Tumble Polishing, or tumbling, is a technique for smoothing and polishing a hard substance. Within the field of metalwork, this is known as “barrelling”, or “barrel polishing”, and is subtly different, but uses the same principles.
For tumbling of rocks as a lapidary technique, a plastic or rubber barrel is loaded with a consignment of rocks, all of similar or the same hardness, some abrasive grit, and a lubricant. Silicon carbide grit is commonly used, and water is a universal lubricant. The barrel is then placed upon slowly rotating rails so that it rotates. The “optimal” speed of rotation depends considerably on the geometry and materials involved.
A well-chosen speed for stone polishing causes the rocks within the barrel to slide past each other, with the abrasive grit between them. The result of this depends on the coarseness of the abrasive, and the duration of the tumble.
Typically, a full tumble polish from rough rock to polish takes around 4-5 weeks, and is done in 4 steps. Initially, the rocks are smoothed with a coarse grit (such as 80 mesh), followed by washing and finer grits (220 then 400 mesh), before the use of a polishing compound (such as cerium oxide or ferric oxide). The precise tumbling time is determined by many factors, including the hardness of the rock and the degree of smoothing desired in the coarser steps.
Sometimes, stone “preforms” are used. This refers to cutting shapes from the rough rock, before tumbling. This gives more control over the final piece, so shapes such as a tear drop can be produced. The technique is still limited to rounded shapes. Preforms may use less time with the coarsest step, or skip it altogether.
During the 1970s, small rock tumblers were a common hobby item, and jewelry decorated with tumbled semi-precious stones was very much in fashion. Likewise, dishes and decorative glass jars filled with tumbled stones (often including common rocks not suitable even for costume jewelry) were frequently used as household ornaments.
Metal tumbling is used for obtaining a specified surface finish, burnishing, burr removal, cleaning, creating radii on edges, deflashing, and breaking runners off of parts in die casting. The process is similar to that explained for rock polishing. The barrel is filled with the parts that are to be barrel. Vanes, typically made of rubber, run along the inside of the barrel. As the barrel turns the vanes catch and lift the parts, which eventually slide down or fall.
Tumbling is an economical finishing process because large batches of parts can be run with little or no supervision by the operator. A full cycle can take anywhere from 6 to 24 hours with the barrel turning at 20 to 38 rpm. In most cases tumbling is most efficient when the barrel is 50% full. Some processes also use a filter system to allow parts or other materials in the cylinder to be separated.
Tumbling can either be a dry, wet, and with or without media. In wet processes a compound, lubricant, or barreling soap is added to aid the finishing process, prevent rusting, and to clean parts. Media is also used to obtain the desired finished project. Media can have any level of abrasiveness and can be made of a variety of synthetic, metallic, and even organic materials. Some of the most common materials are steel ball bearings, aluminum oxide, ceramics and plastics.
Stained glass shards used for mosaic glass are also tumbled. No abrasive is used, to avoid clouding the glass; there is only water as a lubricant. The object of this tumbling is to remove the sharp edges from the glass, so that it may be handled safely. As little as 8 hours of tumbling may be sufficient for tumbled glass.
Tumbling is used to polish and smooth dice for recreational use, but it has the unfortunate effect of making their sides and faces somewhat uneven and thus making the dice less than fair.
These techniques, although they take a long time, involve very little operator intervention and thus are very cheap. Small tumblers (one pound capacity) are available and inexpensive for home/hobbyist use. At the other end of the scale, professionals can use very large barrels to do a lot of work at once. The main disadvantage of tumbling is its limited scope – stones will be smooth and have semi-random shapes (like pebbles from the beach), and metals need to be relatively simple shapes, with no fine work.
- Ball mill – a process with some similarities to tumble polishing
- Raymond Rodebaugh/Alan Silverstein. Big Tumbler, December 17, 1999, p. 10