The study of the shearing behaviour of fine blanking and conventional blanking processes

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The study of the shearing behaviour of fine blanking and conventional blanking processes


Author: Tam, Kam-hay
Title: The study of the shearing behaviour of fine blanking and conventional blanking processes
Degree: M.Sc.
Year: 1995
Subject: Blanking (Metal-work)
Shears (Machine-tools)
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Pages: viii, 125 leaves : ill. ; 30 cm
Language: English
Abstract: This dissertation is prepared to identify the shearing behaviour, in terms of blank quality and surfaces hardness, between the fine blanking and conventional blanking process under different blanking conditions. Materials used in this study is cold rolled mild steel sheet (SPCC), 4.5 mm in thickness. Blanked samples of 48 mm diameter were obtained from both the conventional and fine blanking setup, however, under different blanking conditions. For conventional blanking, 5 different cutting clearances, ie. 3%, 5%, 7%, 10%, and 12%, were selected whilst for fine blanking, the cutting clearances of 0.3%, 0.5%, 0.7% and 1% were chosen. From this study, all samples produced from fine blanking process demonstrated a 100% sheared surface with minimum burrs at the underside as well as relatively small flatness deviation on the blank surfaces. The cut edge was straight, neat and clean with the proof that secondary finishing process, normally, would not be needed. On the other hand, samples from the conventional blanking exhibited a partly sheared and partly fractured finish. The amounts of the sheared surface and the fractured surface were varied in relation to the cutting clearances employed, ie. the larger the cutting clearances employed, the larger was the fractured finish and smaller the sheared finish. The presence of the fractured surfaces gave a dull surface finish and larger angular error as well as burrs to underside of the samples. Flatness deviation of the samples was of larger amount when compared to that of the fine blanked samples and, again, it was found to bear a direct relationship to the cutting clearances. The results of the above blank quality assessment parameters, finally, could draw up a conclusion that the surface finishing of the fine blanking samples was FAR better than that produced from the conventional blanking process and that the cutting clearance was a dominant factor to the shearing finish of the conventional blanking process. Apart from the physical changes of the blank quality, differences in surface hardness of the samples were also identified in this study. Samples from both the conventional and fine blanking process were showing the same signs that the change in hardness was maximum at the point closest to the cutting edge and it decreased with the increase of the distance from the cutting edge. Owing to the fact that higher compressive stress was encountered in the fine blanking process, the maximum increase in hardness of the fine blanked samples was 1.9 of its original hardness whereas the conventional blanked samples had a maximum hardness ratio of 1.5 only. In the investigation of the change in hardness with respect to the cutting depth, it was found that maximum hardness ratio of the fine blanked samples was attained when the cutting depth was at about 70% of the material thickness. On the other hand, the maximum hardness ratio of the conventional blanking samples was limited to the first 35% of the cutting depth. Along with that, for both the conventional and fine blanking process, the relationship of cutting clearances with the change in hardness was not obvious. Nevertheless, the vee-ring pressure was found having a direct effect on the change in hardness such that the hardness ratio was increased with the increasing vee-ring pressure up to 25 tons. However, the increase in hardness was found to be stagnant and level off with further increase of the vee-ring pressure. The effects of the vee-ring projection of the pressure plate was also included in the study. With the missing of the vee-ring projection, the compressive stress of the material was reduced and, in return, the ability of the material to withstand a very high deformation during the shearing operation was greatly weakened. As a result of that, full sheared surface could not be substantiated and fracturing was inevitable. It was found that the sheared surfaces of the samples produced without the application of the vee-ring projection were clearly separated by a fracture band with a width of about 30% of the material thickness. This fractured band was distinguished with dull and rough finish. Very large burr formation, up to 20% of the material thickness, was another characteristics of these samples. With the presence of the fractured band and such a large amount of burr, secondary or even third finishing processes would definitely be required. Change in hardness behaviour of these samples, however, was about the same or that from conventional blanking with maximum increase in hardness of about 150% of its original hardness and was limited to about 35% of the cutting depth.

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