Answer: Rolled screws are produced using a cold forming process; therefore, the steel must be relatively soft (<222 BHN). Rolled screws have minimally higher tensile strength, shear strength, and fatigue resistance compared to machined screws because the natural grain structure is free of interruptions. In addition, the thread finish is higher due to the burnishing effect of the rolling process.
“However”, if heat-treating is required to improve wear resistance, the dense microstructure and smooth surface are lost. Heat-treating releases compression stress imparted by the rolling process causing thread distortion and increased lead error. As a result, it is only possible to control lead accuracy over a certain distance, whereas lead wobble is not directly controllable. In addition, heat-treating causes the surface to oxidize and form a layer of dark scale that must be removed by polishing. As a result, the finish is dependent on the quality of the polishing process.
Machined and ground screws are typically produced using pre-hardened materials eliminating the need for heat-treating. The resulting lead accuracy is entirely dependent on machine accuracy rather than secondary processing.
Pre-hardened machined and ground screws have significantly higher wear resistance than rolled screws which is of particular importance when the screws are used with hardened nut materials. Steel, cast iron, aluminum bronze and manganese bronze nuts will pre-maturely wear unhardened screws.
Since heat-treating is not required with pre-hardened screws, the surface finish of machined and ground screws depends entirely on the quality of the side-cutting wheel or grinding wheel.