Cold drawing machine pull steel to produce a final, shapely part. The process is performed on cylindrical bars of the desired shape.
The dependences of tensile and yield strength on cold work of nickel obtained experimentally allow to build an energy-power mathematical model for wire drawing [45].
When designing a drawing schedule, it is important that the wall reduction ratio (Q value) is high. This is because wall reductions tend to iron, or smooth, the ID surface while diameter reductions roughen it.
The Process
The cold drawing process reduces the cross section of a rod or wire by pulling it through a series of dies. This stretching of the metal increases its length and provides a bright finish. It is often used in place of hot working to provide closer dimensional tolerances and improved mechanical properties.
Before cold drawing begins, metalworking companies must prepare the steel for the process by submerging it in lubricant. This helps the steel bars to slide through the dies without tearing or breaking. The lubricant also helps the steel to retain its strength during the drawing process.
Before the rod or wire is drawn, it must go through a pointing machine that tapers its end so that it can fit into the first die. This step is important because it allows the resulting drawn product to have a more accurate diameter than its original size. This is important because a small deviation in the final size of the wire can affect its tensile strength.
Preparation
In cold drawing, the material is pulled through a die opening, causing stretching or elongation and a reduction in cross sectional area. This requires a high pulling force, which limits the material’s strength. This limits the number of passes and maximum reduction that can be achieved with this process.
Typical raw materials used in drawing include carbon and low-alloy steel, stainless steel and nickel alloys. Before the drawing process begins, the hot rolled rough stock is examined and prepared for cold drawing. This involves removal of abrasive scale and coating with a drawing lubricant.
The lubricant is generally calcium, sodium or aluminum stearate or borax pentahydrate (Neobor). Most drawing is done using the dry drawing process, where the wire comes into intimate contact with the lubricant. A pointing machine is also used to taper the end of the rod to a size that fits the initial dies. The lubricant is used to reduce the friction between the die and the rod. Annealing may be used before, during or between draws depending on the desired final characteristics of the material.
Wire
The first step in preparation for the drawing process is to clean the rod used. This may involve acid pickling, rinsing, drying the coil, and/or electrical butt welding. Once the rod has been cleaned, a lubricant is applied to reduce friction during the drawing process.
The pointing machine then sharpens the end of the rod into a pointed shape, which allows it to fit through the dies of the drawing machine. Once the rod is pointy enough, it enters a block where it is gripped by a pair of pincers.
The tensile strength and yield strength of the rod will limit how much it can be drawn, so it is often annealed after each draw to relieve stress and improve ductility. This is especially important for nickel-based alloys, which are more susceptible to cold work than stainless steel and aluminum. We use built-in Excel functions to develop trend lines and approximation equations for evaluating the change in plastic properties due to cold drawing.
Die Set
The die set is the key to reducing the cross section and shape of drawn wires. It must be sharp enough to cut the strands, yet still capable of drawing the metal down with a constant pressure over a large area.
The parallel bearing portion of the die must be sized to match the desired final diameter. Harder materials typically require shorter bearing lengths to prevent heat buildup that could cause lubrication failure. Softer materials can utilize longer bearing lengths, which help maintain the final wire size for a longer period of time.
Die selection, recutting and inspection are all matters of experience coupled with guidelines that must be understood by shop floor personnel. The inspection process should be a continuous effort to ensure that the dies are being used at their optimal performance. It should also serve as a tool to review the overall history of the dies and make informed decisions on future purchases.
