Barrel Finishing

The rotary barrel, or tumbling barrel, utilizes the sliding movement of an upper layer of workload in the tumbling barrel, as shown in Fig. 1. The barrel is normally loaded about 60% full with a mixture of parts, media, compound, and water. As the barrel rotates, the load moves upward to a turnover point; then the force of gravity overcomes the tendency of the mass to stick together, and the top layer slides toward the lower area of the barrel.

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Fig. 1 Action of media and parts within a rotating barrel

Although abrading action may occur as the work load rises, about 90% of the rubbing action occurs during the slide. In the case of a horizontal barrel that is just over half full, the most effective action occurs to produce the longest slide. As may be judged from Fig. 1, the faster the rotation of the barrel, the steeper the angle of the slide. With all but the shallowest of angles of slide, there is invariably more tendency for tumbling of the load, as well as sliding. The faster the rotation of the barrel, the faster the action will take place. However, the faster the action, the poorer the surface and edge condition will be, and the greater the likelihood of parts being damaged.

Typical equipment for barrel tumbling is shown in Fig. 2 and includes:

• Open-ended, tilted-type barrel (concrete mixer)

• An adaptation known as the bottle-neck barrel

• Horizontal barrel

• Submerged barrel

• Fixtured barrel

Fig. 2 Barrels available in a variety of sizes. (a) Standard open-end, tilting. (b) Bottlenecked. (c) Horizontal octagonal. (d) Triple-action, polygonal. (e) Multiple drums. (f) Multicompartment. (g) End loading. (h) Submerged

The open-ended, tilting barrel is used for light finishing and also for drying. The bottle-neck shape provides essentially the same action, but more effectively, because parts and media remain better mixed. The horizontal barrel, usually of octagonal cross section, is the standard or most used tumbling barrel, capable of achieving a variety of results and maintaining real consistency. Rotational speeds of these barrels range from as low as 4 rpm for large barrels to 60 rpm for small barrels; surface speeds range from about 6 to 60 m/min (20 to 200 sfm); the higher speeds are used for cutting, the slower speeds for burnishing and fine finishing.

Ratio of media to parts ranges from about 3-to-1 to 15-to-1 by volume. Rough work can be loaded only with parts, so that parts self-tumble against each other. Some of the factors to be considered in determining the media to parts ratio are:

• Size and complexity of the workpiece

• Possibility of media lodging in the parts

• Possibility of the parts nesting

• Required quality of final finish

Automation of barrel tumbling equipment is possible. The barrel can rotate in a clockwise direction to deburr and finish the parts. At the end of the process, the barrel rotation is reversed and parts feed out through a scroll, then through a screener into a material handling unit.

The capital cost of barrel tumbling equipment is low and generally maintenance costs are also low. Because barrel tumbling is a very well-established process, the basic conditions are well understood and guidance on best process techniques is readily available. Barrel tumbling is, however, a slow process, almost invariably involving several hours and on occasion several days. The process is space consuming with a high level of work in progress. More modern mass finishing processes offer greater versatility and convenience, with better use of labor and consistency of quality in production. For these reasons, and because higher operator skills are needed, barrel tumbling has been replaced in most modern production facilities.

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