1 \ hk WW AALS Design and Calculation of Piercing and Blanking Die * Calculations for Economic Strip Layout, Calculations of ‘Cutting force and Stripping force, recommending minimum tonnage of a press, Methods of reducing cutting loads on press tools Design aspects of Press tool elements viz. Punches & Methods of mounting punches, types of Die block, Stripper, Pilot, stock guidas, stock stops, Selection and arrangement of Hardware used in Press tools. Setection of steels and its hardness for different elements of Press tools. Centre of prassure, Different types Die sets and its selection, shut height of die, Problems based design of progressive dia ppl 2.1 CALCULATION FOR ECONOMIC STRIP LAYOUT In the blanking die-set design, the first step is to prepare blanking layout i.e. the position of the workpiece in the strip and their orientation with Tespect to each other. This is called as strip layout following are the factors which affect the strip or stock layout. Economy of material Fig. 2.1.1 shows the different ways for arranging the workpiece. As per arrangement in Fig. 2.1.1{a), it can be worked at single row, single pass with a single punch.[Press Too! Design (Mu-Som.6-Mech) (Dest. & Cake. of Pio. & Bla. Die) (2.2) By feeding the material as per Fig. 2.4.10) . there is increase in material utilization upto some extent eo Fig. 2.1.1(c) show a single row, double pass strip. Here strip will have to be passed through the die’s once, turned over and passed through the die’s once, turned over and passed through dies second s a maximum utilization of the material and time, Here, there i reduction in scrap. PPE Iree (Lee c (auFig. 2.1.1 : Strip Layouts (a,b,c) © Another i cs hen tntaeaee Consideration in strip layout is the distance n the nearest point of blanks between the blank and the edges of the strip. Generally thi . ly this. dis i f thickness, Wy this distance is 1.5 times the materi The various terms : ’ Fig. 211.1(4) connected to strip layout are as shown if lerPress Tool Design (MU-Sem.6-Mech) __(Desi. & Calc. of Pig. & Bla. (2-3) (aagPig. 2.1.1(6) (@® The front scrap (back scrap) may be determined by the equations, a = t+0.015h (ii) The distance between successive blank i.e. scrap bridge (b) is given in table. Material thickness (mm) | b(mm) (iii) ‘The feed or advance or the length of one piece of stock required to produce one blank is, f= Web (iv) The number of blanks which can be produce from one length of stock can be found as, ne i where,a = Back scrap on front scrap b = scrap bridge hand = Height and width of the component f = feedon advance3. 4 Press Tool Design Mu-Sem.6-Mech) (Desi. & Cale. of thickness of the material = Height of the strip = Length of the sheet = Number of blank to be produced remaining at the end of one length Zr ) = snp Direction of Material grain ‘This factor is to be: considered if the cut blank have to undergo any subsequent operation such as bending or deep drawing. Strip or called stock yout is whether the stock Another important consideration is the strip lay used will be in the form of strip or coil whereas, the stock strip may be passed through the die more than once, the coiled stock is usually passed through the die only once. Burr direction © Whena sheet metal is cut ina die, of the scrap strip and on the punch side of the blank. ef the burr has to be on the hidden side then the expensive operation of removing the burr need not to be done. aburr is produced on the die side Press used © The stock layout has to be such that it allows the die to be designed within the press capacity. ‘Shear may have to be provided on the punch or die, (0 limit maximum force within the press capacity. Die cost tis higher for © Higher production. © Cutting more than one at a time, especially whe? cute complicated blank shapes or when cutting accurate blank size.Prass Tool Design (MU-Sem.6-Mech) (Desi. & Calc. of Pio. & Bls. Die) (2-8) SS Production required The following guideline may be followed when the production is the a main consideration : @) @) ©) (d) low production thin material (i) strip stock anda single pass layout (ii) cutting of one or more blank at a time low production thick material (i)_ strip stock and a single or double pass layout (ii) cutting one blank at a time High production thin material (3) coiled stock and a single pass layout (i) cutting of one or more blank at atime High production thick material (i) strip stock, and a single or double pass layout (Gi) cutting of more than one blank ata time b> 2.2 CALCULATION OF CUTTING FORCE AND STRIPPING FORCE (A) Cutting forces Cutting force is the force applied on the stock material in order to cut ‘out the blank or slug. This determines the capacity of the press to be used for particular tool. The area to be cut is found by multiplying the length of cut by stock thickness. Formula for calculating the cutting force : The maximum force required to cut a material is given by Fry = sheared area x shearing strength = AX(%,XN)& Press Tool Design (MU-Ser.6-Mech) (Desi. & Calc of Pie. & Bla Die) (2 a — eo ee >>| 2.3. METHOD OF REDUCING CUTTING FORCES ON PRESS TOOL + For cutting operation it has been assumed that, bottom of the punch ang the top of the die block lie in parallel planes. * But, due to this, very high punch force is exerted for a very short tims oa the material, resulting in shock or impulse condition. » Hence, to reduce cutting forces and smooth out the shock impact g heavy loads, following two methods are frequently used, 1, Step the punch length * The method of reducing cutting force is to step punch length Punches or group of punches progressively become shortes by abou One stock material thickness. + This will result in distribution of forces during the blanking q piercing action on the punches interns reducing in total force, © This type is mainly used in piercing of more number of holes ony component. ' (iaenFig.2.3.1 : Step the punch length 2. Grinding the face of the punch or die to a small shear angle * A second method is to grid the face of the punch or die to a smi! shear angle with the horizontal. This has the effect of reducing t Contact area while shearing at one time.r888 Tool Design (MU-Sem.6-Mesh) (Desi. & Cale. of Pia. & Bla_Die) (2-9) * Providing the shear also reduce the shock to the press and smoothens out the cutting operation the shear angle chosen should provide a change in punch from 1 to 1.5 sheet thickness. » Double shear angle is preferred over single shear angie because it does not create laterial forces. © The shear angle for blanking operation will be the die member ‘while as the piercing operation the shear angle will be given on the punch member.ress Tool Design (MU-Sem.6-Mech) __(Desi. & Calc. of ©) ciadyraegytasoF ig. 2.3.2 : Grinding the face of the punch or die to a small Shear angle (a, b, c) %& 2.5.1 Recommending Minimum Tonnage of a Press © Bess tonnage is determined by the sum of all the forces Fequired to cut and form the part. © — Practical experience with presses equipped with tonnage monitor show ‘close agreement between calculated value versus measured value. * — In many cases, the stripping force must be added to the cutting force if a Spring loaded stripper is used because the spring must be compressed while cutting the material, © Fixed on tunnel stripper will keep the press toad to a minimam, bat uwy will not contro] the stock as well as spring loaded ones. «For press tonnage calculation, a rough empirical equation is used for approximation. Required press tonnage ~ hole perimeter (mm) x material thickness (mm) x shear factor, (kp/may 1000 Pxtxs i Tonnage (E(tons)) = 595fal Prass Tool Design (?AU-Sem. §-Mech) (Desi. & Cale. of Pio. & Bia. Die) (2-11) Some Important shear factor (o) Aluminium — 22-25 kP/mm* Mild steel — 40 kP/mm? Stainless ste! - 60 kP/émm? DH 2.4 CENTRE OF PRESSURE ¢ When the irregular shape blank is to be cut, then the summation of shear forces about the centre line of press ram should not be symmetrical. * — Hence, bending moments will be introduced in the press ram because of which undesirable deflection and misalignment are produced, * — Soto avoid this, centre of pressure of the shearing action of the die must be found, © This centre of pressure is the centroid/centre of gravity of the. line perimeter of the blank and not he centroid of the blank area. " Procedure to calculate centre of pressure (a) Draw the outline of the part. (b) Place X and Y axes in a convenient position. (c) The outline of the part is divided into convenient line element and numbered as 1,2,3 and so on. (d)_ The length L,, L., L, of these line element are calculated. (ce) The distance of centroid from the X and Y axes is determined. Let Xp Xp %y and yy. YY; be the distance of centroid of the line clement from the Y and X axes respectively. (f) The centroid of this line element is also calculated.puny som 5-Mgen)_ (Dest BESS of Pio, & Bia Die) (212) 5 Press Tool (g) The distance of ceptre of pressure from each axis 18 determined by sing following foraula * _ th or dyka bose X= Hehe hte ae th yet hist Yoo Reh tht: Wher, X= X distance of centre of pressure Fe ydistance of centre of pressure Perimeter if the blank (asiFig. 24.1 Dias DESIGN ASPECTS OF PRESS TOOL ELEMENTS: 41, Die Block ® [tis the female part of Tt is subjected to extreme made of superior quality of tool press tool which carry the cutting edges pressure and stress conditions, hentt steel Stock stip ee qusaFig. 2.5.1 : Die Block(MU-Sem Bech} (psi y & Cac. ot Pie. 8 Bia Did) (2-13) Th . i Minimum die block thickness depends upon the strength fequired {0 select cuning forces and types and thich material to he cut. went Generally. the die block thickness is calculated by thumb rile. Table show the die block thickness ax per blank perimeter. Table 25.1: Die block thickness as per blank perimeter 2, Strippers © The main function of the stripper is to strip or discard the workpiece from the ponch or the die after the end of the cutting of forming Ee. ‘operation. « = There are two types of strippers ie. fixed or stationary type and spring loaded or movable type- : (a) Fined stripes aii eis anached at a fixed beight over the die block . ‘Tris beight is sufficient to permit the sheet metal 10 Be fed freely between upper die surface and lower plate surface. © The width and lengtn of the stripper plate is same ts die block, the upward motion ‘of the puach, undersides of the strppers plate «For the stripping action. 22 the scrap strip will stacks the cand get stripped off from the puns surface.Seanned with ComScanner Seanted with ComBoacerSeanned with ComScanner Seanted with ComBoacer(Desi. & Calc. of Pie. & Bla. Die) (2.24) fal Press Too! Design (MU-Sem.6-Mech) Selecting the die set : Ten element of die set information must be decided before a die set can be selected. Gj) Make or manufactures Gi) = Type (ii) Size (iv) Material (v) thickness of die holder (vii) type and lengths of bushings (ix) shank diameter Advantages realized when die components are retained ing properly selected die set are : (i) Uniform clearance are miountained around cutting edges fo, producing blanks free of burns (vi) thickness of punch holder (viii) Length of guide post (x) grade of precision (ii) Die life is increased (iii) Storage is facilitated (iv) There is no possibility of loss of loose parts (v) Die can be installed in press in a minimum amount of time Standard die set range from 3 by 3 inches to 45 by 60 inches. Die holder and punch holder thickness range from ! to 34 inches by quarter inches. Sheet metal hardware is quite commonly used in replacement of previously drawn and threaded bosses or instead of additional block. which often had to be attached by quite labor-intensive means or as 2 replacement of various brackets, supports, or standoffs. For heavier work, welded nuts, screws, and pins can be utilized. They will be sufficient for the material of 0.025 to 0.187 inch 0.65 © 4-50 mm] sheet thickness.fs) Riess Tool Design (Mu-SemS-Mach) (Desi & Cate io! Pie 4 Bia Die) (2-25) «In welded nuts, threaded inserts, or pins, there are three Projections located on the surface which is to be attached to the sheet metal parts, these are to ensure the contact and easy fasing of hardware to the shee, metal past. ° For lighter-gauge application, upto 0.0990 in (2.25 ‘mm) sheet thickness, clinching fasteners will prove quite useful, * — Self-clinching studs, nuts and standoffs with a Snap-top adjustment are a Continutation of a wide assortment of this type of hardware. % 2.8.1 Material Used in Press Working * These are different types of metal used in sheet metal work in the forms of sheets and plates. * The specification of metal sheets are given in terms of their gauge number, length and width © Some of the important sheet material are 2s follows: (2) Black Iron * Black iron or uncoated sheets carries no. artificial coating on its surface, but itis cheaper than other type of metal sheets. © Component made from this type of material are pans, tanks, cabinets, store paper etc. (2) Galvanized tron © Zine-coated iron is knows as “galvanized iron”. It is soft iron stee! sheet which makes the surface good looking and rust resistance. © Components made from this type of metal are storage tanks, buckets, fumaces giters, pans, runks etc. (3) Aluminum © Due to low strength of aluminum sheets they are not used in their pure form, hence suitable amount of silicon manganese, copper and iron are added ¢ They are used in the manufacture of doors. windows and building works, aeroplane bodies ete. Eeof Plo. & 1a. Dio} (22n) (4) Stainless steel joy of steel with nickel, chromium, and traces of othe, © This is all resistance and can be welded easily material. It has good corrosive of food containing, equipments, dain © Ibis used in the manufacture plant, chemical plants, bus bodi 5, Hl equipments, food processing tanks ete. (5) Copper © Copper sheet are co’ and relatively good in apperance. ductile and malleable. sed in applications like radiator of automobile, bearing stlier but offers good resistance to COROSioy They are reddish in colour, high ly © They are u! expansion joints, root and hood parts. (6) Tin plate © Ty, plates are used for those iron sheets which are coated with puy tin. © Applications + oil can for automobile, Roofs, food containes furnane fitting cans and pans. 2.8.2 Selection of Material for Press Tool Components The selection of material for press tool component for a gin application depends on which die failure mechanism dominates. > Step 1: Identify the type of wear «This is the most fundamental step Decause it will determine wit wear resistance profile the die material should have e = The following factor are considered to establish the dominss wear to be expected. (i) type of sheet material (i) hardness of sheet material (iii) presence of hard particle in the sheet material» Step currence af chipping or plastic deformation The following factor eterminc the e; Atent of the risk for chipping and Plastic deformation, i, whether hi igh Guctility and or big hardness are (i) ci) Type of operation to be Performed on press too} Thickness and hardness of sheet Material Gil) Geometrical comp! lexity of parts to be produced (a) Wear (b) Chipping Plastic deformation (c) Plastic deformation Cracking (4) Cracking (vay (aaFig, 2.8.1