The most common is cold winding. In this case, wire that has already been heat-treated or worked to its final strength level is coiled into a spring. Because the material is already at peak strength, large wire diameters and small indexes are difficult to achieve. The typical maximum wire diameter for this process is 0.625 inches.
The next process is less common, but still falls under cold winding. In this case, wire is coiled in a soft state and then heat-treated to its final strength condition after coiling. For a given piece of coiling equipment, larger wire diameter and/or smaller indexes can be coiled with this method. This process is used for wire sizes up to .875″ in diameter.
The final process is hot winding. In this case, bars are heated to approximately 1700°F and coiled. Usually, the red-hot spring is quenched in oil and tempered to complete the heat treatment. Coiling at such a high temperature enables spring manufacturers to work with far larger bar sizes than could be coiled at room temperature. This process is generally used for bars up to 1.75″ in diameter.
Which process to use is determined first by the size of wire that must be coiled. Once that is determined, the type of material, final wire strength level, and spring index will drive manufacturing toward a process that is most compatible with the available equipment.