Media to Parts Ratio

The number of parts that can be tumbled (vibratory or barrel) will depend on the dimensions of the part and the desired final finish. Lower ratios of media to parts (no media, 1:1 or 2:1) will lead to rougher finishes and can be used on castings and forgings for aggressive deburring. Moderate and the most common ratios (3:1, 4:1 or 5:1) will minimize part to part interaction and are ideal for moderate deburring and surface finishing. High ratios (6:1, 7:1 and higher) are used for pre-plate and decorative finishes or for fragile parts

The quantity of parts that can be processed in a given system can be determined by first calculating the 'box' size of the part (basically width x length x height). Given the known volume of the tumbler being used (50% for barrels and 90% for vibes, typically) and the box volume of the part, the quantity of parts and media can be calculated for any ratio.

For example, let's assume a 3 cubic foot vibe and a part size of 2" x 3" x 4" (box = 24 cubic inches) and a media to parts ratio of 3 to 1. With the vibe 90% full, the total usable capacity is 2.7 ft3. At a ratio of 3:1, 25% or 0.675 ft3 (1,166 in3) of the usuable capacity will be parts. With the size part noted, the process will be able to tumble about 48 parts per batch (=1166/24). The balance of the capacity (2.025 ft3) will be tumbling media.

Polishing Aluminum Parts

Polishing aluminum parts to a very high, near mirror finish in a mass finishing process is very challenging. The relative softness of aluminum makes the metal very susceptible to scratching and impact marks from the media and the vibratory or barrel tumbling process itself.

Depending on the original surface finish, a two or three step process is recommended. The first step with an abrasive Plastic or Synthetic Media will remove machine marks, scratches, etc. from the part. The second step using a polishing media (Ceramic or Steel) will create a smooth, uniform finish. An optional third step would involve a long cycle time dry tumbling of the part with Walnut Shell Grit or Corn Cob Grit to burnish to a high polish. Additives such as rouge can be added in this final step to improve the finish.

The part pictured was polished in a vibratory tumbler using Steel Media.

Vibratory Tumblers

Vibratory tumblers come in a wide range of sizes and shapes. The configuration of the vibe will determine the tumbling action of the media and parts. Bowl style tumblers will turn the load over and around the circumference of the bowl (3-D tumbling). A tub style tumbler will simply turn the load over (2-D tumbling) but offer the possible for tumbling long parts since there are no 'turns' in the vibe. Tub vibes are also generally more aggressive than bowls.

While most bowl style vibratory tumblers have a similar configuration (think: large doughnut), tub vibes come in a few styles. Some tubs are angular on the bottom with straight sides like a rectangle. This type of set-up is the most difficult to 'rotate' the load and can be very aggressive on the parts. Tubs that are more cylindrical in shape turn the load over very quickly. This can sometimes lead to longer cycle times but is better for sensitive or fragile parts. Vibratory tubs with rounded corners at the bottoms offer an excellent balance of tumbling action and aggressiveness. The systems offered by C & M Topline (the DB series) are an excellent example of this type of vibratory tumbler.

Walnut Shell & Cosmetics

The cosmetic and personal care industry use crushed walnut shell media as an exfoliate in facial, body and foot scrubs. Crushed walnut shell is a hard, fibrous material that is ideal as an abrasive. The grit is extremely durable, angular and multi-faceted, yet considered a soft abrasive with a MOHS hardness of 4-5.

More aggressive formulations will use particle sizes as large as 20-40 mesh. The soft abrasive nature of walnut shell is still viable in flour sizes at 200 mesh and smaller for sensitive, fine scrubs.

The cosmetic grade of walnut shell is either 'natural' or 'irradiated' and has rounded edges (relative to an abrasive blasting grade) for a smoother feel. The irradiation process is performed with gamma rays and sterilizes the walnut shell to provide an indefinite shelf life (no micro-biological growth). 'Natural' product may require an additive in the formulation.

Laser Cut Steel Parts

When steel parts are laser cut, heavy slag can develop on the part. These burrs are typically much thicker and heavier than a normal burr or sharp edge and can sometimes be a large a 1/8" thick. Removing this excess metal requires an extremely aggressive operation. For this piece, part-on-part barrel tumbling was required to knock off the extraneous material.


To keep the cycle time to a minimum (less than 8 hours in this case), a hard abrasive grit, Silicon Carbide, was added to increase the deburring properties as well 'even out' the surface finish. After tumbling the part showed no burrs or slag on the edges and had a uniform, matte finish.

After this ultra-aggressive deburring step, the parts can then be tumbled (barrel or vibratory) to improve the surface finish for further processing.

Surface Finishing Specifications

The Steel Structures Painting Council (SSPC) has designated fifteen different surface preparation standards including simple solvent cleaning (SP-1) to Commercial Blast Cleaning (SP-6) to Industrial Blast Cleaning (SP-14). Some of these standards are simply visual while others designate specific profiles. These standards are applicable to a large variety surfaces. If a specification has not been designated for a surface to be cleaned or blasted, using an SP standard will provide consistent results from part to part and job to job.