DESIGN AND FABRICATION OF A DEEP DRAWING MACHINE: EXPERIMENTAL STUDY OF DRAWING FORCE VS DRAWING STROKE

Transcription

1 DESIGN AND FABRICATION OF A DEEP DRAWING MACHINE: EXPERIMENTAL STUDY OF DRAWING FORCE VS DRAWING STROKE Ahmed Ramahi, ramahi@najah.edu, a_ramahi@yahoo.com. Industrial Engineering Deartment, An-Najah National University, Nablus, Palestine. Yahya Saleh, saleh_yahya@yahoo.com. Industrial Engineering Deartment, An-Najah National University, Nablus, Palestine. ABSTRACT This Paer reresents the work imlemented in designing, fabricating and oerating a model of a chea hydraulic dee drawing machine (DDM), which is currently utilized in the manufacturing rocesses lab in the Industrial Engineering Deartment (IED) at An-Najah National University. The machine is being used in conducting different exeriments related to the dee drawing rocess. As known, dee drawing is a sheet-metal working rocess in which a unch draws a blank sheet into a die cavity to form cu-shaed or box-like arts [1]. This work was carried out in three stages; the first was the design stage, in which all design calculations of the DDM elements were comleted based on the secifications of the roduct (cu) to be drawn. The second was construction stage, in which the DDM elements were fabricated and assembled at the engineering workshos of the university. The last was the oeration and exerimentation stage, in which the DDM was tested by conducting different exeriments. In conclusion, the exerience gained in designing and constructing a mechanical lab equiment was found to be successful in terms of obtaining ractical results that agree with those available in literature, saving money relative to the cost of a similar urchased equiment, as well as enhancing students abilities in understanding the dee drawing rocess in articular and machine elements design concets in general. Keywords: Dee Drawing, Machine Element Design, Dei Design, Machine assembly and Fabrication, Exerimental Investigation of Draw Force and Draw Stroke

2 1. ITRODUCTION Dee drawing is a sheet- metal working rocess used to form cu- shaed or box shaed arts by using a unch that draws a blank into a die cavity. This rocess is carried out by lacing a blank sheet of certain size over the oening of the die and ressing this blank into the die cavity with a unch, as shown in Figure 1, [1]. Tyical roducts made by this rocess are beverage cans, bathtubs, containers of different sizes and shaes, sinks, and automobile anels. In this aer, the basic drawing oeration is studied, which is the drawing of a cu-shaed art with arameters as shown in Figure 1, in this basic oeration a circular blank sheet of a diameter D b and a thickness t, is laced over the die oening of a die having a corner radius R d. Then the blank is held by a blank holder (hold-down ring) with certain force. After that a unch of a diameter D and a corner radius of R is used to unch the blank sheet into the die cavity, thus forming the cu-shaed art. Moreover, the unch moves at a certain velocity V and alies a certain down ward force F to achieve the deformation of the metal, while the blank holder alies holding force F h to revent blank wrinkling. Actually, this aer resents the design and fabrication of a chea Dee Drawing Machine DDM that roduces re-identified cu-shaed roduct, the DDM is now mounted and being used for exerimentation in the Manufacturing Processes Lab in the IE deartment at An-Najah University, the aer resents the detailed design of the DDM main elements including the unch and the die, and the fabrication & assembly of the DDM, it also resents the oeration and testing of the DDM through conducting exeriments on drawing force versus drawing stroke and comare the results with ublished data. Fig. 1: (a) Drawing of a cu_ shaed art: (1) start of oeration before unch contacts work and (2) near end of stroke; and (b) corresonding workart: (1) starting blank and (2) drawn art. Symbols C = clearance, D = blank diameter, D = unch diameter, R d = die corner radius, F h = holding force. R = unch corner radius, F = drawing force, b

3 2. DEEP DRAWING: GENERAL BACKGROUND This section discusses some general concets of the dee drawing rocess including the drawing measures, drawing force and holding force Dee Drawing Measures: One of the most imortant measures of dee drawing oeration is the limiting drawing ratio LDR. Limiting drawing ratio is defined as the maximum ratio of blank sheet diameter to unch diameter that can be drawn under ideal conditions in one stroke without failure [2]. That is, LDR D b max (1) D P An aroximate uer limit on the drawing ratio is a value of 2. [1]. Another measure of drawing is the reduction, Re, which is defined as D D b 1 Re 1 (2) D D LDR b b The reduction is closely related to LDRand its value should be less than 0.5. A third measure of dee drawing oeration is the thickness to diameter ratio t referable that increases [1]. Db t Db ratio to be greater than 1%. As Db, which is often exressed as a ercent. It is t decreases, the tendency for wrinkling In fact, the unch to die clearance is usually some 10% larger than the sheet thickness to accommodate blank thickening during the drawing rocess, [2]. Thus, the clearance ( C ) can be exressed as C 1. 1t (3) The Drawing Force: The force in the unch required to roduce a cu is the summation of the ideal deformation force, the frictional forces, and the force required to roduce ironing. Figure 2 shows the relation between the draw force and the draw stroke [2].

4 Fig. 2: Punch force vs. unch stroke for dee drawing. Curve A tyical of drawing with otimum lank holder ressure The following equation has been develoed to aroximate the total unch force to dee draw a blank of D b, at any stage in the rocess [3], Where: F D D b b D t avg F 1.1. ln 2 h ex B (4) D D 2 F = total unch force, avg. = the average flow stress, d = unch diameter, D = blank diameter, F h = blank holding force, B = force required for bending and unbending blank, t = wall- thickness. = coefficient of the friction. However, equation (4) is somewhat difficult to deal with because of the many variables involved in the oeration and because dee drawing is not a steady state rocess. Hence, an aroximate equation of the maximum unch force ( F ) has been develoed as [2]. Db F Db t ( UTS)( 0.7) (5) D In which: F = maximum drawing force, in lb, (N). t = original blank thickness, in (mm),uts = ultimate 2 tensile strength, (lb/ in ), (M Pa). D, D = are the starting blank diameter and unch diameter in b P (mm), resectively. The drawing force F varies throughout the downward movement of the unch, usually reaching its maximum value at about one-third the length of the unch stroke [1].

5 Blank Holding Force: The holding force F h lays an imortant role in the dee drawing. As a rough aroximation, the holding ressure can be set at value equals of the yield strength of the sheet metal [1]. Thus by multilying the holding ressure by the ortion of the starting area of the blank which is to be held by the blank holder, we can estimate the holding force ( F ) as [1]. F 2 2 h 0.015S y Db ( D 2.2t 2Rd ) (6) h Where F h = maximum holding force in dee drawing, Ib, (N), S y = yield strength of the sheet 2 metal, lb/ in (MPa), R d = die corner radius, in (mm). Tooling and Equiment: A double-action mechanical ress is generally used for dee drawing, hydraulic resses are also used. The double action ress controls the unch and blank holder indeendently and forms the art at a constant seed. Since blank holder force controls the flow of the sheet metal within the die, now resses have been designed with variable blank- holder force. In these resses the blank holder force is varied with unch stroke. The most imortant factor in the die design is the corner radius ( R ) of the die. This radius must have an otimum value since the material is ulled over it. The value for the otimum radius of the die deends uon the rint requirement and the tye of the material being drawn. Obviously, the smaller the die radius, the grater the force needed to draw the cu. The radius of the die may be between four to eight times the thicknesses of the blank [3].That is 4t Rd 8t (7) Practically, it is recommended to start with Rd equal 4t and increase it if necessary. Similarly, the unch nose radius ( R ) is imortant since it shaes the radius of the bottom of roduced cu. If R is too small, the bottom radius of the cu may tear out. It may be necessary to make the radius larger than needed, and reduce its size in subsequent drawing oerations. As a start, a 4 t radius to- blank thickness may be used. [3]. d

6 3. CUP SPECIFICATIONS AND DRAWING & HOLDING FORCE CALCULATIONS The DDM was designed to roduce cu-shaed arts in a single stroke, as stated earlier, the urose of designing the DDM is to rovide the manufacturing rocesses lab at An-Najah University with an aaratus that can demonstrate the dee drawing rocess and also be used by students to erform some basic exeriments related to the dee drawing rocess. Actually, in order to design a roer DDM, it is necessary first to determine the roduct (the cu) secifications, drawing force and holding force. Cu Secifications The roduct of the required DDM is chosen to be a simle cu having a certain inner diameter (d) and deth (h) and to be roduced using a sheet metal of thickness (t). The dimensions of the cu must be selected such that the dee drawing oeration is feasible to roduce the cu in single stroke; to measure the feasibility of the oeration, the LDR, thickness-todiameter ratio (t/d) and the reduction (Re) ercentage must satisfy the feasibility conditions mentioned in section2 of this aer. To do so, It was decided that the thickness of the sheet metal to be used in roducing the cu is t 1 32in. 0. 8mm, hence -based on the recommendations stated in section 2- the corresonding die radius is R d 1 8in. 3. 2mm, unch radius is R 4t in. 3. 2mm, and the clearance ( C ) corresonding to t 0. 8mm is C 1.1t 1.1(0.8) mm. It was also decided that the final cu would have a deth of 20 mm and inner diameter d of 50 mm. The art is shown in figure 3. Now, the blank diameter D b can be calculated using the following formula [3]: D b 2 d 4d( H 0.43r) (8) In which d cu mean diameter, in (mm). H mean high of the cu s shell, r = radius at neutral bend line. Using equation (8) and the cu dimensions of figure 3, D is calculated to be D = 80 mm. With D = 80 mm, one can show that the three drawing feasibility measures are satisfied and the cu can be roduced in a single stroke. 50 mm 0.8 mm 3.2 mm (radius) 20 mm Fig. 3: The Final Cu

7 Determination of Drawing Force and Blank Holding Force The cu is to be roduced from Yellow Brass C (65% Cu, 35%Zn) with UTS 322MPa, S y 98MPa. Using equation (5) with D = 50 mm; one can calculate the drawing force to roduce the cu as F = 36.4 KN. Similarly, from equation (6) F h = 14 KN. So the total drawing force (F d ) to be alied by the DDM equals the summation of F and F h, that is F d = 50.4 KN. For design uroses of DDM elements; the F d shall be multilied by a load factor equals to DESIGN OF THE DEEP DRAWING MACHINE ELEMENTS This section resents the design of selected main elements of the dee drawing machine (DDM). Figure (4) shows a section of the DDM, its elements and the associated legend. Figure (5) is its hoto. 1- Hydraulic cylinder inlet. 2- Hydraulic cylinder outlet 3- Blank holder uer late. 4- Blank holding srings. 5- Blank holder lower late. 6- The unch. 7- The die. 8- The die basement. 9- Machine basement. 10- Machine bench. 11- Lower suort late. 12- Lower Suort late fixing screws. 13- Ejector sring. 14- Ejector. 15- Main suorting rods. 16- Blank holder fixing screws (M10). 17- Arm of hydraulic cylinder. 18- Uer suort late. 19- Hydraulic cylinder fixing screws. 20- Hydraulic cylinder (Piston). 21- Machine basement fixing screws 22- Machine table. Fig. (4): Section of the DDM; Its Elements and the Associated Legend

8 Fig. (5): Photograh of the DDM Design of the Die and the Punch Once the cu secifications have been determined as reviously exlained, one can determine the secifications of the die and the unch being used to roduce that cu. Namely, the unch must have an outer diameter equal to the inner diameter of the cu, i.e. of 50 mm. It also has to be high enough to roduce the required deth (20 mm) of the cu. Hence, the unch was designed to have an outer diameter of 50mm, unch radius ( R ) of 3.2 mm, and a height of 80mm. Die and unch are the mating arts in this rocess; therefore, the internal diameter of the die will be the same as of unch outer diameter lus the comensation of the clearance between them. Figure (6) illustrates the dimensions of the die. Design/ Safety Analysis of the Uer Suort Plate The uer suort late, as its name indicates, is used to suort the DDM by holding the hydraulic cylinder of the machine. Therefore, the design of this late must be based on the maximum force rovided by the hydraulic unit which equals 1.6 F d = 80 KN. Figure (7) shows the dimensions of this late, while Figure (8) is the free body diagram of the late. As shown in figure (8), the loaded art of this late can be aroximated as a fixed suort from both ends with a center load alied by the hydraulic unit. The reactions at A and C are same and equal to 40 KN, and the moments at A, B, and C equal M A = 2090 Nm, M B = 2200 Nm, and M C = 2090 Nm, resectively [4]. Section B (the mid san) is the critical section. Under this loading, the maximum normal stress in this section equals to 27.7 MPa. The late is made of Hot Rolled steel with S y = 170 MPa, Hence, the factor of safety guarding against yielding of the uer late equals 6.

9 Fig. (6): Die Dimensions 300 Φ Φ Fig. (7): Dimensions of Uer Suort Plate (mm)

10 R A F 2 40KN R C F 2 40KN M A A B F max 80KN C M C 220mm Fig. (8): Free Body Diagram of the Uer Suort Plate. Design/ Safety Analysis of the Main Suorting Rods The DDM is suorted by two main rods; these rods are connected between the uer suort late and the lower suort late. The rods will carry equal loads, so what is alied to one will be the same for the other. Figure (9) shows the free body diagram of one main rod. F A = 40 KN M A = 2090 Nm 25 mm 400 mm Fig. (9): FBD of Main Rod One can show that the combined normal stress in the rod due to axial and bending loads is around 160 MPa, and as the rod is made of stainless steel with S Y = 240 MPa, then the factor of safety guarding against yielding of the rod will equal around 1.5.

11 Design of the Blank holder Unit This unit has three main arts; these are the uer and lower blank holding lates and the blank holding srings. The required blank holding force is transmitted from the hydraulic arm through the blank holding srings as a varying load to the blank holder lower late which holds the blank. In order to exert the necessary holding force F h, 7 srings were used; each has stiffness (K) of 32 N/mm. The dimensions of one blank holding sring and the blank holding lower late are shown in figure (10). The Ejector Once the unch comletes its stroke, the blank holding unit is then moved uward and the final cu is ejected uward by the ejector which is connected to a sring with a stiffness ( k 15N / mm) to rovide the required force needed for ejecting the cu. The dimensions of the ejector are shown in figure (11) Fig. (10): Dimensions of a Blank Holding Sring and the Blank Holding Lower Plate (mm) Φ 50 Fig. (11): Dimensions of the Ejector (mm)

12 Design of the Hydraulic System An imortant art of the DDM is the hydraulic unit; the unit that rovides the machine with the ower needed to comlete the drawing cycle. Actually, three issues have to be considered in designing the hydraulic unite; these are the ower required, the factor of safety of the hydraulic cylinder fixing screws, and that of the hydraulic cylinder weld between the cylinder and its flange. A one horse ower hydraulic um is used in the DDM; this ower is enough to drive the unch at a seed of 10 mm/sec. Note that the internal diameter of the hydraulic cylinder is 80 mm, and the diameter of its flange is 180 mm. 5. MANUFACTURING AND ASSEMBLY OPERATIONS This section resents the manufacturing oerations that were needed to roduce DDM main arts (namely the unch and the die). Most of these oerations were metal machining and some welding. Die Manufacturing Oerations The recommended material for dee drawing dies is mostly oil-hardened steel; which we used for roducing the DDM die. A solid circular stock iece of oil-hardened steel was selected with a length 60 mm and a- diameter of 110 mm. The following is the list of manufacturing oerations to roduce the die-with a brief descrition of each oeration- arranged sequentially: 1. External turning rocesses: these are the rocesses that were erformed on the die solid stock iece using the lathe machine to reduce the diameter from 110 mm to 100 mm. 2. Internal turning rocesses: these are erformed using the lathe machine to make a two ste hole into the die. One has inner diameter of 61.8 mm, and the other 51.8 mm. 3. Fillet-making rocesses: This rocess is combination of internal and external turning on the die to make the filet with a radius (R d ) of 3.2 mm by using the lathe machine. 4. Drilling rocesses: these are vertical drilling oerations erformed- by using a vertical drillto make two blind holes each for a deth of 20 mm and a diameter of 10 mm. These holes were used to fix the die with its basement. 5. Threading rocesses: these are the manuals threading rocesses erformed for making the internal threads in the two blind holes. These erformed using a HSS to ta. 6. Surface Finish Processes: these rocesses were erformed for smoothing the external surface, inner surface, and the two faces of the die by using a glass aers while the die was fixed on the lathe machine.

13 Punch Manufacturing Oerations Likewise, the unch was made from the same material of the die; oil hardened steel it started with a solid stock iece with a length of 90 mm and a diameter of 60 mm. The following is the list of all manufacturing rocesses erformed to roduce the unch, Arranged sequentially. 1. External turning rocesses: These are the rocesses done to reduce the outer diameter of the starting stock iece from 60 mm to 50 mm. 2. Drilling rocesses: the rocesses erformed on to make a blind hole of a deth of 20 mm and of diameter of 19 mm. 3. Ventilation-Holes Drilling Processes: these rocesses were erformed to make two holes in the body of the unch; one is vertical and the other is horizontal; these holes were made to revent air from accumulation into the cu and this will increase the force required for drawing. The rocesses were done by vertical drill ress. 4. Fillet Making Processes: these rocesses are similar to those of the die fillet-making. 5. Surface Finish Processes: These were erformed for the same uroses of that of the die. Die and Punch Heat Treatment The mechanical roerties of the unch and the die (esecially the hardness) must be imroved such that these two mating arts can handle the total drawing force without any tye of distortion or deformation, to achieve this urose the die and the unch were heat treated as exlained in the following stes: 1. The unch and the die were reheated to 850 C 0 and left in the oven for a eriod of 30 minutes. 2. The unch and the die were quenched into a mineral oil medium 3. The die and the unch were temered at 250 C 0 for 30 minutes. The hardness of the unch and the die before heat treatment was 300 VHN, after heat treatment the hardness became 900 VHN. The hardness was measured using Vickers hardness tester available in the materials science laboratory at An-Najah N. University. Sequence of Assembly Oerations Once the DDM comonents have been manufactured and reared, these elements were assembled with each other until the whole machine is totally constructed and oerated. The assembly rocess chart figure (12) shows the sequence of assembly oerations of the DDM.

14 Machine basement (I- beam) Lower suort late Main Suorting rods Assemble the U-beam To the lower suort late by fixing screws Assemble (15) to the main suorting rods Die Basement Assemble basement with 16 lower suort late The Die Put the ejector The Ejector inside the die Assemble 16 to die Test the ejector for ejecting Hydraulic system 1 Hydraulic cylinder (iston) Assemble (8) and (9) Uer suort late connect (7) to (8) Hydraulic arm (iston) Blank holder uer late Assemble 10,11,12 The unch The sring Center sring on Blank holder lower late their holes Assembly & Fix Legend: Final Insection for oeration and centricity 1 Final assembly With machine bench Assembly Oeration, Insection or Test Fig. (12): Assembly Oerations Flow Chart

15 Drawing Force (KN) The 7 th Jordanian International Mechanical Engineering Conference (JIMEC 7) 6. DDM OPERATION AND EXPERIMENTATION After the DDM had been designed, manufactured, and assembled, the machine was oerated and tested to ensure aroriate oeration and working. After that, two exeriments were erformed on the DDM, one is to investigate the relation between the drawing force (unch force) and the unch stroke, while the other exeriment is to investigate and study the effect of strain factor on the limiting drawing ratio (LDR). A standard exeriment handout was reared for each of the two exeriments to be used by students in the manufacturing rocesses lab. In here, Figure (13) demonstrates samle result on the relation between drawing force (F = F d F h ; F h is calculated by knowing srings deflection) and drawing stroke for brass cus; which agrees with the general trend shown in figure 2. The data is measured using ressure and dial gages. 7. Conclusions This aer resented the work done and exerience gained in designing, constructing and oerating a hydraulic dee drawing machine which is currently mounted in the manufacturing rocesses lab at An- Najah University and used by students to erform exeriments on dee drawing. We found the exerience gained very useful in terms of obtaining a roerly working DDM, as well as enhancing the students' abilities to understand the concet of dee drawing in articular and the concets of design and construction of machines in general. The direct cost of the DDM was about 2000$; of course this is very chea if comared with a similar machine to be urchased from abroad. REFERENCES 1. Groover, (1996), Fundementals of Modern Manufacturing, Printice-Hall Publ., Schey, (1987), Introduction to Manufacturing Processes, McGRAW-HILL Publ., Hosford and Cadell, (1983), Metal Forming, Mechanics and Metallurgy, Prentice Hall, Shigly, (2008), Mechanical Engineering Design, McGRAW-HILL Publ, 112, Drawing Stroke (mm) Fig. (13): Samle Result: Drawing Force vs Drawing Stroke