Creation of U-Shaped and Skewed Holes by Means of Electrical DischargeMachining Using an Improved Electrode Curved Motion Generator

Paper:

Creation of U-Shaped and Skewed Holes by Means of ElectricalDischarge Machining Using an Improved Electrode Curved

Motion GeneratorTohru Ishida and Yoshimi Takeuchi

Department of Mechanical Engineering, Graduate School of Engineering, Osaka University2-1 Yamadaoka, Suita, Osaka 565-0871, Japan

E-mail: ishidat@mech.eng.osaka-u.ac.jp[Received July 14, 2008; accepted August 22, 2008]

This study deals with the development of a newmethod of machining curved holes for water channelsbuilt in molds, holes which cannot be fabricated us-ing current machining methods in practical use. Sinceholes are generally formed by drilling, water channelsare a series of straight holes. This causes obstructionsto the smooth flow of the coolant, leading to low effi-ciency and unstable performance in the cooling of themolds. To solve these problems, the authors have de-veloped new devices that can machine curved holes,thus enhancing the degree of freedom of water chan-nels. An earlier device consisted of simple mechani-cal parts installed on a die-sinking electrical dischargemachine (EDM). It was able to fabricate an L-shapedcurved hole. However, the controllability of the shapeof the curved holes it produced was unfortunately low.To overcome this weakness, the device has been im-proved and now it can machine curved holes of a va-riety of shapes. Through application experiments, ithas been found that U-shaped and skewed holes, holeswhich cannot be produced using conventional machin-ing methods, can also be created employing the im-proved device.

Keywords: curved hole, electrical discharge machining,water channel, slider crank chain, helical compressionspring

1. Introduction

In the field of machining, drilling generally meansboring straight holes. Most mechanical engineers havetaken the concept for granted, since a method of machin-ing curved holes has not been put into practical use. Con-sequently, even if mechanical designers have designedholes having shapes that can achieve better performancein mechanical products, it has not always been possibleto produce them. As a result, mechanical designers havebeen forced to call for straight holes, even when they areunsuitable. Further yet, they may not even recognize thatit is not suitable to utilize straight holes, due to their un-derstanding that only straight holes can be drilled.

A typical example of straight-line holes being em-ployed unsuitably can be found in the fabrication of waterchannels, i.e., pipelines which run through molds. Reg-ulating the flow rate and temperature of a coolant flow-ing through the channels allows one to control the ther-mal properties of molds, namely, to control the tempera-ture distribution in mold surfaces and also the heat flow inmolding, controls which are necessary in order to preventdefects from occurring in several stages of the moldingprocess. Accordingly, the shape and position of the wa-ter channels in molds are a very important factor in molddesign. In recent years, the great development of com-puter technologies has enabled the simulations of variousphenomena in the molding process. Although these tech-nologies indicate the optimal shapes and positions of wa-ter channels, in most instances these optimal shapes arecurved and their ideal positions are in areas of molds thatare difficult to machine [1]. As the sole practical methodof forming water channels has been to drill a connectedseries of straight holes, it has been impossible to freelycontrol their shape and position so as to accurately pro-duce the ideal water channels proposed.

Another typical example of the unsuitability of straight-line drilling appears in the construction of pipelinesformed in pneumatic components and hydraulic equip-ment. As illustrated in Fig. 1(a), the pipelines, like thewater channels, may have polygonal structures result-ing from their being produced with straight drills. Thepipelines then naturally have several points where straightholes join. These sharp angles in the pipelines causepressure loss in the working fluid running through them.What is more, burrs often form where straight holes join.The burrs can gravely damage the mechanical systemsequipped with such pipelines when the burrs are strippedoff and mixed to the working fluid. Attempting to removethese burrs is a formidable task.

Solving these problems strongly requires the establish-ment of a new method of fabricating curved holes forthe purpose of realizing curved water channels or curvedpipelines, as illustrated in Fig. 1(b). Some methods ofcurved hole machining have been proposed [2, 3]. Aim-ing at achieving curved hole machining with as simplea mechanism as possible, the authors of this paper havealso developed devices consisting of very simple mech-

Int. J. of Automation Technology Vol.2 No.6, 2008 439

Ishida, T. and Takeuchi, Y.

Fig. 1. Comparison of pipeline types.

anisms installed on an EDM, and have proved that thedevices could machine curved holes [4]. In addition, anL-shaped curved hole could be created with one of theaforementioned devices [5]. However, it was difficult todiversify the curvature of the curved hole machined bythem. To overcome this functional problem, slider crankchains were introduced. This brought the effect that theimproved device could produce holes with various cur-vatures [6]. Through application experiments using theimproved device, it has been found that the improved de-vice can create U-shaped and skewed holes, which are im-possible to produce by means of conventional machiningmethods.

2. Improved Device

2.1. Structure of the Improved Device

Figure 2 is a schematic view of the improved devicedesigned in the study. An electrode for electrical dis-charge machining is attached to the main axis of an EDMthrough a shaft and a helical compression spring. Threewires are connected to the electrode inside the spring andarranged at equal angles of 120 degrees. A gate-shapedjig is fixed on the bottom of the working tank of the EDM,and three rotatable disks are mounted on it. The wires areguided to the disks through the inside of the spring. Theend of each wire is fastened to its corresponding disk withthe wire wrapped around it. Similarly, each disk is con-nected to the EDM main axis through a link. That is tosay, the disks, the links, and the EDM main axis constitute

Fig. 2. Schematic view of improved device.

Fig. 3. Initial setting and movement of improved device.

three slider crank chains. Each chain acts as a reciprocat-ing block slider crank mechanism that the EDM main axismoves as a slider. These mechanisms are identical. How-ever, the diameter of the disk on the right side is largerthan those of the two disks on the left side.

The discharge current to machine a workpiece underthe gate-shaped jig is supplied to the electrode through theEDM main axis, shaft, and spring. Except for this conduc-tive path, all other paths between the EDM main axis andthe working tank are insulated. Debris removal is doneby providing working fluid to the discharge gap through aflexible tube and a small center hole on the electrode.

2.2. Movement of the Improved DeviceLet us explain the movement of the improved device

using Fig. 3, a two-dimensional model which providesa simplified view of the device. In the figure, the wirewrapped around the smaller left disk is labeled Wire 1,

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