The guillotine cutting process is also one of the most blanking processes used in the metalworking industry. To get the final product, raw materials need to pass through many processes, being usually the guillotine cutting one of them. This cutting process can be performed manually or automatically and can be integrated as an initial, inter- mediate or finishing step  [17]. The guillotine cutting process   principle,

⁎ Corresponding author.

E-mail address:  fgs@isep.ipp.pt (F.J.G.  Silva).

http://dx.doi.org/10.1016/j.rcim.2016.06.006

Received 26 November 2015; Received in revised form 23 June 2016; Accepted 30 June 2016

0736-5845/ © 2016 Elsevier Ltd. All rights  reserved.

as shown in Fig. 1, consists of positioning the plate between a fixed and a movable blade, which downwards movement penetrates the plate and, when it exceeds the shear tensile strength, the plate is cut.

This cutting principle is transverse to the different types of shearing machines, although the handling characteristics of the blade directly influence the final cut quality. Although the surface quality of guillotine cutting cannot compete with machined ones, this is the most economic- al cutting method to obtain straight shapes   [19,20].

The guillotining process has some typical associated defects, most of them related to frame distortion, blades gap or incorrect cutting angle regulation. Anyway, most of these problems are already fixed or attenuated with some devices already provided by manufacturers as option. It is well known that cut surfaces with higher quality will avoid subsequent finishing operations, as well as that cutting accuracy is different for the plate that remains in the table and the cut strip which, not being hold or fixed by hold down jacks, tends to bend or twist during the cutting process, originating defects as shown in Fig.    2.

Bow is a cutting typical defect resulting from the progressive action of the movable blade in the cutting process. The cut strip is being separated without support, bending under its own weight [20]. The strip bow becomes more pronounced the smaller the cut width and the greater the cut angle. However, reducing the cutting angle can minimize but not completely eliminate the bow [19]. Twist is a defect described as the tendency to roll the cut material trendy a spiral shape. High cutting angles are usually associated to torsion defect, which also results from sheet metal internal stresses. This effect is more pro- nounced in narrow strips, being the last resistant strip-section the one that more easily attains permanent deformation [20]. Camber is a defect resulting from the strip separation, being caused essentially due to  material internal  stresses [19,21,22].

2. Methodology and results

This section is pided into three subsections: firstly, the problem is identified, followed by the initially designed solutions to eliminate it and, finally, the adopted solution to overcome the problem is pre- sented. As previously mentioned, the shearing process has a few typical limitations, but some of them are already solved by shearing machines manufacturers. The problem affects customers that use the guillotine to cut thin plates provided with high length. The second subsection will deal with the main initial ideas thought to eliminate the problem, with some solutions but only one presenting the best cost - benefits relation. Thus, in the third subsection, the adopted solution is described, together with some changes from the initial to the final design.

2.1. The problem

During the thin plate cutting process a very specific problem was detected, which occurs when cutting sheets with smaller thickness than 3 mm and length higher than 700 mm. Because the cutting process is performed with a programmed blade slope, this leads to warpage in the latter cut sheet portion (Fig. 3a), induced by the weight  of  the sheet already cut, which results in high bending stresses at the small area that is not yet cut, as illustrated in Fig. 3b. This occurrence prevents to include the guillotine directly in a processing line because straightening operations are mandatory before sheets pass to another processing step. This weakness is a major embarrassment and annoy- ance when constantly cutting plates with the aforementioned dimen- sional characteristics, leading to a reduction in productivity. Such defects have been reduced with some accessories available on the market. However, the complete elimination of this kind of defect is just expected by the integration of a dynamic holder during the cutting process.