the processing time, 4) the pressure drops, and

5) the temperature.

Furthermore it was evident that there was a shear history effect associated with the usage of certain media. The results indicating such an effect are discussed in section 7 .2.8.

7.1 Preliminary Work

The preliminary results as detailed in sections 7 .1 .1 , 7 .1 .2 , and 7 .1 .3 consist of SEM photomicrographs and graphical representation of (i) the effect of reusing media on the stock removal and surface roughness improvement parameters, and (ii) the effect of repeated medium use on the amount of abrasive grit wear.

7.1.1 AFM Surface Features

SEM photomicrographs were taken of a castle nut, figure 25, and a bearing component, figure 26. The latter was provided in both the unmachined and machined condition which enabled a comparison to be made of the surface topography both before and after AFM processing.

7.1.1.1 Castle Nut Component

The machining objective regarding this component was as an edge deburring operation. This is shown in figure 27(a) and illustrates an inner spur corner that has been deburred by the AFM process. Figure 27(b) depicts a section of the previous figure at a greater magnification and illustrates both the effect the abrasive grit additions have on the surface condition and their direction of flow. Figures 28(a) and 28(b) focus on tw o of the limitations of the AFM process, i.e. the inability to remove inclusions and voids without significantly affecting the component's dimensions. The selective machining capability of the AFM process is shown in figures 29(a) and 29(b), which illustrate the removal of conventional machining marks on different surface locations of the same spur. The ploughing effect of the grit particles can also be seen in figure 30.

7 .1 .1 .2 Bearing Component

The processing of this component required the use of complex tooling since an apex was required to be machined on the flat areas in between the channels, as illustrated in figures 31(a) and 31(b). Figures 32(a) and 32(b), depict the

component surface topography both before and after machining, and illustrate the uni-directional surface pattern generated by the machining process. Figure 33 shows the surfaces on either side of the machined apex and further illustrates the surface pattern generated by the process. The selective machining of one side of the channel wall is shown in figures 34(a) and 34(b) which illustrate the surface characteristics both before and after machining.

7 .1 .2 Medium Reuse

The parameters used to determine the effect of reusing medium on the machining process were the die diameter change, die mass change and surface roughness improvement. All of these parameters are graphically represented in such a way that the results obtained from the three sets of experiments performed, to determine the parameter under consideration, are presented on a single graph. These three sets of experiments, as detailed in section 6 .1 .1 , were such that experiment one was conducted using fresh medium and experiment tw o involved the repetition of experiment one reusing the medium, whilst experiment three consisted of limited repetitions reusing the same medium.

Figures 35 and 36 show the effect of increasing the number of extrusion cycles on the amount of stock removed in the former case with respect to the die diameter and in the latter case with respect to the die mass. Similar results relating to both sets of parameters were obtained that indicated as the number of extrusion cycles increased so the amount of stock removed increased. This trend

was observed even when the same medium was reused during a repeat

experiment. However, it should be noted that there was a decrease in the amount of stock removed the more the medium was reused. This difference was more pronounced as the cumulative number of extrusion cycles increased, and was most evident after the run that consisted of 20 extrusion cycles. In the cases of experiments one and tw o the difference between the respective die diameter measurements after 20 extrusion cycles had been conducted, i.e. 56 and 297 cumulative extrusion cycles respectively, was approximately 0.0080m m .

However, after 70 extrusion cycles, i.e. 241 and 4 82 cumulative extrusion cycles, the difference between the respective die diameter measurements had increased to approximately 0.1900m m .

The effect of increasing the number of extrusion cycles on the surface roughness improvement is shown in figure 37. The results in all cases illustrated that the main improvement in surface roughness was achieved within a limited number of extrusion cycles, afterwhich minimal improvement was achieved. The results from the medium used to conduct the first experiment showed an approximate 4.3//m improvement in the surface roughness after 15 extrusion cycles, afterwhich there was no significant change in this value. Furthermore the majority of the

improvement occurred within the first 5 extrusion cycles. The results of the second experiment showed an identical trend to the first; however the surface roughness improvement achieved was 4/ym, which required 20 extrusion cycles, afterwhich there was minimal improvement. The improvement attained by the third experiment was approximately 3.2//m and was achieved after 10 extrusion cycles; thereafter no significant improvement was attained.

7 .1 .3 Abrasive Grit Wear

These results relate to the tw o assessment methods as detailed in section 6 .1 .2 , viz: