翻译原文-金属板料冲压件多级累进模具的设计.pdf

INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 15, No. 5, pp. 875-881MAY 2014 / 875 KSPE and Springer 2014 Design of the Multi-Stage Progressive Tool for Blanking a Sheet Metal Component Manoj Balakrishnan1,# and Jason Cherian Issac1 1 Department of Mechanical Engineering, Saintgits College of Engineering, Mahatma Gandhi University, Kottayam, Kerala, India, 686532 # Corresponding Author / E-mail: , TEL: +91-9746613228, FAX: +91-481-2430349 KEYWORDS: Progressive tool, Economy factor, Blanking, Centre of pressure, CAD The field of sheet metal stamping had gained wide acceptance and importance especially in mechanical sector where complicated design features which once consumed greater man power and effort became simpler with the introduction of die sets in the 1920s that changed the whole scenario. With regard to the extreme relevance of this area in the present, the current work is focused on developing a multi-stage progressive tool (die) which can be utilized for stamping (blanking) a sheet metal component. In order to arrive at an efficient design, several strip layouts are analyzed. The feasible one that possesses and satisfies a balanced nature of high strip utilization factor and yield is selected. Centre of pressure is determined by distributing the action of punches from the centre of die plate in four quadrants. Thereafter, calculation of forces involved, dimensions of plates, etc., are performed. Based on the calculated values, 2-D drawing of all components including optimum strip layout is performed initially and thereafter 3-D models of components of the designed progressive tool are created. After careful observation at every stage of design, proper remedial actions are taken so that resulting design is feasible both in economical and production aspects. Manuscript received: November 6, 2013 / Revised: January 3, 2014 / Accepted: March 1, 2014 1. Introduction The metal stamping die is a press tool capable of producing large quantities of parts that are consistent in appearance, quality, and dimensional accuracy. Here, a complex work part is split up into a number of simple parts and is gradually finished as it passes sequentially through a number of stations, ejected at the last. Reduced size of components, difficulty to produce the same by conventional machining methods strongly mentions the importance of progressive die system,1 simplifying machining time and improving future assembly process. Zhi-Xin Jia et al.2 by using the design tools showed that the same when constructed on a PC and integrated with a CAD system can dramatically improve the design quality and hence both time and cost can be saved simultaneously. Abbas Vafaeesefat et al.3 had compared the numerical and experimental results of sheet metal flanging using designed tools (including punches, dies and other sub components) and proved that the simulation results were in good agreement with the experimental, enabling the designers to perform the same in order to save materials and time thereby the net cost. Kee Joo Kim et al.4 had developed basic techniques basic techniques in order to apply aluminium sheets for automotive hood. The sheet was subjected to punching action to obtain measurement data in sheet forming for formability evaluations. An integrated modeling and process planning system was developed for bending operations of progressive dies by Li et al.5 based on feature and rule approaches. In order to improve the efficiency of die designers and manufacturers, Choi et al.6 worked on computer-aided design and manufacturing system for irregular-shaped sheet metal products for blanking or piercing and bending operations on knowledge-based rules. Most of the research works mentioned above were either focused on NOMENCLATURE E = Economy factor, in % , = Coordinates of centre of pressure, in mm t = strip thickness, in mm Tmin= Minimum thickness of die plate , in mm XY DOI: 10.1007/s12541-014-0411-0 876 / MAY 2014INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 15, No. 5 design, process planning or automation of the product. But much focus has not been given for optimizing the design of complicated progressive tools with higher number of stations. Hence, the current paper is aimed at design of a multi-stage progressive tool based on possible iterations at and finally arriving at the same. 1.1 Economy Factor (Strip Utilisation factor) Stock material conservation is a decisive factor in press working. All possible means are tried to attain it without sacrificing the required accuracy of the piece part. Economy of any strip layout in percentage is found out by the following formula.7 Economy factor, E in % (1) Significance: Optimum strip layout is found out by placing the part in different positions and the one with the highest percentage of strip utilization is selected. The optimum utilization factor is attained by geometrical considerations so as to minimise scrap (wastage). 2. Design In the design phase, feasible strip layout with maximum strip utilization factor is to be selected among the possible ones. But along with the same the yield for the layouts also has to be considered. Hence by having a proper balance between the two, the feasible layout is selected. Force calculations are performed in order to select a suitable press for the current work as shown in Fig. 1 and then plate thickness values are calculated. Thereafter design of plates, punches and pillars are done which form the super structure of the tool. 2.1 Calculations in strip layout - Arrangement of strips Presently, about five types of strip layouts are possible. But considering the design and production feasibility three of which are analyzed, that include Vertical layout (narrow run), Horizontal layout (wide run) and Inclined layout. 2.1.1 Economy factor calculations for different layouts By calculation, for Vertical layout, E = 53.991%, Horizontal layout, E = 52.286% and that for Inclined layout, E = 47.171%. It is found that, the economy factor (strip utilization factor) for vertical layout is greater than the other two (horizontal and inclined layouts).So in the vertical layout two different possible punch arrangements are considered. But here the pitch length required is higher than the calculated value, for proper arrangement of punches which is not desired. Secondly, the dimension of strip required per blank = 30 mm 40 mm which when substituted in the formula for calculation of economy factor will yield a comparatively lower value even lower than that of horizontal layout. Hence, the only advantage of this arrangement is that the die size will be less. But the net cost for developing the die will be very high. Hence even if the initially obtained value for strip utilization is greater, due to the above mentioned reasons the vertical layout is to be dropped. But confirmation is only after performing yield calculations so as have a quantitative idea of stock material utilization. 2.1.2 Yield calculations for different layouts a. Horizontal layout Density of CRS material= 7860 kg/m3 Mass per strip for blank= density volume = 7860 (26 39.5 1.2) 10-9 = 0.009686 kg = 9.6869.7 g No. of strips per kg.= 1000/9.7 = 103.09103 nos./kg Mass of component= 7860 (496.728 1.2) 10-9 = 0.004685 kg = 4.6854.7 g No. of components per kg.= 1000/4.7 = 212.76213 Unused portion of strip= 213-103 = 110 Unusable stock per kg.= 1000/5 = 200 Yield= 200-110 = 90 nos./kg By calculation, the yield obtained for arrangement of strips in horizontal direction (layout) = 90 nos./kg. b. Vertical layout By calculation, Yield for vertical layout = 25 nos./kg. When pitch is reduced to 29.5 mm, the yield obtained = 31 nos./kg. Variation of yield on reduction of pitch from 30 to 29.5 mm results in a change from 25 to 31 nos./kg. On comparison it is understood that the yield per kg of stock material for horizontal layout is far greater than that of vertical layout, almost three times (90 31). So horizontal layout is chosen and the entire design is done based on this layout. After proper re-arrangement of punches, it is seen that the resulting one is an eight station die. The initial cost of the tool is slightly higher compared to vertical. The fine refined strip layout obtained is as shown in Fig. 3. = Area of the blanknumber of rows() Width of the strippitch() - -100 Fig. 1 Component drawing - 2D Fig. 2 Vertical layout INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 15, No. 5MAY 2014 / 877 2.2 Force calculations Once the feasible strip layout is finalized, a punching press capable of blanking out the sheet metal component under consideration is to be selected. So the shear force (cutting force) required to cut the part from the stock material is calculated followed by the calculation of stripping force. These eventually support in the design of die and stripper plates which are very crucial components in a progressive tool. a. Shear force or cutting force From Ref.7, Fsh= L t Ssh(2) Where, Fsh= Shear force or cutting force, in kgf L= length of cut, in mm (taken from the strip layout) = 206.967 mm t= thickness of stock, in mm = 1.2 mm Ssh= shear strength of stock material (C.R.S), in kg/mm2 = 40 kg/mm2 Fsh= 206.967 1.2 40 = 9934.416 kg = 9.934 tonnes b. Cutting clearance Cutting clearance for CRS from standard chart is given as 4%. ie, cutting clearance = 0.04 mm per side. c. Stripping force (i) From Ref.7, Stripping force, Fstr= (5 to 20) % of cutting force(3) Assuming narrow clearance, Fstr= 0.10 Fsh= 0.10 9934.416 = 993.44 kg = 0.9934 tonnes d. Press force Fpress= Cutting force (Fsh) + Stripping force (Fstr) = 9934.416 + 993.44 = 10927.856 kg = 10.927 tonnes 11 tonnes But from Ref.9, for practical purpose the press selected is 33% greater than the calculated (design) Fpress The capacity of press selected = 11 + (33/100) 11 = 14.63 tonnes 15 tonnes 2.3 Plate thickness calculations a. Minimum thickness of die plate (h) or (tdp) From Ref.8, According to Oehlers findings (4) For a cutting perimeter in the range of (152305) mm, an expansion factor (correction factor) of 1.75 must be considered. Hence, max. die thickness, (tdp)max = 1.75 22 = 38.5 mm By, considering standard dimensions, the thickness of die plate is fixed as tdp = 36 mm b. Minimum thickness of bottom bolster (plate), tbb From Ref.9, (5) Where, Vertical cutting force, V= 97456.620 N Poissons ratio, = 0.3 (for M.S) Permissible hoop stress, f= 60 N/mm2 (for M.S) Hole in press bed, D= 43.1 mm Hole in bolster, d= 39.1 mm Substituting the values, tbb = 38.90839 mm Similarly, the other plate values are calculated and tabulated as shown in Table 1. By calculation of plate thicknesses using standard formulas, the minimum thickness is obtained. Minimum thickness of a plate refers to the least thickness that a plate should possess so as to withstand failure. But if such a dimension is chosen for the final tool, for a small deviation in force, the entire tool structure is under the glimpse of collapse. Also, allowances including grinding have to be provided to the individual plates. Considering the aforementioned, the thickness have to be chosen in such a manner that it shouldnt be very high leading to monetary loss or be very less leading to permanent failure. So an optimum value for the same will serve the purpose. 2.4 Design of plates 2.4.1 Design of die plate and guide plates During punching of sheet metal products, lateral force occurs which might crack the die plate if enough distance is not made available from the outline of die hole to the circumference of the plate. Die margin is a solid cross-section around the die cutting edge. It usually ranges from one to two times the die thickness, T.8 Here minimum die thickness, Tmin= 23 mm. hPmax 3 9934.416 3 21.49722 mm tbb 3V 1+()D2 f D2d2() - - D d - -log= Fig. 3 Fine refined strip layout - Horizontal 878 / MAY 2014INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 15, No. 5 Based on it, for die margin (1), 1.5Tmin, for (2) 1.2Tmin.are taken and for (3), a length of 30 mm is considered from the edge of guide plate. Assuming a factor of 1.3, actual length = 30 1.3 = 39 mm. For die margin (4), near the bending punch hole, a margin of 2Tmin= 2 23 = 46 mm is taken. Considering standard dimensions, width of plate so obtained = 416 mm. Centre of pressure for the resulting die plate is: = 204.5181 mm, = 91.8314 mm But in order to have centre of pressure of the plate to be near to the centre of the plate within a limit of 10 mm, the length of the die plate is extended 20 mm further towards right from the centre and the length is fixed as 436 mm. Hence, the dimensions of the die plate are: 436 mm 176 mm 36 mm. The die plate is chamfered on all edges by a length 3 mm 45. Sub Pillar hole centres are located at a distance of 23 mm (min. die thickness) from the perpendicular edges. It is a stepped hole to provide seating for the sub pillar bush of standard size. 1st hole for screw is located from the centre of sub pillar at a distance of 1.5Tmin= 23 1.5 = 34.535 mm Sub pillar bush: 20H7 (Inner dia), 25h4 (Outer dia), height (h) = 36 mm = thickness of die plate. Main screws (4 nos.) are provided near to the four pillar holes and optional screws (2 nos.) are placed on diagonally opposite sides and dowel pin holes (2 nos.) are provided at a distance of 10 mm from screw hole centre to prevent misalignment. Dowel pins and screws are placed at equal pitch From Ref.7, For pilot holes: Pilot opening (hole) diameter, D = P + 2C(6) Where, P = Pilot diameter = 3.5mm C = normal cutting clearance = 0.04mm Substituting the values of P and C, D = 3.5 + 2 0.04 = 3.58 mm Two pilots position the strip precisely. For this purpose, one pilot hole of 3.5 mm is pierced at the first station and another of 5.05 mm is pierced at the sixth station. Stepped clearance is provided here (irregular profile), since pressure is to be relieved immediately and to prevent touching of sharp edges of work part (blank) and die hole. Pitch holes: By calculation, the value obtained for pitch is 38 mm. As per design, for proper piloting, pitch need to be increased. So it is set at a fixed value = 39.5 mm. Since it is an eight station die, the number of pitch holes provided = 8 (each with 3.5 mm). Lifter holes: A total of 12 lifters are provided with a minimum diameter of which equals 3 mm. Standard lifters are positioned in lifter holes. Tension of the lifter spring is preset by means of screw plug so that the deflection is 2 mm. Lifters should not fall into the pilot holes hence allowed a minimum offset of 3 mm (here). Ejector pin holes: Ejector pins/ Shedders (2 no.s) with half the diameter of lifters are placed symmetrically on stripper insert 1. Significance of inserts in Die Plate: In the present die plate, a total of 6 die inserts are used. If premature wear occurs due to action of any of the punches, only that particular insert need to be changed. Hence the cost of changing the entire die plate can be saved. A normal progressive tool makes use of two Guide plates. But when the length of the die plate exceeds a limit (similar to 400 mm for back plates), the guide plates also need to be split up. The right edge of guide plate (B) is positioned at a distance of 2 mm from the bending punch hole. Thickness, t = 1/3 tdp(min.) = 1/3 238 mm. A length of 2 mm is taken downwards from the cutting edge and hence width of guide plate, W = 28 + 2 = 30 mm. The length of guide plate (B) is taken from the right edge and is extended beneath the pilot hole before the fourth strip, then extended downwards. By measurement length, L = 179 mm. Hence, dimensions of guide plate are: 179 mm 30 mm 7 mm. A spacing of 0.5 mm is provided intentionally between the adjacent plates in order to prevent damage due to close contact between the edges during fastening to die plate. XY Table 1 Plate thickness values S. No. Plate Min.Thickness (t) or (h), mm Optimum thickness, mm 1.Die Plate2336 2.Back plates (die, punch)88 3.Back plate (stripper)1212 4.Bottom Plate3946 5.Guide plate77 6.Stripper plate1232 7.Punch Plate1120 8.Top Plate34.536 Fig. 4 Die margin Fig. 5 Die plate drawing (2D) INTERNATIONAL JOURNAL OF PRECISION ENGINEERING AND MANUFACTURING Vol. 15, No. 5MAY 2014 / 879 The right end bottom portion near to the ejector pins is provided an excess relief from the strip (i) To be at a safe distance from the ejector pins (ii) To provide relief to the ejected parts 2.4.2 Design of screws and dowels From Ref.9, Root area for metric screws, A A= 0.7854(D-1.227P)2(6) For the formula mentioned above, the stripping force (load) is calculated based on the selected press tonnage and is assumed to be 10% of the vertical shearing force. Screws need to be provided for die plate as well as guide plate to with stand the shearing action. So different cases (trials) are considered and optimum value is selected. For Die Plate: When M10 1.5 screw chosen initially, A = 0.7854 (10 1.227 1.5)2= 52.289 mm2 Load capacity at 80 N/mm2 (permissible stress) L = 52.289 80 = 4183.12 N/screw No. of screws required, n = 3.5174 For Guide Plate: After three trials, Trial 4: Choosing M5 0.8 screw No. of screws required, n = 14.514 Considering uniform distribution of screws and reduced load per screw, for guide plates M5 CSK screw- 14no.s (Trial 4) is selected. From Ref.9, For Die Plate: Vertical force (V)= 15000 9.81 = 147150 N Horizontal force (H) = Die clearance x V = 0.04 147150 = 5886 N Area per dowel (Dowel dia. = 10 mm = Screw dia.)(7) = 0.7854 102 = 78.54 mm2 Range of shear stress due to horizontal force for dowel = 5080 N/mm2 No. of dowels at 50 N/mm2 stress = 1.492 For Guide Plate: Dowel dia. = 5 m