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Sheet Metal Forming & Cutting Processes Name Institution of Affiliation Date SECTION 1: SHEET METAL FORMING PROCESSES Task 1(a): Formability of Metals A given metal work piece undergoes plastic deformation without damage occurring to it, this process is called formability. One basic factor that is showing formability and plasticity of materials is the fracture strain, this is determined by the uniaxial tensile test. The strain observed in the test defined by elongation with respect to a references length. Deformation of metallic materials is homogenous up to the point of uniform elongation while strain is localised until the point where fracture occurs. Within the reference length the distribution of deformation is not homogenous which is making the fracture strain not to be considered as an engineering strain. But fracture strain is still a rough indicator of the formability of a material. The typical values of the fracture strain are seven percent of the ultrahigh strength material and over fifty percent for mid-strength steel. One basic typical way for sheet forming application of failure causes is the tearing of material. Neck appears at specific stages of forming which is an indicator of localized plastic deformation. More or less deformation occurs in and around the subsequent neck location in the early stable deformation stage, during the quasi-stable and instable deformation stage, almost all breakage is concentrated in the neck zone. This leads to material failure that is observable through tearing. There are forming-limit curves that show extreme but possible deformation which a sheet material may undergo during any phases of stamping process (Kim & Yang, 2000). The limits largely depend on the mode of destruction and the surface strain ratios. The major surface strain has the lowest range when the plane strain deformation occurs, which means that the minor surfaces strain is zero. Forming limits are specific material property. Normally plane strains values ranges from ten percent of high strength grades and fifty percent of or above this value for mild strength material and for those that have good formability properties. Ductile fracture is a mode of failure which may occur without any tearing after plastic deformation. These fracture occurs as a result of bending or shear deformation in the plane or through the thickness. Void nucleation and expansion on microscopic level may be the cause of the failure mechanism on the materials. When the deformation has exceeded the limits the micro cracks and the macro cracks can now be seen on the work piece. The knowledge of formability of materials is essential for the design and layout of nay of the industrial forming process. Using simulations such as formability criteria of forming limit curves and the finite element method enhance and sometimes are indispensable to specific tool design process. Formability of materials depend on several properties the important properties are such as yield strength, strain hardening rate, and ductility. This properties largely rely on temperature, when the temperature of the workpiece is increases the yield strength and rate of strain hardening progressively reduce and ductility increases. The hot working of metallic materials permits relatively very large amounts of deformation before they crack (Marciniak & Kuczyński, 1967). Task 1(b) (LO1: 1.2, M2, D2): Sheet Forming Processes Sheet metal forming process may include process such as axisymmetric forming process which involve spinning and flow turning, stretch forming, press braking and press die forming. Sheet metals can be cut, bent and stretched into entirely any shape. Two types of operations are performed forming and cutting. Sheet metal working include operations such as cutting operations, bending operations, drawing, other sheet metal forming operations, dies and presses for sheet metal process, and bending of tube stock. Using press die forming, a die is a specialized tool used in cutting and shaping materials mostly by use of a press in industries that do manufacturing. The die is metal block that is used for forming materials like plastic and sheet metals. In the vacuum forming only a one form is used, to create a transparent plastic containers. Vacuum forming is considered a simple thermoforming process and it also uses the same principles as the die forming. The work piece is taken through various stages for the final product to obtain the final form (Namraj & Pravin, 2013). Process; * Blanking: A blanking die produces a flat piece of material by cutting the desired shape in one operation. The finished part is referred to as a blank. Generally a blanking die may only cut the outside contour of a part, often used for parts with no internal features.Three benefits to die blanking are: 1 Accuracy. A properly sharpened die, with the correct amount of clearance between the punch and die, will produce a part that holds close dimensional tolerances in relationship to the part's edges. 2 Appearance. Since the part is blanked in one operation, the finish edges of the part produces a uniform appearance as opposed to varying degrees of burnishing from multiple operations. 3 Flatness. Due to the even compression of the blanking process, the end result is a flat part that may retain a specific level of flatness for additional manufacturing operations. * Broaching: The process of removing material through the use of multiple cutting teeth, with each tooth cutting behind the other. A broaching die is often used to remove material from parts that are too thick for shaving. Bulging: A bulging die expands the closed end of tube through the use of two types of bulging dies. Bulging fluid dies: Uses water or oil as a vehicle to expand the part. Bulging rubber dies: Uses a rubber pad or block under pressure to move the wall of a work piece. * Coining: is similar to forming with the main difference being that a coining die may form completely different features on either face of the blank, these features being transferred from the face of the punch or die respectively. The coining die and punch flow the metal by squeezing the blank within a confined area, instead of bending the blank * Compound operations: Compound dies perform multiple operations on the part. The compound operation is the act of implementing more than one operation during the press cycle. * Compound die: A type of die that has the die block mounted on a punch plate with perforators in the upper die with the inner punch mounted in the lower die set. An inverted type of blanking die that punches upwards, leaving the part sitting on the lower punch instead of blanking the part through * Curling: The curling operation is used to roll the material into a curved shape. A door hinge is an example of a part created by a curling die. * Cut off: Cut off dies are used to cut off excess material from a finished end of a part or to cut off a predetermined length of material strip for additional operations. * Drawing: The drawing operation is very similar to the forming operation except that the drawing operation undergoes severe plastic deformation and the material of the part extends around the sides. A metal cup with a detailed feature at the bottom is an example of the difference between formed and drawn. * Extruding: Extruding is the act of severely deforming blanks of metal called slugs into finished parts such as an aluminium I-beam. Extrusion dies use extremely high pressure from the punch to squeeze the metal out into the desired form. The difference between cold forming and extrusion is extruded parts do not take shape of the punch. * Forming: Forming dies bend the blank along a curved surface. An example of a part that has been formed would be the positive end (+) of an AA battery. * Cold forming (cold heading): Cold forming is similar to extruding in that it squeezes the blank material but cold forming uses the punch and the die to create the desired form, extruding does not. * Roll forming: a continuous bending operation in which sheet or strip metal is gradually formed in tandem sets of rollers until the desired cross-sectional configuration is obtained. Roll forming is ideal for producing parts with long lengths or in large quantities. * Horning: A horning die provides an arbour or horn which the parts are place for secondary operations. * Hydroforming: Forming of tubular part from simpler tubes with high water pressure. * Pancake die: A Pancake die is a simple type of manufacturing die that performs blanking and/or piercing. While many dies perform complex procedures simultaneously, a pancake die may only perform one simple procedure with the finished product being removed by hand. * Piercing: The piercing operation is used to pierce holes in stampings. * Transfer die: Transfer dies provide different stations for operations to be performed. A common practice is to move the material through the die so it is progressively modified at each station until the final operation ejects a finished part. * Progressive die: The sheet metal is fed through as a coil strip, and a different operation (such as punching, blanking, and notching) is performed at the same station of the machine with each stroke of a series of punches. * Shaving: The shaving operation removes a small amount of material from the edges of the part to improve the edges finish or part accuracy. (Compare to Trimming). * Side cam die: Side cams transform vertical motion from the press ram into horizontal or angular motion. * Sub press operation: Sub-press dies blank and/or form small watch, clock, and instrument parts. * Swaging: Swaging (necking) is the process of "necking down" a feature on a part. Swaging is the...
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