Drawing (manufacturing)

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Drawing (metalworking)
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Diagram of bar drawing; the workpiece is pulled from left (tension) rather than pushed from the right (compression).

Drawing is a

plastic deformation over a curved axis. For wire, bar, and tube drawing, the starting stock is drawn through a die to reduce its diameter and increase its length. Drawing is usually performed at room temperature, thus classified as a cold working process; however, drawing may also be performed at higher temperatures to hot work large wires, rods, or hollow tubes in order to reduce forces.[1][2]

Drawing differs from rolling in that pressure is not applied by the turning action of a mill but instead depends on force applied locally near the area of compression. This means the maximal drawing force is limited by the tensile strength of the material, a fact particularly evident when drawing thin wires.[3]

The starting point of cold drawing is

hot-rolled
stock of a suitable size.

Metal

Main article: Wire drawing

Successful drawing depends on the flow and stretch of the material. Steels, copper alloys, and aluminium alloys are commonly drawn metals.[4]

In sheet metal drawing, as a die forms a shape from a flat sheet of metal (the "blank"), the material is forced to move and conform to the die. The flow of material is controlled through pressure applied to the blank and lubrication applied to the die or the blank. If the form moves too easily, wrinkles will occur in the part. To correct this, more pressure or less lubrication is applied to the blank to limit the flow of material and cause the material to stretch or set thin. If too much pressure is applied, the part will become too thin and break. Drawing metal requires finding the correct balance between wrinkles and breaking to achieve a successful part.

Sheet metal drawing becomes

piercing, ironing, necking, rolling, and beading
. In shallow drawing, the depth of drawing is less than the smallest dimension of the hole.

Bar, tube, and wire drawing all work upon the same principle: the starting stock is drawn through a die to reduce its diameter and increase its length. Usually, the die is mounted on a draw bench. The starting end of the workpiece is narrowed or pointed to get the end through the die. The end is then placed in grips which pull the rest of the workpiece through the die.[1]

Drawing can also be used to

anneals may be required.[6] Tube drawing is very similar to bar drawing, except the beginning stock is a tube. It is used to decrease the diameter, improve surface finish, and improve dimensional accuracy. A mandrel may or may not be used depending on the specific process used. A floating plug may also be inserted into the inside diameter of the tube to control the wall thickness. Wire drawing has long been used to produce flexible metal wire by drawing the material through a series of dies of decreasing size. These dies are manufactured from a number of materials, the most common being tungsten carbide and diamond
.

The cold drawing process for steel bars and wire is as follows:

  1. Tube lubrication: The surface of the bar or tube is coated with a drawing lubricant such as phosphate or oil to aid cold drawing.
  2. Push Pointing: Several inches of the lead ends of the bar or tube are reduced in size by swaging or extruding so that it can pass freely through the drawing die. This is done because the die opening is always smaller in size than the original bar or coil section.
  3. Cold drawing, process drawing: In this process, the material is drawn at room temperature. The reduced end of the bar or coil, which is smaller than the die opening, is passed through the die where it enters a gripping device of the drawing machine. The drawing machine pulls ("draws") the remaining unreduced section of the bar or coil through the die. The die reduces the cross section of the bar or coil, shapes its profile, and increases its length.
  4. Finished product: The drawn product, which is referred to as "cold drawn" or "cold finished", exhibits a bright or polished finish, increased mechanical properties, improved machining characteristics, and precise and uniform dimensional tolerances.
  5. Multi-pass drawing: The cold drawing of complex shapes or profiles may involve the workpiece being drawn multiple times through progressively smaller die openings in order to produce the desired shape and tolerances. Material is generally annealed between each drawing pass to increase its ductility and remove internal stresses produced during the cold working.
  6. Annealing: This is a thermal treatment generally used to soften the material being drawn; to modify the microstructure, the mechanical properties, and the machining characteristics of the steel; and to remove internal stresses in the product. Depending on the material and desired final characteristics, annealing may be used before, during (between passes), or after the cold drawing operation.

Glass

Similar drawing processes are applied in glassblowing and in making glass optical fiber.[7]

Plastics

Plastic drawing, sometimes referred to as cold drawing, is the same process as used on metal bars, applied to plastics.[8] Plastic drawing is primarily used in manufacturing plastic fibers. The process was discovered by Julian W. Hill in 1930 while trying to make fibers from an early polyester.[9]

It is performed after the material has been "spun" into filaments; by extruding the polymer

carbonyl oxygens of another chain.[10] Polyethylene terephthalate (PET) sheet is drawn in two dimensions to make BoPET
(biaxially-oriented polyethylene terephthalate) with improved mechanical properties.

See also

References

  1. ^ a b c Degarmo, p. 432.
  2. ^ Kalpakjian, pp. 415–419.
  3. ^ Ganoksin Project. "Rolling and Drawing". Archived from the original on 2014-08-08.
  4. ^ Degarmo, p. 434.
  5. ^ Degarmo, pp. 433–434.
  6. ^ Degarmo, p. 433.
  7. ^ "Optical Fiber". www.thefoa.org. The Fiber Optic Association. Retrieved 17 April 2015.
  8. ^ Degarmo, p. 461.
  9. ^ a b Spinning the Elements – Cold Drawing, Chemical Heritage Foundation, archived from the original on 2001-05-04, retrieved 2008-11-13
  10. ^ a b Menzer, Valerie, Nylon 66, archived from the original on 2005-06-13, retrieved 2008-11-13.

Further reading

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