Metal Forming, Production Engineering II
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Metal Forming, Production Engineering II

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Production Engineering

Production Engineering

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    Metal Forming, Production Engineering II Metal Forming, Production Engineering II Presentation Transcript

    • Lecturer:-Zeradam Y. March, 2014 Ambo University Institute of Technology Department of Mechanical Engineering Production Engineering II (MEng 3162)
    • Outline • Sheet metal forming operation • Applications of sheet metal • Cutting operation Shearing Blanking Punching • Engineering analysis of sheet metal cutting
    • Part I Sheet metal forming  Sheet metal working includes cutting and forming operations performed on relatively thin sheets of metal. Shearing Blanking Punching Bending Drawing Spinning Stretching Cutting Forming
    • Con..  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.
    • Applications Construction equipments Automobile and truck bodies Ship building furnitureRailway Cars Airplane
    • Con.. space industries; Chemical industry;Drink & food industry. Nuclear
    • Con.. Shower cabinets and othersWashing machines Cookers Refrigerator bodies Domestic use
    • Five basic sheet metal operations Cutting  Shearing  Blanking  Punching
    • Con.. Forming  Bending,  Spinning,  Stretching,  Drawing
    • Cutting operation • 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).
    • Con.. c (1) Just before the punch contact the work
    • Con.. • c (2) Punch begins to push into the work, causing plastic deformation
    • Con.. • c (3) Punch compresses and penetrates into the work and causing a smooth cut surface
    • Con.. • c (4) Fracture is initiated at the opposing cutting edges that separate the sheet.  Symbols V and F represents motion and applied force , respectively, t = stock thickness , c = clearance.
    • 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.
    • 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  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.
    • Blanking • Involves 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.
    • Punching • Is 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.
    • Engineering analysis of sheetmetal cutting • One of the important parameters in sheet metal cutting is clearance between the punch and die • The clearance c in a shearing operation is the distance between the punch and die. • Typical clearance in conventional press working range between 4% and 8% of the sheet metal thickness t.
    • Con.. • Measured perpendicular to the direction of the blade movement. • It affects the finish of the cut(burr) and machine’s power consumption.
    • Con.. • If the clearance is too small, then the fracture line tend to pass each other ,causing a double burnishing and large cutting force.
    • con.. • If the clearance is too large ,the metal becomes pinched between the cutting edges and an excessive burr results.
    • Con.. • The correct clearance depends on sheetmetal type and thickness. • The recommended clearance can be calculated by the following formula: C = Act Where: C=Clearance , mm; Ac = Clearance allowance (percentage of the material thickness) t=Stock thickness, mm.
    • 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. Clearance allowance for three sheet metal groups
    • Con.. • The calculated clearance values can be applied to conventional blanking and hole-punching operations to determine the proper punch and die sizes. • The die opening must always be larger than the punch size(obviously). • 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.
    • Con.. • Punch and die sizes for a round blank of diameter Db are determined as; Blanking punch diameter =Db-2c Blanking die diameter =Db • Punch and die sizes for a round hole of diameter Dh are determined as: Hole punch diameter = Dh Hole die diameter = Dh+2c
    • Con.. Angular Clearance  Purpose: allows slug or blank to drop through die  Typical values: 0.25° to 1.5° on each side
    • 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 Where S= Shear strength of the metal, Mpa t = Sthock thickness, mm L= length of the cut edge,mm • In blanking ,punching ,slotting ,and similar operations , L is the perimeter length of the blank or hole being cut.
    • Con.. Example 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.
    • Con.. Other sheet-metal cutting operations
    • Cutoff Is a shearing operation in which blanks are separated from a sheet-metal strip by cutting the opposite sides of the part in sequence.
    • Con.. With each cut, anew part is produced. Distinguish it from a conventional shearing operation. • The cut edges are not necessarily straight
    • Parting  Involves cutting a sheet-metal strip by a punch with two cutting edges that match the opposite sides of the bank.
    • Con..  This might be required because the part outline has an irregular shape.  Parting is less efficient than cutoff in the sense that it results in some wasted materials.
    • Slotting  Is the term sometimes used for a punching operzation that cuts out an elongated or rectangular hole.
    • Perforating  Involves 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.
    • 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.
    • Con..  Seminotching might seem the same as punching and slotting operation.  The difference is that the metal removed by seminotching creats part of the blank outline, while punching and slotting creates holes in the blank.
    • Shaving
    • 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).
    • Con..  Pressure pad applies holding force(Fh) in order to prevent distortion.  The punch then descends with slower than the normal velocity and smaller clearance to provide the desired dimension and cut edge.  The process in usually for small stock thicknesses.
    • Questions ?
    • Part II
    • Bending operation  Bending in sheet metal work is defined as the straining of the metal around a straight 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).
    • Con.. • The metal is plastically deformed so the bend takes a permanent set upon removal of stresses that caused it. • Bending produces little or no change in the thickness of the sheet metal.
    • V-Bending and Edge bending  Bending operations are performed by using punch and die tooling .  The common bending methods are 1. V-Bending ,performed with a V-die.
    • Con.. 2. Edge bending , performed with wiping die.
    • Con..
    • Engineering analysis of bending  The metal of thickness t is bend through an angle called the bend angle α.  This results in a sheet metal part with an included angle α’, where α + α’= 180°
    • 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. • Calculated as the ratio of the neutral axis (measured from the inside bend surface to the material thickness) • Since neutral axis is the theoretical location at which the material is neither compressed nor stretched .we use the following recommended values of K.
    • Con..  Recommended value of Kba R<2t,Kba=0.33 R ≥2t,Kba=0.50  t=stock thickness
    • Bend allowance  The bend allowance (Ab) is the length of the neutral axis between the bend lines, or the arc length of the bend.
    • Spring back  When 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 α’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 α’. Symbol: F=applied bending force.
    • Con.. • Spring back in bending show itself as a decrease in bend angle and an increase in bend radius. Sb= spring back α’= Included angle of the sheet metal (after spring back) α’t =Included angle of the bending tool
    • 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.
    • Bending force
    • Con.. F = Bending force TS = Tensile strength of the sheet metal W = Width of part in the direction of the bend axis. t= stock thickness D=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
    • 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.
    • Other bending operations Flanging
    • Con..
    • Con..
    • Miscellaneous bending operations • Figure: miscellaneous bending operations: (a) channel bending (b) U-bending (c) air bending (d) offset bending (e) corrugating symbol : F=applied force. Bending
    • Part III
    • Drawing  Drawing is a sheet metal forming operation used to make cup shaped ,box-shaped ,or other complex curved, hollow shaped parts.  It is performed by placing a piece of sheet metal over a die cavity and pushing the metal into the opening with a punch.  The blank must be held down flat against the die by blank holder.
    • Con.. ( a ) Drawing of cup shaped part : (1) start of operation before punch contacts work, and (2) near end of stroke
    • 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.
    • Parts produced Ammunition shellsBeverage Sinks Automobile and Other parts
    • 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 .
    • Con..  If the punch and die were to have sharp corners (Rp and Rd=0) a hole punching process wood be accomplished rather than drawing operation.  The sides of the punch and die are separated by a clearance c.  The clearance in drawing is about 10% grater than the stock thickness. C= 0.1t
    • Con..
    • Stages in deformation of the work 1) Punch makes initial contact with work
    • Con.. 2) bending
    • Con.. 3) straightening
    • Con.. 4) Friction and compression
    • Con.. 5) Final cup shape
    • Engineering analysis of drawing Checking for feasibility of the operation First  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 (DR≤2.0)
    • Con.. Second  Reduction r The value of reduction r should be less than 0.50
    • Con.. Third  Thickness to diameter ration  It is desirable the ratio to be greater than 1%.
    • Con.. Example 1
    • Drawing force F = drawing force, N t = original blank thickness TS= tensile strength Db and Dp = starting blank and punch diameter. Constant 0.7 is correction factor to account for friction.
    • Con.. Where Fh=holding force in drawing ,N Y=yield strength of the sheet metal t =starting stock thickness,mm Rd=die corner radius, mm The holding force (Fh) required can be calculated by using the following formula.
    • Con.. 1 Example 2