Stamping punches metal sheet into desired shape

When stamping, a sheet is clamped around the edge and formed into a cavity by a punch. The metal is stretched by membrane forces so that it conforms to the shape of the tools. The membrane stresses in the sheet far exceed the contact stresses between the tools and the sheet, and the through-thickness stresses may be neglected except at small tool radii. The operations associated with stamping are blanking, piercing, forming, and drawing. These operations are done with dedicated tooling also known as hard tooling. This type of tooling is used to make high volume parts of one configuration of part design. By contrast, soft tooling is used in processes such as CNC turret presses, laser profilers and press brakes. All these operations can be done either at a single die station or multiple die stations ?performing a progression of operations, known as a progressive die. The equipment of stamping can be categorized to two types: mechanical presses and hydraulic presses. A mechanical press has a mechanical flywheel to store the energy, transfer it to the punch and to the workpiece. They range in size from 20 tons up to 6000 tons. Strokes range from 5mm to 500mm (0.2-inch to 20-inch) and speeds from 20 to 1500 strokes per minute. Mechanical presses are well suited for high-speed blanking, shallow drawing and for making precision parts. Hydraulic presses use hydraulics to deliver a controlled force. Tonnage can vary from 20 tons to 10,000 tons. Strokes can vary from 10mm to 800mm (0.4-inch to 32-inch). Hydraulic presses can deliver the full power at any point in the stroke; variable tonnage with overload protection; and adjustable stroke and speed. Hydraulic presses are suitable for deep-drawing, compound die action as in blanking with forming or coining, low speed high tonnage blanking, and force type of forming rather than displacement type of forming.

Tooling considerations

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Optimum clearance (total = per side ?2) should be from 20 to 25 percent of the stock thickness. This can be increased to 30 percent to increase die life.

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Punch life can be extended by sharpening whenever the punch edge becomes 0.125mm (0.005 in) radius. Frequent sharpening extends the life of the tool, cuts down on the punch force required. Sharpening is done by removing only 0.025mm to 0.05mm (0.001-inch to 0.002-inch) of the material in one pass with a surface grinder. This is repeated until the tool is sharp. If it is done frequently enough, only 0.125mm to 0.25mm (0.005-inch to 0.010-inch) of the punch material is removed.

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Grinding is to be done with the proper wheel for the tool steel in question. Consult with the abrasive manufacturer for proper choice of abrasive material, feeds and speeds, and coolant.

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After sharpening the edge is to be lightly stoned to remove grinding burrs and end up with a 0.025mm to 0.05mm (0.001-inch to 0.002-inch) radius. This will reduce the chance of chipping.

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Punching Force: Punching can be done without shear or with shear. Punching without shear, this is the case where the entire punch surface strikes the material square, and the complete shear is done along the entire cutting edge of the punch at the same time. Punching Force = Punch Perimeter 譙tock thickness ?Material Shear Strength. Punch Diameter = 25mm (1-inch), Circumference = 78.54mm (3.092-inch) Thickness = 1.5mm, (0.060-inch) Material Shear Strength (Steel) = 0.345 kN/mm2 (25 tons/in2) Punching Force = 78.54?.5 ?.345 (3.09 ?0.060 ?5) = 40.65 kN (4.64 tons) = 4.14 Metric Tons (4.64 US Tons)

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Punching with shear. This is the case where the punch surface penetrates the materiAir Jordan XIII High