Bead Blasting Finish: Choosing Media, Controlling Ra and Protecting Edges

The first prototype came back with a soft matte that made the machined housing look like a production part. The second batch came back matte too, but the Ø6 dowel holes were tight, the anodized color was two shades darker, and one sealing edge had rounded enough to leak.

Bead blasting is usually specified with two words on a drawing. It has more variables than most buyers expect: media, grit size, pressure, nozzle distance, exposure time, masking and what happens afterwards. None of it is hard to control, but it has to be specified.

What blasting actually changes

Bead blasting is a cold mechanical process. Media accelerated by compressed air strikes the surface at 2 to 5 bar and removes material by micro-cutting and peening. The result is a randomized dimple pattern that scatters light, which is what the eye reads as matte.

Three things happen at once. Surface roughness changes. Edges and fine features are eroded slightly. And the surface layer is work-hardened and put into compressive residual stress, which can measurably improve fatigue life.

The blasting does not remove deep tool marks. It superimposes a texture on top of them. A 0.02 mm step from a bad toolpath will still be visible after blasting, just matte.

Media selection

The choice usually comes down to glass bead versus alumina, and the deciding factor is substrate. Glass bead is round and fractures gradually, so it produces a softer, more uniform finish and does not cut deeply. Alumina is angular and hard, removes material far faster, and leaves a sharper texture.

For aluminum, glass bead is almost always right. Alumina can embed particles in soft aluminum, and those inclusions show up as dark speckles after anodizing.

For stainless and titanium, alumina cuts through the passive layer and any heat tint, which glass bead struggles to do.

Ra, tool marks and what blasting actually hides

A machined surface at Ra 0.8 µm blasted with 50 to 100 µm glass bead typically lands between Ra 0.8 and 1.4 µm: the arithmetic roughness barely changes, but the character goes from directional to uniform.

That is why blasting is such an effective cosmetic treatment. A face milled with visible stepover lines reads as machined; the same face blasted reads as a finished product.

It also explains why blasting cannot rescue a bad surface. Blasting adds texture; it does not flatten. A 0.03 mm step or chatter mark will still be visible, because the shadow pattern follows the geometry.

If the goal is to remove tool marks rather than disguise them, the part needs a finer finishing pass or a light polish before blasting.

Blasting before anodizing

Blasting before anodizing is common and works well, with three caveats.

First, the blast texture carries through the anodic layer. A coarse blast produces a visibly coarse matte after anodizing; a fine glass bead produces a satin finish.

Second, the larger surface area absorbs more dye, so color comes out darker than on a polished surface. Parts processed differently will not match.

Third, blasting must be followed by a thorough clean and desmut before anodizing. Embedded media or blasting residue shows up as pits or streaks in the coating.

For powder coating, blasting is usually a pretreatment step that improves adhesion and removes oxide. It should not be so coarse that the texture telegraphs through a 70 µm film, unless that texture is intended.

Masking, edges and precision features

This is where blasting causes real damage.

A nozzle held close to a sharp edge will round it by 10 to 50 µm within seconds. On a cosmetic part that is fine. On a sealing face, a knife edge or a thread, it is a functional defect.

Masking is done with silicone plugs, high-temperature tape, wax or purpose-made caps. The parts of a drawing that usually need protection are:

• Threads, especially fine pitches
• Bores with H7 or tighter fits
• Sealing faces and O-ring grooves
• Datum surfaces used for inspection
• Electrical contact areas
• Any surface that will later be bonded

Masking is labor, and labor is the part of blasting that costs money. A part with twelve masked holes can cost more to blast than to machine.

The alternative is sequence: blast before the finishing operations that establish the critical features. Blast the blank, then machine the bore and the sealing face. That requires the shop to plan the routing, which is why it should be discussed at quoting rather than discovered in production.

Contamination control

Blasting cabinets are a contamination risk if they are not dedicated. Blasting stainless with media that previously ran carbon steel embeds iron particles, and those particles rust.

For stainless and titanium, use iron-free media in a dedicated cabinet. Alumina that has run on steel should never touch a stainless part. Aluminum is more forgiving, but embedded alumina still shows up after anodizing as dark inclusions.

When not to blast

Skip blasting when the surface finish is functional rather than cosmetic: bearing bores, hydraulic sealing surfaces, optical references and gauge faces.

Skip it when the part has thin walls under about 1 mm, because peening can bow them. A 0.8 mm aluminum panel blasted at 5 bar will distort measurably.

Skip it when edge sharpness matters, such as cutting blades, knife edges or valve seats, unless those features are masked.

And skip it if the part needs NDT: blasting hides cracks and porosity under a uniform texture.

Specifying the finish

A complete note reads something like: "Glass bead blast, 50 to 100 µm media, 3 to 4 bar, Ra 0.8 to 1.4 µm, mask all threads and Ø8 H7 bore, blast before anodizing."

That gives the shop the media, the pressure range, the target roughness and the masking. It is more than most drawings carry, and it is the difference between two batches that match and two that do not.

When we quote a part like this at JSHS, blasting and masking are priced as part of the finishing route, and the DFM notes flag any feature where blasting will fight the tolerance. Send the drawing and the cosmetic requirement with your quote request and we will call out the masking before production starts.

Frequently asked questions

Does bead blasting change part dimensions?

It removes 1 to 5 µm per surface and rounds edges by 10 to 50 µm. Negligible on a general tolerance, significant on a press fit or sealing edge.

Can bead blasting hide scratches?

Only shallow ones. A scratch deeper than the blast texture still reads as a line, because the light catches its profile. Deep scratches need polishing or welding before blasting.

What is the difference between bead blasting and shot peening?

Media and intent. Shot peening uses round steel shot at controlled intensity to induce compressive stress for fatigue life. Bead blasting uses glass or ceramic media for appearance, with much looser process control.

Will blasting remove anodizing?

A light cosmetic blast dulls an anodized surface but does not remove the coating. Stripping an anodic layer needs a chemical strip or aggressive mechanical work, and both risk changing dimensions.