JOURNAL OF SCIENCE & TECHNOLOGY • No 94 - 2013 DESIGN AND MANUFACTURE AIR ROTARY TABLE THIET K.E CHE TAO KHI QUAY Vu Toan Thang Ta Thi Thuy Huong Hanoi University of Science and Technology Haiphong University Received March 05, 2013; accepted April 25, 2013 ABSTRACT Air rotary table is widely used in devices, which require high accuracy of rotation such as rotary table of roundness measuring machines, angle measuring devices or angle grating devices This paper presents results of design and manufacturing the air rotary table, with stability of rotary center of 1pm and load capacity up to lOOON in Department of Precise Mechanical and Optical Engineering Hanoi University of Science and Technology Based on studying of structural design, analysis of the factors affecting the air-beartng working capacity, we can get closer to mastering the technology of manufacturing precision equipment such as roundness measuring machine, angle measunng device and other devices which need high precision of rotation Keywords, air rotary table, air bearing, high accuracy rotation, pressure distribution TOM TAT quay duvc st> dung rpng rai cic thiit bi co dp chinh xac quay cao nhw ban quay cOa miy do trdn thiit bi goc va cac thiit bi chia gde Bai bio trinh bay kit qui nghien ct/u thiit ki va chi tao quay vai dd dn dmh tam 1pm, kha nang tai l&n den 1000N tai Bd mdn Ca Chinh_ xic & Quang hoc - Trwang Dai hoc Bich Khoa Hi Not Tren ca sa thiit ki vi phan tich c^c yiu td inh hwang din kha nang hoat ddng cua quay, chung ta cd the lam chu duvc cdng ngh$ chi tao cic thiit bi chinh xac nhw may dd tron, miy goc va cac thiit bi yeu cau chinh xic quay cao INTRODUCTION Air rotary table has a suspend spindle which rotates without mechanical contact and is separated by high pressure thin film between two surfaces from I - bar Air bearing table is designed by different structures for each using purpose It has two main parts: rotary shaft and air-bearing system Because of several following advantages, the air bearing table is usually used for high accurate machines [1] - Virtually zero wear resulting in consistent machine performance and low particle generation - Near zero static friction is, infinite resolution and very high repeatability are possible The errors of surface finish irregularities on the guide are averaged and - Having a squeeze film damping effect resulting in higher dynamic stiffness and better controllability Air bearing table does not use oil lubrication, eliminating the problems associated with oil, it itself lubricates Another reason that the air rotary table is widely used in roundness measuring machine is the high accuracy of rotation Roundness tolerance of mechanical part is only from some to twenty micrometers and the error of rotation is include in measuring data, so in order to improve the accuracy of roundness measurement we need to improve the accuracy of rotation, using the air rotary table is most suitable The advantage in manufacturing technology of air rotary table is not required the high accuracy of absolute dimension, as long as the air bearings and the rotary shaft fit together It is suitable to mechanical manufacturing capabilities in Vietnam JOURNAL OF SCIENCE & TECHNOLOGY No 94 - 2013 Based on comparing several sfructures of air rotary table, high accuracy of rotation and stability of operation was chosen This paper also presents the analyses on form tolerances which effect to operation of air rotary table DESIGN AND ANALYS There are many kinds of structures of air rotary spindle for different using purposes Figure is the first model of the air rotary table, which has been produced in the lab of Department of Precise Mechanical and Optical Engineering Two rows of air-bearing on the inside cylinder surface constrain degrees of freedom, and one on the thrust surface constrains degree of freedom, thus there is only one rotary movement of the shaft [2]rotary shaft Fig Structure of inside rotary shaft and outside air bearing ' -Q- I roii I -Q- Fig Structure of air-rotary table under consideration Fig Structure of outside rotary shaft and inside air bearing This structure is high accuracy of rotation, but there is only one air flat bearing at thrust surface, the load capacity and the stiffness of air rotary table is not high The figure shows that the air bearings fixed on cylinder constrain degrees of freedom and the flat air bearings on surfaces constrain degrees of freedom [3] This model is high load capacity but the stability of rotary axis is not high The structure presented in figure is chosen to be manufactured in this research to improve the load capacity and the rotary precision Two rows of air bearing fixed on cylinder surface constrain degrees of freedom, two rows of air bearing fixed on and under the thrust surfaces constrain degrees of freedom, Thus the rotary shaft is over positioning In order to operate the air rotary table, it needs to improve the precision of manufacture of rotary shaft Assume that the air bearings are fixed on the cylinder surface with constant air clearance and the air clearance between air bearings and JOURNAL OF SCIENCE & TECHNOLOGY • No 94 - 2013 thrust surfaces is maximum of lOnm The total clearance on and under thrust surfaces is 20^m If the air rotary table is able to run, the form and orientation tolerances of rotary shaft are required with strict value shows in figure and figure 5: & >> 0.005 A // 0.005 B / ? O air clearance between air bearings and thrust surface is: z = (20-15-0,63x4)/2=l,24^m (1) And the air rotaty table still runs withoiil mechanical contact With those tolerances we can easily manufacture the rotary shaft in Vietnam * After machining on the grinding machine, the rotary shaft and air bearings are grinded to fit together, using free abrasive particles 0.002 \0\ 0-002 Fig Form and orientation tolerances of rotary shaft - The tolerance of perpendicular between thrust surface and cylinder surface is 5jim Thus the tolerance of run-out is Ojim (Atmax^lOpm) The tolerance of parallel between two thrust surfaces is 5fim (Aifmax=5^m) Fig Run-out and parallel errors of rotary shaft The total error is Atmax +A^max = IS^im, this error is equally distributed by two sides of thrust surfaces If the roughness of air bearings and thrust surfaces is included, the minimum of Fig Allocation of air bearings on rotary shaft If the rotary shaft and air bearing too fit, when the air bearings is supplied a pressured air source, it happens an uneven of air clearance shown in figure Fig Uneven air clearance Using Ansys software to calculate the loading force in case of uneven air clearance: D value is kept by constant, we change the d value to get the loading force Here, D and d are the value of the air clearance in the middle and in the edge (Figure 7) We simulate for an air JOURNAL OF SCIENCE & TECHNOLOGY • No 94 - 2013 bearing which has two supplied sources, groove is H shape, input of pressured air is bar Result shows in figure 8: Load capacity AS raise if d is reduces, but d cannot reduce with small value because of mechanical contact 550 s;: Fig The loading force varies with d In this design model, flat air bearings are used on and under two sides of thrust surfaces These air bearings create the loading force of air rotary table Using air bearings increase the stiffness of the system times [4]: K _ Ag n.Az Ag 6.Az (2) Fig 10 Assembled drawing of air rotary table EXPERIMENTS AND RESULTS K: stiffness of system n: number of air bearings AQ: variation of load The loading capacity of flat air bearing is implemented by raising the pressured through a piston to load on the air bearing, the variation of air clearance is measured by Mitutoyo indicator 0,001 (Figure 11) Az: variation of air clearance The flat air bearing and curve air bearing are designed by load capacity, stiffness and stability The structures of them are shown in figure Fig 11 Diagram to determine load capacity Fig Flat and curve air bearings The assembled drawing of air rotary table of structure Is shown in figure 10 The results show: at air clearance of 10p,m, the load is 115N at 2bar input source, 168N at 3bar, 184N at 4bar, the stiffness is respectively 2,7N/|im, lON/pm, 12N/jim The input of air source is higher, the stiffness of air bearing is higher (figure 12) To an experiment to determine the stiffness of air rotary table in figure 13, using the Mitutoyo dial indicator with l|im resolution to measure the reduction of air clearance JOURNAL OF SCIENCE & TECHNOLOGY • No 94 - 2013 The result in figure 14 shows: if the air rotary shaft is loaded 400N, the reduction of air clearance is 6^m, that means «68N/jim of stiffness The stiffness of single air bearing ii 12N/^m at about lO^im air clearance, 4bar input resource This is suitable with the equation If it raises the load up to lOODN, the shafi rotates heavily and the mechanical contaa happens (figure 15) '•'ig 12 I.Dik/ co/kiciiv ilcpciid I'll oil- clvunnicc iihl iiipiii I'/,lir source liipiil lir prcssiiic i.s •Ihar Ihc /ulary :!i;iri is lillCLl up lOum wilhoul Inacl Fig 15.1'lteck llic Dhixiiiiuiu load L opacity oj air roiiirv uible Fig 13 Expffimenl m JcienninL- ilie siilfiicss of (lir rulary uible iiicrciisiiig tlic load on roUirv' shall \;ikic of ihe Indicalor shows llie rcdiiciion ol ihc ;iir clearance IMN) WaOlx' + 20,029x + 51,134 R' = 0,9971 W V#W Fig- 16 Expcrinieni lo determine ihe fliicliiauvn of rola/T centre, include ihi' roiim/ncss of measuring sample Fig 14 Rclalion bclwecii load and redmiinn of iiir dearuncL- Figure 16 shows the experiment to determine the accuracy of air rotary table, using Mitutoyo standard bore sample with No.!??148 910067 bore diameter of 89.998mm±0,005, Mitutoyo dial indicator 0,001 The sample is adjusted to locate on the JOURNAL OF SCIENCE & TECHNOLOGY No 94 - 2013 :enter of the rotary shaft, rotating the shaft and getting the data of the indicator, result shows the value of fluctuation of rotary center with include of the sample's roundness is 2[xm This result is acceptable because it adds in the roundness of measuring sample CONCLUSION In this research, we have designed and manufactured the air rotary table, which can be operated with lOOON of load and l^m of stability of rotary center Based on these models have been designed and manufactured before, we have analyzed the positions of the air bearings on the rotary shaft to select the suitable structure of air rotating table that is high stability of rotary center and good capacity of load Then we used the software to simulate the load characteristics of the air bearing After manufacturing, the experiments were buih to determine the load capacity and the center stability of the air rotary table; the load on the table increases, the air clearance decreases respectively to stiffness of system (68N/(im) This result has opened up the prospect of manufacturing the high accurate air rotary table in Vietnam REFERENCES New Way Precision Instruction "Air bearmg - application and design", 2003 Vu Toan Thang "Building the measuring method of out-of-roundness in polar coordinate" Doctor Thesis, 2005 Vu Toan Thang "Design and manufacture air rotary table", University Project T2008-08, 2008 Prof Nguyen Tien Tho "Design and manufacture the mechanical moving system of CMM using air bearing", MOET Project B2005-28-163, 2005-2006, Author's address: Vu Toan Thang - Email: thang.vutoan@hust.edu.vn School of Mechanical Engineering Hanoi University of Science and Technology No Dai Co Viet Str., Ha Noi, Viet Nam