In the steel ball production process, the first step is stamping, a key technique for giving the ball its rough shape.
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Stamping involves processing steel wire or bar in machines called presses, inside which the wire or bar is cut into pieces and then passed through equipment called matrices fully automatically and at great speed. The matrices then close on them with a pressure of between 10 and 20 tons, depending on the diameter of the sphere to be formed.
Pressing, also known as cold stamping, transforms the wire or bar into an initial spherical shape, known as a “rough sphere.” Although the sphere acquires its general shape at this stage, a slight excess of material still persists, which must be removed in subsequent processing steps.
After cold stamping, the raw steel balls enter the next stage of the production process called roughing, this step is crucial to remove the excess material that the raw ball has. Roughing thus serves to improve their sphericity, making them closer to the spherical shape, although they are not yet completely round.
The roughing process is accomplished by rolling the balls between two cast iron plates. Throughout this stage the metal of the balls is still relatively soft, so further machining will follow to achieve the hardness and dimensional accuracy needed for high-precision industrial applications.
After Roughing, the production process of steel balls continues with a crucial step for their mechanical properties: heat treatment. This step is crucial to give the balls the hardness, toughness and ability to withstand high loads required for their industrial applications.
Heat treatment of steel balls begins with austenitizing, where the balls are placed in a furnace and heated to high temperatures, typically around 1,560° F (about 850° C). This process allows the crystalline structure of the starting steel to be transformed into a new crystalline structure, called austenite.
Next, the balls undergo a quenching process. This step involves rapid cooling by immersion in a bath of oil or quenching salts. Quenching stops the chemical processes initiated by austenitizing and solidifies the crystalline structure of the steel, giving it a new shape characterized by high hardness, but also brittleness.
Finally, the spheres undergo tempering, a step where they are heated again, but at a much lower temperature, between 340 and 440° F (about 170-230° C). This process relieves the internal stresses created by quenching, improving the balls' resilience and increasing their toughness.
Once the heat treatment is complete, the steel balls move on to the next step called grinding. This process is crucial to ensure that the balls meet the rigorous precision specifications required for their final applications.
During Grinding, the balls are rolled between a cast iron plate and a grinding wheel. The grinding wheel is generally coated with a fine-grained abrasive material, which removes excess material from the surface of the balls in a controlled manner. This process not only brings the size of the balls closer to their final size, but also considerably improves their sphericity.
After the Grinding stage, steel balls reach the final step in the production process: lapping. Lapping is a process that not only further improves the surface finish of the balls but also refines their geometry, allowing them to achieve the tight tolerances required for the most demanding applications.
The lapping process is divided into two similar stages: prelapping and final lapping. In both stages the balls are rolled between two cast iron plates. One plate is fixed, while the other rotates at low speed. This movement allows very fine removal of material from the surface of the balls, resulting in a mirror finish and a geometrically perfect shape.
In between the two lapping stages, eddy current quality control takes place on 100% of the products. Eddy currents are a non-destructive method that, thanks to high-frequency electromagnetic interference, detect surface defects of various kinds.
This control is essential to ensure an excellent level of ball quality.
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After lapping, steel balls undergo the last stage of their production process: washing. This stage is crucial to remove any residue that may have remained on the surface of the balls after previous processing.
The balls are thoroughly cleaned in baths of cleaning solutions. These solutions are designed to remove oils, grease, dust particles and other contaminants that may have adhered to the surface of the balls during previous processes.
Final inspections ensure that each sphere meets the highest quality standards before reaching customers. During the inspection phase, each ball is carefully examined for scratches, imperfections, or size discrepancies. Although all spheres have an identical nominal outside diameter, small variations can still occur.