Q. We are machining a 6-inch, complex-shaped, 6061 aluminum part. We need to further improve the surface finish from the machined 32 Ra to 12 Ra, maintaining a uniform, pleasant finish. What finish system should we consider? S.M.
A. Visual mechanical finish options include a variety of line-grain, satin-matte or bright finishes. Abrasive, blasting and mass finishing all will provide a uniform visual finish, but only abrasive finishing and mass finishing will reduce your finish from 32 Ra to 12 Ra.
The three measurements of surface finish are roughness, waviness and lay. Roughness is the most important and standard of the three, and Ra is the most common of many surface roughness measurements in the U.S. It represents the average of all peaks and valleys in a standardized measured area. Working a surface to reduce Ra measurements requires a finishing system that will reduce the peaks to the valleys. The lower the Ra number, the smaller the peaks, and, therefore, the finer the finish.
Abrasive and mass finishing produce a rubbing action that is efficient at working the surface peaks, reducing surface Ra measurements. Blasting, on the other hand, propels particles at the surface and works both the peaks and valleys. The blast particles may beat down some peaks, slightly improving Ra, but because it works both the peaks and valleys, it is not effective in significantly improving Ra.
Here are my mechanical and visual finishing recommendations:
Abrasive finishing using a 320-grit coated abrasive (disc, belt or non-woven wheel) will reduce the surface from 32 Ra to 7-11 Ra. This method is capable of achieving a uniform straight, swirled, random-lined or bright buffed finish. However, abrasive finishing will be time-consuming by hand and expensive to automate for a complex shaped part.
As stated above, blast finishing on its own will not reduce a surface finish from a 32 Ra to 12 Ra. This method will produce a bright, reflective satin finish with a fine glass or ceramic bead. If a blast finish is visually preferred, it will have to be a two-step process—surface Ra improvement first and blasting second.
Mass finishing is an excellent choice because this process automatically finishes complex shapes. High-energy mass finishing will reduce the 32 Ra to 12 Ra in 30 to 45 min. The vibratory process will reduce it in 2 to 4 hrs. A good media choice is a fast-cut or high-density plastic media. Larger media will produce a large-pattern matte finish, while smaller media will produce a tight-patterned, soft matte finish. A secondary burnish process after plastic media refinement can achieve a bright reflective finish.