Bead blasting impact of surface roughness

It really depends on the supplier, usually the beads are defined by grit size, the higher the grit number, the finer the glass beads. Some suppliers will also classify the beads by course, medium, fine, and very fine.

Hello Mike, to add to Dan’s comments here, specifying coarseness and desired finish in drawings or notes is usually enough for most suppliers to understand what you are going after. For instance, you could add a note such as :  “Media blast with fine glass bead for a satin finish.” 

This does leave some room for interpretation, so if you are concerned with repeatability and consistency for a product line, you can also specify grit range and pressure. For example, I asked for the following for the stand parts seen below : “Media blast with #70-100 glass bead at 50-60psi. Surface to be free of dirt, oils, debris, and blemishes.” 

Adding to Dan’s response, bead blasting can actually improve material properties and has been shown to increase fatigue resistance. This is because it flattens and smoothes out microscopic cracks in the surface, making it harder for cracks to initiate in the first place.

If you want to increase an aluminium part’s resistance to cracking during cold work then you can anneal it. It is also a good idea to anneal aluminium after machining as the machining process will impart mechanical stress to the material which can increase the likelihood of cracks forming once the part is exposed to regular loading. Just two things to keep in mind, do not anneal prior to machining as the material can become too soft to machine, i.e. it could stick to the cutting tools. Secondly, not all grades of aluminium can be annealed.