The Drawing of Wire
The loose end of each coil of rod of wire is hammered or otherwise made smaller for a length long enough to be threaded through the hole in the die and firmly grasped by tongs or “pliers.” The die being firmly fastened, the draw head, of which the pliers are a part, is drawn away until a sufficient length has come through that it can be attached to a revolving block or drum which thereafter continuously pulls the rod through the die and winds it into a coil about the block as it comes, at speeds as high as 400 feet a minute.
As the rod passes through the box in which the die is fastened, besides its lime or sull coating it is sometimes provided and takes up other lubricants, such as soap, grease, or tallow.
The coil of wire is lifted off the drum and tied, or is redrawn to wire of a smaller size.
After from one to several drafts, depending upon conditions, the wire must be annealed for several hours, and again pickled to remove the scale formed, washed, lime-coated and dried. For wire which must undergo many reductions or passes, as must all small wires, several drawings, annealings and cleanings must be gone through with before it is down to the desired size.
In order to work properly and not cut the die too fast, which would rapidly enlarge the hole and cause the rear end of the wire to be of different thickness than the forward end, the rod or wire must be of maximum softness. Either under-cleaning or over-cleaning will make the metal harsh, as, also, will under-baking in the dry ovens. These they endeavor to avoid in preparing the rod.
Other lubricants may be used and certain finishes or colors given to the wires. Such are the various white to red coatings which come from using tin and copper sulphate solutions instead of lime coatings. This is more often done with fine wire than with thicker. Certain “patenting” processes make use of various methods of tempering wires by cooling in air after a last annealing.
Not only are wires ordered to size but many times also of certain temper or grades of hardness and stiffness. These various tempers are determined largely by the chemical analysis of the steel and the number of drafts after the last annealing.
Piano wire is well known to have a very high strength, sometimes as high as 400,000 pounds per square inch. Spring wire is rolled and drawn from higher carbon steel than is ordinary wire, and, through heat treatment, the best properties are imparted to it.