Y.S. Liao

Wear characteristics in turning high hardness alloy steel by ceramic and CBN tools

Abstract The wear behaviour in the turning of AISI 4340 hardened alloy steels by CBN and ceramic tools was studied. Experimental results showed that the main wear mechanism for the CBN tools was the abrasion of the binder material by the hard carbide particles of the workpiece. For the ceramic tools, there was adhesive wear and abrasive wear. It was also found that there was a protective layer formed on the chip–tool interface. For the CBN tool, this was a solution of the binder of the tool material and the work material, while for the ceramic tool, this was from the work material. This layer plays a very important role in the wear behavior of CBN and ceramic tools. Variations of tool wear with the cutting speed and the hardness of the work material are discussed accordingly.

Determination of finish-cutting operation number and machining-parameters setting in wire electrical discharge machining

Abstract Wire electrical discharge machining (WEDM) plays an important role in precision manufacturing. To obtain a precise workpiece with good surface quality, some extra repetitive finish cuts along the rough cutting contour are necessary. An attempt has been made to unveil the influence of the machining parameter (pulse-on time, pulse-off time, table feed-rate, flushing pressure, distance between wire periphery and workpiece surface, and machining history) on the machining performance of WEDM in finish cutting operations. The gap width, the surface roughness and the white layer depth of the machined workpiece surface are measured and evaluated. Based on the Taguchi quality design method and numerical analysis, it is found that the pulse-on time and the distance between the wire periphery and the workpiece surface are two significant factors affecting the machining performance. Mathematical models relating machining parameters and performance are established by regression, and non-linear programming using the Feasible-direction algorithm was used to obtain the optimal machining parameters. A strategy of optimal multi-cut WEDM process planning from rough to finish cutting operations, including the number of machining operations and their corresponding machining-parameters setting for each operation, has been proposed. Experimental results show that the proposed approach can achieve better performance than that achieved by a well-skilled operator. A better surface quality and accurate dimension value can be obtained in less machining time.

Carbide tool wear mechanism in turning of Inconel 718 superalloy

Abstract Wear surfaces of the cutting tools are analyzed to study the wear mechanism of cemented carbide tools in turning in Inconel 718 superalloys. SEM and EPMA analyses indicated that the wear of carbide tools during high speed turning condition (V = 35 m min −1 ) was caused by diffusion of elements (Ni or Fe) in workpiece into tools binder (Co) by a grain boundary diffusion mechanism. This action weakened the bonding strength between carbide particles (WC, TiC, TaC) and the binder (Co). The carbide particles were then detached out of the cemented carbide tool by high flow stresses. The proposed grain boundary diffusion mechanism is also confirmed by theoretical analysis.

A new on-line spindle speed regulation strategy for chatter control

A new on-line control method to suppress regenerative chatter vibration during the machining process by regulating spindle speed is proposed. The dynamic cutting force signal collected from a dynamometer is passed through a low pass filter, and then digitized. The fast Fourier transform is carried out to obtain the corresponding power spectrum. The chatter frequency is identified when the intensity at a certain frequency other than the spindle speed and tooth passing frequency exceeds a critical value. Based on the identified chatter frequency, a new spindle speed is computed by applying the principle of keeping the phase between the present and previous undulations to 90°. The new speed command is executed while the cutting proceeds. It is found from simulation that the chatter vibration can be suppressed by this approach in the shortest time. This method is also verified by experiments through actual cutting of various materials by a computer numerically controlled milling machine. The main feature of this approach is that the feed of the machine tool does not need to be halted during the change of spindle speed. Hence, tool wear can be reduced. Furthermore, no system identification of the machine tool structure is needed, and therefore it has great potential in actual applications.

Study on wear mechanisms in drilling of Inconel 718 superalloy

Abstract The wear mechanisms and the approach to prolong the service life of the TiAlN coated carbide tool in drilling Inconel 718 superalloy are presented. It is found that the coated layer on the cutting edge is gradually abraded-off at the first stage of drill wear due to an excessive friction force on the tool–work interface. This in turns intensifies the friction force and leads to an increase of drilling force. Built-up edge (BUE) is then formed, and chipping starting from the relatively weaker cutting edge takes place subsequently. As a result, many micro-cracks are observed to distribute over the worn area. The subsurface fatigue cracks grow as the drilling process is proceeding. Together with the abrasion of hard carbide particles of the work material, the cutting edges break eventually parallel to the direction of fatigue cracks. At this moment, longer chip forms and cutting process is disturbed to an extent that the process can no longer be effectively continued. Failure of the drill is noted in a very short period of time once the long chips are observed. Finally, drilling experiments with the use of the cutting fluid containing the nano-particle low friction surface modifier are conducted. It is found that the service life of the drill is lengthened significantly and hence the machining cost can be greatly reduced.

Study of wire breaking process and monitoring of WEDM

Wire rupture in the Wire Electrical Discharge Machining (WEDM) process is a serious problem to manufacturers. A new computer-aided pulse discrimination system based on the characteristics of voltage waveform during machining was developed in this paper. With the use of this system, a large amount of sparking frequency data during wire rupture process and under normal working conditions were collected and analyzed. Two symptoms of wire rupture were identified: the excess of arc sparks, and a sudden rise of the total sparking frequency. The governing mechanisms of these two types of wire rupture were found from the SEM and EDAX analyses of the ruptured wire electrode. Furthermore, an index to monitor wire breaking was identified, and its relationships with the metal removal rate and machining parameters were found. Based on the results obtained in the paper, a control strategy to prevent wire from rupturing while at the same time improving the machining speed is proposed.

Wear characteristics of sintered diamond composite during circular sawing

Abstract The wear characteristics of sintered diamond composite segmented sawblades during circular sawing of granite were investigated. Scanning electron microscopy, optical microscopy and energy-dispersive X-ray analysis were used to identify wear mechanisms. The forces acting on the blade for various sawblades were also measured. The results obtained show that fracture of diamond particles can be attributed to impact forces and low cyclic surface fatigue, and wear of the matrix bond is due to flush erosion, cavitation erosion and abrasion. Blades containing diamonds with a greater degree of etching pits result in a relatively higher proportion of macro-fractured or polished (flattened) diamond particles, a larger vertical sawing force and sawing force ratio, and lower grinding ratio than those containing diamonds covered with less etching pits.

Study of the behaviour of diamond saw-blades in stone processing

Abstract The conditions of the diamond on the working surface of the tools in sawing Indian red granite using diamond saw-blades have been studied in this paper, and the effects of the types and the sizes of the diamond on the sawing performance of the saw-blades have been also investigated. SEM and toolmakers microscope analyses of the diamond blades after sawing indicate that the conditions of the worn particles can be classified as follows: whole (without significant wear), polished (flat), micro-fractured, macro-fractured, and pull-out. Experimental results show that sawblades containing higher toughness grits result in a better blade performance, lower sawing forces, and a greater proportion of whole and pull-out grits occurring on the worn surface. When saw-blades containing smaller size grits of the same concentration are used, the blade performance is better, but the sawing forces are relatively larger, and a greater proportion of particle are pulled-out with a small number of polished grits appearing on the working segment surface.

Development of a high precision tabletop versatile CNC wire-EDM for making intricate micro parts

The micro-electrical discharge machining (micro-EDM) process has been proved to be appropriate for making 3D micro parts that are difficult and even impossible to manufacture by other processes. In this paper a high precision tabletop CNC wire electrical discharge machine (wire EDM) designed specifically for machining complex shape micro parts or structures is developed. In the machine developed, a novel micro-wire-cutting mechanism is designed, an approach to control wire tension by magnetic force is proposed and a servo feed control strategy, in accordance with the measured gap voltage, is designed and implemented. To verify the functions and capabilities of the machine developed, several thick micro outer and internal spur gears and rack are machined. It shows that the taper angle along the wall or cavity of a part that appears when other micro-EDM processes are applied can be avoided. A very good dimensional accuracy of 1 µm and a surface finish of Rmax equal to 0.64 µm are achieved. The satisfactory cutting of a miniature 3D pagoda with a micro-hooked structure also reveals that the machine developed is versatile, and can be used as a new tool for making intricate micro parts.

The effects of machining settings on the behavior of pulse trains in the WEDM process

Abstract A pulse discrimination system has been developed to study the characteristics of pulse trains in the WEDM process under various machining conditions. The effects of on-time, off-time and feed on the variations of the proportions of short sparks, are sparks, and normal sparks in the total sparks (defined as short ratio, arc ratio, and normal ratio, respectively); the distribution of the ignition delay time of the normal sparks; the average ignition delay time; and the proportion of machining time in a specific sampling period were investigated. An approximate method for estimating the variation of the average gap width is also introduced. It is found that a short on-time, or a long off-time, or a large feed, results in the decrease of both normal ratio and average ignition delay time, and an increase of short ratio. On-time has the largest effect on the arc ratio, whilst off-time has the least effect. The ignition delay time of the normal sparks displays a reverse J-shape distribution. On-time is the most prominent factor amongst all of the machining settings under consideration affecting this distribution. In addition, the distribution becomes more left skew with a higher peak and a shorter tail under the condition of a shorter on-time setting. Based on the analysis results, the instability during the WEDM process can be attributed to an apparent increase of either arc sparks, short sparks, or both. A strategy to improve machining stability with respect to each case is proposed accordingly.