A novel dynamic holding system for thin metal plateshearing machines(2).pdf

Contents lists available at ScienceDirectRobotics and ComputerIntegrated Manufacturingjournal homepage: /locate/rcimA novel dynamic holding system for thin metal plate shearing machinesL.M.B. Arajo, F.J.G. Silva, R.D.S.G. Campilho, J.A. MatosISEP School of Engineering, Polytechnic of Porto, Rua Dr. Antnio Bernard ino de Almeida, 431, 4200-072 Porto, PortugalARTICLE INFOKeywords:Shear plate machinesCutting processesAggregate toolsSheet metal cuttingDesign for sustainabilityABSTRACTMetal shearing machines are heavy equipments usually linked to low added-value due to the small amount oftechnological devices incorporated. However, this situation can be changed through equipment designerscreativity. Analysing some specic operations, it can be observed that some tools, when coupled to theequipment, should substantially increase the cutting process productivity and the nal product quality.Regarding the thin metal plates shear process, it can be found that in the cutting nal stage, the cut materialweight is suspended by a small material section still requiring to be cut. This leads to strip tip deformation,causing poor quality of the nal product, which cannot stay fully plan. This work was developed around thisproblem, studying the best solution to develop a new tool able to avoid the lack of plateatness after cut. A novelequipment was designed, able to be easily connected to the shearing machine, following the blade movementthroughout cut operation. The system is fully-automated, being operated by a single cut instruction given by themachine operator. This system allows the manufacturing company to increase the added-value of each machine,oering advanced and desirable solutions to the customers, and contributing as well to the company businesssustainability.1. IntroductionAlthough being an old technology, blanking remains nowadays oneof the most used cutting processes in metalworking industry 1. Thistechnology was study many year ago but, recent developments bringsnew challenges such as adapting nite elements model to thesetechnologies in order to predict new materials behaviour, explore theprocesses capacity having as focus to increase the production rate andimplement new devices leading to a quality improvement regarding thenew market requests and expectations. Accordingly, several studieshave been carried out by dierent authors regarding, namely in theeld of the parameters improvement, with Breitling et al. 2 exploringthe competitiveness of the process and studying the blanking speed,concluding that blanking force drops with the increase of the blankingspeed within the range of 14ms1, results whose were corroboratedby Goijaerts et al. 3 within the range of 0.011000 mm s1. Furtherstudies have been carried out by Neugebauer et al. 4 andSubramonian et al. 5 concluding that dynamic eect is higher whenblanking speed increases, using a high speed mechanical press.Mackensen et al. 6 studied the punch inclination angle, concludingthat cutting forces are lower when punch inclination angle rises.However, higher punch inclination angles lead to blank curling. Asreferred by other authors 79, a better geometrical accuracy of theblanked part can be achieved by an optimization of the punch shapeand die clearance. However, it remains clear that parameters such astool wear state, clearance, tool radii and geometry, material geometry,sheet metal thickness, friction, relevant material properties regardingthe cutting (ductility and hardness), sheet metal coating, lubricationuse, stroke rate and blanking speed are the key-factors aecting thesheared edge aspect 10. Thermal eects have been also intensivelystudied in order to correlate the temperature with the blanking force1113 leading to realize that blanking forces fall when materialtemperature upsurges. These studies include pre-heating processes inorder to lead the material temperature to diverse levels and measurethe needed blanking force to cut dierent materials. Many other studieshave been carried out in the analytical and numerical methods eld,trying to get reliable models helping researchers to predict blankingoperations eect regarding dierent materials and cutting conditionswhose are summarized in 10,1416. However, despite these strongeorts, the blanking cutting process still remains an attractive issue toinvestigate due to shear band formed narrowness and the lack of anappropriate fracture criterion.The guillotine cutting process is also one of the most blankingprocesses used in the metalworking industry. To get the nal product,raw materials need to pass through many processes, being usually theguillotine cutting one of them. This cutting process can be performedmanually or automatically and can be integrated as an initial, inter-mediate or nishing step 17. The guillotine cutting process principle,/10.1016/j.rcim.2016.06.006Received 26 November 2015; Received in revised form 23 June 2016; Accepted 30 June 2016Corresponding author.E-mail address: fgsisep.ipp.pt (F.J.G. Silva).Robotics and Computer-Integrated Manufacturing 44 (2017) 242252Available online 15 October 20160736-5845/ 2016 Elsevier Ltd. All rights reserved.MARKas shown in Fig. 1, consists of positioning the plate between a xed anda movable blade, which downwards movement penetrates the plateand, when it exceeds the shear tensile strength, the plate is cut.This cutting principle is transverse to the dierent types of shearingmachines, although the handling characteristics of the blade directlyinuence the nal cut quality. Although the surface quality of guillotinecutting 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, mostof them related to frame distortion, blades gap or incorrect cuttingangle regulation. Anyway, most of these problems are already xed orattenuated with some devices already provided by manufacturers asoption. It is well known that cut surfaces with higher quality will avoidsubsequent nishing operations, as well as that cutting accuracy isdierent for the plate that remains in the table and the cut strip which,not being hold or xed by hold down jacks, tends to bend or twistduring the cutting process, originating defects as shown in Fig. 2.Bow is a cutting typical defect resulting from the progressive actionof the movable blade in the cutting process. The cut strip is beingseparated without support, bending under its own weight 20. Thestrip bow becomes more pronounced the smaller the cut width and thegreater the cut angle. However, reducing the cutting angle canminimize but not completely eliminate the bow 19. Twist is a defectdescribed as the tendency to roll the cut material trendy a spiral shape.High cutting angles are usually associated to torsion defect, which alsoresults from sheet metal internal stresses. This eect is more pro-nounced in narrow strips, being the last resistant strip-section the onethat more easily attains permanent deformation 20. Camber is adefect resulting from the strip separation, being caused essentially dueto material internal stresses 19,21,22.2. Methodology and resultsThis section is divided into three subsections: rstly, the problem isidentied, followed by the initially designed solutions to eliminate itand, nally, the adopted solution to overcome the problem is pre-sented. As previously mentioned, the shearing process has a few typicallimitations, but some of them are already solved by shearing machinesmanufacturers. The problem aects customers that use the guillotine tocut thin plates provided with high length. The second subsection willdeal with the main initial ideas thought to eliminate the problem, withsome solutions but only one presenting the best cost - benets relation.Thus, in the third subsection, the adopted solution is described,together with some changes from the initial to the nal design.2.1. The problemDuring the thin plate cutting process a very specic problem wasdetected, which occurs when cutting sheets with smaller thickness than3 mm and length higher than 700 mm. Because the cutting process isperformed with a programmed blade slope, this leads to warpage in thelatter cut sheet portion (Fig. 3a), induced by the weight of thesheet already cut, which results in high bending stresses at the smallarea that is not yet cut, as illustrated in Fig. 3b. This occurrenceprevents to include the guillotine directly in a processing line becausestraightening operations are mandatory before sheets pass to anotherprocessing 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. Suchdefects have been reduced with some accessories available on themarket. However, the complete elimination of this kind of defect is justexpected by the integration of a dynamic holder during the cuttingprocess.2.2. ApproachFig. 4 shows the 3D model of the conceived platform, able to beassembled on the guillotine structure, which allows giving a real visionabout what can be expected and embodies a good way to detect issuesto be improved. The platform consists of a frame held by fourpneumatic cylinders, in which a guidance system enables the structureto move up and down, as well as to lean, but always without performinghorizontal movements that would result in hitting the guillotines mainframe.The main goal of this device is to be compatible with the newguillotines in production and with most of the guillotines already on themarket, allowing to easily adapt this device to them.2.3. Methodology2.3.1. Dynamic holderThe dynamic holder will support the cut strip during the cuttingprocess and the rst design was a simple plate provided withreinforcement in the middle and square pipes on the top, but thisstructure showed to be very heavy to handle in maintenance operationsand cylinders check, requiring the removal of the whole holding systemfor repairing and checking tasks (Fig. 5a). The modular chassisincludes two removable plates around the central one (Fig. 5b), gainingaccess to the cylinders and decreasing the structures weight, thusfacilitating the assembly and maintenance procedures. The removableplates fastening system is constituted by countersunk bolts and nuts.As the space between holder and ground in the holder rest position hasrestricted access, the nuts were welded to facilitate the cover removaloperation.2.3.2. Guiding systemFirstly, it is necessary to explain why this system needs a guide. InFig. 6 it is pointed out with green arrows the intended freedom degrees,while the red crosses indicate the restricted movements promoted bythe guiding system.The rst developed design is shown in Fig. 7 and the workingprinciple was based on two sets of two positioning bearings at each sideof the holder. Bearing A limits the horizontal movements while bearingB keeps the holding system in the correct vertical path.Hence, at this stage the system needs to be improved because it isreally hard to fabricate and the nal cost will be higher than desired.The design was rethought and now it will consist of applying a centralguide below the holder, using a rod end bearing to keep the holdingsystem in the correct position, giving as well the necessary freedom tothe system tilting movement. The rod end bearing allows the inclina-tion during the cutting process and rotation to extract the strip or keepthe holding system in the rest position. A weak point of this new systemis related to the square shaft and bushing responsible for the verticalmovement, which are dicult to manufacture. Thus, the square shaftFig. 1. Guillotine cutting process scheme 18.L.M.B. Arajo et al. Robotics and Computer-Integrated Manufacturing 44 (2017) 242252243and bushing were replaced by a linear guide. This change makes thissystem completely standard and the nal result is an easy to manu-facture system in which all the components can be easily replacedFig. . Drive systemThe driving system is one of the most important parts of this projectbecause it represents the biggest associated cost. Thus, to choose thecorrect one it is necessary to focus on the required positions:. Upper position. In this position the dynamic holding systemis at the same level as the guillotine table, ensuring the horizontal plateposition and the correct measurement of the strip to be cut, using theback gauge system. After positioning, the hold down jacks immobilizethe plate, and the back gauge automatically retreats 100 mm,preventing the stress generated by the contact between strip andback gauge during the cutting process.. Intermediate position. Starting the cut, a command is givenfor the cylinders to recede from the platform edge at the right position,resulting in the holders tilt. Thousandths of a second later, the sameorder is given to the cylinders at the opposite side. The cylindersmovement coordination is crucial in order to keep the holding systemclose to the strip cut point at each moment.. Discharge position. After the strip has been cut, the two innercylinders are driven to give as much tilt as possible, leading to the stripdischarge operation.. Rest position. This position corresponds to the nal stage ofthe cycle, being used as well when the sheet thickness exceeds 5 mm,since the pneumatic components are only designed to handle thestructure and plates up to 5 mm thickness. It was considered necessaryto implement a structure that allows optimizing the unit, makingpossible the cut for thicknesses higher than 5 mm. Therefore, it wasdecided to include two longitudinal reinforcements that absorb theimpact loads of the strips exceeding 5 mm thickness dropping on theholder. Other established requirements are related to the very connedspace to place the pneumatic cylinders and that components priceshould be as low as possible. After a long and careful market survey, thebest options found are presented as follows.Fig. 3. Traditional defect (a) and high stress concentration (red zone) (b) (For interpretation of the references to color in this gure, the reader is referred to the web version of thisarticle).Fig. 2. Typical defects on guillotine cutting process 19.L.M.B. Arajo et al. Robotics and Computer-Integrated Manufacturing 44 (2017) 2422522442.3.4. Option 1 conventional cylindersThese cylinders present as advantage the high perpendicularresistance to the axis loads, adjustable pneumatic damping and canbe easily found in the market. However, to use this solution it isnecessary to use a “Multi-Position Kit”. This kit will dock the twocylinders coaxially, allowing four possible positions: a fall-back positionand three forward positions, as shown in Fig. 9. The great advantage ofthese cylinders is their price.2.3.5. Option 2 - servo-pneumatic cylindersThis option satises the largest number of requirements initiallydrawn for this project. These cylinders have the particularity to ensuregreater positioning accuracy and enable stopping in several positionsalong its path, contrary to conventional cylinders, which only allow theretreat and forward positions. These are standard cylinders with apositioning control, Fig. 10, which let a position accuracy of around 0.2 mm 24, thus ensuring the exact positioning of the holder at thedesired position. The position versatility that these cylinders can oer,allows an exact following of the holding system during cutting processindependently of cutting angle and this is undoubtedly a hugeadvantage. However, this system has a drawback: their high acquisitioncost. Therefore, solutions must be found that could meet the needs,taking into account the system and budgeting constraints.Regarding these two driving system possibilities, it is possible tohave at least two dierent manufacturing options. Figs. 11 and 12presents the general view of the dierent driving systems. Due to thelongest length of the servo cylinders, their assembly needs to respectthe maximum length available. Thus, their position had to be con-veniently studied to get the cylinders holders into the space available(Fig. 11). The conventional cylinders have a lower length, allowingtheir assembly in parallel to the lateral faces of the machine body(Fig. 12).2.4. ResultsBased on the initial conditions and all work done, the holdingsystem can be moved as follows:Solution 1 - conventional cylinders;Solution 2 - servo-pneumatic cylinders;Solution 3 - mixed version.2.4.1. Option 1 - conventional cylindersThis version consists of using four groups of two pneumaticcylinders assembled with “Multi-Position Kit”. The operation principleis the following: when the system is activated, all cylinders areextended to put the platform in the upper position, allowing achievesthe correct plate placement (Fig. 13).Starting the cutting process, the cylinders of the side where theblade starts to cut begin to retract (Fig. 14).In order to accurately monitor the slope of the blade, the cylinderson the other side also retract, but slowly, allowing that the Z position ofthe holding system is in line with the point where the blade is cuttingthe sheet metal at each moment. At the end of the cutting process, theplatform assumes a horizontal position and each cylinder assembly isretracted (Fig. 15).If it is necessary to continue to cut plates with a smaller thicknessthan 5 mm, the cylinders that support the front part of the dynamicholding system retract, causing the platform tilting and discharging thecut plate (Fig. 16).Fig. 5. Dynamic holding system evolution: (a) First and (b) nal design.Fig. 4. Isometric view (a) and exploded view (b) of preliminary holder version.L.M.B. Arajo et al. Robotics and Computer-Integrated Manufacturing 44 (2017) 242252245After a few seconds, all cylinders are actuated aga