Sheet Metal Plastic Part Notes

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Transcript of Sheet Metal Plastic Part Notes

Sheet Metal Design

Press WorkingAlso called as Chipless Manufacturing and Cold Stamping. Machine Used Press Press Comprises of : Frame supporting Ram & Bed Mechanism to operate ram Ram with Punch Die Block Operations: 1. Cutting Operations(work piece stressed beyond ultimate strength) - Blanking, Punching, Notching, Perforating, Trimming, Shaving, Slitting & Lancing 2. Forming Operations (work piece stressed below ultimate strength) - Bending, Drawing, redrawing & Squeezing

Types of PressesClassified on following: Source of Power: Mechanical - The energy of flywheel is utilized which is transmitted to the work-piece by gears, cranks, eccentrics or levers. The flywheel rotates freely on the crank shaft and is driven from an electric motor through gears or V belts. Hydraulic - In hydraulic press, the ram is actuated by oil pressure on a piston in a cylinder.

Press SelectionFactors to be considered while selecting a press Overall work size, the stock thickness and material, kind of operation to be performed, power required and speed of operation. For punching, blanking and trimming operations usually the crank or eccentric type mechanical press is used. This is due to their small working strokes and high production rates. In these operations, there is sudden release of load at the end of the cutting stroke. This sudden release of load is not advisable in hydraulic presses. So, hydraulic presses are not preferred for these operations. If however these are inevitable, then some damping devices are incorporated in the press design. For coining and other squeezing operations, which require very large forces, knuckle joint mechanical press is ideally suited. Hydraulic presses, which are slower and more powerful, can also be used for these operations. Hydraulic presses are also better adapted to pressing, forming and operations, which are slower processes.

Press WorkingDefinitions of the main components of the die and press : Bed - The bed is the lower part of a press frame, that serves as a table to which a bolster plate is mounted. Bolster plate - This is a thick plate secured to the press bed, which is used for locating and supporting the die assembly. It is usually 5 to 12.5 cm thick. Die set - It is unit assembly which incorporates a lower and upper shoe, two or more guideposts and guidepost bushings. Die - The die may be defined as the female part of a complete tool for producing work in a press. It is also referred to a complete tool consisting of a pair of mating members for producing work in a press. Die block, is a block or a plate which contains a die cavity. Lower Shoe - The lower shoe of a die set is generally mounted on the bolster plate of a press. The die block is mounted on the lower shoe. Also, the guide posts are mounted in it.

Press Working

Press WorkingPunch - This is the male component of the die assembly, which is directly or indirectly moved by and fastened to the press ram or slide. Upper Shoe - This is the upper part of the die set which contains guidepost bushings. Back up plate - Back up plate or pressure plate is placed so that the intensity of pressure does not become excessive on punch hold or the plate distributes the pressure over a wide area and the intensity of pressure on the punch holder is reduced to avoid crushing. Stripper - It is a plate which is used to strip the metal strip from a cutting or non-cutting punch or die. It may also guide the sheet. Knockout - It is a mechanism, usually connected to and operated by the press ram, for freeing a work-piece from a die. Pitman - It is a connecting rod which is used to transmit motion from the main drive shaft to the press slide. Shut Height - It is the distance from top of the bed to the bottom of the slide, with its stroke down and adjustment up.

Cutting OperationsNotching: This is cutting operation by which metal pieces are cut from the edge of a sheet, strip or blank. Perforating: This is a process by which multiple holes which are very small and close together are cut in a flat work material. Trimming: This operation consists of cutting unwanted excess material from the periphery of a previously formed component. Shaving: Edges of a blanked part are generally rough, uneven and unsquare. Accurate dimensions of the part are obtained by removing a thin strip of metal along the edges. This operation is termed as Shaving. Slitting: It refers to the operation of making incomplete holes in a workpiece. Lancing: This is a cutting operation in which a hole is partially cut and then one side is bent down to form a sort of tab or louver. Since no metal is actually removed, there will be no scrap. Nibbling: The nibbling operation which is used for only small quantities of components, is designed for cutting out flat parts from sheet metal. The flat parts range from simple to complex contours. This operation is generally substituted for blanking. The part is usually moved and guided by hand as the continuously operating punch cuts away at the edge of the desired contour.

Forming OperationsBending - In this operation, the material in the form of flat sheet or strip, is uniformly strained around a linear axis which lies in the neutral plane and perpendicular to the lengthwise direction of the sheet or metal. Drawing - This is a process of forming a flat workpiece in to a hollow shape by means of a punch which causes the blank to flow into a die cavity. Squeezing - Under this operation, the metal is caused to flow to all portions of a die cavity under the action of compressive forces.

Press OperationsShearing - Shearing is a sheet metal cutting operation along a straight line between two cutting edges by means of a power shear. Metal sheet is held on top of hardened die, shearing blade cuts downward, usually driven by hydraulic or electric force. Punching & Blanking - Blanking and punching are similar sheet metal cutting operations that involve cutting the sheet metal along a closed outline. If the part that is cut out is the desired product, the operation is called blanking and the product is called blank. If the remaining stock is the desired part, the operation is called punching. Both operations are illustrated on the example of producing a washer:

Blanking & Punching

Cutting of sheet metal is accomplished by a shearing action between two sharp edges. The shearing action is illustrated in the figure:

Punching & Blanking

Blanking & PunchingClearance: Clearance c is the distance between the punch and die. The correct clearance depends on sheet-metal type and thickness t: c = at (Typically 2 to 10% of thickness) where a is the allowance (a = 0.075 for steels and 0.060 for aluminum alloys)

Blanking & PunchingThe usual clearances per side of the die, for various metals, are given below in terms of the stock thickness, t : For brass and soft steel, c = 5% of t For medium steel, c = 6% of t For hard steel, c = 7% of t For aluminium, c = 10% of t

Blanking & PunchingCutting forces Cutting force in all shearing operations is determined by F=StL where S is the shear strength of material, for approximate solutions, S=0.7UTS T is thickness of sheet L is the length of the cut edge

Strip LayoutPreparation of Blanking Layout It is a layout, of position of the work-pieces in the strip and their orientation with respect to one another. Major considerations are Economy of material Direction of material grain Scrap twisting & wedging

The strip layout with maximum material saving may not be the best strip layout, as the die construction may become more complex which would offset the savings due to material economy unless a large number of parts are to be produced.

Forming OperationsSome Design Tips: Dimension the part in a single direction wherever possible Sequential nature of the forming process and introduction of dimensional variation at each bend It is in line with the process and helps to control tolerance accumulation Allow a more generous bend tolerance (+/- .007) as tighter tolerances, while achievable, will result in higher costs Use consistent bend radius for all bends per part ; it minimize setup changes Dimensioning should be done from a feature to an edge. Avoid feature-to-feature dimensions over two or more planes. Feature-to-bend dimensions may require special fixtures or gauging. This also means that tolerances in the title block of a drawing may be unnecessarily restrictive for certain dimensions and angles, while very appropriate for others.

Forming OperationsForming Near Holes When a bend is made too close to a hole, the hole maybecome deformed. Figure "A" shows a hole that has become teardrop shaped because of this problem. To save the cost of punching or drilling in a secondary operation the following formulas can be used to calculate the minimum distance required: For a hole < 1" in diameter the minimum distance "D" = 2T + R (see fig. "B") For a slot or hole > 1" diameter then the minimum distance "D" = 2.5T + R (see fig. "C)

Forming OperationsEdge Distortion An exaggerated example of edge deformation is pictured in figure "A" below. The overhang caused by this distortion can be as large as the material thickness. As material thickness increases and bend radius decreases the overhang becomes more severe. In situations where an overhang is unacceptable the part can be relieved as in figure "B".

Forming OperationsFeature placement restrictions When placing formed features next to one another, care should be taken to allow clearance between features. If the station does not clear a form already placed in the part, the form could be flattened out. An example of good vs. bad placement is illustrated below. Relieving the stripper can overcome this problem in some cases.

Forming OperationsCounter sinks A coun