Manufacturing Engineering,Sheet Metal Forming

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<ul><li><p>Lecturer:-Zeradam Y. March, 2016Ambo University Institute of Technology Department of Mechanical Engineering </p><p>Manufacturing Engineering II (MEng 3182) </p></li><li><p>OutlineSheet metal forming operationApplications of sheet metalCutting operation</p><p>ShearingBlankingPunching Engineering analysis of sheet metal cutting </p><p>*</p></li><li><p>Part ISheet metal forming Sheet metal working includes cutting and forming operations performed on relatively thin sheets of metal. </p><p>Shearing Blanking Punching </p><p> Bending Drawing Spinning Stretching </p><p>Cutting Forming</p></li><li><p>Con..</p><p>Typical sheet metal thicknesses are below 6mm (1/4in). When the thickness exceeds about 6mm,the stock is usually referred to as plate rather than sheet.The sheet or plate stock used in sheet metalworking is produced by rolling.</p></li><li><p>ApplicationsConstruction equipments Automobile and truck bodies Ship building furniture Railway CarsAirplane </p></li><li><p>Con.. space industries;Chemical industry;Drink &amp; food industry.Nuclear </p></li><li><p>Con..</p><p> Shower cabinets and others </p><p>Washing machines Cookers Refrigerator bodies </p><p>Domestic use </p></li><li><p>Five basic sheet metal operations </p><p>Cutting </p><p>Shearing Blanking Punching </p></li><li><p>Con..</p><p> Forming </p><p>Bending, Spinning, Stretching, Drawing</p></li><li><p>Cutting operation</p><p>Cutting of sheet metal is accomplished by a shearing action between two sharp cutting edges.The upper cutting edge (the punch) sweeps down past a stationary lower cutting edge(the die). </p></li><li><p>Con..c</p><p>(1) Just before the punch contact the work</p></li><li><p>Con..c</p><p>(2) Punch begins to push into the work, causing plastic deformation</p></li><li><p>Con..c</p><p>(3) Punch compresses and penetrates into the work and causing a smooth cut surface</p></li><li><p>Con..c</p><p>(4) Fracture is initiated at the opposing cutting edges that separate the sheet.</p><p> Symbols V and F represents motion and applied force , respectively, t = stock thickness , c = clearance.</p></li><li><p>Shearing Is a sheet metal cutting operation along a straight line between two cutting edges.Shearing is typically used to cut large sheets into smaller sections for subsequent press working operations. </p></li><li><p>Con..It is performed on a machine called a power shear, or squaring shear. The upper blade of the power shears is often inclined (to reduce cutting force). The shearing operation constitutes the first stage of any forming process by producing either</p><p>The starting material (cutting out of a sheet) OR Preparing an existing work piece e.g by punching a hole or a series of holes before forming.</p></li><li><p>BlankingInvolves cutting of sheet metal along a closed outline in a single step to separate the piece from the surrounding stock, as shown in figure a the part that is cut out is the desired product in the operation and is called the blank.</p></li><li><p>PunchingIs similar to blanking except that the separated piece is scrap, called the slug. the remaining stock is desired part. the distinction is illustrated in figure b. </p></li><li><p>Engineering analysis of sheetmetal cutting </p><p>One of the important parameters in sheet metal cutting is clearance between the punch and die</p><p>The clearance c in a shearing operation is the distance between the punch and die.</p><p>Typical clearance in conventional press working range between 4% and 8% of the sheet metal thickness t.</p></li><li><p>Con..Measured perpendicular to the direction of the blade movement. </p><p>It affects the finish of the cut(burr) and machines power consumption.</p></li><li><p>Con..If the clearance is too small, then the fracture line tend to pass each other ,causing a double burnishing and large cutting force.</p></li><li><p>con..If the clearance is too large ,the metal becomes pinched between the cutting edges and an excessive burr results.</p></li><li><p>Con..The correct clearance depends on sheetmetal type and thickness. The recommended clearance can be calculated by the following formula:</p><p>C = ActWhere:C=Clearance , mm;Ac = Clearance allowance (percentage of the material thickness) t=Stock thickness, mm.</p></li><li><p>Con..The clearance allowance is determined according to the type of metal 4.5%, 6% or 7.5% of the material thickness. For convenience, metals are classified into three groups with an associated allowance value for each group. </p><p>Clearance allowance for three sheet metal groups</p></li><li><p>Con..The calculated clearance values can be applied to conventional blanking and hole-punching operations to determine the proper punch and die sizes.</p><p>The die opening must always be larger than the punch size(obviously).</p><p>Because of the geometry of the sheared edge, the outer dimension of the part cut out of sheet will be larger than the hole size. </p></li><li><p>Con..Punch and die sizes for a round blank of diameter Db are determined as;</p><p> Blanking punch diameter =Db-2cBlanking die diameter =DbPunch and die sizes for a round hole of diameter Dh are determined as:</p><p> Hole punch diameter = Dh Hole die diameter = Dh+2c</p></li><li><p>Con..Angular ClearancePurpose: allows slug or blank to drop through dieTypical values: 0.25 to 1.5 on each side</p></li><li><p>Cutting force Estimates of cutting force are important because this force determines the size (tonnage) of the press needed. Cutting force F in sheetmetal working can be determined by </p><p>Where S= Shear strength of the metal, Mpa t = Sthock thickness, mm L= length of the cut edge,mm </p><p>In blanking ,punching ,slotting ,and similar operations , L is the perimeter length of the blank or hole being cut. </p></li><li><p>Con..Example</p><p>A round disk of 150mm diameter is to be blanked from a strip of 3.2mm cold rolled steel whose shear strength=310MPa. Determine (a) the appropriate punch and die diameters, and (b) blanking force. </p></li><li><p>Lesson IIOutline</p><p>Other sheet-metal cutting operations Sheet Metal Bending Operation </p></li><li><p>Con..</p><p>Other sheet-metal cutting operations </p></li><li><p>CutoffIs a shearing operation in which blanks are separated from a sheet-metal strip by cutting the opposite sides of the part in sequence. </p></li><li><p>Con..With each cut, anew part is produced.Distinguish it from a conventional shearing operation.The cut edges are not necessarily straight</p></li><li><p>Parting Involves cutting a sheet-metal strip by a punch with two cutting edges that match the opposite sides of the bank. </p></li><li><p>Con..</p><p>This might be required because the part outline has an irregular shape.</p><p>Parting is less efficient than cutoff in the sense that it results in some wasted materials. </p></li><li><p>SlottingIs the term sometimes used for a punching operation that cuts out an elongated or rectangular hole. </p></li><li><p>PerforatingInvolves the simultaneous punching of a pattern of holes in sheet metal.The hole pattern is usually for decorative purposes, or allow passage of light, gas, of fluid.</p></li><li><p>Notching and Seminotching Notching involves cutting out a portion of metal From the side of the sheet or strip.Seminotching removes a portion of metal from the interior of the sheet. </p></li><li><p>Con..Seminotching might seem the same as punching and slotting operation. </p><p>The difference is that the metal removed by seminotching creats part of the blank outline, while punching and slotting creates holes in the blank.</p></li><li><p>Shaving Shaving is a shearing operation performed with very small clearance to obtain accurate dimensions and cut edges that are smooth and straight, as pictured in figure (a).Shaving is typically performed as a secondary or finishing Operation on apart that have been previously cut.</p></li><li><p>Fine blanking Is a shearing operation used to blank sheet metal parts with close tolerances and smooth, straight edges in one step, as illustrated in figure (b). </p></li><li><p>Con..Pressure pad applies holding force(Fh) in order to prevent distortion. </p><p>The punch then descends with slower than the normal velocity and smaller clearance to provide the desired dimension and cut edge. </p><p>The process in usually for small stock thicknesses.v</p></li><li><p>Part II</p></li><li><p>Bending operation Bending in sheet metal work is defined as the straining of the metal around a straight (bend) axis. (figure a). During bending the metal on the inside of the neutral plane is compressed ,while the metal on the outside of the neutral plane is stretched (figure b). </p></li><li><p>Con..The metal is plastically deformed so the bend takes a permanent set upon removal of stresses that caused it.</p><p>Bending produces little or no change in the thickness of the sheet metal. </p></li><li><p>V-Bending and Edge bending Bending operations are performed by using punch and die tooling .The common bending methods are V-Bending ,performed with a V-die.</p></li><li><p>Con..2. Edge bending , performed with wiping die. </p></li><li><p>Con..Edge bending is limited to bend of 90 or less. More complicated wiping dies can be designed for bend angles greater than 90.Because of the pressure pad, wiping dies are more complicated than V-dies and generally used for high production work. </p></li><li><p>Engineering analysis of bending </p><p>The metal of thickness t is bend through an angle called the bend angle . This results in a sheet metal part with an included angle , where + = 180</p></li><li><p>Con..R=Inside Bend radius W= The bend is made over the width of the work piece W.Kba= k factor ( the location of the neutral axis in the material.Since neutral axis is the theoretical location at which the material is neither compressed nor stretched .we use the following recommended values of Kba.</p></li><li>Con..Recommended value of KbaR</li><li><p>Bend allowance The bend allowance (Ab) is the length of the neutral axis between the bend lines, or the arc length of the bend. Where: = bend angle </p></li><li><p>Spring backWhen the bending pressure removed at the end of deformation ,elastic energy remains in the bent part ,causing it to recover particularly to its original shape. This elastic recovery is called springback. Figure: (1) during the operation ,the work is forced to take the radius Rt and included angle t determined by the bending tool (punch in v-bending) (2) after the punch is removed ,the work springs back to radius R and included angle . Symbol: F=applied bending force.</p></li><li><p>Con..Spring back in bending show itself as a decrease in bend angle and an increase in bend radius. </p><p>Sb= spring back= Included angle of the sheet metal (after spring back)t =Included angle of the bending tool</p></li><li><p>Compensation for springback Can accomplished by several methods one of these methods is over bending. Due to this elastic recovery, it is necessary to over-bend the sheet a precise amount to achieve the desired bend radius and bend angle . This is called over bending. </p></li><li><p>Bending forceBending Force: The force required to perform bending depends on the Geometry of the punch and die and the strength, thickness, and length of Sheetmtal. The maximum bending force can be estimated by means of the following equation. </p></li><li><p>Con..Where F = Bending forceTS = Tensile strength of the sheet metalW = Width of part in the direction of the bend axis.t= stock thicknessD=die opening dimension Kbf= Is a constant for differences encountered in actual bending process. Its value depends on type of bending: For V-bending, Kbf=1.33; and for edge bending, Kbf=0.33</p></li><li><p>Con..Die opening dimension a) V-die b) Wiping Die</p></li><li><p>Con..Example:A sheet metal blank is to be bent as shown in figure below. The metal has a modulus of elasticity=205(103) MPa, yield strength=275MPa, and tensile strength=450MPa. Determine (a) the starting blank size and (b) the bending force if a V-die is used with a die opening dimension = 25mm.v</p></li><li><p>Other bending operationsFlanging </p></li><li><p>Con..</p></li><li><p>Con..</p></li><li><p>Miscellaneous bending operationsFigure: miscellaneous bending operations: (a) Channel bending (b) U-bending</p><p>(c) Air bending (d) Offset bending (e) Corrugating symbol : F=applied force. </p></li><li><p>Part III</p></li><li><p>Drawing Drawing is a sheet metal forming operation used to make cup shaped ,box-shaped ,or other complex curved, hollow shaped parts. </p><p>It is performed by placing a piece of sheet metal over a die cavity and pushing the metal into the opening with a punch. </p><p>The blank must be held down flat against the die by blank holder. </p></li><li><p>Con..</p><p>( a ) Drawing of cup shaped part :(1) start of operation before punch contacts work, and (2) near end of stroke </p></li><li><p>Con..(b) Workpart: (1) starting blank, and (2) drawn part . Symbols: C=clearance, Db=blank diameter, Dp= punch diameter, Rp=punch corner radius, F=drawing force,Fh=holding force.</p></li><li><p>Parts produced Ammunition shellsBeverageSinksand Other parts 12</p></li><li><p>Mechanics of drawing A blank diameter Db is drawn into a die by means of a punch of diameter Dp. The punch and die must have a corner radii, given by Rp and Rd .</p></li><li><p>Con..If the punch and die were to have sharp corners (Rp and Rd=0) a hole punching process would-be accomplished rather than drawing operation. </p><p>The sides of the punch and die are separated by a clearance c.</p><p>The clearance in drawing is about 10% grater than the stock thickness. C= 0.1t </p></li><li><p>Con..The punch applies a downward force F to accomplish the deformation of the metal, and a downward holding force Fh is applied by the blankholder, as shown in the sketch.</p></li><li><p>Stages in deformation of the work 1) Punch makes initial contact with work2) bending </p></li><li><p>Con..3) straightening 4) Friction and compression 5) Final cup shape </p></li><li><p>Con..</p></li><li><p>Engineering analysis of drawing Checking for feasibility of the operationFirst Drawing ration: The ratio of blank diameter Db to punch diameter Dp. Un upper limit on the drawing ration is a value of 2.0 (DR2.0)</p></li><li><p>Con..Second Reduction r</p><p>The value of reduction r should be less than 0.50</p></li><li><p>Con..Third Thickness to diameter ration </p><p>It is desirable the ratio to be greater than 1%. </p></li><li><p>Con..Example 1A drawing operation is used to form a cylindrical cup with inside diameter=75mm and height = 50mm. The starting blank size=138mm and the stock thickness=2.4mm.based on these data, is the operation feasible?</p></li><li><p>Drawing forceF = Drawing force, Nt = Original blank thicknessTS= Tensile strengthDb and Dp = Starting blank and punch diameter.</p><p>Constant 0.7 is correction factor to account for friction. </p><p>Forces: The drawing force required to perform a given operation can be estimated by the formula:</p></li><li><p>Con..Where Fh=holding force in drawing ,NY=yield strength of the sheet metalt =starting stock thickness,mmRd=die corner radius, mmThe holding force (Fh) required can be calculated by using the following formula. </p></li><li><p>Con..Example 2For the drawing operation of Example 1 determine(a) drawing force and (b) holding force, given that the tensile strength of the sheet metal(low-carbon steel)=300MPa and yield strength=175MPa.The die corner radius=6mm.</p></li><li><p>Part IVStretch Forming</p></li><li><p>Con..Stretch forming: is a sheet metal deformation process in which the sheet is internationally stretched and simultaneously bent in order to achieve shape change. Figure below shows stretch forming for relatively simple gradual bend.</p></li><li><p>Con..The workpart is gripped by one or more jaws on each end and then stretched and bent over a die containing the desired form.The metal is stressed in tension to a level above its yield point.When the tension loading is released, the metal has been plastically deformed.The combination of stretching and bending results in relatively little springback in the part</p></li><li><p>Con..Where; F=Stretching Force, N L=length of the sheet in the direction perpendicular to stretching(mm) t =Instantaneous stock thickness, mm Yf=flow stress of the metal, Mpa</p></li><li><p>Con..Yf=flow stress, Mpa; K=Strength coefficient , n =str...</p></li></ul>

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