DEVELOPMENT OF A MICROCONTROLLER BASED ROTATING ELECTRODE ATTACHMENT FOR RAM TYPE ELECTRO DISCHARGE MACHINE Valaki J.B., Barot R.S. and Karhadkar G.D. L.D.College of Engg. Ahmedabad , BVM Engineering college, V.V.Nagar, Faculty of Tech. & Engg, Baroda,
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ABSTRACT: Since demonstration of application of spark erosion to material removal from hard & conductive materials by lazarenkos in 1943, the development of EDM machine has gone through considerable changes in design and construction to suit specific requirements and overcome certain limitations. Rotating electrode concept has been introduced with the claim of improved material removal rate, surface finish, dimensional and geometrical accuracies (Tapering, corner rounding, skewing, parallelism etc.) and spark stability and marketed as an attachment for RAM type EDM. Authors in this present paper discusses design , development & fabrication issues of Rotating electrode attachment controlled by dedicated microcontroller to be marketed it as a cost effective accessory. Keywords: Rotating Electrode, Material Removal Rate (MRR), Dimensional and Geometrical Accuracies, Spark Stability, Dedicated Microcontroller, Cost Effective Accessory. 1. PROBLEM STATEMENT: Once upon a time, Electro discharge machining had been considered as a novice process in the field of material removal having applications limited to removal of broken taps from the drilled holes. Now it is rapidly gaining importance for high tech materials processing due to the technologies made available for the EDM users. The new technologies not only enhanced the functional capabilities of the process but also improved the process efficiency, which once upon a time was the main barrier for accepting the EDM. Rotating electrode is one such technology which has opened new avenues towards developing multi-axis CNC EDM like CNC Ball Milling. Rotating electrode attachment marketed by few of the companies, are costing almost 30-50 % of the EDM machine itself. The high cost of the attachment itself and lack of awareness of the machine and its technologies have been the reason for ignorance to use the EDM machine for the shop floor application. Many researches have been done on various aspects of EDM using rotary electrode. But none of them have been found working on cost effective solution for the rotating electrode attachment. So in this paper emphasize have been made to develop a cost effective solution for rotating electrode attachment assisted by dedicated micro controller to obtain various speeds required to have optimum speed selection for various electrode/ work piece material combinations. 2. LITERATURE REVIEW: Many researches have been done using rotary electrode for RAM type EDM. From the literature reviewed, it is seen that many researcher have given the introductory information regarding the components of the rotary electrode attachment. Even the literature of the companies manufacturing the rotary electrode attachment is not giving any clear cut idea of what system they have used. [Chakravarti & Mohan] have used electric motor and V-belt to transmit power to electrode. But has not given any idea about the system used. [Biing, Che, Han & Yan] have developed ball burnishing attachment for their study but have not given any guideline for developing it.
3. DEVELOPMENT OF ROTARY ELECTRODE ATTACHMENT: RAM type EDM machines, because of its constructional features became a general purpose machine rather than a special purpose machine. As all the RAM type EDM machines possess same type of head architecture, to develop an attachment, some considerations must be looked at to make the attachment compatible with all the RAM type EDM machines. Common RAM type EDM head architecture is shown in fig. -1.
Fig.-1 Machine head architecture of RAM type EDM Like -
Maximum height available between work piece and machine head. Maximum weight which can be hanged with the machine head without affecting the sensitive control system of the machine. Path of the current within the machine head Type of flushing commonly employed in RAM type EDM machines Maximum travel available for table or machine head.
These considerations portray a broad idea about the dimensions, weight, and type of arrangement for the attachment. There can be so many ideas to give rotation to the electrode of EDM machine but keeping the basic architecture of the machine in the mind, the ideas can be sort listed to best feasible ones. That is, either a motor could be mounted inline with the machine head axis or by means of some transmission system, eccentricity can be provided between machine head and the motor shaft. The concept of this eccentricity looks something like fig.-2.
Fig. 2: concept of rotating electrode attachment
This idea lacks practicality. Many requirements must be fulfilled to make it a practically feasible solution. Like
Current should be passed from stationary rod to rotating electrode. Bearing heating should be prevented to maintain axis alignment as this is very important while setting machine for a new job Rigidity should be provided to the system. I.e. deflection of the electrode should be minimum to reduce the radial run out. Precise rotation should be provided to the electrode.
The concept which is conceived for providing the rotation to the electrode is just a brief idea. To turn the idea in to the reality, it needs some in-depth study and analysis of the design requirements and manufacturability requirements. To meet the above mentioned requirements that a rotating electrode attachment should possess to be compatible with the ram type EDM machine; some modifications are required in the concept. Modifications, which are required to be made, are divided in to different categories. 3.1. MODIFICATIONS FOR PASSING CURRENT FROM STATIONARY ROD TO ROTATING ELECTRODE. Because of the conductivity difference between copper rod and steel bearing, there will be temperature rise at the interface, which will subsequently lead to uneven thermal expansion of copper rod and bearing. It adversely affects the axis alignment and increase the radial run out of the electrode. One well known fact is that current always flows through least resistance path. To prevent the heating of the bearing, the current can be bypassed from bearings by means of high conductivity carbon brushes [fig. -3], which are widely being used in alternators and generators for the same purpose. The graphite because of its high thermal conductivity and high lubricity provides least resistance path for the current to flow. Flexible connectors readily available in the market along with the common brass lucks are used as the current bypassing media [fig.-4].
S Fig.-3 Schematic of carbon brush
Fig.4: Carbon brush and flexible connector with brass lucks
The current will pass through the least resistance path so temperature rise at the interface can be minimized and hence the electrode deflection and radial run out. 3.2 .MODIFICATIONS FOR PROVIDING RIGIDITY TO THE SYSTEM Because of the torque to be transferred by the gears from the motor shaft to the rotating rod, the shaft will act as a simply supported beam at one end and load applied at other end. It has inherent characteristic of getting deflected. To minimize the deflection, the shaft can be considered as a spindle and by mounting the two bearings at optimum spacing; we can minimize the deflection of the rotating rod. Minimum deflection of spindle supported at two points can be calculated by the formula given below.
D4 / 3 L 1/ 3 K
………………(1)
Where L Length constraint in mm D Average diameter of the rod in mm K Constant for normal machining = 0.05 Precision machining = 0.1 For anti friction bearings, the following condition must be satisfied
optimum Where
L 2.5 …………….(2) C
Bearing ratio
L Distance between two bearings in mm C Load application from one bearing in mm
By using these formulas, we can find out the optimum spacing between two bearings to provide maximum rigidity to minimize the deflection of rotating electrode. Also for the same internal and external dimensions, the roller bearing has much higher load carrying capacity than ball bearing. So roller bearing is preferable for this purpose. Arrangement of roller bearing mounted at Optimum spacing on rotating copper rod is shown in the fig. 5.
Fig.-5: optimum spacing between roller bearing 3.3. MODIFICATIONS FOR FITTING THE BEARINGS IN THE STATIONARY ROD, MOTOR MOUNTING, TRANSMISSION SYSTEM ETC… Roller bearings are very sensitive to the type of fitting. If they are tight fitted, the bearing can not rotate and if bearing is loosely fitted, it tends to deflect from its axis radially and laterally. Best fit can be given by selecting proper tolerance to the shaft and hole. To give precise rotation to the
electrode, timer belt & pulley system can be used as torque transmission system. To provide initial tension in the timer belt, lateral movement is provided to motor. The developed rotating electrode attachment is shown in the fig.-6.
Fig. 6: rotating electrode attachment
4. DEVELOPMENT OF MICRO-CONTROLLER: Application of microcontrollers in manufacturing has revolutionized the way in which the processes are to be controlled. The sophisticated functional capabilities of microcontrollers, have almost replaced the bulky electrical & hydraulic circuitry. Microcontroller is a “one chip solution “of a microprocessor, developed for a specific task only. So the unnecessary hard wares and specifications can be avoided and hence the system becomes compact, cheap and easy to troubleshoot. It possesses on one chip all the hardware’s required by a microprocessor. Large scale integration of microcontroller chip has added more functional capabilities & sophistications to do complex mathematical computations. Many input and out put pins makes it possible to control many parameters at a time. Timers and counters provide high sensitivity and the intelligence which is not possible in case of hydraulic and electrical control systems. Due to the following reasons, microcontrollers provide superior technological solutions for the bulky systems available for the same purpose Wide range of speed control with high degree of accuracy, so no need of costly & bulky gear train for the drive systems. Compact & cheap hardware system Reliability of IC chip to work in dynamic shop floor conditions Low power consumption Ease in program change due to flash memory[EEPROM]
Many inputs & outputs, timers & counters, provide multifunctional capabilities and intelligence to the control system. The microcontroller system for rotating electrode attachment is divided in to main three parts. That is power supply circuit, speed input and display (push button and LED) circuit, and stepper motor driver circuit. Fig.7 & 8 given below depicts the complete outline of microcontroller circuit [axleson].
Fig. 7: outline of the power supply for microcontroller
Fig. 8: outline of the microcontroller system Micro controller requires 5 V regulated DC power supply for its working. So the 230 V AC power supplies which is available at source is to be converted, by means of a step down transformer, in to 12 V AC. Then full wave rectifier converts it in to 12 V pulsed DC. Voltage regular, LN 7805, provides 5 V regulated DC output. The selection of speed can be done means of input switches, which are connected with the input port of the microcontroller. As soon as the microcontroller gets the 5 V DC power, the program which is already fed in to the memory of the microcontroller gets activated and starts generating the pulses of given duty cycle. These pulses are amplified by means of suitable circuit known as stepper motor driver card. 5. CONCLUSION: Cost effective solution of this rotating electrode system will assist EDM users to take full advantage of higher MRR , better surface finish, better dimensional and geometrical accuracies of work piece & electrode, better process stability, and hence efficiency. Microcontroller replaces the bulky gear train with simple belt drive. Microcontroller rotates the electrode at optimum speed according to various process parameters settings. By setting the optimum process parameters, huge database can be created for the newly developing intelligent controllers. The attachment provides initial breakthrough to develop Multi axis CNC EDM. Multi axis CNC EDM will solve the problem of one electrode for one geometrical feature.
6. REFERENCES: Mohan, B., Rajadurai, A., & Satyanarayana, K.G. (2000). Effects of SiC and rotation of Electrode on electric discharge machining of Al-SiC Composites., Journal of materials & manufacturing Process,124(2002), 297-304. Biing Hwa Yan,Che Chung Wang,Han Ming Chow & Yan Cherng Lin (2000) “ feasibility Study of rotary electrical discharge machining with ball burnishing for AL 2O3 / 6061AL composite”international journal of machine tools and manufacture 40, 1403-1421. Schumacher, M., Writch, B. (1990) About the role of debris in the gap during electrical discharge machining, Annals of CIRP, vol. 39/1,( 197) Junkar, M., Roethel, F. & Znidarsic, M.(1991) , the influence of dielectric fluid on EDM process control, international machinery monitoring & diagnostic conference, dec. 9-12, USA,20-24 Atkinson, A., Ghoreishi, M. 2002). Comparative experimental study of machining characteristics in vibratory, rotary and vibro-rotary electro discharge machining” Journal of materials processing technology, 120(2002) 374“The microcontroller idea book” 2nd edition
By Jan axelson, Penram International Publishing (India) Pvt. Ltd. ,
