There are two types of rock tumblers - rotary tumblers and vibratory tumblers. Rotary tumblers have a barrel-shaped container into which the operator loads rough rocks, abrasive grit and water. The barrel is then sealed and mounted on a small machine that rotates the barrel. Inside the barrel the rocks are tumbled with the abrasive grit and water. The tumbling motion of the rocks rounds their sharp edges and smooths their surface. The tumbling process is done in several steps using various grades of abrasives and usually takes a few weeks to complete.
How Do Rotary Rock Tumblers Work?
As explained above a rotary tumbler works by tumbling rock material in a barrel. The tumbling action wears the rocks into rounded shapes and smooths their surfaces.
There are typically five steps in the tumbling process.
Step 1 - Coarse Grind
In the first step of the tumbling process rough rocks are placed into the barrel with coarse silicon carbide abrasive (60/90 grit) and water. The rocks are tumbled from several days to a week with the coarse grit. During this step the rocks are rounded. The rocks are then inspected. If they are nicely rounded they progress to Step 2. If not they are tumbled with coarse grit a second time. Some people manually sort the rocks, removing those that are ready for Step 2.
Step 2 - Fine Grind
In this second step the rounded rocks are placed in the barrel with fine silicon carbide abrasive (120/220 grit) and water and tumbled for several days. During this step very little shaping is done. The fine grit smooths the surface of the rocks and remove scratches left by the 60/90 grit.
Step 3 - Pre Polish
In the pre polish step the smooth, rounded rocks are placed in a barrel with 500 grit aluminum oxide or
500 grit silicon carbide and water. They are again tumbled for several days. This step removes scratches left by the 120/220 grit abrasive and prepares the surface of the rocks for the final polish. Almost no shaping takes place during this step. Plastic pellets are sometimes added during this step to cushion the rocks from impact and to make up for lost volume.
Step 4 - Polish
In this step the rocks are placed in the barrel with a polishing powder such as aluminum oxide, tin oxide, cerium oxide or tripoli. The tumbling action during this step buffs the surface of the rocks to a high luster. Plastic pellets are often added during this step to cushion the rocks from impact and make up for lost volume.
Step 5 - Burnishing
If the rocks have a brilliant luster after the final polish you can skip burnishing. However, the final polishing step sometimes leaves a thin hazy film on the surface of the rocks. For the burnishing step rocks and plastic pellets are placed in the barrel with a very generous amount of non-abrasive bar soap, such as Ivory Soap (we buy a small bar and grate about 1/4 of it in a three-pound tumbler). The rocks are tumbled for a few hours and then removed. Burnishing usually removes the polishing film and yields brilliantly polished rocks.
We have much more detailed instructions for the grinding and polishing steps of the rock tumbling process. These include tips, grit measurments and times for different sizes of tumblers.
Types of Rotary Rock Tumblers
Several different companies make rotary rock tumblers. The most frequently advertised brands include: Lortone, Thumler's and Chicago. There are also rotary rock tumblers made especially for kids.
The three companies listed make multiple tumbler models. The three decisions that you must make in buying a tumbler are: 1) How many barrels? 2) How many pounds? and, 3) Which brand? We will explore these questions below.
One Barrel or Multiple Barrels?
Most rotary rock tumblers have one, two or three barrels. Which should you purchase? If you are tumbling large amounts of a single type of material the most efficient method is to use one large barrel rather than two small ones. The labor required to measure grit, load, seal, open, and clean is much higher for two small barrels than for one large barrel. However, if you have two pounds of Material A and two pounds of Material B the two barrel tumbler will be a better choice - especially if plan to tumble materials of different hardness.
Two barrels also work better if you are experimenting with different types of materials or different polishing compounds. The downside of some two barrel tumblers is that they sometimes do not run well with just one barrel. We have experience with dual-barrel tumblers and some of them run best when both barrels are in action.
Large barrels can tumble large quantities of rock and accommodate large pieces of rough. If you live in an area where you can collect your own rough or have lots of rough rock then a large capacity tumbler will be a good choice. Or, if you want to produce large 3" and up tumbles then you will need a large barrel to produce them.
If you must buy all of your rough and are on a limited budget then a small capacity tumbler might be a good choice. You will then be able to tumble small loads and not be forced to wait until you accumulate enough rock to load a large barrel. Small tumblers also make sense for people who use a tumbler to do the polishing step of cabochons, beads or other preformed stones.
Thoughts on Long-Term Ownership
Some tumblers are made to be thrown away if any problem arises and others are made so that the most vulnerable parts can be replaced. If you use your tumbler regularly you will eventually need a new belt, your barrels will wear thin and the bearings will become worn. Most dealers sell replacement belts and barrels for Lortone and Thumler's tumblers. Some dealers sell replacment motors, bearings, drive shafts and other parts for these tumblers.
Most replacement parts are reasonably priced and almost anyone can diagnose a problem and make a repair. If you think that you will be tumbling for a long time or you are buying a second tumbler because you wore your first one out then it might be a great idea to buy one from a dealer that stocks replacement parts or at least buy a model for which replacement parts are available.
Rotary tumblers are more popular than vibratory tumblers because they have the ability to produce rounded rocks. They are simple machines that will last a long time if cared for and can easily be repaired with parts become worn.
This video compares the features of several different rotary rock tumblers by Lortone and Thumler's. Small-capacity tumblers are less expensive and provide a low-cost way for beginners to try the hobby. Large-capacity tumblers are typically used by people who find their own rough or produce large quantities of tumbled stone for craft projects. RockTumbler.com and Geology.com are owned by the same company.
Photograph of a Lortone Model 3A rotary rock tumbler. This machine has a single three pound barrel. It runs on household electricity and is build to last through several years of regular use. The machine is about 9.5 inches in length and 6 inches wide. The barrel is about 5 inches high and 4.5 inches in diameter. It has a capacity of about three pounds of rock, however the barrel is filled about 2/3 with rock for tumbling so you will be tumbling about two pounds of rock at a time.
Photograph of a Lortone six pound tumbler barrel filled about 2/3 full with tumbling rough. The capacity of the barrel is six pounds of crushed rock. However, the barrel is not tumbled completely full because that would leave little room for a tumbling action to occur. Note that the inside of this barrel is not perfectly round - instead it is a decagon (a ten-sided shape). This shape keeps the rock from sliding as the barrel turns and encourages a tumbling action.
Photographs of a Lortone Model QT6 rotary rock tumbler. This machine has a single six pound barrel. It runs on household electricity and is build to last through several years of regular use. The view on the left shows the tumbler with barrel in place. The view on the right shows the red shafts that the tumbler barrel runs on. This machine is about 11 inches in length and 8.5 inches wide. The barrel is about 5 inches high and 8 inches in diameter. It has a six-pound capacity barrel that can process about four pounds of rock.
Photograph of a Thumlers Model A-R2 rotary tumbler kit that includes grit, polish, plastic pellets and rough rock. This tumbler has two three-pound barrels. The base is about 8 inches wide and 11 inches deep. Each barrel can process about two pounds of rough rock.
Photograph of a Chicago Rock Tumbler made by Chicago Electric Power Tools. It has a single three-pound barrel that will process about two pounds of rock. Chicago also makes a machine that turns two three-pound barrels.
The barrle size of a tumbler determines how much rock can be processed and what sizes of rocks can be processed. Here are three Lortone tumbler barrels. From left to right: six pound barrel (5" high, 8" diameter), three pound barrel (5" high, 4.5" diameter) and one and a half pound barrel (3.5" high, 4.5" diameter).