Shaped tube electrolytic machining
Several methods have been employed to drill holes. Most of the time only mechanical methods have been thought up. These mechanical methods have a lot of disadvantages. Some of them being production of heat while drilling and also there was use of a lot of energy. Moreover, to drill both small and large holes is very difficult especially in hard and brittle materials using traditional drilling methods (Robert, H. & Allen, D, 1994). In some areas such as space, automobile, electronic sand computer (printed circuit boards), medical (Surgical implants), optics and miniature manufacturing provide the need for drilling holes in hard and brittle materials. Therefore, shaped tube electrolytic machining (STEM) developed to take care of this situation. The above mentioned industries need complex, accurate and good position holes. This machining type was developed for drilling holes with big depth to diameter ratio.
Shaped tube electrolytic machining mainly used to drill holes which either has the same or different sizes. Holes, which are grouped, are drilled parallel to each other (Robert, H. & Allen, D, 1994). They can as well be drilled at compound angles as directed by guide bushings. Since this method uses an electrolyte, it is applied in materials which is corrosion free or which cannot be dissolved by the electrolyte. This paper describes shaped tube electrolytic machining process, technology and the parts which are produced using this process.
Shaped tube electrolytic machining (STEM) is a quick, less expensive machining process that do not involve and mechanical stress(Sharma S,& Jain, V.2002).STEM was invented by the Electric Aircraft Engine Group, to drill holes which could not be drilled using any ordinary methods. These holes initially were drilled using electrochemical machining (ECM). But the disadvantage of ECM was that it produces precipitates which are insoluble and this interferes with the smooth flow of the electrolyte. STEM is an improved form of ECM in that it uses an acid electrolyte that enables there moved metals to go into solution (Robert, H. & Allen, D, 1994). By so doing, there is no formation of precipitates which blocks the flow of electrolyte.
While drilling a hole of uniform diameter using a shaped-tube electrolytic machining process, a conductive tube is advanced towards the workpiece and electrolyte passed through the tube which is in contact with the workpiece. This will pass a fixed electric current in between the tube and the workpiece. This fixed electric current should be maintained using a sensor. The sensed fixed electric current is compared to a reference signal and making adjustment according to the comparison of the two. This ensures that the conductive material is uniformly charged throughout the hole. Acids especially nitric and hydrochloric are mostly preferred in STEM (Sharma S, & Jain, V.2002). However, some researches have it that neutral salt electrolyte together with a small percentage of acid electrolytes some times are used. This aids in avoiding the formation of sludge in the electrolyte. The process works on the principle that holes are drilled by deplating in a controlled manner of an electrically conductive material. The process of deplating takes place inside an electrolytic cell. The cell is formed by metal side negatively charged, and the workpiece positively charged, and separated by an electrolyte which flows in between them (Robert, H. & Allen, D, 1994).The positively charged side which is known as the cathode, is made of a tube which is resistant to acid. An example of a material which can be used is titanium that is shaped according to geometry of the desired hole.
The entire length of the tube is made to be straight and insulated except at the tip. Furthermore, the acid electrolyte, which is under pressure, is passed through the tube up to the tip. The electrolyte then returns a gap which created on the outside of the coated tube to the top of the workpiece. The rate at which the electrolytes are fed is equal to the rate at which the workpiece is being dissolved. The voltage required in STEM process is between 5v and 15v direct current (DC) (Sharma S, & Jain, V.2002).As a result of the use of low voltage, more conductive acid electrolyte is used. Uniform wall thickness during repetitive production using STEM method is ensured by lack of mechanical conduct. High integrity holes are realized through the dissolution of the molecule by molecule of the produced materials application. The tool does not wear out since there is no mechanical or thermal load on it. Consequently, the dimension and accuracy of all the articles are the same entirely, and it makes the process more reliable and productive.
While dealing with parts for jet engines and gas turbines, the application of STEM are machining of the following: Turbine blade cooling holes, Turbine vane cooling holes, Turbine disk cooling holes, oil passages, Fuel nozzles and any holes where electrical discharge machining (EDM) recast is not allowable(Sharma S,& Jain, V.2002). Some other applications include; starting holes for wire EDM cuts this is when the length of cut is above 100 mm. producing several sets of holes in metals which are corrosion resistant. It is used also in drilling of holes in bearings to allow oil to pass. Recently; shaped-tube electrolytic machining processes have also been applied in making of precision extrusion dies for producing ceramic honey comb structures.STEM process is highly accurate and can be used to produce several holes which are identical and have high quality.
Model formation
STEM process has got the following dimensional accuracies: dimensional tolerances can be held to ±0.0002” (±0.005mm), and another dimensional tolerance can be held to ±0.002” (±0.05mm.) and finally, Surface finishes of 15µ inch Ra can be achieved in a single pass.
Consider a situation whereby a tool of outer radius is given as r1 is drilling a hole of radius r2 at a steady state. The feed rate at which the tool is moving is given by an f.let assume that a voltage of V is applied between the tool and the workpiece, then the rate of change of hole radius given by dr/dt=EJs/FPa. Where, E is gram equivalent weight of workpiece material it is machining time; F is Faraday’s constant, ρa density of workpiece material. Js is the current density at any radius r and is given by Js=Is/Area=Is/2ΩrL.Furthermore, Is=V/Rg=2ΩLVKe/ln (r2/r1). (Sharma S, & Jain, V.2002).
While applying the above equation, we assume that resistance of the electrolyte in the side gap is the same as the resistance of the electrolyte enclosed between two concentric cylindrical electrodes of length L. Here L is the magnitude of the BTL and κe is the effective conductivity of the electrolyte (Sharma S, & Jain, V.2002). On the other hand, while using two concentric cylinder electrodes, then the equation assumes that there is uniformity of current all over the length of the electrodes. This assumption is supported by two reasons. First Is being primarily emanating only from the BTL. Secondly, the side current distribution should be uniform. Combining the above two equations, and integrating between the limits r=r1.the resulting equation can be used to predict DAROC (g) =KVKet/FPa.
Conclusion
In conclusion, since the invention of STEM, several improvements have been made to ensure the production of micro-holes as well as good depth to diameter ratio of the holes. Different shapes of the holes have also been produced using this process. Such shapes include but not limited to elliptical and rectangular holes and holes with contoured surfaces (Sharma S, & Jain, V.2002). These improvements have come about as a result of controlling the process parameters carefully. There is also making use of complex tooling and good electrolyte manufacturing. Better operating practices are still need to be in place in order to take care of the environmental safety. The process involves the use of acid electrolyte, which is corrosive and toxic in nature. There is also the generation of toxic fumes in the process of drilling a hole.
Presently, there is progress in a shaped-tube electrolytic polishing process. Despite all these demerits of STEM, it still remains one of the most outstanding methods of drilling. It provides reduced surface roughness of the hole and therefore, smooth holes are produced. (Sharma S, & Jain, V.2002). The STEM process is nowadays mostly used since it is easy and reliable.
References.
Robert, H. & Allen, D, (1994), Manufacturing Processes Reference Guide, New York: Industrial Press Inc.
Sharma S,& Jain, V. (2002).Electrochemical drilling of inconel super alloy with acidified NaCl electrolyte.New York: Industrial Press Inc.
