The media does the work. It transfers energy to the part. The rest of the machine only sets it in motion.

That is why the media choice decides the result, not the machine.

Four things matter: material, structure, shape and size.

Material

The rule is simple and comes down to hardness.

Ceramic media — for harder parts: steel, cast iron, ceramic, glass.

Plastic media — for softer parts: aluminium, brass, bronze, wood, plastic.

Ceramic media on an aluminium part does not finish it. It hammers it.

Structure

Structure means how much abrasive the media contains.

Coarse structure — high grinding capacity. Removes material quickly.

Fine structure — low grinding capacity. Works slowly, but leaves a smooth surface.

The codes are the fastest shortcut when you have a quote in front of you. In ceramic media, the coarse structures are RAH, RMB, RX and RXX — those cut. The fine ones are RP, RCP and RF — those smooth.

Notations differ between manufacturers, but the logic holds: the more abrasive in the media, the faster it removes material and the rougher the surface it leaves. The choice follows how much material has to come off, not how good the code sounds.

Shape

The shape of the part determines the shape of the media. This is where most mistakes happen.

Media with many edges — more aggressive. Used when edges are the target: deburring, radiusing.

Media with many faces — less aggressive. Used when surfaces are the target: grinding, smoothing, polishing.

A correct choice has two effects: uniform processing and, just as important, no media lodging inside the parts. A part with holes and the wrong media shape means an hour spent picking stones out with tweezers.

Size

Large media — heavier, more aggressive. Shorter cycle time, but a rougher surface.

Small media — lighter, gentler. Finishing operations, longer cycle time.

It is topped up, not replaced

Media is not replaced. It is topped up.

It is normal for the bowl to hold a mix of different sizes — some new, some worn. You add the consumed quantity daily, so the level stays constant.

That is how you get a constant effect from one day to the next. Empty the bowl and refill it with new media only, and the first batch comes out different from the last.

Dry finishing

There is a version without water. It runs on vegetable granulate — ground walnut shell, corn cob — sometimes together with a polishing paste.

What it does: brings up gloss, dries the part, removes fine marks left by wet operations. What it does not do: it removes no material and corrects no geometry.

So it is not an alternative to deburring, it is a finishing stage. It is used when the part has to come out of the machine dry, when moisture is a problem, or when you want a gloss the wet medium will not give.

Burnishing with steel media

Steel media — balls, satellites, pins. It does not cut, it compacts.

The surface comes out bright, and the surface layer work-hardens: it becomes harder and takes on compressive stress. It is the same mechanism as shot peening, at a different scale and for a different purpose.

It runs in water, with a dedicated compound. Steel media is kept clean and dry between batches, otherwise it rusts and stains the parts.

When it is worth it: parts that have to both look good and hold up better under variable loading.

How to choose vibratory finishing media