Modern Sheet-metal Forming Processes Used in the Automotive Industry - Case Study Example

Modern sheet-metal forming processes used in the automotive industry Introduction Sheet- metal parts are usually made through forming material in cold conditions. However, many sheet metal parts are formed in a hot condition as the material has a lower resistance to deformation when heated. Initial materials very often used are strips or blanks, and are formed on presses using appropriate tools, the shape of the tool generally determining the shape of the part. Sheet-metal forming processes are used for both serial and mass production. Their characteristics include high levels of productivity, high efficiency in the use of material, easy servicing of machines, ability to use workers without advanced basic skills and other economic advantages. Parts that are made using sheet metal have numerous attractive qualities, which include excellent accuracy of dimension, adequate strength, light weight, and a wide range of possible dimensions ranging from miniature parts in electronics to the large parts of airplane structures. All sheet-metal forming processes can be divided into two major groups: cutting processes that include shearing, blanking, notching, piercing, and so on; and plastic deformation processes, which include bending, stretch formation, deep drawing, and other various forming processes. The cutting group of processes involves cutting the material by subjecting it to sheer stresses between punch and die or between the blades of a shear. The punch and die may be any shape, and the cutting contour may be open and closed. Shearing: this involves cutting of flat material forms from sheets, plates or strip. The type of blade or cutter used, whether the cutter is rotary or straight, may classify the process. Blanking: this involves cutting the material to a closed contour by subjecting it to sheer stresses between punch and die. The work part is usually the slug while the remainder is scrap. Punching: this is a cutting operation in which various shaped holes are sheared in blanks. The sheared slug is discarded while the material that surrounds the punch is produced. Parting: this consists of cutting the piece into several pieces or removing pieces of scrap of various shapes from deep drawn pieces. The operation of parting results in production of some scrap, unlike cutoff. Lancing: in this process, a single line cut is made partway across the work material. There is no scrap as no material is removed. Shaving; this is a cutting operation that improves the quality and accuracy of blanked parts through removing a thin strip of metal along the edges. The plastic deformation group of processes involves partial or total plastic deformation of the work material. Bending; this consists of straining flat sheets or strips of metal uniformly around a linear axis. Metal on the outside of the bend is stressed in tension beyond the elastic limit. Metal on the inside of the bend is compressed. Twisting; the process of straining flat strips of metal around a longitudinal axis. Curling: a rounded, folded back or beaded edge is formed on thin metal parts or strips for stiffening and for providing a smooth, rounded edge. Deep drawing: the process of forming a flat piece of metal blank into a cylindrical part using a punch that forces the blank into a die cavity. Spinning; the process of forming work pieces from a circular blank or from a length of tubing. Stretch forming: this is producing contoured parts by stretching a metal sheet, strips or profile over a shaped block form. Necking; the process by which the top of a cup may be made smaller than its body. Bulging: a process involving placing a tubular, conical or curvilinear part in a split female die and expanding it. Flanging; a whole making process performed on flat stock. Bending Bending is a process by which metal can be molded into any shape through plastically deforming the material. It usually refers to deformation about one axis, and the material is stressed above its yield strength but below it’s ultimate tensile strength. The surface area of the material is not altered much. It is a flexile process by which a wide variety of shapes can be produced using standard die sets. The material is placed on the die, then positioned in place using stops and gages, and held in place with hold-downs. The upper part of the press, which is the ram with the desired shaped punch descends and forms a v-shaped bend (Boljanovic 2004, p234-312). Bending is done by use of press brakes that normally have a 20 to 200 tons capacity to accommodate 1m to 4.5m stock. However, larger or smaller presses can be used for specializes applications. Programmable back gages together with multiple die sets are currently available, making economical processes (Boljanovic 2004, p 334-336). There are two types of bending: Air bending and bottoming. Air bending is done with the punch touching the work piece. As the punch is released, the work piece has less bend than that on the punch. This is known as spring-back, and its amount depends on the material, thickness, grain and temper. The spring-back usually has a range of 5 to 10 degrees and the same angle is used in both the punch and the die for minimal set-up time. The inner radius of the bend is similar to the radius of the punch. In bottoming, the punch and the work piece bottom on the die. This makes for a controlled angle with little spring back, with the tonnage required more than in air bending (Kuljanic 2005). Advantages of bending include cost effectiveness when used for between low and medium quantities, as it does not require large amounts of tooling. Deep drawing Deep drawing is the process in which sheet metal is stretched into the required part shape. A tool is pushed downward on the sheet metal, forcing it into a die cavity in the desired part’s shape. The tensile force applied causes the sheet to deform into a cup-shaped part plastically. Deep drawn parts have a depth that is equal to more than half of the diameter of the part. These parts may have a wide variety cross sections ranging from straight, tapered to curved walls, cylindrical and rectangular parts being the most common. Deep drawing works most effectively with ductile materials like aluminum, brass, copper and mild steel. Automotive bodies, fuel tanks, utensils and kitchen sinks are examples of parts formed using deep drawing (Brown 2001). The process requires a blank, blank holder, punch and a die, the blank being a piece of sheet metal, usually in the shape of a disc or rectangle that is pre-cut from stock material to be formed into the part. The blank is then clamped down using the blank holder on top of the die, which contains a cavity shaped in the required part’s shape. The punch moves downwards into the blank and stretches the material into the die cavity. The punch movement is usually powered using a hydraulic so that enough force can be applied to the blank. Since both the die and the punch experience wear due to the forces applied to the sheet metal and therefore they are made of tool steel or carbon steel. The process of part drawing sometimes takes place in a series of operations that are called draw reductions whereby in each step, the punch forces the part into a different die, each time stretching the part to a greater depth. The sheet metal portion clamped under the blank holder can form a flange around the part to be trimmed off (Kuljanic 2005). Advantages of deep drawing include production in high volumes, as unit cost reduces considerably as the unit count increases. This is because once the tooling and the dice are created, the process continues with little input or downtime. In comparison to other manufacturing processes, tool construction costs in deep drawing are much lower even in smaller production volumes, proving to be the most cost effective sheet metal forming process. Another advantage includes the functionality of the final product. Deep drawing is ideal for products requiring considerable strength and minimal weight. It is also suitable for product geometries that cannot be achieved using other manufacturing processes (Kuljanic 2005). Deep drawing is most suitable for creating cylindrical end products in that a circular metal blank can be easily draw down into a three dimensional object with a single drawn ratio, with minimal production time and cost. Squares, rectangles and other complex shapes can cause slight complications, but can still be created efficiently and easily. As the complexity of the geometric shape increases, the draw ratio number and costs of production will increase (Kuljanic 2005, p337-502). Spinning as a plastic deformation process Spinning is considered one of the modern methods of sheet metal forming process in the world today. It is applied in the formation of cylindrical parts and shapes of a metal. The modern items manufactured using spinning process includes satellite discs, Rocket nose cones, hubcaps musical instruments and cookware among others. Spinning is also useful to artisans and architectural work in the manufacture of items especially in lighting decorative items. This is done by rotating pieces of sheet metal with forces being applied on the sides to give the metal a cylindrical shape. The disc formed is then subjected to rotation at a very high speed with rollers pressed against the sheet. The shape to be formed is done in accordance with the requirements of items desired. Spinning requires certain tools like the mandrel, blank and the roller to produce the desired shapes, and is done either on a lathe or manually (Todd & Allen 2004). There are basically two methods used in spinning, that is, Shear spinning and the conventional spinning. Shear spinning involves the roller being applied to bend the blank against the mandrel but subjected it to high force as it rotates. This enables it stretch the material on top of the mandrel. The conventional spinning on the other hand is where the roller is pushed against the blank to fit against the contour of the mandrel. The materials commonly used include aluminum, copper, steel and brass. These items are mostly preferable because of their strength and ability to withstand high temperatures (Todd & Allen 2004, p465-502). Advantages of Spinning The American society of metals (1969) observed that spinning method has various advantages over the other classical methods in that, a number of operations can be performed at one go. The method is also much cost effective compared to other methods in that its parameters and parts like the blank, mandrels and rollers can be replaced quickly at less cost. Spinning method have low production cost as this makes it preferable over the classical methods such as stamping, casting, hydro forming and forging among others. Spinning also minimizes wastage of material compared to the classical methods. Future Trends Engineering profession has discovered that the conventional methods used in sheet metal forming processes ate costly, take tool long and complex in nature. It is therefore important to come up with a method that solves all this problems. The future trend in sheet metal formation has come up with an invention of using the asymmetric incremental sheet forming process. This method does not involve the use of die and uses very simple tools. It can also be used for mass production in the manufacture of a variety of components. The engineers see this method as the alternative since it is cost effective and flexible. It is applied in the field of aerospace, automobile industry, biomedical and recycling of panels among others (Jesweit et al 2005). Conclusion In conclusion, according to the engineering community, sheet metal forming usually undergoes different processes depending on the design of the item to be formed. However, plastic deformation process is commonly used in the modern engineering especially in the automobile industries due to its ability to produce complex parts, economy of materials, and accuracy associated with plastic deformation. Engineers therefore consider spinning, bending and deep drawing as the more modern method used in sheet metal formation. Works cited American Society for Metals, (1969), Metals handbook: Technology and engineering. London: The society Boljanovic, V. (2004). Sheet metal forming processes and die design. London: Industrial Press Inc. Print Brown, J. (2001). Modern manufacturing processes. New York: Industrial Press Inc. Print Groover, M. (2001).Fundamentals of Modern Manufacturing: Materials, Processes, and Systems. London: John Wiley and Sons. Print J. 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