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2. Four-Roll Bender Configurations and Bending Sequences
Figure 1 shows the configuration of the operational roll and plate specimen of a four-roll horizontal bender. Generally, in such benders the top roll is normally connected to a drive system so that the continuous conveyance of a bent plate can be performed. At this instance the two side rolls and the bottom pinch roll are free to rotate but are powered to translate along their predetermined directions in their respective positional planes. The positional plane of either the pre-bending active or the pre-bending inactive (subsequently referred to as pre-active or pre-inactive, respectively) side roll powered direction depends upon the geometric setting of Xo and -Xo, and also upon the inclination of the preliminary positional angle, ~ and -~o. For a set geometry of a four-roll bender, the positional powering centre, Op (Figures 6 and 7) of the preactive and the preinactive side rolls remains constant. However, the instantaneous contact centre, Oo, and the instantaneous contact angle, ~o, of the bent plate and the active side roll, varies as bending proceeds. When in operation gripping of a plate is performed by moving the axis of the bottom roll centre towards that of the top roll. For a particular fourroll horizontal bender with the top pinch roll rotational driven both clockwise and anticlockwise, the nomination of the pre-active and pre-inactive side roll is arbitrary, unless specific plate handling features are allocated to the bender. To assist further discussion, the r.h.s, side roll is referred to as the pre-active side roll and the I.h.s. side roll as the preinactive side roll. Figure 2 shows the detailed operational sequences for thin plate four-roll bending. In principle, they consist of some or all of the following modes: (i) edge setting preparation, (ii) edge prebending, (iii) pre-bending continuous bending, (iv) roller swapping bending, (v) steady continuous bending and (vi) completion roller bending. The combination of the bending modes depends mainly upon the material flexural rigidity and the bender capacity, and partly upon the size and accuracy required of the finished tubular sections. It sometimes even depends upon the personal preference of the operators.
1. Introduction
Large cylindrical tubular sections are widely used in heavy engineering and civil construction environments. A relatively economical method of production of various sizes of tubes is to weld cylindrical sections bent from plates. In the metal bending industry, one of the most versatile techniques for the production of large diameter tubes is the continuous four-roll plate bending process. The advantage of the technique over the other processes of stampbending, stretch-bending and pressbrake bending, is to reduce the setting up time, to reduce cost in tooling investment, to minimise the length of the straight end-edge remaining in the finished product (subsequently, lowering the machining cost and material wastage), and to achieve much greater finished dimensional accuracy and better circularity of an accomplished cylindrical section. In addition, it only occupies a relatively small space on the shop floor. Generally, the completed tube formed has only a single welded seam. By Data Products Components(HK) Ltd. Sime Darby Industrial Estate Centre, 420 Kwung Tong Rd, Kowloon, Hong Kong.
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M. Hua et aL / Journal o f Materials Processing Technology 48 (1995) 159-172
continuous four-roll plate bending [5] was initiated at Aston University, Birmingham, England. Some preliminary results from bending aluminium plates, with thicknesses of 8mm and
operating a roller bender at its designed capacity, a comprehensive range of tubediameters with a minimum just larger than that of the top roll of the bender, for different plate thicknesses and widths, can be produced. Despite the versatility of the process, there is limited understanding of the bending mechanisms and available literature concerning the process is limited. Although Winshop [1] discussed various configurations of the threeand four-roll plate benders, he did not provide any insight into the bending mechanisms. Roggendorf and Haeusler [2] only traced the historical development of the Haeusler roller benders before 1979, whilst Basset and Johnson [3] and Hansen and Jannerup [4] initiated merely the theoretical analysis for three-roll pyramid benders. Consequently, the operation of the process is heavily dependent on the experience of skilled operators and working by "template" or "trial and error" remains common practice. In order to gain an understanding of the bending process and to introduce C.N.C. operation, a study of Departmentof MechanicalEngineering, Universityof Adelaide,SouthAustralia5005