Handbook of Advanced Ceramics Machining Episode 6 ppsx

A mirror finish surface was obtained when ELID grinding wasperformed with a #4000 mesh-size wheel or finer.. The effects of annealing temperature on the strength of ground silicon nitrid

finish parameters Ra, Rz, and Rmaxwere measured after the grinding ations A mirror finish surface was obtained when ELID grinding wasperformed with a #4000 mesh-size wheel or finer Figure 5.32 and Figure5.33 show a significant improvement in surface finish when grinding using

oper-a #4000 mesh wheel compoper-ared with #2000 mesh wheel Better surfoper-ace finishwas obtained with the SRBSN material than Si3N4, especially when usingrougher wheels However, with finer wheels (#4000) almost the same sur-face finish was obtained with both materials The results are shown inFigure 5.34 [27] To produce a mirror surface finish by ELID grinding, athree-step operation was required The silicon nitride specimens were firstground with a #325 mesh-size wheel These specimens were further ground

TABLE 5.2

Grain Size of Used Diamond Grinding Wheels

Mesh Size Grain Size (mm) Average Grain Size (mm)

Wheel: cup type CIFB-D, diameter 200 mm

0.02 0.04 0.06 0.08 0.1 0.12

The effect of wheel grit size on surface finish.

with a #600 grit-size wheel and finally ground with a #4000 mesh-sizewheel The mechanism of material removal in ceramic grinding is acombination of microbrittle fracture and micro- or quasiplastic cuttingmechanism [28,29] The quasiplastic cutting mechanism, typically referred

Material silicon nitride Wheel: cup type CIFB-D, diameter 200mm

[v = 21.5 m/sec, f = 80 mm/min, t = 1 µm/pass]

Wheel: cup type CIFB-D, diameter 200 mm [v = 21.5 m/sec, f = 80 mm/min,

t = 1 µm/pass]

FIGURE 5.34

The effect of wheel grit size on surface finish.

Highly Efficient and Ultraprecision Fabrication of Structural Ceramic Parts 137

to as ductile-mode grinding, results in grooves on the surface that arerelatively smooth in appearance By a careful choice of grinding parametersand control of the process, ceramics can be ground predominantly in thismode [30,31] On the other hand, the microbrittle fracture mechanismresults in surface fracture and fragmentation Ductile-regime grinding

is preferred since no grinding flaws are introduced if the machining isperformed in this mode Surfaces ground in the brittle fracture mode willhave significant amounts of surface fragmentation On the other hand, asurface produced by the ductile mode will contain little or no surfacefragmentation These two modes can be easily differentiated by observingthe surfaces under SEM and AFM

5.4.8.1 SEM and AFM Studies

The ground surface topography was analyzed by SEM and AFM to mine the brittle- to ductile-mode transition The specimens were sputtercoated with Au–Pd to enable easier SEM imaging The brittle fractureportion of the work piece will be represented by a ‘‘white frosted’’ area

deter-in the SEM micrograph Specimens ground with a #325 grit wheel showed

a significant white frosted area confirming that the material was inantly removed by brittle fracture The SEM micrographs also showedthat with increasing grit size (finer grain size), the amount of surfacefragmentation decreases When ELID grinding was performed with a

predom-#4000 grit-size wheel or a finer, SEM micrographs did not show anysurface fragmentation This shows that under the material is ground inthe ductile mode [27]

Ground surfaces were also observed under AFM The observed surfacearea was 18  18 mm2 The change in surface topography can be observedusing AFM The AFM surface topography also shows that the material waspredominantly removed in the ductile mode when ELID grinding was doneusing a #4000 mesh wheel or finer The surface finish obtained from theAFM study is presented in Table 5.3 [27]

TABLE 5.3

Surface Roughness of Si3N4by AFM

Wheel Mesh R a (nm) R max (nm) R rms (nm) R z (nm)

5.4.9 Flexural Strength of Silicon Nitride

Significant research has been conducted to study the effect of grindingparameters on the strength of ground ceramic specimens Various investiga-tors have also studied the effect of grinding direction on the strength ofceramics Figure 5.35 shows the relationship between the annealing tempera-ture and bending strength of alumina ceramics at room temperature Theresults show that before annealing specimens ground in the transversedirection have a bending strength 60% lower than that of the specimensground longitudinally The bending strength of the specimens ground inthe transverse direction increased with the annealing temperature At about12008C, the strength was approximately equal to that of the specimensground in the longitudinal direction [32] The effects of annealing temperature

on the strength of ground silicon nitride specimen have not been studied.Therefore, experiments were conducted to study the effect of annealing andELID finish grinding on the bending strength of silicon nitride specimens

The material used in the experiment was silicon nitride SN235 tured by Kyocera The size of the specimens was 3  10  50 This is therecommended size for the four-point bending test A large number of Si3N4specimens were fixed on a plate with wax and mounted on the table of thesurface grinding machine Specimens were ground in longitudinal (PG) and

manufac-in transverse (TG) direction usmanufac-ing a #140 grit-size BB diamond wheel Thefollowing grinding conditions were used: (a) wheel velocity, 1200 m=min,(b) table speed, 20 m=min; (c) traverse pitch, 1 mm, (d) depth of cut, 5 mm,and (e) spark out, 3 passes The total depth of cut was around 70 mm

FIGURE 5.35

Effects of annealing on four-point bending strength.

Highly Efficient and Ultraprecision Fabrication of Structural Ceramic Parts 139

Four-point bend tests were performed at room temperature and at 14008C.All bending tests had an upper span of 10 mm and a lower span of 30 mm.

An Instron-type universal tester was used for bending tests, with a constantcross head of 0.2 mm=min Figure 5.36 shows the dimensions and coordinatesystem of the beam specimen with ground surface in a four-point bend test.Bending test results are presented in Figure 5.37 A significant reduction

in bending strength was noticed when specimens were transversely groundcompared with those ground longitudinally PG and TG specimens wereannealed at 12008C for 2 h and the bending strength was determined Thestrength of the heat-treated TG specimens increased significantly as shown

in Figure 5.37 [33] There was no significant change in the strength of the PGground specimens after the annealing process As the TG specimens havethe lowest strength, it was decided to study the effect of ELID finish

Grinding direction

grinding on the bending strength of TG ground specimens These specimenswere ground using ELID grinding and a #6000 grit-size (average grain size

¼ 3.15 mm) SD cast iron-bonded wheel The diameter of the wheel was

150 mm and the width was 10 mm The following grinding conditions wereused: wheel speed, 1200 m=min; table speed, 20 m=min; traverse pitch,0.6 mm; depth of cut, 0.5 mm; and total depth of cut, 40 mm

The power supply used in the experiment was EPD-10 A, with a capacity

of 90 V, 10 A The following ELID conditions were used: Eo, 60 V; Ip, 10 A;on-time and off-time, 2 msec [square wave] Noritake AFG-M grinding fluidwith 2% of water was used in the experiment The bending strength of theELID ground specimens was determined (TGE) These ELID ground speci-mens were annealed and the bending strength of these specimens was alsodetermined The results are presented in Figure 5.37 A significant improve-ment in the bending strength of Si3N4 specimens was achieved whenELID grinding was applied The bending strength of the specimens wasdetermined at 14008C The results are presented in Figure 5.38 The max-imum bending strength at the elevated temperature was found with theTGE specimens [33]

Previous investigations have shown that during grinding of ceramics twosets of flaws are introduced One set is parallel to the grinding direction,

The effect of ELID grinding on the strength of Si 3 N 4

Highly Efficient and Ultraprecision Fabrication of Structural Ceramic Parts 141

which is fairly elongated, while the second is perpendicular to the grindingdirection and smaller in size [34] This has resulted in a significant reduction

in the strength of the silicon nitride ceramic when ground in the transversedirection As explained earlier, ductile-mode grinding was performed withthe application of fine ELID grinding When TG work pieces were finishELID ground with a #6000 grit-sized wheel, the grinding mode was ductile.The flaws produced by initial grinding were removed by the fine ELIDgrinding The TGE specimens therefore do not contain any significantmicrocracks This may be the reason for the significant improvement inthe bending strength of the TGE specimens

5.5 Conclusions

In this paper, the application of ELID grinding for effective and precisiongrinding of various structural ceramic materials is described ELID techno-logy has been successfully applied for surface grinding using a machiningcenter, a horizontal surface grinder, a vertical rotary surface grinder, and forcylindrical grinding on a turning center The results are as follows:

1 Compared with conventional grinding, there is a significant

reduc-tion in normal grinding force with ELID grinding Therefore, ELID

grinding is recommended for heavy material removal grinding,

low-rigidity machines, and low-low-rigidity work pieces

510 MPa SD—51

670 MPa SD—48

540 MPa SD—60

FIGURE 5.38

The effect of ELID grinding on the strength of Si 3 N 4 (at elevated temperature).

2 The full potential of ELID grinding, that is, reduced grinding force,

can be used only after it has been stabilized However, the newly

proposed modified ELID dressing can provide reduced and almost

constant grinding force immediately at the start of grinding

3 Compared with conventional grinding, a reduction in G-ratio was

found when ELID grinding was performed The G-ratio can be

improved by optimizing the ELID current

4 A mirror surface was achieved on silicon nitride materials when

ELID grinding was performed using a #4000 grit-size wheel The

finish ELID technology will find wide application in the optical and

semiconductor industries such as mirror finishing of silicon wafers,

many kinds of ceramics, ferrite, and glass

5 SEM and AFM studies reveal that the work piece was

pre-dominantly ground in the ductile mode when ELID grinding was

performed with a #4000 grit-sized wheel or finer

6 The bending strength of transversely ground Si3N4specimens can be

improved by annealing at 12008C

7 A significant improvement in the bending strength of Si3N4 was

achieved when finish ELID grinding was performed

5.6 Acknowledgments

The authors express their sincere thanks to the industrial members of theELID research project for their financial support Part of the research projectwas supported by the U.S Department of Energy Special thanks to Fuji Die

Co Ltd., Toyko, Japan, for supplying grinding wheels and Ikegami Mold ofAmerica and RIKEN authorities for their financial support The authors alsothank Ms Joyce Medalen for preparing the manuscript

References

1 Jahanmir, S., Ives, L.K., Ruff, A.W., and Peterson, M.B., ‘‘Ceramic Machining:Assessment of Current Practice and Research Needs in the United States,’’ NISTspecial publication 834, Gaithersburg, MD, 1992

2 Malkin, S and Hwang, T.W., ‘‘Grinding Mechanisms for Ceramics,’’ Annals of theCIRP, Vol 45, No 2, 1996, pp 569–580

3 Nakagawa, T., Suzuki, K., and Uematsu, T., ‘‘Three Dimensional Creep FeedGrinding of Ceramics by Machining Center,’’ Proceedings ASME, WAM, PED, 17,

1985, pp 1–7

Highly Efficient and Ultraprecision Fabrication of Structural Ceramic Parts 143

4 Nakagawa, T and Suzuki, K., ‘‘Highly Efficient Grinding of Ceramics and HardMetals on Machining Center,’’ Annals of CIRP, Vol 35, No 1, 1986, pp 205–210.

5 Fuji ELIDer, Catalog from Fuji Die Co Ltd., Tokyo, Japan, 1996

6 McGeough, J.A., Principles of Electro Chemical Machining, Chapman and Hall,

1974, pp 162–195

7 Welch, E., Yi, Y., and Bifano, T., ‘‘Electro Chemical Dressing of Bronze BondedDiamond Grinding Wheels,’’ Proceedings of the International Conference on Machin-ing of Advanced Materials, NIST publication 847, 1993, pp 333–340

8 NICCO Creep Feed Grinders: Catalog from Carl Citron Inc., NJ

9 Ohmori, H and Nakagawa, T., ‘‘Mirror Surface Grinding of Silicon Waferswith Electrolytic In-Process Dressing,’’ Annals of CIRP, Vol 39, No 1, 1990,

12 Bandyopadhyay, B.P., Ohmori, H., and Takahashi, I, ‘‘Efficient and Stable ing of Ceramics by Electrolytic In-Process Dressing (ELID),’’ Journal of MaterialsProcessing Technology, Elsevier, Vol 66, 1997, pp 18–24

Grind-13 Vickerstaff, T.J., ‘‘Diamond Dressing—Its Effect on Work Surface Roughness,’’Industrial Diamond Review, Vol 30, 1970, pp 260–267

14 Davis, C.E., ‘‘The Dependence of Grinding Wheel Performance on Dressing cedure,’’ International Journal of Machine Tool Design Research, Vol 14, 1974, pp 33–52

Pro-15 Konig, W and Meyen, H.P., ‘‘AE in Grinding and Dressing: Accuracy andProcess Reliability,’’ SME, 1990, MR, pp 90–526

16 Malkin, S and Murray, T., ‘‘Mechanics of Rotary Dressing of Grinding Wheels,’’Journal of Engineering for Industry, ASME, Vol 100, 1978, pp 95–102

17 Koshy, P., Jain, V.K., and Lal, G.K., ‘‘A Model for the Topography of DiamondGrinding Wheels,’’ Wear, Vol 169, 1993, pp 237–242

18 Syoji, K., Zhou, L., and Mitsui, S., ‘‘Studies on Truing and Dressing of GrindingWheels, 1st Report,’’ Bulletin of the Japan Society of Precision Engineering, Vol 24,

No 2, 1990, pp 124–129

19 Suzuki, K., Uematsu, T., Yanase, T., and Nakagawa, T., ‘‘On-Machine discharge Truing for Metal Bond Diamond Grinding Wheels for Ceramics,’’Proceedings of the International Conference on Machining of Advanced Materials,NIST 847, July 1993, pp 83–88

Electro-20 Wang, X., Ying, B., and Liu, W., ‘‘EDM Dressing of Fine Grain Super AbrasiveGrinding Wheel,’’ Journal of Materials Processing Technology, Elsevier, Vol 62,

1996, pp 299–302

21 Piscoty, M.A., Davis, P.J., Saito, T.T., Blaedel, K.L., and Griffith, L., ‘‘Use ofIn-Process EDM Truing to Generate Complex Contours on Metal BondSuperabrasive Grinding Wheels for Precision Grinding Structural Ceramics,’’ Pro-ceeding of International Conference on Precision Engineering, Taipei, Taiwan, 1997,

pp 559–564

22 Ohmori, H., Takahashi, I., and Bandyopadhyay, B.P., ‘‘Ultra Precision Grinding

of Structural Ceramics by Electrolytic In-Process Dressing (ELID) Grinding,’’Journal of Materials Processing Technology, Elsevier, Vol 57, 1996, pp 272–277

23 Ohmori, H., Takahashi, I., and Bandyopadhyay, B.P., ‘‘Highly Efficient Grinding

of Ceramic Parts by Electrolytic In-Process Dressing (ELID) Grinding,’’ Materialsand Manufacturing Processes, Marcel Dekker, Vol 11, No 1, 1996, pp 31–44

24 Bandyopadhyay, B.P., Ohmori, H., and Makinouchi, A., ‘‘Efficient and PrecisionGrinding Characteristics of Structural Ceramics by Electrolytic In-Process Dress-ing (ELID) Grinding,’’ Proceedings of 1998 Japan–U.S.A Symposium on FlexibleAutomation, Otsu, Japan, July 1998, pp 305–311

25 Bandyopadhyay, B.P., ‘‘Application of Electrolytic In-Process Dressing for HighEfficiency Grinding of Ceramic Parts: Research Activities 1995–1996,’’ORNL=SUB=96-SV716=1 Report, February 1997

26 Ohmori, H., Takahashi, I., and Bandyopadhyay, B.P., ‘‘Ultra-precision Grinding

of Structural Ceramics by Electrolytic In-Process Dressing (ELID) Grinding,’’Journal of Materials Processing Technology, Elsevier, Vol 57, 1996, pp 272–277

27 Bandyopadhyay, B.P., Ohmori, H., and Takahashi, I., ‘‘Ductile Regime Finish Grinding of Ceramics with Electrolytic In-Process Dressing (ELID) Grind-ing,’’ Materials and Manufacturing Processes Journal, Marcel Dekker, Vol 11, No 5,

pp 287–290

31 Namba, Y., Yamada, Y., Tsuboi, A., Unno, K., and Nakao, H., ‘‘Surface Structure

of Mn–Zn Ferrite Single Crystals Ground by an Ultra Precision SurfaceGrinder with Various Diamond Wheels,’’ Annals of CIRP, Vol 41, No 1, 1992,

pp 347–351

32 Matsuo, Y., Ogasawara, T., Kimura, S., Sato, S., and Yasuda, Y., ‘‘The Effects ofAnnealing on Surface Machining Damage of Alumina Ceramics,’’ Journal of theCeramic Society of Japan, (Intl Edition), Vol 99, No 5, 1991, pp 371–376

33 Bandyopadhyay, B.P and Ohmori, H., ‘‘The Effect of ELID Grinding on theFlexural Strength of Silicon Nitride,’’ to be published in the International Journal

of Machine Tools and Manufacture, Pergamon press

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Highly Efficient and Ultraprecision Fabrication of Structural Ceramic Parts 145

Electrolytic In-Process Dressing Grinding

of Ceramic Materials

H Ohmori and K Katahira

CONTENTS

Abstract 148

Key Words 148

6.1 Introduction 148

6.2 ELID Grinding Technique 149

6.2.1 The Concept of ELID 149

6.2.2 ED Truing Technique 151

6.2.3 Electrical Behavior during Predressing 151

6.2.4 The ELID Grinding Mechanism 154

6.3 Efficient and Precision ELID Centerless Grinding of Zirconia Ceramics 155

6.3.1 Experimental Setup for ELID Centerless Grinding of Zirconia Ceramics 155

6.3.2 Results of ELID Centerless Grinding of Zirconia Ceramics 157

6.4 ELID Grinding Characteristics for the Machining of Optical Surface Quality for Ceramic Spherical Lens Molds 160

6.4.1 Mechanism of ELID CG-Grinding 160

6.4.2 Experimental Setup for ELID CG-Grinding 162

6.4.3 ELID CG-Grinding of Ceramic Spherical Lens Molds 162

6.5 ELID Grinding Characteristics and Surface Modifying Effects of Aluminum Nitride Ceramics 164

6.5.1 Experimental Setup for ELID Grinding of AlN Ceramics 164

6.5.2 Observation of the ELID Ground Surface 167

6.5.3 Surface Modifying Effect by ELID Grinding 168

6.5.4 Analysis of the Modified Surface 174

Acknowledgments 176

References 176

147

ABSTRACT Efficient precision grinding techniques for ceramics arerequired in order to mass-produce ceramic parts New grinding techniquesfor ceramics that employ fine bonded superabrasive wheels and high-stiffnessgrinding machines have been designed in order to achieve high grindingefficiency and produce high-quality ceramic parts Ceramics areextremely hard to machine using conventional methods Mirror finishing

of these hard, brittle materials requires the use of diamond abrasives.Grinding in particular, compared with lapping and polishing, can be used

to efficiently produce various geometric forms A novel grinding ogy, known as electrolytic in-process dressing (ELID) that incorporatesin-process dressing of metal-bonded grinding wheels, provides dressing

technol-of the metal-bonded wheels during the grinding process, while maintainingcontinuous protrudent abrasive from superabrasive wheels This paperdescribes the highly efficient and precise ELID grinding method anddiscusses the ELID grinding process and the grinding characteristics ofceramic materials

KEY WORDS ELID, electrolytic in-process dressing, ceramics, electro-dischargetruing (ED truing), metal-bonded diamond grinding wheels, cast-iron-bondeddiamond grinding wheel, efficient grinding, mirror finish grinding, ductile modegrinding, brittle mode grinding, surface modification

6.1 Introduction

Interest in advanced ceramics has increased significantly in recent years due

to its unique physical properties and the significant improvement in themechanical properties and reliability The advantages of ceramics over othermaterials include: (a) high hardness and strength, the retention thereof atelevated temperatures, (b) chemical stability, and (c) superior wear resist-ance Until now, mirror surface machining of ceramics has been performedmainly by polishing or lapping However, such machining techniques can-not always be described as highly efficient or productive, and the adverseeffects of scattered abrasive particles on equipment, the treatment of wasteliquids, and other problems remain to be investigated

On the other hand, grinding in particular, compared to lapping andpolishing, can be used to efficiently produce geometric forms New ceramicgrinding techniques that employ fine bonded superabrasive wheels andhigh-stiffness grinding machines have been designed to achieve high grind-ing efficiency and high-quality ceramic parts Dr Ohmori pioneered a novelgrinding technique that incorporates in-process dressing of metal-bondedsuperabrasive wheels known as ELID [1–8], Japanese Patent No 1,947,329 [9]

This technique provides in-process dressing of metal-bonded wheels, ing the grinding process, for continuous protrudent abrasive from super-abrasive wheels Moreover, ELID grinding has recently been found to bringabout improvements in the surface characteristics of the workpiece beingprocessed [10–14], and this technique produces superior resistance

dur-to corrosion and wear, as well as other advantages The present chapterdescribes the basics of the ELID grinding technique and presents a discussion

on the ELID grinding process and the grinding characteristics of ceramicmaterials

6.2 ELID Grinding Technique

6.2.1 The Concept of ELID

ELID grinding was first proposed in 1988, and a number of reports ing the advantages of this process have been published [1–14] The basicconstruction of the ELID grinding system is shown in Figure 6.1 Theessential elements of the ELID system include: (a) a metal-bonded grindingwheel, (b) an electric power source, and (c) an electrolytic coolant The mostimportant feature is that there is no requirement for a special machine

describ-The principle of ELID grinding is shown in Figure 6.2 describ-The wheel serves

as the positive electrode The negative electrode is installed opposite thegrinding surface of the wheel The clearance between these two electrodes isset at 0.1–0.3 mm DC-pulse voltage is supplied between the two electrodes

in order to electrolytically remove only the metal bond of the wheel, ing efficient and automatic dressing of the wheel This dressing is continuedeven during the grinding work in order to prevent reduced wheel sharpnessfrom wear, thereby realizing highly efficient mirror surface grinding

allow-ELID grinding consists of the following steps:

1 Truing, for example, using a SiC wheel of grit size #100 or an

aluminum oxide stick is required in order to reduce the initial

eccentricity

2 The truing of tough metal-bonded wheels is very difficult and time

consuming for coarse-grit wheels of larger diameter A new efficient

electrodischarge truing (ED truing) has been developed and is

(Cast) Iron bonded super-abrasive wheel

Grinding wheel

ELID technique

Grinding Fluid

Chemical solution type fluid Electrode

Negative electrode ( −ve)

Work Chuck

FIGURE 6.2

The principle of ELID grinding.