How a Cold Drawing Machine Works
Cold drawing is an industrial process that reduces the diameter of rods, bars and tubes to obtain a required section size. The process involves sizing and stretching of the metal as it is drawn through dies with controlled pressure and lubrication. The resulting section is work hardened and has a smooth surface that can be further refined in a sizing machine. In addition to reducing the section size, a cold drawing machine can also shape the metal into round, hexagonal, square or profiled shapes. The process can be used for a variety of industries that require precise sizing and strict tolerances.
The process begins with the submersion of raw steel bar stock in a lubricant. This is done to allow the metal to pass more easily through the die. The lubricant is added to help prevent the material from sticking or developing internal stresses that can cause cracking and breakage.
After the lubrication is applied, the lead ends of the steel bar are trimmed to be smaller than the rest of the stock. This allows the lead end to travel through the die first, which makes it easier for it to pass through the die and complete the drawing process. Depending on the type of steel and the application, the lubricated bar is subjected to a series of cold draws until it achieves the desired size and section.
In cold drawing, the optimum die angle varies with the diameter-to-thickness ratio (D/t). In general, low-angle dies tend to thin the wall; high-angle dies thicken it. In order to get the correct die angle, a drawing schedule must be carefully designed. The optimum angle must be balanced against the draw force required to avoid excessive deformation. Too much thinning and the metal will break, too little thinning and the steel will not reach its final dimensions.
A well-designed draw schedule will provide a high degree of quality and consistency, as the process is under constant control. Using strain gauges to monitor the drawing forces and power consumption, the optimum draw angle is determined for each production run. A computer program can then use these results to calculate the exact drawing forces and power consumed.
Mathematical modeling of the drawing process can be carried out using a number of software packages, such as QForm, ABAQUS and DEFORM. The adequacy of these mathematical models depends on the accuracy of the parameters specified, especially the friction coefficient and the die half-angle. The choice of these values determines the accuracy of the calculation methods and, consequently, the validity of the obtained calculation results.