of radial as well as lateralcracks are considered to contribute greatly to the materialremoval process. As shown in Fig. 2, the initiation andpropagation of these radial and lateral cracks at the end leadto chipping of the brittle material. A critical load Pcforinitiating a radial crack is given by 7:Pc aK4ICH3V(1)where a is a dimensionless factor related to the indentergeometry, KICis the fracture toughness of workpiece, andHVis the Vickers hardness of the workpiece material.For the size of the median or radial crack Crand lateralcrack CL, respectively, the following equations have beenderived 8:Cr x1P1=2H1=4VK1=3IC CL x2PKICC18C193=4(2)where P is the load applied, and x1and x2are proportionalconstants.It is generally regarded that the depth of the lateral crackChis proportional to P=HV1=2:Ch x3PHVC18C191=2(3)where x3is a proportionality constant 9.It is concluded from these results that the size of themedian/radial or lateral crack grows with an increase inthe load and with a decrease in the fracture toughness ofthe workpiece material. Investigations on indentation des-cribed so far provide useful information for understandingthe practical diamond drilling process of ceramics.2.2. The relationship between the material removal rateand various parametersAccording to the test results of ceramics in indentation, amodel of material removal caused by a single abrasive isproposed, as shown in Fig. 3. The model takes into accountthe linear tangential motion of the abrasive along the sur-face. Assuming that an individual abrasive grain follows alinear path with a constant depth of cut, the volume ofworkpiece material removed by an abrasive grain (indenter)under a normal load P is proportional to the dimensions ofthe lateral crack and the length of travel d. The volume ofFig. 1. Schematic diagram of the diamond tool drilling process: (1) chuck,(2) water, (3) workpiece, (4) tool, (5) water jacket.Fig. 2. Localized deformation and fracture of ceramics due to indentation. Fig. 3. Schematic model of chip formation.Q.H. Zhang et al. / Journal of Materials Processing Technology 122 (2002) 232236 233ntsremoved workpiece material V0for one grain is obtained as(see Fig. 4):V0 2CLChd (4)where CLis the length of the lateral crack, Chis the depth ofthe lateral crack, d is the acting distance of the grain.Then, the material removal rate for one grain is given by:MV0 4pCLChor (5)where o is the rotational speed of the tool, and r is the radiusof the grains track.Assuming that the density of the effective cutting grains isl, the number of effective grains in area dA (see Fig. 4) is:n ldA 2lpr dr (6)Then, the material removal rate of the tool is:MVZR2R18p2lCLChor2dr 83p2lCLChoR32C0 R31(7)The number of effective grains in the terminal face of thetool is 10:N lA K1d206ugpC18C192=3pR22C0 R21 (8)where A pR22C0 R21, R2is the external radius of thediamond drilling tool, R1is the internal radius of thediamond drilling tool, K1is proportionality constant; ugisthe concentration of abrasive grains, and d0the meandiameter of the abrasive grains. Then:l K1d206ugpC18C192=3(9)The load acting on a single abrasive grain is:P WN Wd20pK1C18C196ugpC18C19C02=3R22C0 R21C01(10)Substituting Eqs. (2), (3), (9), (10) into Eq. (7):MV83p3=4x1x2od20K1C18C191=46ugpC18C19C01=6C2 KC03=4ICHC01=2VW5=4R32C0 R31R22C0 R215=4(11)Eq. (11) can be simplified to:MV Kd1=20uC01=6gKC03=4ICHC01=2VoW5=4(12)where K is a proportional constant.According to Eq. (12), the material removal rate MVwillbe increased with the increase of the applied static load W,the rotational speed of the toolo, and the size of the abrasivegrains d0.3. Experimental procedureExperiments were performed on a drilling machine. Thediamond drilling tool, which was especially designed toaccept coolant (see Fig. 1), was a special diamond wheelwith a external radius of R2 10 mm and an internal radiusof R1 6 mm. Three types of diamond drilling tool wereprepared, their grits being 80, 120, and 160, and theirconcentrations were being 100%.A 99.5% Al2O3ceramic was selected as the workpiecematerial and water was selected as the coolant.The MRR is measured through a dial gauge with anaccuracy of 0.001 mm. The depth of drilling per minutecan be measured with a dial gauge, and then the MRR canbe calculated (The MRR is the cross-sectional area of thedrilling tool multiplied by the depth of drilling per minute.).The surface roughness is measured using a Talysurf 40(England) surface measuring instrument with a relativeaccuracy of 5%.4. Experimental results and discussion4.1. The effect of the static loadTest results show that the material removal rate tends toincrease with the increase of the static load, as shown inFig. 5, which is similar to Eq. (12).The surface roughness is found to be slightly affected bythe applied static load, increasing with the increase of thestatic load. The present experiments were conducted underFig. 4. Schematic diagram of the terminal face of the drilling tool.Fig. 5. The effect of the static load.234 Q.H. Zhang et al. / Journal of Materials Processing Technology 122 (2002) 232236ntsthe following conditions: o 720 rev/min, C 80. Whenthe static load W is 25, 35, 45, 55, 65 N, the surfaceroughness (Ra) is 0.0040, 0.0040, 0.0042, 0.0045, and0.0045 mm, respectively.4.2. The effect of the rotational speed of the drilling toolFig. 6 shows the effect of the rotational speed of the drillingtool on the MRR. An increase of the rotational speed of thedrilling tool causes an increase in the MRR. As there are someother factors affecting the MRR, for example the flushing ofswarf and the self-sharpening of abrasive grains, the MRR isnot proportional to the rotational speed of the drilling tool.It is very important for high material removal rate to flushaway the swarf. With the increase of the rotational speed ofthe drilling tool, a lot of swarf is formed, and flushing it awaybecomes increasingly more difficult. This will also affectthe self-sharpening of the abrasive grains. Generally speak-ing, self-sharpening of abrasive grains include two compo-nents, self-sharpening through progressive abrasive grainfragmentation and self-sharpening through progressive bonderosion. When the swarf is not completely flushed away,bond erosion and abrasive grain fragmentation becomedifficult, resulting in the dulling of the drilling tool: thusthe MRR will be affected. Additionally, the flushing ofswarf is related to the pressure of the coolant. In the presentexperiments, the pressure of the coolant is 2:0 C2 104Pa.Under this condition, loading occurred when the rotationalspeed of the drilling tool was 1200 rpm.The surface roughness decreases with an increase of therotational speed of the drilling tool. The present experimentswere conducted under the following conditions: W 55 N,C 80. When the rotation speed o is 520, 720, 900 and1120 rev/min, the surface roughness (Ra) is 0.0055, 0.0045,0.0042, and 0.0042 mm, respectively.4.3. The effect of the grain sizeAccording to Eq. (12), the material removal rate willincrease with the increase of the grain size, and this isconfirmed by the test results, as shown in Fig. 7.The size of the abrasive grain can greatly affect thesurface roughness. The surface roughness depends mainlyon the size of the abrasive grains. The greater is the grainsize, the rougher is the finished surface. The present experi-ments were conducted under the following conditions:W 55 N, o 720 rev/min. When the C is 80, 120, and160, the surface roughness (Ra) is 0.0045, 0.0040, and0.0020 mm, respectively.5. ConclusionsThe basic mechanism of the diamond tool drilling ofceramics has been studied, and the effect on the materialremoval rate has been explored. The test results showthat any increase in terms of the static load applied, therotational speed of the drilling tool, and the size of abrasivegrains, results in an increase of the material removal rate.The relationship between the roughness of the finishedsurface and each parameter is given: the finished surfaceroughness increase with increase of the static load, decreaseof the rotational speed, and increase of the abrasive grainsize.References1 R.W. Davidge, Mechanical Behavior of Ceramics, CambridgeUniversity Press, Cambridge, 1979.2 T. Warren Liao, Flexural strength of creep feed ground ceramics:general pa