Investigating the effect of magnetic field application on machining efficiency in the semi-dry electric discharge machining process

To obtain a master's degree

Mechanical Engineering - Manufacturing and Production

Abstract:

The electrical discharge machining process is one of the advanced machining processes that machining hard and high-strength parts such as ceramics and heat-treated steels is one of the important applications of this process. Despite the unique applications of this machining method, low chipping rate, high surface roughness, high tool wear rate and environmental problems caused by this process are among the problems and limitations of this machining method. In this regard, the semi-dry electric discharge machining process is introduced in order to overcome the limitations of the normal electric discharge machining process, and using the Taguchi test design method, experiments to investigate the effect of various input parameters on the output characteristics of this process, such as the chipping rate, tool wear rate, and surface roughness are designed and carried out, and the optimal values ??of the chipping rate, tool wear rate, and surface roughness are determined along with the conditions to reach these optimal values. Variance analysis is also used to determine the most important factors affecting the output characteristics of this process. Also, the effect of the tool material (copper and brass) on the machining performance of this process is investigated and this process is compared with conventional and dry electric discharge machining processes in order to determine the advantages of these processes in comparison with each other and finally, the effect of applying a magnetic field to the gap distance in this process is investigated. The results obtained from this research showed that the semi-dry electric discharge machining process produces the lowest surface roughness and is the most favorable process for polishing operations, while the normal electric discharge machining process is the best process for roughing operations. Also, by applying a magnetic field to the gap in this process, the chipping rate is increased and a lower surface roughness is obtained. Optical micro-images of the machined surfaces also show that better surface health is obtained in the semi-dry electric discharge machining process with the help of a magnetic field compared to the semi-dry electric discharge machining process without a magnetic field.

Key words: semi-dry electric discharge machining, Taguchi test design method, magnetic field, chip removal rate, tool wear rate, surface roughness.

1-1- Introduction

Machining, shaping and casting processes are among the most widely used processes used in the production of industrial parts, each of these processes has specific characteristics, advantages and disadvantages [1.] Machining process, removing chips from the workpiece to produce a specific geometry with a certain degree of accuracy and The smoothness of the surface. One of the characteristics of the machining process that distinguishes it from other processes is the reduction of the weight and volume of the workpiece due to its removal [2.] [

The antiquity of the removal methods goes back to ancient times and when early people cut and produced the parts they needed by hand or with tools made of bone, wood or stone. Until the 17th century, the required parts such as wagons, boats and basic appliances were made by these simple and primitive tools and very simple mechanical methods. With the advent of water, steam and then electricity as new energy sources and with the use of improved alloy steels in the manufacture of tools, in the 18th and 19th centuries, the machine tool industry emerged and various machine tools such as lathes, internal lathes, face lathes, drilling machines, milling machines, grinding machines, honing [1], boring [2] and special machines were used in the production of various parts. [3.] were used.[

In all traditional machining processes[3] a tool harder than the workpiece must be present and this tool must penetrate to a certain depth in the workpiece. In addition, the tool and the workpiece must move in relation to each other in order for the chipping operation to be performed.Traditional machining processes can be divided into the following two categories [2:[

Machining through cutting such as turning and milling

Machining through wear such as grinding

With the advancement of technology and various industries such as automobile manufacturing, aerospace and nuclear reactor construction in the 19th and 20th centuries, the need Materials with high strength, hardness, toughness and high strength-to-weight ratio, such as high-temperature resistant alloys, ceramics, carbides, stellites[4] (cobalt-based alloys) and fiber-reinforced composites, became a basic need. Increasing the hardness of the workpiece material in traditional machining processes leads to problems such as cutting speed reduction and as a result not being economical, so that in some cases, the tool cannot chip hard workpiece materials. On the other hand, it is not possible to obtain a tool material that is hard enough to chip high-strength materials. Also, other needs such as better surface smoothness, greater geometric accuracy, higher production rate, the ability to machine complex shapes, machining on very small scales, machining points of the workpiece that are difficult to access and cannot be machined with traditional methods, creating holes with a low entry angle, non-circular and curved holes and holes without pleats in hard and high strength materials, lead to the growth and development of new machining processes. It was called advanced machining processes [5] (modern or non-traditional) [4.] [

In advanced machining processes, the cutting tool does not cut the surface of the workpiece, but energy is used directly to remove the material from the workpiece. The scope of application of advanced machining processes is determined by the workpiece properties such as electrical conductivity, thermal conductivity and melting temperature [5.] Advanced machining processes can be divided into mechanical, thermoelectric, electrochemical and chemical categories. Electrical discharge machining [6], ultrasonic machining [7] and electrochemical machining [8] are examples of advanced machining processes. In the following, the machining process with electrical discharge is briefly reviewed [4.] 1-2- The history of electrical discharge machining process Sir Joseph Prestily in 1768 first experienced the wear of metal with spark discharges (spark) [9]. More than a hundred years passed until this work was put into practice, until in 1943, two Russians named B. R&N I. Lazarenko, concluded that spark discharge can be used to machine new materials that are difficult to shape with previous methods [6.] 1-3-Shaving mechanism of electrical discharge machining process Two metal electrodes, one of which is a predetermined shape (tool) and the other is a workpiece, in a dielectric fluid such as oil. They are submerged. A series of voltage pulses, often in the form of a rectangle with a frequency of 5 KHZ, is applied between two electrodes that are separated by a small distance (0.5-0.01 mm). The use of these voltage pulses in such a short distance causes the electrical breakdown of the dielectric locally. This phenomenon occurs in a channel with an approximate radius of 10 (plasma channel)[10]. The cause of this electrical breakdown is the acceleration of electrons exiting the cathode towards the anode due to the effective electric field. These electrons collide with the neutral atoms of the dielectric and create more positive and negative ions, which are accelerated towards the cathode and anode, respectively. When electrons and positive ions reach the anode and cathode, they lose their kinetic energy as heat, so it is possible to reach a temperature of 8000-12000 °C in the electrodes. Therefore, with very short duration sparks, the local temperature of electrons reaches more than their natural melting point. Due to the evaporation of the dielectric, the pressure in the plasma channel increases rapidly to about 200 bar. Although this high pressure prevents vaporization of the superheated metal, nevertheless, when the voltage is removed at the end of the pulse, the pressure suddenly drops and the superheated metal vaporizes, thus removing the metal from the electrodes.