Ultra-precision surface finish of the hardened stainless mold steel using vibration-assisted ball polishing process

A vibration-assisted spherical polishing system driven by a piezoelectric actuator has been newly developed on a machining center to improve the burnished surface roughness of hardened STAVAX plastic mold stainless steel and to reduce the volumetric wear of the polishing ball. The optimal plane surface ball burnishing and vibration-assisted spherical polishing parameters of the specimens have been determined after conducting the Taguchi's L₉ and L₁₈ matrix experiments, respectively. The surface roughness R_a=0.10 μm, on average, of the burnished specimens can be improved to R_a=0.036 μm (R_max=0.380 μm) using the optimal plane surface vibration-assisted spherical polishing process. The improvement of volumetric wear of the polishing ball was about 72% using the vibration-assisted polishing process compared with the non-vibrated polishing process. A simplified kinetic model of the vibration-assisted spherical polishing system for the burnished surface profile was also derived in this study. Applying the optimal plane surface ball burnishing and vibrated spherical polishing parameters sequentially to a fine-milled freeform surface carrier of an F-theta scan lens, the surface roughness of R_a=0.045 μm (R_y=0.65 μm), on average, within the measuring range of 149 μm×112 μm on the freeform surface, was obtainable.     

A Taguchi and experimental investigation into the optimal processing conditions for the abrasive jet polishing of SKD61 mold steel

This study introduces an abrasive jet polishing (AJP) technique in which the pneumatic air stream carries not only abrasive particles, but also an additive of either pure water or pure water with a specified quantity of machining oil. Taguchi design experiments are performed to identify the optimal AJP parameters when applied to the polishing of electrical discharge machined SKD61 mold steel specimens. A series of experimental trials are then conducted using the optimal AJP parameters to investigate the respective effects of the additive type and the abrasive particle material and diameter in achieving a mirror-like finish of the polished surface. The Taguchi trials indicate that when polishing is performed using pure water as an additive, the optimal processing parameters are as follows: an abrasive material to additive ratio of 1:2, an impact angle of 30°, a gas pressure of 4 kg/cm², a nozzle-to-workpiece height of 10 mm, a platform rotational velocity of 200 rpm, and a platform travel speed of 150 mm/s. Applying these processing parameters, it is found that the optimal polishing effect is attained using #8000SiC abrasive particles and a 1:1 mixture of water-solvent machining oil and pure water. The experimental results show that under these conditions, the average roughness of the electrical discharge machined SKD61 surface is reduced from an original value of R_a=1.03 μm (R_max: 7.74 μm) to a final value of R_a=0.13 μm (R_max: 0.90 μm), corresponding to a surface roughness improvement of approximately 87%.     

3D graphical evaluation of micron-scale protrusion topography of diamond grinding wheel

A new graphical evaluation of micron-scale wheel protrusion topography is proposed by using 3D coordinate data derived from contact measuring of 180 diamond grinding wheel. The objective is to quantify 3D distribution of grain protrusion height, gain rake angle and grain relief angle on wheel working surface in dressing. First adaptive measuring was conducted on the base of topographical curvature to identify grain cutting edge in 3D space, second grain protrusion mode was established by polar coordinate transfer so as to ascertain datum plane of grain protrusion, then linear approximation graphics was conducted to display wheel protrusion topography, finally distributions of gain rake angle and grain relief angle were investigated with reference to grain protrusion height. Analytical results show that higher outer grains have more and shaper cutting edges, but lower layer grains retain approximately original crystal forms. In wheel protrusion topography, grain protrusion heights, grain rake angles and grain relief angles are dispersedly distributed in the range 0–28 μm, −45.0° to −89.1° and 1.2–73.1°, respectively, which can be increased by dressing. It is concluded that 3D grain protrusion attitudes distributed on wheel working surface can be quantified by 3D graphical evaluation method.     

Surface and subsurface integrity in diamond grinding of optical glasses on Tetraform ‘C’

In order to investigate the surface and subsurface integrity of diamond-ground optical glasses, a Tetraform ‘C’ machine tool featuring high close-loop stiffness was used to conduct the ultra-precision machining of fused silica and fused quartz assisted with electrolytic in-process dressing (ELID). An acoustic emission (AE) sensor and a piezoelectric dynamometer were used to monitor the grinding process to correlate the processing characteristics with the generated surface and subsurface integrities, which were characterized by atomic force microscope (AFM), scanning electronic microscope (SEM), and nano-indentation technique. Experimental results showed that for optical glasses the fracture toughness value can be used to predict the machinability while its bigger value always means a better surface and subsurface integrity. During the grinding process of optical glasses, the smaller amplitude and RMS values of AE signal, as well as the smaller grinding forces and the ratio of normal force to tangential force, correspond to a better surface and subsurface integrity. With selected machining parameters and a 6–12 μm grain-sized diamond-grinding wheel, nanometric quality surfaces (Ra<5 nm) with minimal subsurface damage depth (< 0.5 μm) can be generated for fused quartz on Tetraform ‘C’.     

A novel method of composite electroplating on lap in lapping process

A lapping machine with the composite electroplating to dress the lap in-process has been developed. Mirror surface lapping operations on silicon wafers were conducted by plating tin with 0.3 μm Al₂O₃ particles on the lap surface in-process. During the lapping process, the cathode current density ranged from 0.28 to 1.68 A/dm². Results showed that the removal rate of the silicon wafer for this process ranges from 0.12 to 0.18 μm/min, which is about 2.5–3.5 times higher than those without dressing in-process. This indicates that this novel method can produce sharp new edges on the lap surface continuously. It is found that the growth rate for the coating thickness of the lap is almost equal to its wear rate at the cathode current density of 0.28 A/dm². Hence, it is not necessary to replace and dress during the lapping by using this method. The silicon wafer can be lapped to a surface roughness, R_a=2 nm in this novel method.     

A new apparatus for finishing large size/large batch silicon nitride (Si3N4) balls for hybrid bearing applications by magnetic float polishing (MFP)

A new apparatus was designed and built for the finishing of large size/large batch silicon nitride (Si₃N₄) balls by magnetic float polishing (MFP) technology for hybrid bearing applications. The polishing chamber is so designed that during polishing it can self-align with the upper part of the polishing chamber connected to the spindle. In situ machining of the upper part of the chamber is performed on the machine tool in which the apparatus is located, in order to achieve high accuracy and geometric alignment of the system. The finishing methodology consists of mechanical polishing followed by chemo-mechanical polishing. Boron carbide (B₄C), silicon carbide (SiC), and cerium oxide (CeO₂) are the three abrasives used in this investigation. Three stages are involved in polishing, namely, 1. a roughing stage to remove maximum material without imparting any damage to the surface, 2. an intermediate stage of semi-finishing to control the size and improve sphericity, and 3. a final finishing stage to obtain best surface finish and sphericity while maintaining the final diameter. Taguchi method was applied for the roughing stage to optimize the polishing conditions for the best material removal rate. Level average response analysis has indicated that a load of 1.5 N/ball, an abrasive concentration of 20%, and a speed of 400 rpm would give a high material removal rate using B₄C (500 grit) abrasive. A groove is formed on the bevel of the upper part of the chamber which plays different roles, some beneficial and other not so beneficial, in each stage. In the roughing stage, it is preferable, though not essential, to machine the groove after each run to maintain high material removal rates. It is, however, necessary to remove the groove formed at the end of the roughing stage. In the intermediate or semifinishing stage, sphericity can be significantly improved by not machining the groove. Thus groove, in this case, facilitates in the improvement of sphericity. Before the beginning of the final finishing stage, machining the groove is necessary for rapid improvement in the surface finish. A batch of 46, 3/4 in. Si₃N₄ balls was finished to a final diameter of 0.7500 in. with an average sphericity of 0.25 μm (best value of 0.15 μm) and an average surface finish, Ra of 8 nm (best value of 6.7 nm) with an actual polishing time of <30 h. This technology is easy to implement in industry and does not entile high capital investment.     

Study on ultrasonic-assisted lapping of gears

Ultrasonic-assisted lapping of gears is firstly proposed and compared with conventional lapping in material removal process and mechanism. The material removal mechanisms of the ultrasonic lapping include hammering, impacting and acoustic cavitation. The experiments showed that the material removal rate of ultrasonic lapping is nearly three times that of the conventional lapping in the same condition, and the ultrasonic lapping can produce a better tooth surface quality (Ra=0.2 μm and the section height c=1.2 μm) than the conventional lapping (Ra=0.33 μm and c=3.2 μm). Then a set of parametric experiments for the ultrasonic lapping was conducted with the Taguchi experimental design. The results of this set of experiments reveal that the optimum conditions for a high removal rate in the ultrasonic lapping experiments of spiral-bevel gears are of brake torque, 0.12 Nm; pinion rotational speed, 600 rpm; and slurry concentration with 20%. The contributions by percentage of torque, speed and concentration to the removal rate are 8.13, 19.26 and 68.11, respectively.     

Application of micro-EDM combined with high-frequency dither grinding to micro-hole machining

This research presents a novel process using micro electro-discharge machining (micro-EDM) combined with high-frequency dither grinding (HFDG) to improve the surface roughness of micro-holes. Micro-EDM is a well-established machining option for manufacturing geometrically complex small parts (diameter under 100 μm) of hard or super-tough materials. However, micro-EDM causes the recast layer formed on the machined surface to become covered with discharge craters and micro-cracks, resulting in poor surface quality. This affects the diameter of the micro-hole machined and undermines seriously the precision of the geometric shape. The proposed method that combines micro-EDM process with HFDG is applied to machining high-nickel alloy. As observed in SEM photographs and surface roughness measurement, HFDG method can reduce surface roughness from 2.12 to 0.85 μm Rmax with micro-cracks eliminated. Our results demonstrated that micro-holes fabricated by micro-EDM at peak current 500 mA followed by HFDG at 40 V can achieve precise shape and good surface quality after 6–8 min of lapping.     

Straightening of micro wires using the direct wire heating and pulling method

In this study, we have developed a novel micro wire straightener using the direct wire heating and pulling (DWHP) method. The straightener can remove the bend of the micro wire (< 200 μm) by heating it with the direct current, which flows through the wire in the glass chamber and simultaneously giving it the appropriate tension. A tension meter was attached to control the tension of the micro wire (tungsten). In order to avoid surface oxidization of the wire, we supplied inert gas (argon) into the glass chamber during the heating process, and examined the effect of the gas flow rate. The effects of the tension and the current applied to the micro wires (tungsten) were investigated experimentally. With Results from various experiments and parametric studies, we could obtain desired straightness (≈1 μm/1000 μm) with a tension of 500–600 gf and an approximate electric current of 1.5 A.     

Effect of various parameters on the surface roughness of an aluminium alloy burnished with a spherical surfaced polycrystalline diamond tool

The effect of various parameters on the surface roughness of an aluminium alloy burnished with a spherical surfaced polycrystalline diamond tool are studied experimentally with a theoretical analysis. Problems in selecting the optimum burnishing parameters and some burnishing mechanisms are discussed. With suitable parameters employed, the new no-chip finishing process developed can eliminate or reduce the cutting marks left on the workpiece surface by diamond cutting tools, with its surface roughness reduced to Ra=0.026 μm from the original 0.5 μm.     

Vitreous bond silicon carbide wheel for grinding of silicon nitride

This study investigates the grinding of sintered silicon nitride using a SiC wheel with a fine abrasive grit size and dense vitreous bond. The difference of hardness between the green SiC abrasive and sintered Si₃N₄ workpiece (25.5 vs. 13.7 GPa) is small. Large grinding forces, particularly the specific tangential grinding forces, are observed in SiC grinding of Si₃N₄. The measured specific grinding energy is high, 400–6000 J/mm³, and follows an inverse relationship relative to the maximum uncut chip thickness as observed in other grinding studies. The SiC wheel wears fast in grinding Si₃N₄. The G-ratio varies from 2 to 12. Two unique features in SiC grinding of Si₃N₄ are the trend of increasing G-ratio at higher material removal rate and the excellent surface integrity, with 0.04–0.1 μm R_a and no visible surface damage. For a specific material removal rate, surface cracks along the grinding direction are generated on the ground surface. The problem of chatter vibration was identified at high material removal rates. Periodic and uneven wheel loading marks and clusters of workpiece surface cracks across the grinding direction could be observed at high material removal rates. This study demonstrates that the SiC grinding wheel can be utilized for precision form grinding of Si₃N₄ to achieve good surface integrity under a limited material removal rate.     

Servo scanning 3D micro-EDM based on macro/micro-dual-feed spindle

Using the end discharge of micro-rod-shaped electrode to scan layer by layer, micro-electrical discharge machining (EDM) can fabricate complex 3D micro-structures. During the machining process, the discharge state is broken frequently due to the wear of the tool electrode and the relative scanning motion. To keep a favorable discharge gap, the feed spindle of the tool electrode needs the characteristics of high-frequency response and high resolution. In this study, an experimental system with a macro/micro-dual-feed spindle was designed to improve the machining performance of servo scanning 3D micro-EDM (3D SSMEDM), which integrates an ultrasonic linear motor as the macro-drive and a piezoelectric (PZT) actuator as micro-feeding mechanism. Based on LabVIEW and Visual C++ software platform, a real-time control system was developed to control coordinately the dual-feed spindle to drive the tool electrode. The micro-feed motor controls the tool electrode to keep the favorable discharge gap, and the macro-drive motor realizes long working range by a macro/micro-feed conversion. The emphasis is paid on the process control of the 3D SSMEDM based on macro/micro-dual-feed spindle for higher machining accuracy and efficiency. A number of experiments were carried out to study the machining performance. According to the numerical control (NC) code, several typical 3D micro-structures have been machined on the P-doped silicon chips. Our study results show that the machining process is stable and the regular discharge ratio is higher. Based on our fundamental machining experiments, some better-machined effects have been gained as follows. By machining a micro-rectangle cavity (960 μm×660 μm), the machined depth error can be controlled within 2%, the XY dimensional error is within 1%, the surface roughness R_a reaches 0.37 μm, and the material removal rate is about 1.58×10⁴ μm³/s by using a tool electrode of Φ=100 μm in diameter. By machining multi-micro-triangle cavities (side length 700 μm), it is known that the machining repeatability error is <0.7%.     

Selective laser melting of Fe-Ni-Cr layer on AISI H 13 tool steel

An attempt to fabricate Fe-Ni-Cr coating on AISI H13 tool steel was performed with selective laser melting. Fe-Ni-Cr coating was produced by experimental facilities consisting of a 200 W fiber laser which can be focused to 80 μm and atmospheric chamber which can control atmospheric pressure with N₂ or Ar. Coating layer was fabricated with various process parameters such as laser power, scan rate and fill spacing. Surface quality and coating thickness were measured and analyzed. Three different surface patterns, such as type I, type II and type III, are shown with various test conditions and smooth regular pattern is obtained under the conditions as 10 μm of fill spacing, 50–350 mm/s of scan rate and 40 μm of fill spacing, 10–150 mm/s of scan rate. The maximum coating thickness is increased with power elevation or scan rate drop, and average thickness of 10 μm fill spacing is lower than that of 40 μm fill spacing.     

Residual stresses, distortion and surface roughness produced by grinding thin wall ductile iron plates

This work deals with grinding effects on thin wall ductile iron plates. Residual stresses, shape distortion and surface roughness were measured on thin wall plates of different nodule count, ferritised and afterwards dry ground under several grinding conditions. In all cases, tensile residual stresses are maximum at the surface, and their profile decreases with depth until becoming compressive. No phase transformations can be observed at depths of up to 30 μm below surface, although plastic deformation is visible through nodules and grains enlargement. Distortion increases when the depth of cut and nodule count increase and the workspeed decreases. The mean stresses of the profile tensile zone also increase when the nodule count increases. Surface roughness improves slightly as nodule count increases and workspeed decreases. This tendency is more noticeable when depth of cut decreases. The arithmetic mean roughness (Ra) values obtained were always below 0.8 μm.     

Error correction in hydrostatic spindles by optimal bearing tuning

In this paper a high precision grinding wheel is considered as a rigid rotor mounted on two hydrostatic bearings. The equations for small perturbations of the wheel on the bearings are derived in the form of a multi-input, multi-output transfer function matrix, enabling the frequency response function of the wheel to be determined. Thereafter an optimisation algorithm is proposed which considers speed, load and dimensions of the spindle, and computes optimal stiffness and damping of the bearings. The dynamic characteristics of the bearings, tuned for minimum radial displacement of the spindle, is achieved maximising thereby the accuracy of the grinding process. Simulation results show that by stiffness coarse adjustment, and fine adjustment of the damping in the bearings, a spindle with 35 μm manufacturing error, can produce components with 3 μm accuracy.     

Optical fiber polishing automation with on-line force sensing

This study proposes a bare fiber polishing control strategy and force sensing mechanism to improve the performance of fiber polishing in optical communication. By analyzing the force and problems encountered during polishing, this study successfully measured the force with the required precision. Finally, the experimental results demonstrate a yield improvement from 20% to 80%, for a fiber tip offset within 1.5 μm.     

Mirror surface grinding for brittle materials with in-process electrolytic dressing

Use of a diamond wheel with superabrasive is required for mirror-like surface grinding of brittle materials. However, conventional dressing methods cannot apply to the diamond wheel with superabrasive. Recently, an electrolytic dressing method was developed for use with a cast iron-bonded diamond wheel and superabrasive. This technique can replace lapping and polishing. Using electrolytic dressing, surface roughness of the workpiece was improved significantly, and the grinding force was very low and the continuity of the grinding force was also improved. The purpose of this study was to achieve mirror-like surface grinding of ferrite with electrolytic dressing of a metal-bonded diamond wheel. For application of ultraprecision grinding for brittle material, superabrasive, air spindle, and in-process electrolytic dressings were used. Additionally, the effects of pick current and pulse width on ground surface were investigated, and suitable dressing conditions for ferrite were determined.     

Laser polishing of parts built up by selective laser sintering

In this work, a surface finish method for parts built-up by selective laser sintering (SLS) is presented. One of the main drawbacks of the SLS technique is the high surface roughness of resulting parts. Therefore, parts have to be polished to be valid for operation conditions. Polishing processes are usually based on manual abrasive techniques. However, in the present paper, a surface polishing method based on laser irradiation is presented. The laser beam melts a microscopic layer on the surface, which re-solidifies under shielding gas protective conditions, resulting in a smoother surface.Laser-polishing tests for lines, planar surfaces and inclined planes have been performed, with satisfactory results in all the cases. The experimental tests were carried out on sintered test parts with an initial roughness of 7.5–7.8 μm Ra. The tested material is a commercial alloy denominated LaserForm ST-100^©, composed by sintered stainless steel and infiltrated bronze that it is used mainly for the constitution of injection moulds. Experimental results present final surface roughness below 1.49 μm Ra, which represent an 80.1% reduction of the mean roughness. Finally, a complete analysis of test probes and its metallurgical composition is presented. Considering that the material presents a non-homogeneous structure, the polished surfaces present slightly higher hardness values and are more homogeneous than the initial ones. Thus, polished surfaces do not present any heat affected zone or cracks, which could cause failure during the part operation.     

Determination of micro-scale plastic strain caused by orthogonal cutting

An electron beam lithography technique has been used to produce microgrids in order to measure local plastic strains, induced during an orthogonal cutting process, at the microscopic scale in the shear zone and under the machined surface. Microgrids with a 10 μm pitch and a line width less than 1 μm have been printed on the polished surface of an aluminium alloy AA 5182 to test the applicability of the technique in metal cutting operations. Orthogonal cutting tests were carried out at 40 mm/s. Results show that the distortion of the grids could successfully be used to compute plastic strains due to orthogonal cutting with higher accuracy compared to other techniques reported in the literature. Strain maps of the machined specimens have been produced and show high-strain gradients very close to the machined surface with local values reaching 2.2. High-resolution strain measurements carried out in the primary deformation zone also provide new insight into the material deformation during the chip formation process.     

Design of a six-axis high precision machine tool and its application in machining aspherical optical mirrors

The paper presents the design of a six-axis machining system and its application in fabricating large off-axis aspherical mirrors with sub-aperture lapping techniques. The new system is based on computer-controlled optical surfacing (CCOS), which combines the faculties of grinding, polishing, and on-machine profile measuring, has the features of conventional loose abrasive machining with the characteristics of a tool having multiple degrees of freedom moving in planar model. And a novel dual touch-trigger probe profiler is designed, which is composed of a probe, model METRO-MT60 made by HEIDENHAIN Co., is integrated into the system for measuring the shape accuracy of the tested aspherical surface, another probe modeled METRO-MT12 is designed as a calibrating device for minimizing the cosine error caused by assembly inaccuracy. The new CNC machining system with two kinds of moving coordinate systems, dual tool activities and on-machine measuring is presently developed based on the new concept. The general material removal function during machining is analyzed on the basis of the Preston hypothesis. Further, an alignment test of the measuring profiler is carried out using a leveling rule as a specimen. The accuracy of the optical surfaces measured by the dual probe profiler is found to be within 1 μm PV after removing cosine error and error compensating, achieves to the resolving power of the profiler is about 0.2–0.5 μm, so the developed system can be applied to the shape accuracy measuring of aspheric fabrication with micro precision during fine grinding process according to the calibrating results. Finally, the manufacturing experiments are carried out by virtue of an off-axis oblate ellipsoid mirror with rectangular aperture as 770 mm×210 mm and centered 127 mm. The accuracy of the aspherical mirror improved from the initial form error of 17.648 μm rms to the final one of 0.728 μm rms after grinding for 200 h.