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9/1/2011Sheet Metaly Product has light weight andversatile shape ascompared to forging/castingSheet Metal Operationpy Most commonly used– low carbon steel sheet (cost,strength,g formability)yy Aluminium and titanium for aircraft and aerospacey Sheet metal has become a significant material for,‐ automotive bodies and frames,‐ office furnitureBy S K Mondal‐frames for home appliancesPiercing (Punching) and BlankingPiercing (Punching) and Blankingy Piercing and blanking are shearing operations.y In blanking, the piece being punched out becomesthe workpiece and any major burrs or undesirablefeatures should be left on the remaining strip.y In piercing (Punching), the punch‐out is the scrapand the remaining strip is the workpiece.y Both done on some form of mechanical press.Clearance (VIMP)y Die opening must be larger than punch and known as‘clearance’.y PunchingPunch size of holeDie punch size 2 clearanceyRemember: In punching punch is correct size.y BlankingDie size of productPunch Die size ‐2 clearanceyRemember: In blanking die size will be correct.1

9/1/2011ClearanceContd .BlankingPunchingExampleDetermine the die and punch sizes for blanking a circulardisc of 20‐mm diameter from a sheet whose thickness is 1.5mm.Shear strength of sheet material 294 MPaPunching Force and Blanking ForceFm ax LtτThe punching force for holes which are smaller than the stockthickness may be estimated as follows:Fmax π dtσ3Capacity of Press for Punching and BlankingPress capacity will be Fmax CdtMinimum Diameter of PiercingπPiercing pressure, τs πd.t Strength of punch, σc 4 d2[Where C is a constant and equal to 1.1 to 1.75 dependingupon the profile]2

9/1/2011ExampleEstimate the blanking force to cut a blank 25 mm wideand 30 mm long from a 1.5 mm thick metal strip, if theShear on Punchy To reduce shearing force, shear is ground on the face ofthe die or punch.ultimate shear strength of the material is 450 N/mm2.y It distribute the cutting action over a period of time.Also determine the work done if the percentagey Shear only reduces the maximum force to be applied butpenetration is 25 percent of material thickness.total work done remains same.Force required with shear on PunchF Fmax (tp) Lτ t(tp) SSWhere p penetration of punch as a fractionS shear on the punch or die, mmExampley A hole, 100 mm diameter, is to be punched in steel plate5.6 mm thick. The ultimate shear stress is 550 N/mm2 .With normal clearance on the tools,, cuttingg is completepat 40 per cent penetration of the punch. Give suitableshear angle for the punch to bring the work within thecapacity of a 30T press.ExampleA washer with a 12.7 mm internal hole and an outsidediameter of 25.4 mm is to be made from 1.5 mm thickstrip. The ultimate shearing strength of the material ofthe washer is 280 N/mm2.((a)) Find the total cuttingg force if both ppunches act atthe same time and no shear is applied to either punchor the die.(b) What will be the cutting force if the punches arestaggered, so that only one punch acts at a time.(c) Taking 60% penetration and shear on punch of 1mm, what will be the cutting force if both punches acttogether.3

9/1/2011Fine BlankingEnergy and Power for Punching and BlankingIdeal Energy (E in J) maximum force x punch travel Fmax ( p t )(Unit:Fmax in kJ and t in mm othrwise use Fmax in J and t in m)Where p is percentage penetration required for ruptureE NIdeal power in press ( P inW ) 60[Where N actual number of stroke per minute]Fine Blanking ‐ dies are designed that have smallclearances and pressure pads that hold the materialwhile it is sheared. The final result is blanks that haveextremely close tolerances.Actual Energy ( E in J ) Fmax ( p t ) CWhere C is a constant and equal to 1.1 to 1.75 depending upon the profileE NActual power in press ( P inW ) 60 ηWhereE is actual energy and η is efficiency of the pressy Slitting ‐ moving rollers trace out complex paths duringcutting (like a can opener).y Perforating: Multiple holes which are very small andgare cut in flat work material.close togethery Notching: Metal pieces are cut from the edge of a sheet,strip or blank.y Trimming ‐ Cutting unwanted excess material from theperiphery of a previously formed component.y Shaving ‐ Accurate dimensions of the part are obtained byy Lancing – A hole is partially cut and then one side is bentdown to form a sort of tab or louver. No metal removal, noscrap.removing a thin strip of metal along the edges.y Squeezing ‐ Metal is caused to flow to all portions of a diecavity under the action of compressive forces.4

9/1/2011Dinkingy Steel Rules ‐ soft materials are cut with a steel stripshaped so that the edge is the pattern to be cut.y Nibbling ‐ a single punch is moved up and down rapidly,y Used to blank shapes from low‐strength materials, such asrubber, fiber, or cloth.y The shank of a die is either struck with a hammer or mallet orthe entire die is driven downward by some form ofpmechanical press.each time cutting off a small amount of material. Thisallows a simple die to cut complex slots.Elastic recovery or spring backy Total deformation elastic deformation plasticElastic recovery or spring backContd.y More important in cold working.deformation.y It depends on the yield strength. Higher the yieldy At the end of a metal working operation, when thestrength, greater spring back.pressure is released, there is an elastic recovery and thetotal deformation will get reduced a little. Thisphenomenon is called as "spring back".y To compensate this, the cold deformation be carriedbeyond the desired limit by an amount equal to thespring back.Punch and Die materialyCommonly used – tool steelyFor high production ‐ carbidesPunching Press5

9/1/2011Bolster plateBolster plateContd.y When many dies are to run in the same press at differenttimes, the wear occurring on the press bed is high. Thebolster plate is incorporated to take this wear.y Relatively cheap and easy to replace.y Attached to the press bed and the die shoe is thenattached to it.Punch plateStrippery Used to locate and hold thepunch in position.y This is a useful way ofmounting,especiallyforsmall punches.StripperContd.y The stripper removes the stock from the punch after apiercing or blanking operation.Ps KLtKnockouty Knockout is a mechanism, usually connected to andoperated by the press ram, for freeing a work piece froma die.Where Ps stripping force, kNL perimeteri t off cut,t mmt stock thickness, mmK stripping constant, 0.0103 for low‐ carbon steels thinner than 1.5 mm withthe cut at the edge or near a preceding cut 0.0145 for same materials but for other cuts 0.0207 for low‐ carbon steels above 1.5‐mm thickness 0.0241 for harder materials6

9/1/2011PitmanDowel piny It is a connecting rod which is used to transmit motionfrom the main drive shaft to the press slide.Drawingy Drawing is a plastic deformation process in which a flatsheet or plate is formed into a three‐dimensional partwith a depth more than several times the thickness ofthe metal.Drawingy As a punch descends into a mating die, the metalassumes the desired configuration.Drawingy Hot drawing is used for thick‐walled parts of simplegeometries, thinning takes place.y Cold drawing uses relatively thin metal, changes thethickness very little or not at all, and produces parts in awide variety of shapes.7

9/1/2011Blank SizeD d 2 4dhWhen d 20rBlank Holding Forcey Blank holding force required depends on the wrinklingDrawing Force D P πdts C d Draw Clearancey Punch diameter Die opening diameter – 2.5 t.tendency of the cup. The maximum limit is generally tobe one‐third of the drawing force.Deep drawingy Drawing when cup height is more than half the diameter istermed deep drawing.y Easy with ductile materials.Stresses on Deep Drawingy In flange of blank: Bi‐axial tension and compressiony In wall of the cup: simple uni‐axial tensiony Due to the radial flowflo of material,material the side wallsalls increase inthickness as the height is increased.y A cylindrical vessel with flat bottom can be deep drawn bydouble action deep drawing.y Deep drawing ‐ is a combination of drawing and stretching.8

9/1/2011Deep Drawabilityy The ratio of the maximum blank diameter to thediameter of the cup drawn . i.e. D/d.y There is a limiting drawing ratio (LDR), after which thepunch will pierce a hole in the blank instead of drawing.y This ratio depends upon material, amount of frictionpresent, etc.y Limiting drawing ratio (LDR) is 1.6 to 2.3Limiting Drawing Ratio (LDR)y The average reduction in deep drawingd 0.5Dd Reduction 1 100% 50%D Thumb rule:Exampley A symmetrical cup of 80 mm diameter and 250 mmheight is to be fabricated on a deep drawing die.How many drawing operations will be necessary ifno intervening annealing is done.y Also find the drawing forceFirst draw:Reduction 50 %Second draw:Reduction 30 %Third draw:Reduction 25 %Fourth draw:Reduction 16 %Fifth draw:Reduction 13 %Draw Die Designy Progressive diesy Compound diesy CombinationCbi ti diesdiProgressive diesPerform two or more operations simultaneously in a singlestroke of a punch press, so that a complete component isobtained for each stroke.Compound diesAll the necessary operations are carried out at a singlestation, in a singleg stroke of the ram. To do more than one setof operations, a compound die consists of the necessary setsof punches and dies.Combination diesA combination die is same as that of a compound die withthe main difference that here non‐cutting operations such asbending and forming are also included as part of theoperation.9

9/1/2011Lubricationy In drawing operation, proper lubrication is essential for1.2.3.4.To improve die life.To reduce drawing forces.To reduce temperature.To improve surface finish.Defects in Drawing ‐ wrinkley An insufficient blank holder pressure causes wrinkles todevelop on the flange, which may also extend to the wallof the cup.Defects in Drawing ‐ FractureDefects in Drawing ‐earingy Further, too much of a blank holder pressure and frictiony While drawing a rolled stock, ears or lobes tend to occurmay cause a thinning of the walls and a fracture at theflange, bottom, and the corners (if any).because of the anisotropy induced by the rollingoperation.Defects in Drawing – miss strikeDefects in Drawing – Orange peely Due to the misplacement of the stock, unsymmetricaly A surface roughening (defect) encountered in formingflanges may result. This defect is known as miss strike.products from metal stock that has a coarse grain size.y It is due to uneven flow or to the appearance of theoverly large grains usually the result of annealing at toohigh a temperature.temperature10

9/1/2011Stretcher strains (like Luders Lines)y Caused by plastic deformation due to inhomogeneousSurface scratchesy Die or punch not having a smooth surface, insufficientyielding.y These lines can criss‐cross the surface of the workpiece andlubricationmay be visibly objectionable.objectionabley Low carbon steel and aluminium shows more stretcherstrains.SpinningSpinningy Spinning is a cold‐forming operation in which arotating disk of sheet metal is shaped over a maleform, or mandrel.y Localized pressure is applied through a simpleround‐ended wooden or metal tool or small roller,which traverses the entire surface of the partSpinning1. A mandrel (or die for internal pieces) is placed on arotating axis (like a turning center).2. A blank or tube is held to the face of the mandrel.3. A roller is pushed against the material near thecentert off rotation,t tiandd slowlyl l movedd outwards,td pushinghithe blank against the mandrel.4. The part conforms to the shape of the mandrel (withsome springback).5. The process is stopped, and the part is removed andtrimmed.11