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附錄一
A novel dynamic holding system for thin metal plate shearing machines
L.M.B. Araújo, F.J.G. Silva , R.D.S.G. Campilho, J.A. Matos
ABSTRACT
Metal shearing machines are heavy equipments usually linked to low added-value due to the small amount of technological devices incorporated. However, this situation can be changed through equipment designers' creativity. Analysing some specific operations, it can be observed that some tools, when coupled to the equipment, should substantially increase the cutting process productivity and the final product quality. Regarding the thin metal plates shear process, it can be found that in the cutting final stage, the cut material weight is suspended by a small material section still requiring to be cut. This leads to strip tip deformation, causing poor quality of the final product, which cannot stay fully plan. This work was developed around this problem, studying the best solution to develop a new tool able to avoid the lack of plate flatness after cut. A novel equipment was designed, able to be easily connected to the shearing machine, following the blade movement throughout cut operation. The system is fully-automated, being operated by a single cut instruction given by the machine operator. This system allows the manufacturing company to increase the added-value of each machine, o?ering advanced and desirable solutions to the customers, and contributing as well to the company business sustainability.
1. Introduction
Although being an old technology, blanking remains nowadays one of the most used cutting processes in metalworking industry [1]. This technology was study many year ago but, recent developments brings new challenges such as adapting finite elements model to these technologies in order to predict new materials behaviour, explore the processes capacity having as focus to increase the production rate and implement new devices leading to a quality improvement regarding the new market requests and expectations. Accordingly, several studies have been carried out by di?erent authors regarding, namely in the field of the parameters improvement, with Breitling et al. [2] exploring the competitiveness of the process and studying the blanking speed, concluding that blanking force drops with the increase of the blanking speed within the range of 1–4 m s?1, results whose were corroborated by Goijaerts et al. [3] within the range of 0.01–1000 mm s?1. Further studies have been carried out by Neugebauer et al. [4] and Subramonian et al. [5] concluding that dynamic e?ect is higher when blanking speed increases, using a high speed mechanical press. Mackensen et al. [6] studied the punch inclination angle, concluding that cutting forces are lower when punch inclination angle rises. However, higher punch inclination angles lead to blank curling. As referred by other authors [7–9], a better geometrical accuracy of the blanked part can be achieved by an optimization of the punch shape and die clearance. However, it remains clear that parameters such as tool wear state, clearance, tool radii and geometry, material geometry, sheet metal thickness, friction, relevant material properties regarding the cutting (ductility and hardness), sheet metal coating, lubrication use, stroke rate and blanking speed are the key-factors a?ecting the sheared edge aspect [10]. Thermal e?ects have been also intensively studied in order to correlate the temperature with the blanking force [11–13] leading to realize that blanking forces fall when material temperature upsurges. These studies include pre-heating processes in order to lead the material temperature to diverse levels and measure the needed blanking force to cut di?erent materials. Many other studies have been carried out in the analytical and numerical methods field, trying to get reliable models helping researchers to predict blanking operations e?ect regarding di?erent materials and cutting conditions whose are summarized in [10,14–16]. However, despite these strong e?orts, the blanking cutting process still remains an attractive issue to investigate due to shear band formed narrowness and the lack of an appropriate fracture criterion.
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,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 di?erent 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 di?erent 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 e?ect 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 divided 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 a?ects 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.
2.2. Approach
Fig. 4 shows the 3D model of the conceived platform, able to be assembled on the guillotine structure, which allows giving a real vision about what can be expected and embodies a good way to detect issues to be improved. The platform consists of a frame held by four pneumatic cylinders, in which a guidance system enables the structure to move up and down, as well as to lean, but always without performing horizontal movements that would result in hitting the guillotine's main frame.
The main goal of this device is to be compatible with the new guillotines in production and with most of the guillotines already on the market, allowing to easily adapt this device to them.
2.3. Methodology
2.3.1. Dynamic holder
The dynamic holder will support the cut strip during the cutting process and the first design was a simple plate provided with reinforcement in the middle and square pipes on the top, but this structure showed to be very heavy to handle in maintenance operations and cylinders check, requiring the removal of the whole holding system for repairing and checking tasks (Fig. 5a). The modular chassis includes two removable plates around the central one (Fig. 5b), gaining access to the cylinders and decreasing the structure's weight, thus facilitating the assembly and maintenance procedures. The removable plates fastening system is constituted by countersunk bolts and nuts. As the space between holder and ground in the holder rest position has restricted access, the nuts were welded to facilitate the cover removal operation.
2.3.2. Guiding system
Firstly, it is necessary to explain why this system needs a guide. In Fig. 6 it is pointed out with green arrows the intended freedom degrees, while the red crosses indicate the restricted movements promoted by the guiding system.
The first developed design is shown in Fig. 7 and the working principle was based on two sets of two positioning bearings at each side of the holder. Bearing A limits the horizontal movements while bearing B keeps the holding system in the correct vertical path.
Hence, at this stage the system needs to be improved because it is really hard to fabricate and the final cost will be higher than desired. The design was rethought and now it will consist of applying a central guide below the holder, using a rod end bearing to keep the holding system in the correct position, giving as well the necessary freedom to the system tilting movement. The rod end bearing allows the inclina-tion during the cutting process and rotation to extract the strip or keep the holding system in the rest position. A weak point of this new system is related to the square shaft and bushing responsible for the vertical movement, which are di?cult to manufacture. Thus, the square shaft and bushing were replaced by a linear guide. This change makes this system completely standard and the final result is an easy to manu-facture system in which all the components can be easily replaced Fig. 8.
Fig. 1. Guillotine cutting process scheme [18]
Fig. 2. Typical defects on guillotine cutting process [19].
2.3.3. Drive system
The driving system is one of the most important parts of this project because it represents the biggest associated cost. Thus, to choose the correct one it is necessary to focus on the required positions:
2.3.3.1. Upper position. In this position the dynamic holding system is at the same level as the guillotine table, ensuring the horizontal plate position and the correct measurement of the strip to be cut, using the back gauge system. After positioning, the hold down jacks immobilize the plate, and the back gauge automatically retreats 100 mm, preventing the stress generated by the contact between strip and back gauge during the cutting process.
Intermediate position. Starting the cut, a command is given for the cylinders to recede from the platform edge at the right position, resulting in the holder's tilt. Thousandths of a second later, the same order is given to the cylinders at the opposite side. The cylindersmovement coordination is crucial in order to keep the holding system close to the strip cut point at each moment.
2.3.3.3. Discharge position. After the strip has been cut, the two inner cylinders are driven to give as much tilt as possible, leading to the strip discharge operation.
Rest position. This position corresponds to the final stage of the cycle, being used as well when the sheet thickness exceeds 5 mm, since the pneumatic components are only designed to handle the structure and plates up to 5 mm thickness. It was considered necessary to implement a structure that allows optimizing the unit, making possible the cut for thicknesses higher than 5 mm. Therefore, it was decided to include two longitudinal reinforcements that absorb the impact loads of the strips exceeding 5 mm thickness dropping on the holder. Other established requirements are related to the very confined space to place the pneumatic cylinders and that components price should be as low as possible. After a long and careful market survey, the best 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 figure, the reader is referred to the web version of this article).
.
Fig. 4. Isometric view (a) and exploded view (b) of preliminary holder version.
2.3.4. Option 1 – conventional cylinders
These cylinders present as advantage the high perpendicular resistance to the axis loads, adjustable pneumatic damping and can be easily found in the market. However, to use this solution it is necessary to use a “Multi-Position Kit”. This kit will dock the two cylinders coaxially, allowing four possible positions: a fall-back position and three forward positions, as shown in Fig. 9. The great advantage of these cylinders is their price.
2.3.5. Option 2 - servo-pneumatic cylinders
This option satisfies the largest number of requirements initially drawn for this project. These cylinders have the particularity to ensure greater positioning accuracy and enable stopping in several positions along its path, contrary to conventional cylinders, which only allow the retreat and forward positions. These are standard cylinders with a positioning control, Fig. 10, which let a position accuracy of around ± 0.2 mm [24], thus ensuring the exact positioning of the holder at the desired position. The position versatility that these cylinders can o?er, allows an exact following of the holding system during cutting process independently of cutting angle and this is undoubtedly a huge advantage. However, this system has a drawback: their high acquisition cost. 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 to have at least two di?erent manufacturing options. Figs. 11 and 12 presents the general view of the di?erent driving systems. Due to the longest length of the servo cylinders, their assembly needs to respect the maximum length available. Thus, their position had to be con-veniently studied to get the cylinder's holders into the space available (Fig. 11). The conventional cylinders have a lower length, allowing
their assembly in parallel to the lateral faces of the machine body (Fig. 12).
2.4. Results
Based on the initial conditions and all work done, the holding system can be moved as follows:
? Solution 1 - conventional cylinders;
? Solution 2 - servo-pneumatic cylinders;
? Solution 3 - mixed version.
2.4.1. Option 1 - conventional cylinders
This version consists of using four groups of two pneumatic cylinders assembled with “Multi-Position Kit”. The operation principle is the following: when the system is activated, all cylinders are extended to put the platform in the upper position, allowing achieves the correct plate placement (Fig. 13).
Starting the cutting process, the cylinders of the side where the blade starts to cut begin to retract (Fig. 14).
In order to accurately monitor the slope of the blade, the cylinders on the other side also retract, but slowly, allowing that the Z position of the holding system is in line with the point where the blade is cutting the sheet metal at each moment. At the end of the cutting process, the platform assumes a horizontal position and each cylinder assembly is retracted (Fig. 15).
If it is necessary to continue to cut plates with a smaller thickness than 5 mm, the cylinders that support the front part of the dynamic holding system retract, causing the platform tilting and discharging the cut plate (Fig. 16).
Fig. 5. Dynamic holding system evolution: (a) First and (b) final design.
Fig. 6. Intended freedom degrees and restricted movements on the system. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article).
Fig. 7. First approach of the guiding system.
Fig. 8. (a) Second approach of the guiding system and (b) final concept.
After a few seconds, all cylinders are actuated again putting the holding system in the upper position. Otherwise, all cylinders recede to put the holding system in the rest position (Fig. 17).
The great advantage of this option is that it uses conventional
cylinders that are easily found in the market at reasonable prices. As disadvantages, they present a very limited operation, since they only consent two positions, extended or retracted, not allowing control intermediate positions. In this case, a careful adjustment is also needed
Fig. 9. (a) Assembly of two cylinders with multi-position kit and (b) possible assembly positions [23].
Fig. 10. Servo-pneumatic cylinder DNCI/DDPC provided by FESTO [24].
Fig. 11. Pneumatic servo cylinders positioning.
Fig. 12. Conventional cylinders positioning.
Fig. 13. Conventional cylinder – upper position.
Fig. 14. Conventional cylinder – intermediate position.
Fig. 15. Conventional cylinder – final cutting stage.
in order to match the cylinders’ movement and the holder position with the descending blade and the cutting point.
2.4.2. Option 2 – servo-pneumatic cylinders
The operation principle is similar to that described in option 1. However, this option uses four servo-pneumatic cylinders. In the upper position, all cylinders are extended (Fig. 18).
Starting the cutting process, the cylinders at the cutting side start to retract, following the angle of the blade (Fig. 19).
At the end of the sheet metal cutting process, cylinders put the holding system in a horizontal position, slightly below the blade (Fig. 20).
If the operator wishes to continue cutting sheets smaller than 5 mm thickness, the cylinders that support the holding system in the frontretract, leading to the platform tilt, which in turn leads to the sheet discharge by gravity (Fig. 21).
If it is necessary to turn o? the guillotine or to cut plates higher than 5 mm thickness, all cylinders retract, placing the platform in the rest position (Fig. 22).
This system presents many benefits such as stop at intermediate positions or controlling the holder movement speed between stages. Other advantage of this option is the frontal discharge capability, because it is possible to give the order to raise the
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