Machining and work-hardening steel creates burrs and scale that needs to be removed. The part in the middle of the picture above shows the ‘excess’ material in threads as well as the darkening caused by scale formation in the heat treating process. While barrel or vibratory tumbling can be used to remove these burrs and create a smooth surface finish, threaded areas often need to maintain the square edges and dimensionality on the part.
The other two parts were blasted with different types of abrasive blast media to show how each media can accomplish the deburring task but create a totally different surface finish. The part on the left was blasted with a Medium Glass Bead (70-100 mesh) and resulted in a satin-like surface finish with clean edges. The part on the right was blasted with 120 Mesh Aluminum Oxide Grit and has a more matte, gray surface finish.
While Glass Bead can often result in a more aesthetically pleasing finish, the micro-profile achieved with Aluminum Oxide Grit is often required for future processing of the parts (i.e., coatings application).