Mass Finishing Consumable Materials

For the vast majority of mass finishing processes, equipment is loaded with components to be finished, media, compound, and water. Media are solid stones or chips. Compounds are the materials that dissolve in water to form solutions to facilitate or modify the action of media against components.

There are very few occasions when media can be used with no water and, therefore, no compound. There are some occasions when the parts themselves act as the media; this is a self-tumbling process.

Clearly, the selection of media and compounds for the mass finishing operation is as important as the selection of the correct tooling for any forming operation, and an understanding of the materials available is essential for effective use of equipment.

Mass Finishing Compounds. The correct use of compound-water solutions is vital to good and consistent mass finishing processing in any equipment. The compound solution is used to:

• Develop and maintain cleanliness of parts and media during the process

• Control pH, foam, and water hardness

• Emulsify oily soils

• Remove tarnish or scale

• Control part color

• Suspend soils

• Control lubricity

• Prevent corrosion

• Provide cooling

• Ensure effluent meets EPA, OSHA, and plant standards

Compounds may be liquid or powder. Addition of compound to the mass finishing equipment may be made in three ways:

• Batch: This is the simplest technique, used in closed machines, barrels, small centrifugal barrels, and vibrators with no drains. The machine is charged with compound and water, which is flushed away at the end of the process cycle.

• Recirculation: The solution is mixed in a tank and pumped into the process, allowing it to drain back into the tank for reuse. This process is simple and has the basic benefit that there is no continuous drainage, and the effluent can be treated on a batch basis. The major disadvantage of this system is that the solution deteriorates during its life and, therefore, results vary. For this reason, recirculation should be avoided if at all possible.

• Flow-through: This system pumps fresh solution into the machine, allows it to act, and then drains it out. Modern compound solutions can be very dilute and economical. For example, a 0.3 m (10 ft3) machine normally uses less than 4 L (1 gal) of compound per shift. When using flow-through systems with liquid compounds, automatic addition through a compound metering pump is possible, further reducing waste and improving consistency of the process.

Because the use of a flow-through compound solution system permits close control and is readily automated, liquid compounds are normally preferred for consistent results. When mass finishing in closed-batch systems, powdered compounds can be economical. Powdered compounds can include loose abrasives that enhance cutting capabilities of media. Abrasives are seldom recommended in vibratory systems.

Finishing Media. The media in the mass finishing operation are equivalent to the tooling used in any machining operation. The functions of media are to abrade or burnish edges and surfaces of components to be finished and to keep parts separate from one another to avoid or limit any part-on-part impingement. Media may be selected from any of the following materials:

• Natural media: Stones that have been quarried, crushed, and graded were the original media for mass finishing operations. Natural media have been largely replaced by synthetic materials that are harder with longer life; greater consistency of cut, wear, and dimension; and greater variety of capabilities.

• Agricultural materials: Sawdust, corncob, and walnut shells are frequently used in mass finishing machines for drying. Mixed with fine abrasives, these materials are suited for some fine polishing operations, particularly in the jewelry industry. Wood pegs may be coated with fine abrasives or waxes for edge radiusing and finishing of some wooden and plastic components.

• Synthetic random media: Fused and sintered aluminum-oxide media, crushed and graded, are available in a number of grades, both heavy cutting and fine finishing. Generally they are much tougher than natural media and more consistent.

• Preformed ceramic media: Porcelain or other vitreous material is mixed with abrasives and formed into shapes, then fired to vitrify. These media are available in a large range of shapes, sizes, grades of abrasive, proportions of abrasive to binder, and types of binder to enable selection of material to suit virtually every application. The consistency of quality can be ensured. This type of material is the present standard for the mass finishing industry.

• Preformed resin-bonded media are abrasives bonded into polyester or urea-formaldehyde resins. Like the ceramic materials, these are available in a broad range of shapes and sizes with different types, grades, and quantities of abrasive to meet a range of applications. Plastic media are somewhat softer than ceramic, and for a given degree of abrasion they usually have shorter life, but they also have somewhat lower cost and lower density. These softer materials achieve better preplate finish than other media and are better suited for handling soft metals.

• Steel: Hardened steel preformed shapes are available in a variety of shapes and sizes and are well suited for burnishing. Steel applications also include cleaning and light deburring. The basic benefit is that these media wear very little. Although there is a high initial investment, they are not consumable and do not need reclassification. Steel pins and tacks used with abrasive compounds can be useful means of removing somewhat inaccessible burrs.

Final selection of the best media must be made on a trial and error basis, but the following factors affecting selection should be understood and considered before any form of testing is started:

Shape and size of media

• To achieve uniform edge and surface finishes

• To avoid jamming in holes and recesses

• To achieve ease of separation

• To achieve shortest cycle time

Availability and cost of media

• From consistent supplier

• For economical cost per pound or unit volume

For consistent quality

Ability and versatility of media

• For minimum wear and reclassification

• For handling a range of products within a given machine

• For minimum break-in requirements

• For cushioning action between parts

Optimum media-to-part ratio is another consideration for choosing the best media (Table 2).

Table 2 Typical media-to-part ratios for vibratory and tumbling mass finishing processes

Media to part ratio, by volume

Commercial application

0:1

No media, part-on-part, used for beating off burrs, no media for cutting, sometimes suitable for burnishing

1:1

Equal volumes of media and parts, forgings, and castings; crude, very rough surfaces

2:1

More gentle, more separation, still severe part-on-part damage is possible

3:1

About minimum for nonferrous parts, considerable part-on-part contact, fair to good for ferrous metals

4:1

Probably average conditions for nonferrous parts, fair to good surfaces, good for ferrous metals

5:1

Good for nonferrous metals, minimal part-to-part contact

6:1

Very good for nonferrous parts, common for preplate work on zinc with plastic media

8:1

For higher-quality preplate finishes

10:1 to 15:1 or more

Better, used for very irregularly shaped parts or parts that tangle or bend

No contact

Absolutely no part-on-part contact, one part per machine or compartment, part fixturing

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