Research on ECM process of micro holes with internal features

Micro holes with internal features are widely used as spray holes and cooling holes nowadays, which are usually required to be with high aspect ratio and shape accuracy, as well as good surface quality. An electrochemical machining (ECM) process is presented to machine these micro holes with diameter <200 μm. A quantitative relation between micro-hole diameter and machining parameters including voltage, duty ratio and feedrate is obtained through orthogonal experiments. According to the designed shape of internal features, change rules of machining parameters for varied diameters in different depth are obtained, and then micro holes with internal features are shaped precisely. Taking reverse tapered hole as an example, ECM experiments by varying parameters of voltage, duty ratio and feedrate (called varying voltage machining, varying duty ratio machining and varying feedrate machining, respectively) are carried out. Micro holes with inlet diameter of 178 μm and taper angle of 1.05° are shaped on a 1.0 mm-thick workpiece of 18CrNi8. The deviation of inlet is <3 μm and the taper-angle error is <0.1° in varying voltage machining. The corresponding dimensional accuracy of taper angle is improved by 51% than that of varying duty ratio machining under the same efficiency. The machining efficiency of varying voltage machining is increased by 36% compared to the efficiency in varying feedrate machining. In addition, the micro holes with complex features of funnel shape and bamboo shape are machined.     

Research on improvement of machining accuracy of micro-rods with electrostatic induction feeding ECM

Micro-rods were machined by electrochemical machining using the electrostatic induction feeding method, with which ultra-short current pulse duration of several tens of ns can easily be obtained. A tungsten plate and stainless steel (SUS304) rod were used as the tool electrode and workpiece, respectively. To improve the machining accuracy, the machining characteristics when the workpiece is fed in the axial and radial directions were investigated using NaCl aqueous solution as the electrolyte. When fed in the axial direction, the machinable length of the micro-rods was found to peak at the optimum feed speed because of the influence of pitting corrosion and collision between electrodes. When the workpiece was fed in the radial direction, the influence of pitting corrosion decreased, however, the micro-rod was shortened with increasing feed distance in the radial direction because of the stray current flowing through the end of the micro-rod. The simulation results of the material removal process agreed qualitatively with experimental results. Next, machining characteristics were compared between the electrolytes, NaCl and NaNO₃ aqueous solutions, by feeding the workpiece in the axial direction. It was found that the influence of pitting corrosion was eliminated with the NaNO₃ aqueous solution, and there was no machinable length limitation with suitable feed speeds. In addition, the taper angle and gap width were smaller with the NaNO₃ aqueous solution, compared with those of the NaCl aqueous solution. Stainless steel micro-rods of 100 μm in diameter with a high aspect ratio of 20 were fabricated with the NaNO₃ aqueous solution. According to the preliminary research results, the machinable minimum diameter of the micro-rods was investigated and micro-rods with an average diameter of 9 μm and length of 78 μm were machined successfully.     

Uncertainty analysis of slot die coater gap width measurement by using a shear mode micro-probing system

A shear mode micro-probing system was constructed for gap measurement of a precision slot die coater with a nominal gap width of 90 μm and a length of 200 mm. A glass micro-stylus with a nominal tip ball diameter of 52.6 μm was oscillated by a tuning fork quartz crystal resonator with its oscillation direction parallel to the measurement surfaces. An on-line qualification setup was established to compensate for the influences of the uncertainty sources, including the water layers on the measurement surfaces. The measurement uncertainty of the measured gap width was estimated to be less than 100 nm.     

Precision machining of micro tool electrodes in micro EDM for drilling array micro holes

Micro electro discharge machining (micro EDM) is suitable for machining micro holes on metal alloy materials, and the micro holes can be machined even to several microns by use of wire electro discharge grinding (WEDG) of micro electrodes. However, considering practicability of micro holes <Φ100 μm in batch processing, the controllable accuracy of holes’ diameter, the consistency accuracy of repeated machining and the processing efficiency are required to be systematically improved. On the basis of conventional WEDG method, a tangential feed WEDG (TF-WEDG) method combined with on-line measurement using a charge coupled device (CCD) was proposed for improving on-line machining accuracy of micro electrodes. In TF-WEDG, removal resolution of micro-electrode diameter (the minimum thickness to be removed from micro electrode) is greatly improved by feeding the electrode along the tangential direction of wire-guide arc, and the resolution is further improved by employing negative polarity machining. Taking advantage of the high removal resolution, the precise diameter of micro-electrode can be achieved by the tangential feed of electrode to a certain position after diameter feedback of on-line measurement. Furthermore, a hybrid process was presented by combining the TF-WEDG method and a self-drilled holes method to improve the machining efficiency of micro electrodes. A cyclic alternating process of micro-electrode repeated machining and micro holes’ drilling was implemented for array micro holes with high consistency accuracy. Micro-EDM experiments were carried out for verifying the proposed methods and processes, and the experimental results show that the repeated machining accuracy of micro electrodes was less than 2 μm and the consistency accuracy of array micro holes was ±1.1 μm.     

Size effects in finish machining of porous powdered metal for engineered surface quality

Porous tungsten is conventionally machined with the aid of a plastic infiltrant to achieve acceptable surface finish. For dispenser cathode application, both high surface porosity and low surface roughness are necessary. Cryogenic machining has already been demonstrated to be capable of eliminating the need for plastic infiltration by greatly reducing smearing of pores. In order to address the problem of undesirable brittle fracture during cryogenic machining, the importance of uncut chip geometry is investigated. The value of critical chip thickness, beyond which brittle fracture occurs, is found to be closely linked to the microstructure of the workpiece material. While machining with very low uncut chip thickness leads to ploughing and spalling of the workpiece surface, ductile mode machining of porous tungsten with cryogenic cooling is found to yield excellent surface quality. When the maximum uncut chip thickness is approximately equal to the average ligament diameter of 80% density porous tungsten (d ≈ 8–9 μm), ductile mode machining is possible under both dry and cryogenic conditions. Changes in shock compaction behavior of the workpiece material, leading to altered physical properties, is hypothesized to be the underlying mechanism of ductile mode machining of porous tungsten.     

A long-stroke 3D contact scanning probe for micro/nano coordinate measuring machine

This paper presents a long-stroke contact scanning probe with high precision and low stiffness for micro/nano coordinate measuring machines (micro/nano CMMs). The displacements of the probe tip in 3D are detected by two plane mirrors supported by an elastic mechanism, which is comprised of a tungsten stylus, a floating plate and two orthogonal Z-shaped leaf springs fixed to the outer case. A Michelson interferometer is used to detect the vertical displacement of the mirror mounted on the center of the floating plate. An autocollimator based two dimensional angle sensor is used to detect the tilt of the other plane mirror located at the end of the arm of the floating plate. The stiffness and the dynamic properties are investigated by simulation. The optimal structural parameters of the probe are obtained based on the force-motion model and the constrained conditions of stiffness, measurement range and horizontal size. The results of the performance tests show that the probe has a contact force gradient within 0.5 mN/μm, a measuring range of (±20 μm), (±20 μm), and 20 μm, respectively, in X, Y and Z directions, and a measurement standard deviation of 30 nm. The feasibility of the probe has preliminarily been verified by testing the curved surface of a convex lens.     

Micro wire electrochemical machining with an axial electrolyte flow

Micro wire electrochemical machining is a useful technique to produce high-aspect-ratio slit micro-structures. To improve processing stability, the axial electrolyte flow is adopted to renew electrolytes in the machining gap. A wire electrochemical micro-machining system with an axial electrolyte flow unit is developed. A mathematical model of tool feed rate is presented. To investigate the influence of electrolyte flow on processing stability and machining efficiency, comparative experiments were carried out. The influence of applied voltage and electrolyte concentration on machining accuracy is studied and the parameters such as electrolyte flow rate and applied voltage are optimized. Low initial machining gap is applied to decrease the stray current machining in the initial machining period. With the optimal parameters, the high-aspect-ratio micro spline and curved flow channel with the slit width of 160?μm have been fabricated on 5-mm-thick stainless steel (0Cr18Ni9). The width of the slit is uniform and the aspect ratio is 31.     

Ultrasonic vibration assisted grinding of hard and brittle linear micro-structured surfaces

In this paper, a novel ultrasonic vibration assisted grinding (UVAG) technique was presented for machining hard and brittle linear micro-structured surfaces. The kinematics of the UVAG for micro-structures was first analyzed by considering both the vibration trace and the topological features on the machined surface. Then, the influences of the ultrasonic vibration parameters and the tilt angle on the ground quality of micro-structured surfaces were investigated. The experimental results indicate that the introduction of ultrasonic vibration is able to improve the surface quality (The roughness SR_a was reduced to 78 nm from 136 nm), especially in guaranteeing the edge sharpness of micro-structures. By increasing the tilt angle, the surface roughness can be further reduced to 56 nm for a 59% improvement in total. By using the preferred UVAG parameters realized by orthogonal experiments, a micro cylinder array with surface roughness of less than 50 nm and edge radius of less than 1 μm was fabricated. The primary and secondary sequence of the grinding parameters obtained by the orthogonal experiments are as follows: feed rate, tilt angle of workpiece, depth of grinding, vibration frequency and amplitude. The spindle speed in the range of 1000 rpm–3000 rpm does not significantly affect the machined micro-structured surface roughness. Finally, more micro-structures including a micro V-groove array and a micro pyramid array were machined on binderless WC as well as SiC ceramic by means of the UVAG technique. The edge radius on the V-grooves and pyramids are both less than 1 μm, indicating the feasibility of UVAG in machining hard and brittle micro-structured surfaces for an improved surface quality.     

An investigation of accuracy,repeatability and reproducibility of laser micromachining systems

Component technologies of laser micro machining systems are key factors affecting their overall performance. The effects of these technologies on accuracy, repeatability and reproducibility (ARR) in different implementations of such systems have to be investigated to quantify their contributions to the overall processing uncertainty, especially those with the highest impact on beam delivery sub-systems. The aim of this research was to evaluate the capabilities of state-of-the-art machining platforms that were specially designed and implemented for laser micro structuring and texturing. An empirical comparative study was conducted to quantify the effects of key component technologies on ARR of four state-of-the-art systems. In particular, the capabilities of the optical and mechanical axes were investigated when they were utilised separately or in combination for precision laser machining. Conclusions are made about the positional accuracy of the mechanical and optical axes and the importance of their proper calibration on the systems’ overall performance is discussed. It is shown that the laser machining platforms can achieve repeatability and reproducibility better than 2 μm and 6 μm, respectively.     

A precision CNC turn-mill machining center with gear hobbing capability

With ever increasing demand for small parts with complex shapes and high dimensional accuracy, many traditional machine tools have become ineffective for machining these miniature components. Typical examples include dental implants, parts used in mechanical watch movements, and parts used in medical endoscopes. This paper introduces our PC-controlled CNC turn-mill machining center. It has 5 axes, an automatic bar feeder, an automatic part collection tray, and a tool changer. It features a special control algorithm for the synchronization of its axes that produces not only higher accuracy but also makes the machine easier to use. In addition, a volumetric error compensation algorithm is implemented to improve accuracy. Based on experiments, the machining error is ±3 μm for turning, ±7 μm for milling and the maximum profile error is less than ±7.5 μm for gear hobbing.     

A 3-axis precision positioning device using PZT actuators with low interference motions

For expected applications of fast tool servo (FTS) and vibration machining, a 3-axis positioning device with low interference motions is proposed in this paper. The positioning device was composed of a XY stage and a Z-axis stage, which were actuated by piezoelectric (PZT) actuators combined with specially-designed symmetric flexure hinges. Through fundamental experiments, when the applied voltage was 50 V, the displacements along the X-, Y-, and Z-axes were measured as 6.35 μm, 6.61 μm, and 10.12 μm, respectively, with the corresponding small percentages of interference displacement of 3.80%, 4.02%, and 3.30%. In addition, the resonant frequencies were obtained as 1.06 kHz, 0.65 kHz, and 0.54 kHz. To examine control performances, a real-time control system considering hysteresis effect of PZT actuators was implemented by the field-programmable gate array (FPGA) modules to conduct tracing controls for sinusoidal waveform, 3D Lissajous motion, and 3D spiral motion. The tracing errors along 3-axis actuations were under 30 nm. The performances of a 3-axis positioning device were well demonstrated. Future work is to perform machining examinations on a machine tool.     

Application of Monte Carlo simulation for estimation of uncertainty of four-point roundness measurements of rolls

Large-scale rotors in the paper and steel industry are called rolls. Rolls are reground at regular intervals and roundness measurements are made throughout the machining process. Measurement systems for roundness and diameter variation of large rolls (diameter <2000 mm) are available on the market, and generally use two to four sensors and a roundness measurement algorithm. These methods are intended to separate roundness of the rotor from its movement. The hybrid four-point method has improved accuracy, even for harmonic component amplitudes. For reliable measurement results, every measurement should be traceable with an estimation of measurement uncertainty. In this paper, the Monte-Carlo method is used for uncertainty evaluation of the harmonic components of the measured roundness profile under typical industrial conditions. According to the evaluation, the standard uncertainties for the harmonic amplitudes with the hybrid method are below 0.5 μm for the even harmonics and from 1.5 μm to 2.5 μm for the odd harmonics, when the standard uncertainty for the four probes is 0.3 μm each. The standard uncertainty for roundness deviation is 3.3 μm.     

The design and optimization of micro polycrystalline diamond ball end mill for repairing micro-defects on the surface of KDP crystal

Micro-milling is a promising approach to repair the micro-defects on the surface of KH₂PO₄ (KDP) crystal. The geometrical parameters of micro ball end mill will greatly influence the repairing process as a result of the soft brittle properties of KDP crystal. Two types of double-edged micro ball end mills were designed and a three-dimensional finite element (FE) model was established to simulate the micro milling process of KDP crystal, which was validated by the milling experiments. The rake angle of −45°, the relief angle of 45° and the cutting edge radius of 1.5–2 μm were suggested to be the optimal geometrical parameters, whereas the rake angle of −25° and the relief angle of 9° were optimal just for micro ball end mill of Type I, the configuration with the rake angles ranging from 0° to 35°, by fully considering the cutting force, and the stress–strain distribution over the entire tool and the cutting zone in the simulation. Moreover, the micro polycrystalline diamond (PCD) ball end mills adopting the obtained optimal parameters were fabricated by wire electro-discharge machining (WEDM) and grinding techniques, with the average surface roughness Ra of tool rake face and tool flank face ∼0.10 μm, and the cutting edge radius of the tool ∼1.6 μm. The influence of tool's geometrical parameters on the finished surface quality was verified by the cutting experiments, and the tool with symmetric structure was found to have a better cutting performance. The repairing outlines with Ra of 31.3 nm were processed by the self-fabricated tool, which could successfully hold the growth of unstable damage sites on KDP crystal.     

Measurement and statistical analysis toward reproducibility validation of AZ4562 cylindrical microlenses obtained by reflow

This paper presents the statistical analysis applied into the shape of microlenses (MLs) for validating the high-reproducibility feature of their fabrication process. The MLs were fabricated with the AZ4562 photoresist, using photolithography and thermal reflow processes. Two types of MLs arrays were produced for statistical analysis purposes: the first with a cross-sectional diameter of 24 μm and the second with a cross-sectional diameter of 30 μm, and both with 5 μm spacing between MLs. In the case of 24 μm diameter arrays, the measurements showed a mean difference in diameter of 2.78 μm with a standard deviation (SD) of 0.22 μm (e.g., 2.78 ± 0.22 μm of SD) before the reflow, and 2.34 ± 0.35 μm of SD after the reflow. For the same arrays, the mean difference in height obtained was, comparatively to the 5.06 μm expected, 0.76 ± 0.10 μm of SD before the reflow and 1.91 ± 0.15 μm of SD after the reflow, respectively. A mean difference in diameter of 2.64 ± 0.41 μm of SD before the reflow, and 1.87 ± 0.34 μm of SD after the reflow was obtained for 30 μm diameter MLs arrays. For these MLs, a mean difference in height of 0.71 ± 0.12 μm of SD before the reflow and 2.24 ± 0.24 μm of SD after the thermal reflow was obtained, in comparison to the 5.06 μm of height expected to obtain. These results validate the requirement for reproducibility and opens good perspectives for applying this fabrication process on high-volume production of MLs arrays.     

Subpixel edge location method proposed to improve the performance of optical fiber spherical coupling probe during dimensional measurement of micro-cavities with high aspect ratio

A subpixel edge location method based on orthogonal Jacobi–Fourier moments is proposed in this paper to improve the performance of optical fiber spherical coupling probe during dimensional measurement of micro-cavities with high aspect ratio. The effectiveness of the proposed method is proved through the performance test of a micro-hole measuring machine with optical spherical coupling probe. Test results indicate that a blind micro-hole of 400 μm in diameter can be experimentally measured at the depth of 2000 μm with a repeatability of 40 nm and an extremity resolution of 42 nm.     

Micro electrical discharge machining using high electric resistance electrodes

In micro electrical discharge machining (EDM), because the material removal per single pulse discharge mainly determines the minimum machinable size of a micro EDM, decreasing the material removal per single pulse discharge is important. In this study, in order to decrease the material removal per single pulse discharge, high electric resistance materials such as single-crystal silicon are used for electrodes. Analytical results show that when the electrode resistance increases, the peak value of the discharge current decreases, whereas the pulse duration increases. In addition, the discharge energy decreases when increasing the resistance. Silicon is used as a tool electrode, and the effect of resistivity of the silicon tool electrode on the diameter of discharge craters generated on the stainless steel workpiece is examined. Experimental results reveal that with increasing silicon electrode resistivity, the diameter of discharge craters decreases. Because the diameter of discharge craters can be decreased to 0.5 μm, improved finished surfaces of R_z 0.03 μm are obtained.     

Electrogenerated chemical polishing of copper

Stress free polishing method is preferred for a damage free surface of copper with ultra-flatness and ultra-smoothness. Such a surface offers a perfect substrate for integrated circuits and micro-electromechanical systems fabrication. A new polishing method, called electrogenerated chemical polishing (EGCP), is proposed based on the principle of the scanning electrochemical microscope (SECM) and the diffusion controlled chemical reaction. Roughness of a Cu surface is reduced from 100.5 nm to 3.6 nm by the proposed method. To demonstrate the planarization capability of this new method, a patterned Cu surface with an array of micro-columns is planarized with a peak-valley (PV) value from 4.7 μm to 0.059 μm.     

Development of a vacuum-compatible hydrodynamic spindle using an ionic liquid as a lubricant

The importance of beam machining and extreme ultraviolet lithography technologies in the area of precise and fine machining used for high-density optical discs, integrated circuits and patterned media of hard disc drives (HDDs) is rapidly increasing.In this paper, a very simple vacuum-compatible rotary spindle is proposed that uses an ionic liquid as a lubricant with a very low vapor pressure. The usefulness of the proposed spindle lubricated by an ionic liquid was experimentally confirmed by measuring the partial pressures of outgassed products during rotation of the spindle in the vacuum chamber, measuring the accuracy of movement of the rotary table and machining circular grooves by an electron beam in a scanning electron microscope (SEM). It was found that the proposed spindle could be used in vacuum, and the partial pressures of outgassed products were almost the same as those of a clean, empty vacuum chamber. In addition, it was confirmed that by using the proposed spindle, circular grooves with diameters of 200 and 400 μm, 450 nm width and 40 nm depth could be machined on a photoresist surface coated on a silicon wafer in vacuum of an SEM.     

Effect on crystalline structure of AISI M2 steel using tungsten–thorium electrode through MRR,EWR, and surface finish

To investigate on the crystalline structure of AISI M2 steel by using tungsten–thorium electrode in electrical discharge machining (EDM) process was studied. Furthermore, the investigation were carried out for finding the value of material removal rate (MRR), electrode wear rate (EWR) and surface roughness (SR) of tool steel material depending upon three variable input process parameters. On the basis of weight loss, the value of MRR and EWR were calculated at optimized process parameter. Subsequently, surface topography of the processed material were examined through different characterization techniques like scanning electron microscopy (SEM), Optical surface profiler (OSP) and Atomic force microscopy (AFM), respectively. In XRD study, broadening of the peak was observed which confirmed the change in material properties due to the homogeneous dispersion of the particles inside the matrix. Lowest surface roughness and MRR of 0.001208 mg/min was obtained. Minimum surface roughness was obtained 1.12 μm and 2.18427 nm by OSP and AFM study, respectively. Also, minimum EWR was found as 0.013986 mg/min.     

Influences of lubrication conditions and blank holder force on micro deep drawing of C1100 micro conical–cylindrical cup

Lubrication conditions and blank holder force (BHF) are two key processing parameters in deep drawing. This is more obvious in micro forming because of the miniaturization of the specimen size. Micro conical–cylindrical cups with internal conical bottom diameter of only 0.4 mm were well formed. The influences of lubrication conditions and BHF on micro deep drawing of micro conical–cylindrical cups were investigated using a micro blanking–deep drawing compound mold. Pure copper C1100 with a thickness of 50 μm, which was annealed at 450 °C for 2 h in vacuum condition, was chosen as the specimen material. The experiments were conducted on a universal testing machine with a forming velocity of 0.05 mm/s under 4 kinds of lubrication conditions and BHF. The experimental results showed that a micro conical–cylindrical cup with internal conical bottom diameter of only 0.4 mm was well formed, and the limiting drawing ratio (LDR) reached 2.1. The polyethylene (PE) film, which decreased the drawing force and increased the drawing ratio (DR), was superior to castor oil, petroleum jelly and dry friction, and can be chosen as a proper lubricant for micro deep drawing. The rim of the micro cup seriously wrinkled when BHF was less than 4.2 N. The bottom of the micro cup cracked when the BHF was larger than 5.6 N.