Ductile Mode Cutting of Brittle Materials

In this chapter, ductile mode cutting mechanism of brittle material is analysed theoretically and systematically. The coexisting crack propagation and dislocation extension in the chip formation zone are examined based on an analysis of cutting geometry and forces in the cutting zone, both on Taylor’s dislocation hardening theory and strain gradient plasticity theory. Ductile chip formation is a result of large compressive stress and shear stress in cutting zone, of which shields the growth of pre-existing flaws by enhancing material’s yield strength and suppressing its stress intensity factor K_I. Large compressive stress in cutting zone is obtained by satisfying two conditions: (a) very small undeformed chip thickness, and (b) undeformed chip thickness being smaller than tool cutting edge radius. Experimental verification shows that thrust force F_t is much larger than cutting force F_c in cutting of brittle material, which indicates that a large compressive stress is generated in cutting zone to enhance material’s yield strength by dislocation hardening and strain gradient, and shields the growth of pre-existing flaws by suppressing its stress intensity factor K_I. Thereafter, ductile mode cutting of brittle material is achieved when two conditions are satisfied, such that work material is able to undertake a large cutting stress in cutting zone without fracturing.

Ductile mode cutting characteristics and material removal mechanism of brittle material are discussed using tungsten carbide as an example work material in this chapter. Grooving and milling tests are designed and conducted to investigate their cutting modes. Experimental results indicate that there is a transition from ductile mode cutting to brittle mode cutting in grooving of tungsten carbide when depth of cut is increased from zero to a certain value. SEM observations on machined work surfaces show that there are three cutting modes in grooving of brittle material as depth of cut being increased: ductile mode cutting, semi-brittle mode cutting and brittle mode cutting. Cutting modes is identified and classified by machined surface texture and chip formation. In ductile mode cutting of tungsten carbide, thrust force F_t is much larger than cutting force F_c, which results in a large compressive stress in cutting zone. Large compressive stress and shear stress could shield the growth of pre-existing flaws in work material by suppressing its stress intensity factor K_I, such that K_I < K_C making work material is able to undertake a large cutting stress without fracturing to achieve ductile mode cutting. SEM and EDS examinations on cutting tools indicate that tool wear mainly occurs on flank face and tool wear mechanisms are dominated by diffusion, adhesion and abrasion in cutting of tungsten carbide.

Although the demand for industrial applications for brittle material growing rapidly, the manufacturing of brittle material for making precise components is very challenging due to its poor machinability and brittleness. In this chapter, theoretical analyses are given based on brittle material’s mechanical properties as the functions of temperature and on critical conditions for ductile mode chip formation in cutting of brittle material. An energy model for ductile mode chip formation in cutting of brittle material is developed, in which critical undeformed chip thickness for ductile chip formation in cutting of brittle material is predicted from material’s mechanical properties, or tool geometry and cutting conditions used. Experiments are conducted on conventional grooving of tungsten carbide material to verify the proposed model for predicting critical undeformed chip thickness, which shows a substantial agreement between the predicted value and experimental results.

In this chapter, fundamental aspects of molecular dynamics simulation and several considerations are accounted for to attain accurate results that are comparable with experimental works. The multitude of potential functions is discussed with emphasis on those frequently used in modelling of brittle material. Several example theoretical models for nanometric machining of silicon and silicon carbide are presented, in view of these materials being regularly used across widespread application of industries. Different interatomic potentials used for these materials, along with the possible output data, are reviewed. Finally, the stress distribution is discussed by considering the interactive atomic forces and an empirical relationship of the tool-workpiece contact geometry.

In wafer fabrication, most machining processes such as slicing, edge grinding, finishing, lapping, polishing, back thinning and dicing, are based on grinding or/and abrasive process, which always generate micro cracks and subsurface damage. In this chapter, theoretical analysis on ductile mode cutting of silicon wafer shows that machined silicon surface with free of fracture and nanometer scale surface roughness can be achieved when dislocation dominates its chip formation rather than crack propagation. Nanometric cutting of silicon wafers using an ultra-precision CNC lathe with single crystal diamond cutters are carried out to investigate the tool edge radius effect on critical undeformed chip thickness and verify ductile mode cutting performance of silicon wafer. Machined workpiece surfaces and used diamond tools are examined using a scanning electron microscope (SEM), transmission electron microscopy (TEM) and atomic force microscope (AFM). Experimental results from the nanometric cutting tests indicate that in cutting of silicon wafers, there is a critical undeformed chip thickness, at or below which chip formation is under ductile mode cutting generating continuous chips. And critical undeformed chip thickness differs when cutting of silicon wafers using diamond cutters with different tool edge radius. Larger diamond tool edge radius, larger critical undeformed chip thickness. But there is an upper bound for diamond tool edge radius, above which chip formation is changed from ductile mode to brittle mode even though undeformed chip thickness remains to be smaller than tool edge radius. Experimental results are found to well substantiate the analytical findings and nanometric ductile mode cutting of silicon wafer is successfully achieved under certain cutting conditions.

Recently, the industrial application of glass has increased enormously because of its excellent and unique mechanical, physical, chemical and optical properties. However, machining of glass is still a major problem for the manufacturing industry since it is very brittle and high hardness. In this chapter, grooving and cutting tests of soda-lime glass are conducted to evaluate its cutting performance using an ultra-precision lathe with a single crystal diamond tool. The machined workpiece surface topography, chip formation and surface roughness are examined using a SEM, AFM and white light interferometer. Tool wear is measured using OMIS. Experimental results indicate that ductile mode cutting of soda-lime glass is achieved when the undeformed chip thickness being less than a critical value. Ultrasonic vibration assisted cutting is employed to improve ductile mode cutting performance of soda-lime glass. Continuous layer chip and smooth surface are obtained in ultrasonic vibration assisted cutting of soda-lime glass, which largely improve its machinability in ductile mode cutting. But extremely short tool life is the main constrain for realizing the ultrasonic vibration assisted ductile mode cutting of glass in industry.

Cutting experiments are carried out to evaluate the cutting performance of tungsten carbide under nanometer scale chip thickness using a 5-axis CNC machining centre with CBN tools. The cutting forces are measured using a three-component dynamometer. Machined workpiece surface topography, chip formation, and tool wear are examined using an OMIS and SEM. Tool flank wear VB_max is also measured using the OMIS. Surface roughness is measured using a stylus profiler. Experimental results indicate that radial force F_x is much larger than tangential force F_z and axial force F_y. Under different cutting conditions, three types of surfaces of machined workpiece are achieved: ductile mode cutting surface, semi fractured surface and fractured surface. Continuous chips and discontinuous chips are formed under different cutting conditions. Surface roughness increases monotonically when the depth of cut and feed rate being increased. Tool wear occurs mainly on the flank face in ductile mode cutting of tungsten carbide and tool wear mechanisms are dominated by abrasion, adhesion and diffusion wear. SEM observations on machined workpiece surfaces and chip formation indicate that ductile mode cutting is mainly determined by undeformed chip thickness when the tool cutting edge radius is fixed. Ductile mode cutting of tungsten carbide is achieved when undeformed chip thickness is less than a critical value.

Calcium fluoride is one of the favourite optical materials in the advanced optics applications  owing to its superior optical transmission and mechanical properties. While micromachining techniques have evolved to enhance the production process for optical components, this chapter will focus on the fundamental aspects to achieve ductile mode machining by single point diamond turning and the essential surface characterisation techniques are also covered. The anisotropic characteristics will be examined along with the numerical simulation tools to evaluate the ductile–brittle transition. Methods to enhance the machinability integrated with single point diamond turning are also discussed in this chapter.

It has been well known that ultrasonic vibration assisted cutting (UVC) is able to improve the machining performance for various material removal processes. For machining of brittle material, UVC has also been proven useful in improving surface integrity and increasing tool life by significantly increasing critical undeformed chip thickness for ductile-to-brittle transition. This chapter presents an analytical model to predict critical undeformed chip thickness in UVC of brittle material, based on the variation of specific cutting energy for prediction of ductile-to-brittle transition in nano-machining. Vibration parameters are taken into consideration in addition to work material intrinsic properties, tool geometry and machining parameters in predicting critical undeformed chip thickness. A series of cutting tests on single crystal silicon workpiece, using a single crystal diamond tool with different nominal cutting speeds, are conducted to verify the proposed theoretical model.

In this chapter, ultrasonic vibration assisted cutting is conducted to investigate the effect of various cutting conditions such as vibration mode and amplitude, diamond type, cutting speed, feed rate and depth of cut, on ductile mode cutting of tungsten carbide such as critical depth of cut, cutting force, chip formation, tool wear and surface integrity. Cutting forces are measured using a three-component dynamometer, critical depth of cut is measured using a stylus profilometer, machined surface integrity and chip formation are examined using an SEM, and tool wear is examined using an OMIS. It is found that critical depth of cut for the transition from ductile mode cutting to brittle mode cutting in 1D ultrasonic vibration assisted grooving is several times larger than that in the conventional grooving. Lower thrust directional amplitude in 2D ultrasonic vibration leads to less brittle fracture generated on the machined surface of tungsten carbide, and 1D ultrasonic vibration with no thrust directional vibration leads to minimum brittle fracture and less diamond tool wear. Nano-polycrystalline diamond with isotropic mechanical properties does not perform better than single crystal diamond as tool material in terms of tool flank wear in ultrasonic vibration assisted turning of tungsten carbide. Radial cutting force F_x is much larger than tangential cutting force F_z and axial cutting force F_y. Cutting speed has no significant effect on ductile chip formation mode. Ductile mode cutting is achieved when maximum undeformed chip thickness is smaller than a critical value. And the larger critical depth of cut for 1D ultrasonic vibration assisted grooving of tungsten carbide implies that ultrasonic vibration could be used to improve ductile mode cutting performance of brittle material.

The growing necessity for advanced manufacturing technologies brings forth initiatives to incorporate thermal effects in micromachining of difficult-to-machine brittle material. While conventional heating methods exist in the macroform and are infeasible for micro-cutting applications, the fundamental concepts of heating are miniaturized to enable ductile mode micro-cutting of brittle single crystals. In general, ductile–brittle transition of these hard materials undergoes a significant improvement with thermal assistance, but the fundamental cause is material specific and could be attributed to the thermal softening effect, phase transition, and slip system activation. In addition, several aspects of the most promising technological advancements in thermally assisted machining, micro-laser assisted machining, are discussed in relation to machining conditions and tool wear. Temperature measurement techniques are also covered with the emphasis on its importance in ensuring proper thermal control of the machining system.

In this book, ductile mode cutting of brittle material is presented and discussed systematically in terms of fundamentals, engineering applications and hybrid ductile mode cutting techniques, which is summarized as the following aspects.