Critical aspects on the behavior of material from the mechanical tool-workpiece interaction in single point diamond turning

Critical Aspects on the Behavior of Material from the Mechanical Tool-Workpiece Interaction in Single Point Diamond Turning

Renato Goulart Jasinevicius

Jaime Gilberto Duduch

Arthur José Vieira Porto

Benedito Morais Purquério

Departamento de Engenharia Mecânica

Escola de Engenharia de São Carlos

13560-970 São Carlos, SP Brazil

[e-mail]

Abstract

Some material aspects such as grain size, purity and anisotropy exert an important influence on surface quality, especially in single point diamond turning. The aim of this paper is to present and discuss some critical factors that can limit the accuracy of ultraprecision machining of non-ferrous metals and to identify the effects of them on the cutting mechanism with single point diamond tools. This will be carried out through observations of machined surfaces and chips produced using optical and scanning electron microscopy. Solutions to reduce the influence of some of these limiting factors related with the mechanism of generation of mirror-like surfaces will be discussed.

Keywords: Single Point Diamond Turning, Ultraprecision, Non-ferrous Metal, Surface Quality, Scanning Electron Microscopy.

Introduction

Ultraprecision diamond turning is the process that uses a single point diamond tool to produce surfaces with mirror-like surface finish on a specially designed machine tool with high stiffness and high positioning resolution. The field of application comprises both reflection and transmission optical elements. Material that can be diamond turned include metals, ceramics, optical glasses, semiconductors crystals and polymers. Research into this process is still of increasing interest because of the possibility of replacing with advantage conventional machining processes, such as lapping and polishing, traditionally used for the fabrication of optical components (Ikawa et al. 1991). It is worth mentioning that this process is capable of producing surfaces with complex shape such as aspherics with submicrometre accuracy with surface finish of the order of a few nanometres (Ra < 10nm), low surface and subsurface damage with high removal rates (Franse, 1990).

In ultraprecision machining, the typical cutting conditions, i.e., feedrate and nominal depth of cut range from 5-15 m m/rev and 1-10 m m, respectively. This order of magnitude implies that material removal and surface generation process be governed by the micro interaction between the diamond cutting edge and the workpiece material (König and Spenrath, 1991). In machining of polycrystalline materials, the cross section of the chip produced will b within the domain of a single crystal, as shown in Fig. 1 (a) and (b), since the dimensions of the machining conditions are typically smaller than the average size of the crystal grains. It is possible to observe that a single grain shows several grooves left by the tool.

Polycrystalline materials, which are considered isotropic, continuum and homogeneous in conventional machining (Von Turkovich, 1981; Shaw, 1984), will be treated as a series of single crystals with random crystallographic orientations and anisotropic properties in ultraprecision machining. This anisotropy can be responsible for the changes in the dynamic behaviour of the cutting process, that is, when the tool cutting edge passes from a crystal grain to another, different mechanical properties starting from the grain boundary are found (Sato et al., 1991; Moriwaki et al., 1993,Yuan et al., 1994). These physical variations are considered responsible for the microvibrations of the tool tip both in the crossfeed and infeed directions.

Due to deformation stress fields, which result from the micro-interaction mechanism between the cutting edge and material during the machining process, tensile and/or compressive stresses are introduced in the machined surface and subsurface. For exemple, there are stresses introduced by the sliding of the tool clearance face on the newly formed surface due to the elastic recovery of the material (Horio et al., 1992). This is considered the third zone of deformation in the tool/workpiece-material interaction field (Liu and Dornfeld, 1996).

Different stress states along with the elastic recovery that takes place in each crystal grain after the tool passes will promote small topographic alterations which result in small steps of the order of few nanometres on the machined surface affecting the surface quality of the workpiece (Moriwaki and Okuda, 1989).

The sharpness and the surface roughness of the tool cutting edge are extremely important factors in achieving high resolution in terms of thickness of cut, optical quality surface finish and low level of surface and subsurface damage (Jasinevicius et al., 1993).

The objective of this article is to present and discuss the influence of impurities, hard inclusions, surface scratches and polycrystallinity in the single point diamond turning of non-ferrous metal alloys. Based on observations of optical and scanning electron micrographs, the machined surfaces as well as the chips produced were assessed and the effects of the above mentioned factors on the cutting mechanism identified. It was observed that the microstructure plays an important role in the performance of the cutting process and in the function of the surfaces generated. Solutions to reduce some of these limiting factors related with the mechanism of generation of mirror-like surfaces will be discussed.

Factors of Influence of Non Ferrous Metals in Single Point Daimond Turning

Impurities, Hard Inclusions and Surface Scratches

The size and number of hard particles (impurities, inclusions, etc.) in metals and alloys can be considered as a limiting factor in achieving any improvement in terms of surface finish in ultraprecision machining. These impurities are, generally, residuals of pyrometallurgical process and /or of refining steps applied to obtain metals and alloys (Ohmori and Takada, 1982). These particles are extremely hard and are found in the soft metal matrix. There are some materials, for example, aluminum alloys which have high levels of silicon, so that small silicon inclusions are precipitated in the ductile matrix. These inclusions are difficult to cut and will rather be drawn or dragged causing small scratches within the cut grooves. This is shown in Fig. 2.

Hard particles can also be observed on the surface of the chip removed. Fig. 3(a) shows some of these inclusions encrusted in the thicker part of the chip. In the thin edge of the chip, it is possible to detect small holes which demonstrates that, during that pass, the tool was not able to extract the inclusions, and removed the ductile metal in the vicinity of them, this can be observed in Fig. 3(b). It is worth to mention that, because of the high strength and hardness of these particles, severe damage can be caused to the tool cutting edge. Microfractures generated by a fatigue mechanism can affect drastically the surface finish of the workpiece, since the achievement of mirror-like surfaces is directly related to the high fidelity reproduction of the roughness of the cutting edge onto the workpiece surface, so if the cutting edge shows any imperfection, this will be reproduced on the machined surface. (Ikawa and Shimada, 1977; Evans, 1991).

The Effect of Policrystalinity

In policrystalline materials, each crystal grain is randomly oriented and, consequently, its mechanical properties will change as a function of the crystallographic orientation. This phenomenon of differing properties can affect several machining aspects such as cutting forces (Liang et al., 1994), chip formation mechanism (starting from the grain boundary) (Ueda and Iwata, 1980; Moriwaki et al., 1993) and, finally, the surface finish (Sato et al., 1991).

Due to the high deformation resulting from the chip formation mechanism, strains are introduced into the surface of the workpiece. Because of the variation of mechanical properties of adjacent grains, which statistically have different crystallographic orientations, different levels of deformation are introduced in each crystal grain, after the tool passes. This new stress state will cause uneven elastic and plastic deformation within each crystal grain. As a result, the crystal grains can present small topographic variations, i.e., difference in height within a grain starting from the grain boundary as Shown in Fig. 4. This can degrade the surface finish and in optical applications the function.

Some explanations for this are the sharpness of the tool cutting edge (Ohmori and Takada, 1982; Evans et al. 1987) and the depth of cut: nanometre level tool sharpness and reduced depth of cut implies smaller topographic variations in the height of each crystal (Moriwaki and Okuda, 1989). Some authors attribute the cause for this phenomenon to the elastic behaviour of the workpiece material. The elastic module in each crystal grain is dependent on the crystallographic orientation (Fuchs et al., 1989, Callister, 1994); consequently, the surface of each crystal will deform in a particular manner when submited to strain stress, demonstrating different levels of plastic and elastic deformation. In other words, the stress fields generated by the interaction of the tool tip/crystal grain would be statistically different from one crystal to another.

Another way to explain this phenomenon is related to the plastic behaviour of the material. The plastic deformation in nonferrous metals such as copper and aluminum, which have face centered cubic structure (fcc), will be more pronounced in the crystallographic direction with the lower elastic module (Carr et al., 1991); because of this, when a tool is cutting a crystal with a higher elastic module, the elastic recovery will be higher; then after the tool passes, this crystal grain will recover and tend to return to a topographic position on the surface different from the crystal grains in the vicinity, which is likely to have a different elastic module (Moriwaki et al., 1993). This topographic difference in the height of the crystal grains takes place due to differential work hardening induced by the sharpness of the tool cutting edge. When the depth of cut is approximately equal to the cutting edge radius, the material will be removed following the same mechanism as a cutting tool with highly negative rake angle (König and Spenrath, 1991). In this case, the negative rake angle would be formed by the round part of the cutting edge in contact with the workpiece material. Despite of this mechanism, it was also considered the plastic deformation caused by rubbing or burnishing from the interaction of clearance face and the newly surface formed due to the elastic recovery as well as the plowing which are mechanisms that prevail at this dimensions of cut and govern the material removal process instead of deformation and shear.

The damaged surface layer by work hardening, showing amorphous characteristics (the grain boundaries turn to be less evident on the machined surface and on the free surface of the chip), will be removed by successive cuts. When the cut takes place within the work hardened layer, the mechanism of deformation and shear are independent of crystal orientation and cutting direction (Moriwaki et al., 1993). According to Yuan et al. (1996) the relationship between the level of the cutting edge sharpness is of extreme relevance to the mechanism of deformation that occurs during cutting and to the surface integrity of the surface. The minimum thickness of cut and the thickness of the work hardened surface layer is directly proportional to the tool edge sharpness.

In single point diamond turning of ductile polycrystalline materials, the cutting forces and the chip formation mechanism are normally dependent on the crystallographic orientation (Sato et al., 1991), crystal grain size (Furukawa and Moronuki, 1988) and can change starting from the grain boundary (Liang et al., 1994). However, during machining, several grains must be cut simultaneously along the width of cut, determined by the nominal depth of cut and the tool nose radius. Because of this, the equilibrium position for the width of cut might occur at an intermediate position to avoid the influence of chattering in the cutting mechanism (Sugano et al., 1987). Figs. 5 (a) e (b) exemplify what was mentioned showing the change in the lamellar structure on the free surface of the chip, where the lamella spacing changes from one grain to another. The lamellar structure formed in the chips is a function of the nominal depth of cut, cutting edge radius (Nakayama and Tamura, 1968, Von Turkovich and Black, 1970; Blake and Scattergood, 1986), and crystallographic orientation (Black, 1979, Ueda and Iwata, 1980). These factors are directly related to the shear angle formed during the chip formation process. According to Black (1979), the shear angle can also be related to the stacking fault energy, which is related to the material purity. Pure materials, such as single crystals, present higher stacking fault energy and lower shear angles during cutting.

The importance of lessening the anisotropy effects during machining, e.g., the use of single point diamond tools with larger nose radius, positive rake angles and material with fine grain size (which can result in more homogeneous mechanical properties), is due to the fact that these differences in mechanical strength in each grain can promote microvibrations in the tool/material interface, which can affect the surface roughness of the workpiece, this is shown in Fig. 6.

Discussion

Microvibrations caused by the polycrystalline aggregate microstructure during ultraprecision machining can be detected and monitored by cutting force signals. Some researches demonstrated that the origins of these vibrations are due to the change in the crystallographic orientation which forms the polycrystalline aggregate (Lee 1990, 1994). The variation in the cutting force signals starts from the grain boundary (Liang et al., 1994). The reduction in the crystal grain size is pointed out as an alternative to attenuate the vibrations generated by the change in crystallographic orientation of the crystal grains since, in this case, the tool will be cutting a larger number of grains simultaneously, minimizing the microvibrations due to the crystallographic change (Furukawa and Moronuki 1988). The grain refining implies in better homogeneity in the mechanical properties of the material. In addition, another solution is to obtain more homogeneous properties of the polycrystalline aggregate through manufacturing operations such as cold rolling. This will provide texture to the crystal grains, e.g., grain with preferred orientation. This can attenuate the variations which are dependent on the orientation of the grains such as shear angle, cutting forces (Lee and Zhou, 1993). According to Sato et al., 1983, experimental results showed that the cutting mechanism in anisotropic cold rolled materials, which have definite preferred orientations such as 90% cold rolled materials, is similar to that of a single crystal having the same orientations as those of the anisotropic material. Work hardened materials have larger yield strength and, because of this, the shear angle will increase during cutting, which is desirable because this will ease the chip formation as well as reduce cutting forces. During the cutting of a single crystal it has been proposed that crystal rotation axes be selected in such mode that the shear angle show the least variation (König and Spenrath 1991, Lee 1990).

Another important aspect to be considered is the use of single crystals or of alloys with low quantities of impurity in the matrix. The problem of surface finish damage caused by hard inclusion in the ductile matrix can be avoided using negative rake angle tools. This tool geometry will benefit the removal of inclusions without drag them on the surface. In addition, negative rake angle tools do not favour the material removal and, consequently, increases tool forces. In this circumstance, the depth of the work hardened layer will increase due to the higher deformation imposed by the interaction between the tool rake face and the material being cut during machining. This is not desirable for optical components because, according to Hurt and Decker (1984,1985), the work hardening damages the reflectivity of the surface since this parameter, is directly related to the conductivity of the material and, work hardened materials present higher resistivity, which in inversely proportional to conductivity.

The machining condition is another factor of great relevance to the mechanism of surface generation. The cutting conditions must be such chosen in such a manner that they act as predominant factors in the surface roughness results, when compared to natural influences such as, elastic recovery, microvibrations, etc.. These aspects are due to inherent responses of the material and can not be eliminated but, its effects can be minimized. The diagnosis, for example, if the tool feed rate is well marked on the workpiece surface, can be evaluated through a spectral analysis from the measured surface roughness profile (Porto et al., 1995).

Metal deposition on the surface as an alternative to eliminate undesirable features on the machined surface can not be viewed as a solution to better the final surface finish of the workpiece. Metal deposition will reveal all imperfections encountered on the surface since the deposited layer is always constant.

Concluding Remarks

The primary materials effects on the cutting mechanism and the improvement of single point diamond turned surface quality were discussed. It was demonstrated that in ultraprecision machining the material has to be considered non-homogeneous and anisotropic. Because of the reduced cutting dimensions, the material microstructure plays a fundamental role in the performance of the cutting process and in the achievement of machined surfaces with optical quality. The responses given by the machined surfaces offer some insights to the microstructure and composition of the materials to be used in this process. This is important because there may be non-reversible deleterious damage, not only in the surface but also in the tool cutting edge caused by the microstructure of the material used in manufacturing of high quality surfaces. The observation of the machined chips produced by the cutting process can provide interesting and complementary information on the tool/material interaction. Material mechanical properties, such as hardness, yield strength, etc. and its influence on the cutting mechanism and surface generation process are aspects that shall be studied and understood in ultraprecision machining. This is a theme for future works.

The surface roughness of single point diamond turned workpieces are influenced by the effect related to the material (grain boundaries, impurities, crystal grain size and exposed grains) and by the level of vibrations and chatter of the machine-tool. These effects, however, can be attenuate by using appropriate tool geometry, materials with uniform size and distribution of grains, material texture in the case of polycrystalline materials and cutting parameters. It should be mentioned that the removal rate of this process can be considered higher and faster in relation to traditional optical manufacturing processes, which could be replaced with some advantages by single point diamond turning. One of the most important characteristics of this process is the possibility of directly determine and more rapidly conform components with complex shape accuracy and ultra-smooth surface finish of the fabricated surfaces through the CNC and allowing the fabrication of anespherics surfaces

The application of products manufactured using this technology are broad and comprises the mechanical, optical and electronic industry (Duduch et alli, 1995). The application of ultraprecision machine tools to the manufacturing of microcomponents has been considered of great interest. Microstructures with extremely low dimensions such as few micrometer can be fabricated with ultraprecision machine tools with the advantage of low cutting time and low cutting tool wear (Weck et al., 1997). The list of materials that can be machined with single point diamond tools with mirror-like finish are not constrained solely to non-ferrous metals. It has been broadened to polymers, brittle materials such as silicon, germanium and other IR materials, optical glasses as well as amorphous metals. Ferrous materials are still a challenge due to the high affinity to the carbon of the cutting tools and the Fe element, which can react forming carbides.

Acknowledgments

The authors would like to acknowledge that this work was supported by CNPq, FAPESP, CAPES.

Presented at DINAME 97 - 7th International Conference on Dynamic Problems in Mechanics, 3 - 7 March 1997, Angra dos Reis, RJ, Brazil. Technical Editor: Agenor de Toledo Fleury.

• Black, J.T., Nov. 1979, "Flow Stress Model in Metal Cutting" Trans. ASME, J. of Engg. for Industry, Vol. 101, pp. 403-415.
• Blake, P.N., Scattergood, R.O., 1986, "Chip Topography of Diamond Turned Ductile Metals". Proc. SPIE, Vol.676, pp.96-103.
• Callister Jr., W.D., 1994, "Materials Science and Engineering - an Introduction", Third edition, ed. John Wiley and Sons, 811p.
• Carr, J., Narayan, C., Kim, J., 1991, "Single Point Diamond Machining Induced Damage on Single Crystal Copper, Aluminum and Gold", Proc. of the 6th ASPE Annual Conf., Santa Fé N.M., USA, pp.96-99.
• Duduch, J.G., Gee, A.E., Porto, A.J.V. , Jasinevicius, R.G., Carpinetti, L.C.R., Sept. 1995, "Ultraprecision Manufacturing Technology and the Mechanical and Opto-Electronic Industry", Annals of the First Brazil Int. Congress of Industrial Engg., Vol. 3, pp.1574-1578, São Carlos, Brazil.
• Evans, C., Polvani, R., Postek, M., Rhorer, R., 1987, "Some Observations on the Tool Sharpness and Subsurface Damage in Single Point Diamond Turning, Proc. of the SPIE, Vol. 802, pp.52-66.
• Evans, C., 1991, "Cryogenic Diamond Turning of Stainless Steel" Annals of the CIRP, Vol.40(2):571-575.
• Franse, J., 1990, "Manufacturing Techniques for Complex Shapes with Submicron Accuracy". Rep. Prog. Phys. Vol.53, pp. 1049-1094.
• Fuchs, B.A., Syn, C.K., Velsko, S.P., 1989, "Diamond Turning of L-Arginine Phosphate, A New Organic Nonlinear Crystal" Appl. Opt., Vol.28, pp. 4465-4472.
• Furukawa, Y., Moronuki, N., 1988, "Effect of Material Properties on Ultra Precise Cutting Processes" Annals of the CIRP, Vol. 37(1): 113-116.
• Horio, K., Kasai, T., Ogata, Y., Kobayashi, A., 1992, "A Study on Damaged Layer Remaining in Diamond Mirror Cut Surface", Annals of the CIRP, Vol.41/1/pp.137-140.
• Hurt, H.H., Decker, D.L., 1984, "Tribological Considerations of the Diamond Single-Point Tool". Proc. of the SPIE, Vol. 508, pp.126-131.
• Hurt, H.H., 1985, "Defects Induced in Optical Surfaces by the Diamond-Turning Process". Proc. SPIE, Vol.525, pp.16-21.
• Ikawa, N. Shimada, S., july 1977, "Cutting Tool for Ultraprecision Machining", Proc. of 3rd Int. Conf. on production Engg., 3, Kyoto, Japan, pp. 357-364.
• Ikawa, N., Donaldson, R. R., Komanduri, R., König, W., Mckeown, P.A., Moriwaki, T., Stowers, I. F., 1991, "Ultraprecision Metal Cutting -The Past, the Present and the Future". Annals of the CIRP, Vol. 40(2):587-594.
• Jasinevicius, R.G., Porto, A.J.V., Purquério, B.M., Duduch, J.G., 1993, "A Review of Monocrystalline Diamond Tools for Ultraprecision Machining" J. of the Braz. Soc. of Mechanical Sci. Vol. 15(4):405-411, (In Portuguese).
• König, W., Spenrath, N., 1991, "The Influence of the Crystallographic Structure of the Substrate Material on Surface Quality and Cutting Forces in Micromachining" 2^nd Prog. in Prec. Engg., pp. 141-152.
• Lee, W.B., Jan. 1990, "Prediction of microcutting force variation in ultra-precision machining", Prec. Engg., Vol.12 (1):25-28.
• Lee, W.B., Zhou, M., 1993, "A Theoretical Analysis of the Effect of Crystallographic Orientation on Chip Formation in Micromachining", Int. J. Mach. Tools Manufact., Vol. 33(3):439-447.
• Liang, Y., Moronuki, N., Furukawa, Y., April 1994, "Calculations of the Effect of Material Anisotropy on Microcutting Processes". Precision Engineering, Vol. 16(2): 132-138.
• Liu, J.J., Dornfeld, D.A., 1996, "Modeling and Analysis of Acoustic Emission in Diamond Turning", Journal of Manufacturing Science and Engineering, Vol.116(2):199-207.
• Lucca, D.A., Seo, Y.W., Rhorer, R.L., 1994, "Aspects of Surface Generation in Orthogonal Ultraprecision Machining", Annals of the CIRP, Vol. 43(1):43-46.
• Moriwaki, T., Okuda, K., 1989, "Machinability of Copper in Ultraprecision Micro Diamond Cutting", Annals of the CIRP, Vol. 38(1):115-118.
• Moriwaki, T., Okuda, K., Shen, J.G., 1993, "Study of Ultraprecision Orthogonal Microdiamond Cutting of Single Crystal Copper", JSME Int. J., Series C, Vol.36(3): 400-406.
• Nakayama, K., Tamura, K., Feb. 1968, "Size Effect in Metal Cutting Force", Trans. ASME, J. of Engg. for Industry, pp.119-126.
• Ohmori, G., Takada, S.; March 1982, "Primary Factors Affecting Accuracy in Ultraprecision Machining by Diamond Tool", Bull. Japan Soc. of Prec. Engg., Vol. 16(1): 3-7.
• Porto AJ.V., Carpinetti, Duduch, J.G., Jasinevicius, R.G., Tsunaki, R.H., Oct. 1995, "Diagnostic of Diamond Turning Machining through FFT Analysis of Finished Surfaces" Tenth ASPE Annual Meeting, Austin, Texas, USA, pp.219-222.
• Sato, M., Kato, Y., Apki, S., Ikoma, A., May 1983, "Effects of Crystal Orientation on the Cutting Mechanism of the Aluminum Single Crystal" Bull. of the JSME, Vol.26(215):890-896.
• Sato, M., Yamazaki, T., Shimizu, Y., Takabayashi, T., 1991, "A Study on the Microcutting of Aluminum Single Crystals", JSME Int. J., Series III, Vol.34(4): 540-545.
• Shaw, M.C., 1984, "Metal Cutting Principles", Oxford Press.
• Sugano, T., Takeuchi, K., Goto, T., Yoshida,Y., 1987, "Diamond Turning of an Aluminum Alloy for Mirror", Annals of the CIRP, Vol. 36(1):17-20.
• Yuan, Z.J., Lee, W.B., Yao, Y.X., Zhou, M., 1994, "Effect of Crystallographic Orientation on Cutting Forces and Surface Quality in Diamond Cutting of Single Crystal" Annals of the CIRP, Vol. 43(1):39-42.
• Yuan, Z.J., Zhou, M., Dong, S., 1996, "Effect of Diamond Tool Sharpness on Minimum Cutting Thickness and Surface Integrity in Ultraprecision Machining", J. of Mat. Proc. Technol. Vol. 62, pp.327-330.
• Ueda, K., Iwata, K., 1980, " Chip Formation Mechanism in Single Crystal Cutting of b-Brass", Annals of the CIRP, Vol. 29(1):41-46.
• Von Turkovich, B. F., Black, J. T., 1970, "Micro-Machining of Copper and Aluminum Crystals", Trans. of the ASME, J. of Engg. for Industry, vol. 92 (1) : 130-134, Feb.
• Von Turkovich, B.F., Field, M., 1981, "Survey on material behavior in machining". Annals of the CIRP, Vol. 30 (2):533-540.
• Weck, M., Fisher, S. Vos, M., 1997, "Fabrication of Microcomponents Using Ultraprecision Machine Tools" Nanotechnology, Vol. 8, pp.145-148.

Publication Dates

• Publication in this collection
  20 Nov 2002
• Date of issue
  Sept 1999