Bead blasting propels fine glass beads at a surface under air pressure. Because the beads are round rather than angular they peen the surface instead of cutting it, which produces a uniform satin appearance without the aggressive texture of an angular abrasive. It is the standard way to make a machined part look finished.
What it actually does
The impacts flatten and blend the peaks left by the cutting tool, hiding the directional tool marks that make adjacent machined faces look different from each other. It also lightly work hardens and compressively stresses the surface, which is why shot peening, a heavier version of the same idea, is used deliberately to improve fatigue life.
Bead against grit and sand
Angular abrasives such as aluminium oxide cut the surface, remove material and leave a rougher, more aggressive texture, which is what you want before a coating that needs a key. Glass beads peen and clean without removing much material, leaving a smoother satin finish. Silica sand is largely obsolete for health reasons and is regulated as a respirable crystalline silica hazard.
Where it sits in the sequence
After machining and deburring, before anodising or painting. Blasting after anodising would damage the coating; anodising after blasting produces the familiar uniform matte anodised appearance that most machined aluminium products have. Where a part is only partly blasted, the boundary has to be masked, and a masked boundary is visible, so put it on an edge.
Practical notes
It rounds sharp edges slightly, which is usually welcome and occasionally not. It can embed media in soft materials, so aluminium parts destined for cleanroom or medical use are washed carefully afterwards. Blast media breaks down and must be replaced. And it removes a small amount of material, so it should not be applied to a surface already at its final tolerance.
Questions people ask about bead blasting
What does bead blasting do?
It propels glass beads at the surface to peen and blend the tool marks, producing a uniform satin finish without removing much material.
Bead blasting or grit blasting?
Beads for a smooth satin appearance with little material removal. Angular grit for cutting, cleaning and creating a key before a coating.
Does bead blasting come before or after anodising?
Before. Blasting produces the uniform surface that anodising then colours, and blasting an anodised part would damage the coating.
Does it affect dimensions?
Slightly. It removes a small amount of material and rounds sharp edges, so it should not be applied to a feature already at final tolerance.