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Generic aspects of tool design for electrochemical machining
Journal of Materials Processing Technology 149 (2004) 384–392
Generic aspects of tool design for electrochemical machining J.A. Westley a , J. Atkinson a,∗ , A. Duffield b a
Manufacturing Division, Department of Mechanical Engineering, University of Manchester Institute of Science and Technology, P.O. Box 88, Sackville Street, Manchester M60 1QD, UK b Hampson Aerospace, Pegasus House, Bromford Gate, Bromford Lane, Birmingham B24 8DW, UK
Abstract This paper concerns some of the generic aspects of tooling design for the electrochemical machining (ECM) process. The work described was carried out at Earby Light Engineers Ltd. (ELE) in Lancashire, UK. This company had little previous experience of the ECM process and the design of ECM tooling. Thus, an aim was to develop a design brief for future production of ECM tooling. The objectives were to study electrolyte flow, the problems occurring during development and to derive generic design solutions arising. In addition, it was required to identify factors, such as insulation requirements and machined face considerations, that could relate to other ECM components. These observations would then be made use of when producing subsequent ECM electrodes. The findings are derived from work carried out in adapting new electrodes for a casting gate removal process. © 2004 Elsevier B.V. All rights reserved. Keywords: Electrochemical machining; ECM tooling; Electrolyte flow
1. Introduction There are many industrial uses of ECM, examples include electrochemical forming and cooling hole drilling in high-pressure turbine blades. A selection of turbine blades produced by ECM is shown in Fig. 1. The present study concerns the design of tooling for the removal of casting gates on gas turbine blades. Certain turbine blades for land-based gas turbine engines are produced by the investment casting processes [4]. The shape of these components is very complex and it is possible for the molten material not to flow into some areas of the mould. In order to avoid this deficiency, extra pouring gates are included in the design to ensure that the metal reaches all sections of the mould. After solidification the casting gate can be removed by grinding, milling or other conventional machining methods. However, these processes introduce heat into the component. This in turn, can alter the granular configuration of the material and can induce tensile residual stresses [3]. Such stresses within the castings can cause fatigue failure in service. Due to the fact that ECM is essentially a “stress-free” process, it is frequently used to remove casting gates in order to reduce the possibility of fatigue failure in the blade. Fig. 2 shows ∗ Corresponding author. Tel.: +44-161-200-3815; fax: +44-161-200-3803. E-mail address: [email protected] (J. Atkinson).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.02.046
an example of a central root casting gate (marked “A”) in a gas turbine blade. A schematic of a cross-section of this component and the tooling used to machine it is shown in Fig. 3.
2. Development of ECM tooling The process of electrochemical machining is simply described as the electroplating process in reverse. For the process to successfully achieve the removal of material from the workpiece, a suitable electric circuit must be assembled. The workpiece is the positively charged anode electrode, the tool is the negatively charged cathode electrode and the electrolyte solution is the medium that completes the circuit [1]. Although, both the tool and the workpiece are electrodes, in this text only the tool will be referred to as the electrode. When designing the tooling for the removal of the casting gate there are several factors that need to be taken into consideration. The process develops an equilibrium inter-electrode gap and a constant equilibrium current. However, a suitable initial gap must be allowed to permit the process to rapidly achieve equilibrium. At this point the process will operate at maximum efficiency. Electrolyte solution flow is vitally important to the system. The flow of the solution through the system conveys the disassociated electrons away from the anode and prevents them from joining to the cathode [1].
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Fig. 1. ECM turbine blades.
Fig. 2. Central root casting gate in a gas turbine blade.
The design of the cathode and the position and features of the anode can cause the electrolyte solution to encounter many flow restrictions that are generic to all ECM operations [2]. These generic problems cause electrolyte solution flow problems including turbulence and laminar flow which can cause malformations of the work surface as well as stray machining which will over-machine certain areas of the workpiece. The temperature of the solution is also a critical factor since the conductivity of the electrolyte solution changes as a function of temperature. For example, sodium chloride increases its conductivity by 100% when the temperature is increased from 24 to 71 ◦ C [6].
Fig. 3. Schematic of casting gate removal tooling.
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The controlling parameters of the process are difficult to predict when designing a new electrode for a newly designed blade. It is therefore advisable to produce a primary tool from a soft material, such as brass, to facilitate any subsequent alterations [5]. When the electrode is seen to perform satisfactorily, a hard material, such as copper–tungsten, should be used to manufacture the production electrode, as this has a high melting point and will resist damage from electrical sparks. The electrode cannot be the exact length of the casting gate to be removed as this would restrict the electrolyte solution causing it to exit via the least inhibitive route. This would leave parts of the tool with very little solution. This restriction would cause short circuiting problems as the cathode and anode would be exposed to one another. To overcome this, the electrode should be manufactured 0.5 mm less than the length of the casting gate. In order to produce an electrode that would best conform to the blade shape but could be easily adjusted during development, Naval Brass (specification: Cu: 62%; Zn: 37%; Sn: 1%) was chosen as this is soft enough to allow repairs and alterations to be easily performed on the tool. The dimensions of the finished component drawing were entered into a CMM machine to produce a digital profile of the required workpiece. This digitised profile was then entered into the controller of a CNC machine. The brass tool was then machined in accordance with the digitised data. The brass tool was then altered as needed in order to achieve the best surface finish and pocket sizes. The knowledge gained from developing the electrode was then used in developing electrodes for subsequent components. The lessons learned from this exercise are detailed in Sections 2.1 and 2.2. 2.1. The practical problems of ECM tooling development One of the main considerations for ECM tooling is the flow of the electrolyte solution. If the electrolyte is restricted as it exits from the flow slot, there is a very high risk of a spark occurring between the electrode and the workface [7]. This is because there will be no solution to carry away the disassociated electrons from the workpiece and therefore no machining will occur. The machining gap will then get smaller and smaller until the tool and workface are so close together that sparks will occur between the two. There are several phenomena that can cause sparking; however in cases of severe flow restriction, the electrode and workpiece may touch. This will cause a short circuit, and as the brass has a relatively low melting point of 915 ◦ C, the heat at the electrode/workpiece contact point will cause the material to melt or vaporise (boiling point of 2560 ◦ C) and therefore damage the electrode. When using a brass electrode, or an electrode of an equally low melting point material, it is possible for a spark to cause areas of the electrode to be melted as the heat generated by the spark will increase the temperature of the tool. The extent of damage can result in costly and time-consuming
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Fig. 4. Spark damage caused by electrolyte flow restriction.
repairs. Fig. 4 shows an example of a soft electrode damaged by sparking arising from restricted electrolyte flow. The damaged area has been marked in the figure. Various aspects of the tool shape can cause electrolyte flow restrictions [6]. For example if the face of an electrode requires a peak, this will provide a high spot for the electrolyte solution to reach. The solution will take the path of least resistance, therefore, it will flow around the peak. This will result in a high-risk point for sparking or contact. Any valleys in the electrode face will provide a hole for electrolyte solution to collect in. The electrolyte solution will flow into the valley but, due to the shape of the recess, the flow will continue to circulate the electrolyte around the hole. Fluid eddy currents will form in the electrolyte solution and the pattern of the currents will be subsequently machined onto the workface. 2.2. Overcoming the problems in practice Most electrolyte flow restriction problems can be overcome [8]. The final purpose of the component will determine the degree of flexibility allowed for dimensional variation from the design drawing of the finished component. One way of reducing restriction of flow is to increase the amount of electrolyte solution passing over the electrode face. This can be achieved by a combination of two things. First, if the amount of electrolyte solution reaching the electrode is insufficient then the electrolyte solution flow from the electrolyte solution tank must be increased. This is accomplished by increasing the electrolyte pumping pressure. Second, increasing the size of the electrolyte exit slot and maintaining the pressure of the electrolyte solution will allow more electrolyte to flow from the tool. The problem of peaks on the electrode face can be difficult to overcome as the solution will always adopt the easiest route. However, the height of a peak should be reduced as much as possible thereby providing a smoother incline for the electrolyte to flow over. In some cases it is possible that a small amount of post-ECM polishing of the component may be permitted. If this is the case, then the peak can be reduced as much as possible to allow an unrestricted electrolyte flow and then the precise dimensions of the recess in the workface can be polished into the casting using dimensions
from the finished component drawing once machining is completed. A valley in the electrode face that retains electrolyte and allows excess material to be machined from the workface due to stray machining, is not an easy problem to overcome. Unlike the removal of metal from the peaks on an electrode face, the extra metal removed from the work surface by a valley cannot be replaced by means of a post-ECM operation. Valleys in the electrode face should be avoided whenever possible, but when this is not an option, the valley will have to be blended into the electrode face as much as the finished component drawing will allow. This will reduce the build-up of electrolyte solution and decrease the excess machining.
3. Case study ELE had carried out some initial development work for a similar project several years previously. Although, the work had progressed no further than development trials, some tooling had been designed and manufactured. The component fixture designed for the previous trials had proved to be too complicated and a great deal of time was taken loading and unloading the component. In consequence, a new fixture was designed as well as simple setting blocks and electrolyte manifold setting pieces. Fig. 5 shows the final holding fixture configuration for the turbine blades. It was decided that the tooling for each component in this study would be treated individually and developed in succession. The information detailed in the following paragraphs was gathered during the development of this tooling. 3.1. Electrolyte flow This study concentrated on the development of tooling for four components. For each component, the primary tool was manufactured from Naval Brass (specification: Cu: 62%; Zn: 37%; Sn: 1%) and was expected to undergo many repairs and alterations during development. In order to ensure the accuracy of the CNC program for producing the copper–tungsten tool, a second brass electrode was to be manufactured once the primary electrode was proven to be producing a satisfactory workface. After testing the accuracy of the CNC program, the final production electrode was manufactured. Each of the four electrodes was a different shape and size resulting in a variety of different problems to be overcome for each. The tools produced by the manufacturer were very angular. The edges of the electrodes had been milled and were at 90◦ to the electrode face. The electrolyte exit slots were also machined perpendicular to the face. These aspects of the tools were common on all four electrodes and therefore any alterations performed on the first development tool would be generic to the remaining electrodes. These generic findings are discussed later. Amongst the tooling designed for an earlier development trial was an electrolyte manifold used to transport the
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Fig. 5. Turbine blade clamping fixture.
electrolyte solution from its storage tank to the electrode face. From the previous work with this tooling it was known that the manifold delivered sufficient electrolyte solution to the electrode working gap. Therefore, it was decided that this manifold would be used for the new tooling. This meant that the new electrodes had to be made to locate in the same position as those from the earlier development work. Fig. 6 shows a schematic of the electrode used in the previous developments. This tool was manufactured from copper–tungsten and, as can be seen, the sides of the electrode have been machined at 90◦ to each other. In consequence, the right-angled sides caused restrictions to the electrolyte flow. The V-slot configuration of the electrolyte exit slots caused a problem known as Crow’s Feet, this phenomenon is discussed later. 3.2. Peaks and valleys Fig. 7 shows the smallest of the brass gate removal electrodes used in the case study. This electrode has one distinct peak—highlighted at A and one distinct valley—highlighted at B.
Fig. 6. Schematic of V-slot ECM electrode.
The height of the peak and the sharpness of its crest caused a definite restriction resulting, during one machining cycle, in a large spark, which caused the crest to melt. In this case the customer permitted the addition of a post-ECM polishing operation to the production cycle of the turbine blade and so it was possible to polish the peak on the tooling to a smooth rise to allow the electrolyte a clear passage. This reduced the likelihood of sparking at this point. The valley retained a small amount of electrolyte, which was circulated during machining. This resulted in small impressions of the eddy currents being machined into the casting. After consultation with the customer it was found that the valley was not essential and so it was filled with weld material and the electrode face re-polished to exclude the valley. If the customer had insisted on the valley feature, then it would have been very difficult to achieve the dimensional accuracy required in the component. 3.3. Electrode edges A 90◦ angle between the electrode face and the side wall of the tool caused flow problems for the electrolyte solution. The solution would leave the exit slot and flow across the face of the tool, if the edges of the tool are at 90◦ to the side
Fig. 7. Peaks and valleys.
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Fig. 8. Sketch of linear flow.
face, the electrolyte solution would continue to flow linearly away from the tool. A sketch of this action is shown in Fig. 8. If this linear flow is allowed to happen, as the tool moves closer to the workface, stray machining would occur along the edges of the casting gate causing oxidation markings. Fig. 9 shows a turbine blade with oxidation markings along the edge of the gate. By putting a 5 mm radius on the edges of the electrode face where it meets the side walls of the tool, the electrolyte solution is encouraged to flow up the side wall of the tool and away from the casting gate. A sketch of this flow action is given in Fig. 10. The addition of a radius of 5 mm to the edge of the tool face prevents any stray machining occurring along the edge of the workface and reduces the likelihood of oxidation markings occurring on the machined surface. 3.4. Electrolyte flow slots The design of the slot through which the electrolyte solution exits is an important factor in determining the characteristics of the final machined face. The flow of the solution as it exits from the tool causes an imperfection on the machined face known as a “pip” or a “cusp”, this phenomenon
Fig. 9. Oxidation markings.
Fig. 10. Sketch of non-linear flow.
Fig. 11. Narrow electrolyte slot.
is described in Section 3.5. It is the size and shape of the exit slot that determine the dimensions of the imperfection. Figs. 11 and 13 show the same electrode but with two different electrolyte slots. Figs. 12 and 14 show, diagrammatically, the respective workface profiles. The size of the radius at the slot edges determines the way in which the electrolyte solution flows over the electrode face. If the radius is not great enough, the electrolyte will not flow evenly over the surface of the electrode and this will result in sparks or short circuits occurring. The uneven flow can also result in ridges being machined into the workpiece. For the present case, a 5 mm radius was found to produce the best results.
Fig. 12. Narrow pip.
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Fig. 16. Cast of electrode slot.
Fig. 13. Wide electrolyte slot.
Fig. 14. Wide pip.
3.5. Machined face considerations The pressure at which the electrolyte is forced out of the electrode is usually in the region of 13–17 bar. This means that the shape of the electrode, both internally and externally, can have an effect on the machined surface. The two main effects of this restriction are “mirroring” and “pipping”. The electrolyte flows through the exit slot at such a pressure that the inside face of the electrode affects the flow pattern of the electrolyte. In Fig. 15, the machined face shows deep ridges that have been caused by the uneven surface on the inside face of the electrode. A slot drilling operation had been employed to manufacture the electrolyte exit slot. This had resulted is a stepped formation on the inside of the slot, a metal cast of which is shown in Fig. 16.
Fig. 15. Ridges on the workpiece.
As the electrolyte solution exits the slot it is separating into streams. The uneven flow is separating into lines rather than flowing in a continuous sheet. Due to this, during machining, the workpiece material is removed in the same uneven formation. This resulted in a mirror image of the interior of the electrode being machined into the work surface. This reproduction of the electrode imperfections is known as Mirroring. The effect is shown in Fig. 15, the ridges on the inside of the slot (shown by the casting in Fig. 16) have been mirrored into the workface. Mirroring can be overcome by ensuring that the internal surface of the electrolyte slot is smooth and even. Unfortunately, in this case, the land available for machining the ridged interior of the exit slot was very restricted and if the internal face had been altered to make it flat the electrode face would have been destroyed. Due to this lack of material on the electrode face, the tool had to have a layer of weld material added to increase the land available. The welding of the interior face had to be achieved by welding from the working face. This meant that all detail on the working face of the electrode was lost and the shape had to be re-formed after the interior of the slot had been welded. An unavoidable aspect of machining using slotted electrodes is “pipping”. This is also known as “cusping”. A pip is formed on the machined face at the centre of the electrolyte exit slot. When the solution exits from the tool it divides to flow over the entire workface. The nature of the fluid flow results in a small area in the centre of the exit slot being starved of electrolyte solution. No machining occurs in this area and so a small pip of unmachined material remains. The height and depth of the pip are determined by the size of the electrolyte slot. A narrow slot will produce a tall, narrow pip and a wide slot will produce a short, wide pip. These are compared diagrammatically in Fig. 17.
Fig. 17. Pipping configurations.
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Fig. 18. Crow’s feet.
glass fibre filled epoxy-resin composite material. This material is supplied is sheets that must be machined to the appropriate size. Since the material is anisotropic it can be difficult to use in tooling design as it is very weak and brittle when machined against the grain. Due to this difficulty, the production of small areas of insulation is very expensive. Small repairs will quite often require a complete re-make of the insulation. To help reduce the cost of development tooling and to implement repairs quickly and efficiently on site, a cheap, effective and malleable substance is often used as insulation. In this case a “Chemi-Metal” commonly used for small repairs of car body parts was utilised. Chemi-Metal is a non-conductive metal filled polyester resin manufactured by Plastic Padding in Sweden. Fig. 21 shows one of the brass electrodes with the Greenglass insulation inserted. The area of the figure marked shows a repair that was made using Chemi-Metal. 4. Results and discussion
Fig. 19. Theoretical insulation requirements.
As can be seen in Fig. 6, the copper–tungsten electrode manufactured for the previous trials at ELE had “Vs” machined on the edge of the electrolyte exit slots as it was hoped this would allow better electrolyte flow. However, when this electrode was used in production it was discovered that the roots of the “V-slots” retained electrolyte. During machining, small eddy currents formed in the roots of the V-slots and as a result “Crow’s Feet” were machined into the finished surface. Fig. 18 shows an example of Crow’s Feet at the right hand end of the machined face.
This study concentrated on developing the primary electrodes for four turbine blades. During development, a radius was applied to all the edges of the electrodes, including those of the electrolyte solution exit slots. One tool was welded to enable the interior of the electrolyte solution exit slot to be machined flat. Insulation was added to areas of the electrodes that would pass areas of the turbine blades that were not to be machined. All peaks and valleys on the working
3.6. Insulation requirements The configuration of a root pocket for a turbine blade is such that when machining the gate, the electrode will have to be inside the pocket. This is shown in the sketch of Fig. 19. When the electrode is inside the pocket it is possible for stray machining to occur and the underside of the pocket can be machined away (undercut), a schematic of this process is shown in Fig. 20. It is also possible that there will be insufficient electrolyte flowing between the side of the tool and the workpiece. This could permit sparks to occur between the tool and the workpiece. Undercutting is not acceptable, and the problem is overcome by the addition of insulation to the areas of the electrode that will face the metal of the blade, which must remain unmachined. Fig. 19 represents a set-up for the insulation of an electrode to prevent undercutting of the turbine blade. Any material that does not conduct electricity can provide insulation. In this case a variety of Tufnol, 10G40, commonly known as Greenglass was utilised. Greenglass is a
Fig. 20. Schematic of undercutting.
Fig. 21. Electrode insulation.
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Fig. 23. Schematic of a bias anode. Fig. 22. Production stage of an exit guide vane.
faces of the electrodes were reduced or removed. These were originally design features in the component drawings but, following discussions with the customer, it was decided that the machined face would be altered to match the original design after the ECM process was complete. The observations made in this study are relevant to the development of tooling for other ECM operations. In the production of ECM forming electrodes, for example, some of the finished edges of the component are often required to be straight. An example of the various production stages of such a component is given in Fig. 22, this is an exit guide vane for a jet engine. However, as discussed earlier, a radius is required on the edge of the electrode to aid the electrolyte flow over the tool. In order to aid the production of straight edges on the component, the forming electrodes should have an absolute minimum radius applied to the electrode edges. The actual radius will depend on the particular task but will certainly be less than 1 mm. Unfortunately, the ECM process will also produce a certain amount of over-machining that will reduce the formed straight edges into curves. To overcome this problem, a bias anode is used. The bias anode consists of a circuit of high electrically resistant metal, such as platinum. This is machined into the holding fixture at the point where the straight edge is required. The platinum circuit is then linked to a separate power supply that is set at a voltage high enough to attract the freed electrons away from the workpiece edges during machining, Fig. 23 depicts a schematic of a bias anode. By attracting the electrons away from the workpiece, the material at the edges is not removed and straight edges are achieved. The practical observations (Fig. 24) of the study with reference to the overall shape of the electrode are true for any form of ECM electrode. Peaks, valleys, V-slots and other shapes on the electrode face will cause flow restriction problems and subsequently sparking and contact. It is also true that pipping will occur on any machined surface using a slotted electrode. All electrodes that will pass the workface must have sufficient insulation to prevent undercut machining, as shown
Fig. 24. Tooling with bias anode.
in Figs. 19 and 20. Electrodes with separately moving parts, such as forming tools must also be insulated to prevent sparking occurring between two metal faces of the tool. If the faces are very close together it is likely that they would need to be coated with a waterproof non-conductive substance such as silicon grease to prevent any sparking. However, this protective coating would have to be re-applied after every machining cycle. 5. Conclusions Clearly, many practical aspects of ECM tool design are generic. If these aspects are taken into account during the design stage of the tooling most of the problems found during development can be overcome. In the case of machining a central casting gate pocket, the size of the electrode should be 5 mm shorter than the length of the gate. This allows 2.5 mm at each end of the electrode to prevent any restriction to the electrolyte solution as it flows over the tool face. When developing any electrode for a new operation, the electrode will invariably require adjustment. It is therefore advisable to manufacture the electrode from a soft material that can be easily altered, in this case Naval Brass was used and found to suit this purpose very well. The interior of the electrolyte exit slot should be as flat as possible. Any indentations or ridges on the interior will be
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mirrored in the workface by the flow of the high-pressure electrolyte. The path of the electrolyte flow should be as unrestricted as possible. All of the electrode edges should have a radius. This study found a 5 mm radius to work well. The size of the exit slot will be governed by the amount of pipping permitted. However, the edges of the exit slot must also have a radius on them to help prevent restriction as the electrolyte solution exits from the electrode. Electrolyte exit slots should be of a continuous form without V-slots or enlargements. Configurations such as that shown in Fig. 6 should not be used. Peaks and valleys on the electrode face should be avoided as they cause electrolyte flow restrictions. If they must be included in the design, they should be as smooth as possible. Finally, insulation should be considered. At any point where the electrode is likely to be opposite to the area of the component face that is not to be machined, insulation is needed to prevent undercutting. For development purposes, a non-conductive chemical metal can be used; this will help reduce the tooling cost. When the finished tooling is manufactured a hard non-conductive material should be used.
Acknowledgements The authors are grateful to the Teaching Company Directorate and Earby Light Engineers Ltd. for funding this work.
References [1] J.A. McGeough, Principles of Electrochemical Machining, Chapman & Hall, London, UK, 1974. [2] R.M. Wolosewicz, Introduction to Electrochemical Machining, Anocut Ltd., Chicago, USA, 1969. [3] Anon., Turbine Engines, Rolls-Royce plc, Derby, UK, 1998. [4] Anon., The Investment Casting Process, Howmet Castings (Exeter) Ltd., Exeter, UK, 1995. [5] A.E. De Barr, D.A. Oliver, Electrochemical Machining, Macdonald & Co. Ltd., Surrey, UK, 1968. [6] G.F. Benedict, Non-traditional Manufacturing Processes, Marcel Dekker, New York, USA, 1987. [7] C.R. Allison, How electrolytes influence the ECM process, Am. Soc. Tool Manuf. Eng. 64 (1963) 64–79. [8] J.F. Wilson, Practice and Theory of Electrochemical Machining, Wiley/Interscience, New York, USA, 1971.
Generic aspects of tool design for electrochemical machining J.A. Westley a , J. Atkinson a,∗ , A. Duffield b a
Manufacturing Division, Department of Mechanical Engineering, University of Manchester Institute of Science and Technology, P.O. Box 88, Sackville Street, Manchester M60 1QD, UK b Hampson Aerospace, Pegasus House, Bromford Gate, Bromford Lane, Birmingham B24 8DW, UK
Abstract This paper concerns some of the generic aspects of tooling design for the electrochemical machining (ECM) process. The work described was carried out at Earby Light Engineers Ltd. (ELE) in Lancashire, UK. This company had little previous experience of the ECM process and the design of ECM tooling. Thus, an aim was to develop a design brief for future production of ECM tooling. The objectives were to study electrolyte flow, the problems occurring during development and to derive generic design solutions arising. In addition, it was required to identify factors, such as insulation requirements and machined face considerations, that could relate to other ECM components. These observations would then be made use of when producing subsequent ECM electrodes. The findings are derived from work carried out in adapting new electrodes for a casting gate removal process. © 2004 Elsevier B.V. All rights reserved. Keywords: Electrochemical machining; ECM tooling; Electrolyte flow
1. Introduction There are many industrial uses of ECM, examples include electrochemical forming and cooling hole drilling in high-pressure turbine blades. A selection of turbine blades produced by ECM is shown in Fig. 1. The present study concerns the design of tooling for the removal of casting gates on gas turbine blades. Certain turbine blades for land-based gas turbine engines are produced by the investment casting processes [4]. The shape of these components is very complex and it is possible for the molten material not to flow into some areas of the mould. In order to avoid this deficiency, extra pouring gates are included in the design to ensure that the metal reaches all sections of the mould. After solidification the casting gate can be removed by grinding, milling or other conventional machining methods. However, these processes introduce heat into the component. This in turn, can alter the granular configuration of the material and can induce tensile residual stresses [3]. Such stresses within the castings can cause fatigue failure in service. Due to the fact that ECM is essentially a “stress-free” process, it is frequently used to remove casting gates in order to reduce the possibility of fatigue failure in the blade. Fig. 2 shows ∗ Corresponding author. Tel.: +44-161-200-3815; fax: +44-161-200-3803. E-mail address: [email protected] (J. Atkinson).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.02.046
an example of a central root casting gate (marked “A”) in a gas turbine blade. A schematic of a cross-section of this component and the tooling used to machine it is shown in Fig. 3.
2. Development of ECM tooling The process of electrochemical machining is simply described as the electroplating process in reverse. For the process to successfully achieve the removal of material from the workpiece, a suitable electric circuit must be assembled. The workpiece is the positively charged anode electrode, the tool is the negatively charged cathode electrode and the electrolyte solution is the medium that completes the circuit [1]. Although, both the tool and the workpiece are electrodes, in this text only the tool will be referred to as the electrode. When designing the tooling for the removal of the casting gate there are several factors that need to be taken into consideration. The process develops an equilibrium inter-electrode gap and a constant equilibrium current. However, a suitable initial gap must be allowed to permit the process to rapidly achieve equilibrium. At this point the process will operate at maximum efficiency. Electrolyte solution flow is vitally important to the system. The flow of the solution through the system conveys the disassociated electrons away from the anode and prevents them from joining to the cathode [1].
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Fig. 1. ECM turbine blades.
Fig. 2. Central root casting gate in a gas turbine blade.
The design of the cathode and the position and features of the anode can cause the electrolyte solution to encounter many flow restrictions that are generic to all ECM operations [2]. These generic problems cause electrolyte solution flow problems including turbulence and laminar flow which can cause malformations of the work surface as well as stray machining which will over-machine certain areas of the workpiece. The temperature of the solution is also a critical factor since the conductivity of the electrolyte solution changes as a function of temperature. For example, sodium chloride increases its conductivity by 100% when the temperature is increased from 24 to 71 ◦ C [6].
Fig. 3. Schematic of casting gate removal tooling.
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The controlling parameters of the process are difficult to predict when designing a new electrode for a newly designed blade. It is therefore advisable to produce a primary tool from a soft material, such as brass, to facilitate any subsequent alterations [5]. When the electrode is seen to perform satisfactorily, a hard material, such as copper–tungsten, should be used to manufacture the production electrode, as this has a high melting point and will resist damage from electrical sparks. The electrode cannot be the exact length of the casting gate to be removed as this would restrict the electrolyte solution causing it to exit via the least inhibitive route. This would leave parts of the tool with very little solution. This restriction would cause short circuiting problems as the cathode and anode would be exposed to one another. To overcome this, the electrode should be manufactured 0.5 mm less than the length of the casting gate. In order to produce an electrode that would best conform to the blade shape but could be easily adjusted during development, Naval Brass (specification: Cu: 62%; Zn: 37%; Sn: 1%) was chosen as this is soft enough to allow repairs and alterations to be easily performed on the tool. The dimensions of the finished component drawing were entered into a CMM machine to produce a digital profile of the required workpiece. This digitised profile was then entered into the controller of a CNC machine. The brass tool was then machined in accordance with the digitised data. The brass tool was then altered as needed in order to achieve the best surface finish and pocket sizes. The knowledge gained from developing the electrode was then used in developing electrodes for subsequent components. The lessons learned from this exercise are detailed in Sections 2.1 and 2.2. 2.1. The practical problems of ECM tooling development One of the main considerations for ECM tooling is the flow of the electrolyte solution. If the electrolyte is restricted as it exits from the flow slot, there is a very high risk of a spark occurring between the electrode and the workface [7]. This is because there will be no solution to carry away the disassociated electrons from the workpiece and therefore no machining will occur. The machining gap will then get smaller and smaller until the tool and workface are so close together that sparks will occur between the two. There are several phenomena that can cause sparking; however in cases of severe flow restriction, the electrode and workpiece may touch. This will cause a short circuit, and as the brass has a relatively low melting point of 915 ◦ C, the heat at the electrode/workpiece contact point will cause the material to melt or vaporise (boiling point of 2560 ◦ C) and therefore damage the electrode. When using a brass electrode, or an electrode of an equally low melting point material, it is possible for a spark to cause areas of the electrode to be melted as the heat generated by the spark will increase the temperature of the tool. The extent of damage can result in costly and time-consuming
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Fig. 4. Spark damage caused by electrolyte flow restriction.
repairs. Fig. 4 shows an example of a soft electrode damaged by sparking arising from restricted electrolyte flow. The damaged area has been marked in the figure. Various aspects of the tool shape can cause electrolyte flow restrictions [6]. For example if the face of an electrode requires a peak, this will provide a high spot for the electrolyte solution to reach. The solution will take the path of least resistance, therefore, it will flow around the peak. This will result in a high-risk point for sparking or contact. Any valleys in the electrode face will provide a hole for electrolyte solution to collect in. The electrolyte solution will flow into the valley but, due to the shape of the recess, the flow will continue to circulate the electrolyte around the hole. Fluid eddy currents will form in the electrolyte solution and the pattern of the currents will be subsequently machined onto the workface. 2.2. Overcoming the problems in practice Most electrolyte flow restriction problems can be overcome [8]. The final purpose of the component will determine the degree of flexibility allowed for dimensional variation from the design drawing of the finished component. One way of reducing restriction of flow is to increase the amount of electrolyte solution passing over the electrode face. This can be achieved by a combination of two things. First, if the amount of electrolyte solution reaching the electrode is insufficient then the electrolyte solution flow from the electrolyte solution tank must be increased. This is accomplished by increasing the electrolyte pumping pressure. Second, increasing the size of the electrolyte exit slot and maintaining the pressure of the electrolyte solution will allow more electrolyte to flow from the tool. The problem of peaks on the electrode face can be difficult to overcome as the solution will always adopt the easiest route. However, the height of a peak should be reduced as much as possible thereby providing a smoother incline for the electrolyte to flow over. In some cases it is possible that a small amount of post-ECM polishing of the component may be permitted. If this is the case, then the peak can be reduced as much as possible to allow an unrestricted electrolyte flow and then the precise dimensions of the recess in the workface can be polished into the casting using dimensions
from the finished component drawing once machining is completed. A valley in the electrode face that retains electrolyte and allows excess material to be machined from the workface due to stray machining, is not an easy problem to overcome. Unlike the removal of metal from the peaks on an electrode face, the extra metal removed from the work surface by a valley cannot be replaced by means of a post-ECM operation. Valleys in the electrode face should be avoided whenever possible, but when this is not an option, the valley will have to be blended into the electrode face as much as the finished component drawing will allow. This will reduce the build-up of electrolyte solution and decrease the excess machining.
3. Case study ELE had carried out some initial development work for a similar project several years previously. Although, the work had progressed no further than development trials, some tooling had been designed and manufactured. The component fixture designed for the previous trials had proved to be too complicated and a great deal of time was taken loading and unloading the component. In consequence, a new fixture was designed as well as simple setting blocks and electrolyte manifold setting pieces. Fig. 5 shows the final holding fixture configuration for the turbine blades. It was decided that the tooling for each component in this study would be treated individually and developed in succession. The information detailed in the following paragraphs was gathered during the development of this tooling. 3.1. Electrolyte flow This study concentrated on the development of tooling for four components. For each component, the primary tool was manufactured from Naval Brass (specification: Cu: 62%; Zn: 37%; Sn: 1%) and was expected to undergo many repairs and alterations during development. In order to ensure the accuracy of the CNC program for producing the copper–tungsten tool, a second brass electrode was to be manufactured once the primary electrode was proven to be producing a satisfactory workface. After testing the accuracy of the CNC program, the final production electrode was manufactured. Each of the four electrodes was a different shape and size resulting in a variety of different problems to be overcome for each. The tools produced by the manufacturer were very angular. The edges of the electrodes had been milled and were at 90◦ to the electrode face. The electrolyte exit slots were also machined perpendicular to the face. These aspects of the tools were common on all four electrodes and therefore any alterations performed on the first development tool would be generic to the remaining electrodes. These generic findings are discussed later. Amongst the tooling designed for an earlier development trial was an electrolyte manifold used to transport the
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Fig. 5. Turbine blade clamping fixture.
electrolyte solution from its storage tank to the electrode face. From the previous work with this tooling it was known that the manifold delivered sufficient electrolyte solution to the electrode working gap. Therefore, it was decided that this manifold would be used for the new tooling. This meant that the new electrodes had to be made to locate in the same position as those from the earlier development work. Fig. 6 shows a schematic of the electrode used in the previous developments. This tool was manufactured from copper–tungsten and, as can be seen, the sides of the electrode have been machined at 90◦ to each other. In consequence, the right-angled sides caused restrictions to the electrolyte flow. The V-slot configuration of the electrolyte exit slots caused a problem known as Crow’s Feet, this phenomenon is discussed later. 3.2. Peaks and valleys Fig. 7 shows the smallest of the brass gate removal electrodes used in the case study. This electrode has one distinct peak—highlighted at A and one distinct valley—highlighted at B.
Fig. 6. Schematic of V-slot ECM electrode.
The height of the peak and the sharpness of its crest caused a definite restriction resulting, during one machining cycle, in a large spark, which caused the crest to melt. In this case the customer permitted the addition of a post-ECM polishing operation to the production cycle of the turbine blade and so it was possible to polish the peak on the tooling to a smooth rise to allow the electrolyte a clear passage. This reduced the likelihood of sparking at this point. The valley retained a small amount of electrolyte, which was circulated during machining. This resulted in small impressions of the eddy currents being machined into the casting. After consultation with the customer it was found that the valley was not essential and so it was filled with weld material and the electrode face re-polished to exclude the valley. If the customer had insisted on the valley feature, then it would have been very difficult to achieve the dimensional accuracy required in the component. 3.3. Electrode edges A 90◦ angle between the electrode face and the side wall of the tool caused flow problems for the electrolyte solution. The solution would leave the exit slot and flow across the face of the tool, if the edges of the tool are at 90◦ to the side
Fig. 7. Peaks and valleys.
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Fig. 8. Sketch of linear flow.
face, the electrolyte solution would continue to flow linearly away from the tool. A sketch of this action is shown in Fig. 8. If this linear flow is allowed to happen, as the tool moves closer to the workface, stray machining would occur along the edges of the casting gate causing oxidation markings. Fig. 9 shows a turbine blade with oxidation markings along the edge of the gate. By putting a 5 mm radius on the edges of the electrode face where it meets the side walls of the tool, the electrolyte solution is encouraged to flow up the side wall of the tool and away from the casting gate. A sketch of this flow action is given in Fig. 10. The addition of a radius of 5 mm to the edge of the tool face prevents any stray machining occurring along the edge of the workface and reduces the likelihood of oxidation markings occurring on the machined surface. 3.4. Electrolyte flow slots The design of the slot through which the electrolyte solution exits is an important factor in determining the characteristics of the final machined face. The flow of the solution as it exits from the tool causes an imperfection on the machined face known as a “pip” or a “cusp”, this phenomenon
Fig. 9. Oxidation markings.
Fig. 10. Sketch of non-linear flow.
Fig. 11. Narrow electrolyte slot.
is described in Section 3.5. It is the size and shape of the exit slot that determine the dimensions of the imperfection. Figs. 11 and 13 show the same electrode but with two different electrolyte slots. Figs. 12 and 14 show, diagrammatically, the respective workface profiles. The size of the radius at the slot edges determines the way in which the electrolyte solution flows over the electrode face. If the radius is not great enough, the electrolyte will not flow evenly over the surface of the electrode and this will result in sparks or short circuits occurring. The uneven flow can also result in ridges being machined into the workpiece. For the present case, a 5 mm radius was found to produce the best results.
Fig. 12. Narrow pip.
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Fig. 16. Cast of electrode slot.
Fig. 13. Wide electrolyte slot.
Fig. 14. Wide pip.
3.5. Machined face considerations The pressure at which the electrolyte is forced out of the electrode is usually in the region of 13–17 bar. This means that the shape of the electrode, both internally and externally, can have an effect on the machined surface. The two main effects of this restriction are “mirroring” and “pipping”. The electrolyte flows through the exit slot at such a pressure that the inside face of the electrode affects the flow pattern of the electrolyte. In Fig. 15, the machined face shows deep ridges that have been caused by the uneven surface on the inside face of the electrode. A slot drilling operation had been employed to manufacture the electrolyte exit slot. This had resulted is a stepped formation on the inside of the slot, a metal cast of which is shown in Fig. 16.
Fig. 15. Ridges on the workpiece.
As the electrolyte solution exits the slot it is separating into streams. The uneven flow is separating into lines rather than flowing in a continuous sheet. Due to this, during machining, the workpiece material is removed in the same uneven formation. This resulted in a mirror image of the interior of the electrode being machined into the work surface. This reproduction of the electrode imperfections is known as Mirroring. The effect is shown in Fig. 15, the ridges on the inside of the slot (shown by the casting in Fig. 16) have been mirrored into the workface. Mirroring can be overcome by ensuring that the internal surface of the electrolyte slot is smooth and even. Unfortunately, in this case, the land available for machining the ridged interior of the exit slot was very restricted and if the internal face had been altered to make it flat the electrode face would have been destroyed. Due to this lack of material on the electrode face, the tool had to have a layer of weld material added to increase the land available. The welding of the interior face had to be achieved by welding from the working face. This meant that all detail on the working face of the electrode was lost and the shape had to be re-formed after the interior of the slot had been welded. An unavoidable aspect of machining using slotted electrodes is “pipping”. This is also known as “cusping”. A pip is formed on the machined face at the centre of the electrolyte exit slot. When the solution exits from the tool it divides to flow over the entire workface. The nature of the fluid flow results in a small area in the centre of the exit slot being starved of electrolyte solution. No machining occurs in this area and so a small pip of unmachined material remains. The height and depth of the pip are determined by the size of the electrolyte slot. A narrow slot will produce a tall, narrow pip and a wide slot will produce a short, wide pip. These are compared diagrammatically in Fig. 17.
Fig. 17. Pipping configurations.
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Fig. 18. Crow’s feet.
glass fibre filled epoxy-resin composite material. This material is supplied is sheets that must be machined to the appropriate size. Since the material is anisotropic it can be difficult to use in tooling design as it is very weak and brittle when machined against the grain. Due to this difficulty, the production of small areas of insulation is very expensive. Small repairs will quite often require a complete re-make of the insulation. To help reduce the cost of development tooling and to implement repairs quickly and efficiently on site, a cheap, effective and malleable substance is often used as insulation. In this case a “Chemi-Metal” commonly used for small repairs of car body parts was utilised. Chemi-Metal is a non-conductive metal filled polyester resin manufactured by Plastic Padding in Sweden. Fig. 21 shows one of the brass electrodes with the Greenglass insulation inserted. The area of the figure marked shows a repair that was made using Chemi-Metal. 4. Results and discussion
Fig. 19. Theoretical insulation requirements.
As can be seen in Fig. 6, the copper–tungsten electrode manufactured for the previous trials at ELE had “Vs” machined on the edge of the electrolyte exit slots as it was hoped this would allow better electrolyte flow. However, when this electrode was used in production it was discovered that the roots of the “V-slots” retained electrolyte. During machining, small eddy currents formed in the roots of the V-slots and as a result “Crow’s Feet” were machined into the finished surface. Fig. 18 shows an example of Crow’s Feet at the right hand end of the machined face.
This study concentrated on developing the primary electrodes for four turbine blades. During development, a radius was applied to all the edges of the electrodes, including those of the electrolyte solution exit slots. One tool was welded to enable the interior of the electrolyte solution exit slot to be machined flat. Insulation was added to areas of the electrodes that would pass areas of the turbine blades that were not to be machined. All peaks and valleys on the working
3.6. Insulation requirements The configuration of a root pocket for a turbine blade is such that when machining the gate, the electrode will have to be inside the pocket. This is shown in the sketch of Fig. 19. When the electrode is inside the pocket it is possible for stray machining to occur and the underside of the pocket can be machined away (undercut), a schematic of this process is shown in Fig. 20. It is also possible that there will be insufficient electrolyte flowing between the side of the tool and the workpiece. This could permit sparks to occur between the tool and the workpiece. Undercutting is not acceptable, and the problem is overcome by the addition of insulation to the areas of the electrode that will face the metal of the blade, which must remain unmachined. Fig. 19 represents a set-up for the insulation of an electrode to prevent undercutting of the turbine blade. Any material that does not conduct electricity can provide insulation. In this case a variety of Tufnol, 10G40, commonly known as Greenglass was utilised. Greenglass is a
Fig. 20. Schematic of undercutting.
Fig. 21. Electrode insulation.
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Fig. 23. Schematic of a bias anode. Fig. 22. Production stage of an exit guide vane.
faces of the electrodes were reduced or removed. These were originally design features in the component drawings but, following discussions with the customer, it was decided that the machined face would be altered to match the original design after the ECM process was complete. The observations made in this study are relevant to the development of tooling for other ECM operations. In the production of ECM forming electrodes, for example, some of the finished edges of the component are often required to be straight. An example of the various production stages of such a component is given in Fig. 22, this is an exit guide vane for a jet engine. However, as discussed earlier, a radius is required on the edge of the electrode to aid the electrolyte flow over the tool. In order to aid the production of straight edges on the component, the forming electrodes should have an absolute minimum radius applied to the electrode edges. The actual radius will depend on the particular task but will certainly be less than 1 mm. Unfortunately, the ECM process will also produce a certain amount of over-machining that will reduce the formed straight edges into curves. To overcome this problem, a bias anode is used. The bias anode consists of a circuit of high electrically resistant metal, such as platinum. This is machined into the holding fixture at the point where the straight edge is required. The platinum circuit is then linked to a separate power supply that is set at a voltage high enough to attract the freed electrons away from the workpiece edges during machining, Fig. 23 depicts a schematic of a bias anode. By attracting the electrons away from the workpiece, the material at the edges is not removed and straight edges are achieved. The practical observations (Fig. 24) of the study with reference to the overall shape of the electrode are true for any form of ECM electrode. Peaks, valleys, V-slots and other shapes on the electrode face will cause flow restriction problems and subsequently sparking and contact. It is also true that pipping will occur on any machined surface using a slotted electrode. All electrodes that will pass the workface must have sufficient insulation to prevent undercut machining, as shown
Fig. 24. Tooling with bias anode.
in Figs. 19 and 20. Electrodes with separately moving parts, such as forming tools must also be insulated to prevent sparking occurring between two metal faces of the tool. If the faces are very close together it is likely that they would need to be coated with a waterproof non-conductive substance such as silicon grease to prevent any sparking. However, this protective coating would have to be re-applied after every machining cycle. 5. Conclusions Clearly, many practical aspects of ECM tool design are generic. If these aspects are taken into account during the design stage of the tooling most of the problems found during development can be overcome. In the case of machining a central casting gate pocket, the size of the electrode should be 5 mm shorter than the length of the gate. This allows 2.5 mm at each end of the electrode to prevent any restriction to the electrolyte solution as it flows over the tool face. When developing any electrode for a new operation, the electrode will invariably require adjustment. It is therefore advisable to manufacture the electrode from a soft material that can be easily altered, in this case Naval Brass was used and found to suit this purpose very well. The interior of the electrolyte exit slot should be as flat as possible. Any indentations or ridges on the interior will be
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mirrored in the workface by the flow of the high-pressure electrolyte. The path of the electrolyte flow should be as unrestricted as possible. All of the electrode edges should have a radius. This study found a 5 mm radius to work well. The size of the exit slot will be governed by the amount of pipping permitted. However, the edges of the exit slot must also have a radius on them to help prevent restriction as the electrolyte solution exits from the electrode. Electrolyte exit slots should be of a continuous form without V-slots or enlargements. Configurations such as that shown in Fig. 6 should not be used. Peaks and valleys on the electrode face should be avoided as they cause electrolyte flow restrictions. If they must be included in the design, they should be as smooth as possible. Finally, insulation should be considered. At any point where the electrode is likely to be opposite to the area of the component face that is not to be machined, insulation is needed to prevent undercutting. For development purposes, a non-conductive chemical metal can be used; this will help reduce the tooling cost. When the finished tooling is manufactured a hard non-conductive material should be used.
Acknowledgements The authors are grateful to the Teaching Company Directorate and Earby Light Engineers Ltd. for funding this work.
References [1] J.A. McGeough, Principles of Electrochemical Machining, Chapman & Hall, London, UK, 1974. [2] R.M. Wolosewicz, Introduction to Electrochemical Machining, Anocut Ltd., Chicago, USA, 1969. [3] Anon., Turbine Engines, Rolls-Royce plc, Derby, UK, 1998. [4] Anon., The Investment Casting Process, Howmet Castings (Exeter) Ltd., Exeter, UK, 1995. [5] A.E. De Barr, D.A. Oliver, Electrochemical Machining, Macdonald & Co. Ltd., Surrey, UK, 1968. [6] G.F. Benedict, Non-traditional Manufacturing Processes, Marcel Dekker, New York, USA, 1987. [7] C.R. Allison, How electrolytes influence the ECM process, Am. Soc. Tool Manuf. Eng. 64 (1963) 64–79. [8] J.F. Wilson, Practice and Theory of Electrochemical Machining, Wiley/Interscience, New York, USA, 1971.