تعیین آزمایشی ویسکوزیته محیط (سیال) ماشینکاری جریان ساینده Experimental determination of viscosity of abrasive flow machining media

۱ Introduction Abrasive flow machining is a finishing process in which a small quantity of material is removed by flowing semi-solid, abrasive-laden putty (called media) over the machined surface. The media is a rubber-like material of a high viscosity and can be deformed by applying a little pressure. Two vertically opposed cylinders extrude abrasive media back and forth through the passage formed either by the workpiece and tooling, or by the workpiece alone. The semi-solid abrasive media is forced through the workpiece or through the restricted passage formed by the workpiece. Abrasive particles act as cutting tools, resulting in a multi-point cutting process. The material removal rate is quite low. The process is employed for both metals and non-metals. Abrasive flow machining is suitable to automate finish operations that ask for high cost and are labour intensive. It is also employed for finishing operations in aerospace, automotive, semiconductor and medical component industries. Specifically, it is very useful for finishing the surfaces of extrusion dies, nozzles of a flame cutting torch and airfoil surface of an impeller, deburring of aircraft valves bodies and removing of recast layer after electric discharge machining. The characteristics of the media, including its viscosity, abrasive concentration and temperature determine the aggressiveness of action of the abrasives during the abrasive flow machining process. The media used in abrasive flow machining is a pliable material and is resilient enough to act as a self-forming grinding stone when forced through a passageway (Rhoades, 1989). It consists of a base and abrasive grits of two or three sizes. The base material is visco-elastic and made up of an organic polymer and hydrocarbon gel. The composition of the base material determines its degree of stiffness. The stiffest medium is used for small holes. High stiffness of the media results in pure extrusion, while soft media leads to a faster flow in the centre instead of along walls. It is reported that media with a greater stiffness finishes a passageway more uniformly, while a less stiff media results in a greater radius of the passage opening (Williams and Rajurkar, 1989, 1992). Experience with various types of media and abrasives have been reported in the literature (Inasaki et al., 1993; Loveless et al., 1994; Perry, 1989). There is clear evidence that the viscosity of the media plays a very important role in abrasive flow machining. While limited attempts have been reported in the literature (Davies and Fletcher, 1995; Fletcher et al., 1990; Hall et al., 1992), a systematic approach towards the evaluation of the viscosity of media has not been addressed. In an earlier study (Jain et al., 2001), the authors reported the development of a capillary viscometer for viscosity measurement of the AFM media. The advantage of the design was its simplicity, specifically the possibility of making measurements at steady state. However, it had two drawbacks: Firstly, the instrument was bulky, requiring a large amount of material. Secondly, the static load required to maintain flow was quite large, being in the range of 100–۱۵۰ kg. Thirdly, traces of oil in the media sample led to considerable scatter in the measured data. The apparatus developed in the present work requires smaller samples and smaller loads, but the technique relies on the temporal response of the media, and hence requires measurements as a function of time. The compactness of the apparatus has an inherent advantage when the viscosity is to be obtained as a function of bulk temperature. The main objectives of the present work can now be summarised as follows: • design and fabrication of a set-up for viscosity measurement of the abrasive media • study changes in viscosity with abrasive concentration and temperature • compare the results obtained with those from a capillary viscometer.