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Raw material comes in wire form. This material is first visually inspected for quality and lack of any defects. Diameter is verified and the raw material certificate is reviewed.
The cold heading machine cuts off a specified length of the wire material into a cylindrical slug. Two hemispherical halves of the heading die then form the slug into a roughly spherical shape. This forging process is performed at room temperature and a slight amount of excess material is used to ensure that the die cavity is completely filled. Cold heading is performed at very high speeds with cycle time averaging about one large ball per second. Smaller balls are headed at rates between two to four balls per second.
Excess material that forms around the perimeter of the precision steel ball is removed during the flashing process. The balls are passed numerous times between two grooved cast iron plates removing small amounts of excess material as they roll.
Parts are then heat-treated using a quench and temper process. A rotary furnace is employed to ensure that all parts are exposed to the same conditions. After the initial heat treatment, the parts are immersed in an oil reservoir. This rapid cooling (oil quenching) produces martensite, a steel phase which is characterized by high hardness and superior wear properties. Subsequent tempering operations further decrease internal stresses until the final specified hardness range of the bearing is reached.
Grinding is performed both before and after heat treatment. Finish Grinding (also known as Hard Grinding) brings the ball closer to its final requirements. The grade of a precision metal ball is a measure of its overall precision; the lower the number, the more precise is the ball. Ball grade encompasses diameter tolerance, roundness (sphericity) and surface roughness - also called surface finish. Precision ball manufacturing is a batch operation. Lot size is determined by the size of the machinery used for the grinding and lapping operations.
Lapping is similar to grinding but involves a significantly lower material removal rate. Lapping is performed using two phenolic plates and a very fine abrasive slurry such as diamond dust. This final manufacturing process greatly improves surface roughness. Lapping is required for high precision or super precision ball grades.
A cleaning operation then removes any processing fluids and residual abrasive material from the manufacturing process. Customers requiring more stringent cleanliness requirements, such as those serving micro-electronics, medical or food industries, can take advantage of Hartford Technologies more sophisticated cleaning options.
After the primary manufacturing process, every lot of precision steel balls undergoes multiple in-process quality control checks. A visual inspection is performed to check for defects such as rust or dirt.
Roller gauging is a 100% sorting process that separates both under-size and over-size precision steel balls. Please check out our separate video on the roller gauging process.
Each lot of precision balls is inspected to ensure grade requirements for diameter tolerance, roundness and surface roughness. During this process, other relevant characteristics such as hardness, and any visual requirements are also evaluated.
At the end of this process journey, precision balls are packed dry in plastic bags. Vapor corrosion inhibitor paper is included for materials which are prone to oxidation. In some cases, VCI bags are used. Precision balls can also be coated with dry to touch oil. And finally, carton weight and labels are specified to meet exacting customer requirements.
Hartford Technologies manufactures over a billion precision balls in many materials, sizes and grades for industries worldwide. Contact us today and let’s get the ball rolling!
