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Once prepared, the work piece is pointed at one end, allowing that end to be inserted through the die. This end is then mechanically gripped so that the rest of the work can be pulled through. At certain points in the process, the drawn product may require straightening. Straightening rolls can be employed as part of the manufacturing process. Metal drawing can be either a discrete or continuous operation and can be very economically efficient for certain applications. In commercial industry, this process provides stock material for machining operations and for the manufacture of such items as fences, coat hangers, nails, screws and bolts. Metal wire drawing plays a huge roll in the manufacturing industry in the production of cable and electrical wire.
Drawing Dies
Metal drawing dies, in manufacturing industry, are usually made of cemented carbides or tool steels. Mandrels for tube drawing are often made of similar materials as the die. Occasionally diamond die are employed to form extremely thin wire. As the work transverses the mold it passes through different sections. The die's first section is a bell curved opening. This area does not contact the work, but helps filter lubricant into the mold and allows for adequate entry of the work into the mold without damage from die edges. Next, the forming of the work occurs in the approach section. The approach angles down the cross sectional area, connecting with the next section, the bearing surface. Bearing surface, also known as land, holds the precise geometric cross section for a length of the draw. This acts as a sizing operation, ensuring tight tolerances. The last section is the exit zone, this is a steeply angled section similar to the entry zone. Exit zones are used to protect drawn work from the edges of the die.
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Defects In Metal Drawing
Defects that occur in metal drawing manufacture are similar to those that occur while manufacturing by extrusion. Controlling metal flow is essential in preventing defects. Mold characteristics and friction play a critical roll in the process.
Internal Cracking: Internal breakage may occur in drawn products, particularly along the centerline. This is due to improper metal flow creating high internal stresses. Causes may be high die angles or low friction.
Surface Defect: A wide variety of surface defects can be observed in metal drawing manufacture. Seams, scratches and cracks are all possible defects on the surface of drawn product. Excessive force on the surface of the work during the drawing operation, (such as from friction), can be the cause of breakage. Also, many metal drawing operations form at very high speeds, sufficiently designed entry and exit zones need to be provided in order to avoid damage to the work material from the die. For more detailed information on internal breakage and surface defects see extrusion defects.