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Most rock tumbler problems have one of three causes: the rock, the process or the machine. Dull stones usually trace back to a rushed coarse stage or grit contamination. Leaks usually trace back to grit on the barrel rim. Slipping belts and hot motors usually trace back to overloading, oil or dirt.
Use this guide to diagnose the problem, then fix it. The fixes follow Lortone's, Kingsley North's and Harbor Freight's instructions and the troubleshooting guides from Rocktumbler.com, a long-running dealer for Thumler's.
Quick Diagnosis Table
| Symptom | Most likely cause | First fix |
|---|---|---|
| Stones shine wet, dull dry | Not finished; scratches remain | Go back to pre-polish |
| Fine scratches in polish | Coarse grit contamination | Clean everything, redo pre-polish and polish |
| Pits on finished stones | Coarse stage too short; porous or included rock | Return to coarse with fresh grit |
| Chips and bruises | Underfilled barrel; fragile stones | Add ceramic media; reach 1/2 to 2/3 full |
| Soft stones shrank to nothing | Mixed hardness or coarse too long | Sort by Mohs; shorten coarse |
| Barrel leaking at lid | Grit on rim; lid not seated; worn gasket | Scrub rim and lid; reseat; replace gasket |
| Barrel bulging | Gas from the load | Vent carefully; check twice a day |
| Barrel not turning | Belt slipped or broken; barrel wedged | Check belt and barrel position |
| Motor very hot or smells | Blocked airflow, overload, dry bearings | Unplug; check cooling, load and bearings |
| Loud growling or rattling | Overload; loose screws; dry bearings | Reduce load; tighten; oil bearings |
Results Problems
Dull Shine
This is the most common complaint, and it's almost never the polish's fault.
- Do the wet-dry test. Kingsley North's rule is that a finished stone looks the same wet and dry. If it only shines wet, it isn't done.
- Check for scratches under a bright light. Fine scratches mean grit contamination or a short pre-polish.
- Check the coarse stage. Most dull results start there. Kingsley North calls coarse the most important step, and Rocktumbler.com says its team usually runs coarse for at least two weeks.
Fixes:
- Go back to pre-polish for another week, then polish again.
- If you see pits or deep scratches, go all the way back to coarse.
- Clean the barrel, lid, seal and stones thoroughly between every stage. Lortone recommends washing rocks and barrel parts with mild soap and water.
- Use the right dose: Lortone's 3A manual calls for 6 level tablespoons of polish in a 3 lb barrel.
- Try a different polish. Rocktumbler.com and most kits use aluminum oxide; cerium oxide is a common alternative for quartz-family stones. Our rock tumbler grit guide covers both.
Scratches in the Final Polish
One grain of coarse grit can scratch a whole batch. Sources of contamination:
- The barrel rim and lid seal. Scrub with a toothbrush.
- Cracks and pits in stones, which hold grit and release it later. Cull or re-grind those stones.
- Porous rock. Rocktumbler.com says pores trap grit and carry it into the polish step.
- Plastic pellets carried between stages. Rocktumbler.com says grit embeds in plastic, so pellets should stay with one grit size.
- New ceramic media in the polish. Break it in during earlier stages first.
Pitting
Pits that survive to the polish mean one of two things. Either the coarse stage was too short, or the rock has natural pits, cavities or inclusions. Rocktumbler.com notes that garnet often pits because of mineral inclusions, and that rocks with voids should stay out of the barrel. Return pitted stones to coarse with fresh grit. If pits remain after two or three coarse runs, the stone probably won't polish.
Chips, Bruises and Broken Stones
- Underfilled barrel. Rocktumbler.com says below about half full, rocks are violently tossed around and can break, bruise or chip. Fill to the level your maker recommends: Lortone says 1/2 to 2/3, and Kingsley North and Harbor Freight allow up to 3/4.
- Fragile material. Obsidian, glass, crystalline quartz and feldspars bruise or split. Rocktumbler.com uses at least 50% media for especially fragile rough. See our ceramic media guide.
- Fractured rough. Cracked stones break apart and their sharp edges damage others. Cull them before tumbling.
- Stones too big for the barrel. Keep stones under about half the barrel diameter.
Soft Stones Disappeared
If softer stones shrank to almost nothing, they were either tumbled with harder rock or run too long in coarse. Rocktumbler.com's rule: tumble stones of similar Mohs hardness together, and cut about 1-1/2 coarse days for each Mohs point below 7. Its example is obsidian (5 to 6) tumbled with agate (7): the obsidian is ground tiny before the agate is shaped. Our guide to the best rocks for tumbling groups stones by hardness, and our timeline calculator adjusts stage times.
Stones Barely Changed
If a week of coarse made little difference, the barrel may be overfilled. Rocktumbler.com says past about 2/3 to 3/4 full there's no room to tumble, rocks barely change and grit doesn't break down. Too much water also slows grinding; Kingsley North says water should just reach the bottom of the top layer of stones.
Barrel Problems
Leaks
Rocktumbler.com lists the usual causes, roughly from most to least common:
- Grit or debris on the sealing surfaces
- Lid not seated straight
- Rough edges or cracks on the lid
- Lid not secured tightly
- Damaged gasket
- Worn-out lid
- Damaged barrel
Fixes: Empty and rinse the barrel, then scrub the rim and lid edge with a toothbrush. Reseat the lid so it sits flat. Lightly sand a rough lid edge with extra-fine sandpaper, and replace a cracked lid or holed gasket. Snug the retainer without overtightening, which can deform the lid. Harbor Freight's manual says to watch for leaks in the first 10 minutes of each run, which is when they're easiest to fix.
To reduce lid wear, Rocktumbler.com suggests keeping the fill below 2/3 with loosely packed rock and avoiding very large, angular, hard rough over about 2 inches.
Gas and Bulging Barrels
Some materials release gas as they tumble. Rocktumbler.com lists man-made glass (especially colored glass), volcanic glass such as obsidian and Apache tears, metallic ores and metals, blast furnace slag and nephrite. Adding soap, detergent, borax or clay can also produce gas.
Signs: a bulging or hard barrel bottom, mud seeping at the lid, or muddy bubbles around the lid edge.
What to do:
- Check gas-prone loads at least twice a day.
- Vent early in every run. Harbor Freight's manual vents at 12 and 24 hours, Kingsley North at 24 hours, and Lortone says to open the barrel occasionally.
- Vent safely. Put the barrel in a tub or outdoors, wear eye protection, and open the lid slowly, pointed away from you.
- Use a vented lid for glass and obsidian. Rocktumbler.com drills a tiny hole in a spare lid and covers it with tape, peeling the tape back to vent.
Rocktumbler.com says baking soda is sometimes suggested but hasn't worked reliably in its experience.
Machine Problems
Barrel Not Turning
- Belt off or broken. Check that the belt is on both pulleys and not cracked.
- Barrel wedged between the shafts or rubbing the frame. Reposition it.
- Overloaded barrel, too heavy for the motor. Reduce the load.
- Motor not running. Check the outlet and switch; see the motor section below.
Belt Slipping, Stretching or Breaking
Belts are the most common wear part on any rotary tumbler. Rocktumbler.com says genuine Thumler's belts last several years on clean, well-kept machines, while cheap substitute belts broke within weeks in its tests.
Causes: oil or grit on the belt, too loose or too tight, misaligned pulleys, rough pulley edges, age and heat, overloading, and wrong-size belts.
Fixes:
- Clean the belt in a few drops of mild dish soap and hot water, and wipe the pulleys. Avoid solvents.
- Set tension so the belt gives slightly under light finger pressure but turns the barrel without slipping. On many tumblers you adjust by loosening the motor nuts and sliding the motor. Rocktumbler.com notes that Thumler's A-R1, A-R2, A-R6, A-R12 and Model B don't need tension adjustments.
- Align the pulleys and smooth any rough spots with a stroke of fine sandpaper.
- Keep one spare belt, stored between about 50 and 75°F (10 and 25°C), per Rocktumbler.com.
Harbor Freight's troubleshooting section lists a loose, slipping or misaligned belt as a cause of overheating on its dual-drum, and its replacement V-belt is part 93835.
Hot Motor
Some heat is normal. Lortone says it's normal for its motor to run hot, and Rocktumbler.com says most tumbler motors are air-cooled and hot to the touch. Thermally protected motors shut themselves off if they overheat.
It's a problem if you smell hot plastic, oil or burning, see smoke, or the outlet or plug is hot or deformed. Unplug the tumbler if you can do so safely. Rocktumbler.com says that if there's smoke at the outlet, leave the building and call for help.
Common causes and fixes:
- Blocked airflow. Don't cover the tumbler or run it in a hot shed or direct sun.
- Overloading, including heavy materials like hematite or a barrel too big for the machine.
- Friction: a wedged barrel, a rubbing lid, or dirty, dry bearings.
- Dust in the motor. Blow it out with compressed air once it's unplugged and cool.
- Lubrication. Follow your manual. Lortone calls for one drop of oil on the shaft bearings every 30 days and says its motor doesn't need oiling. Never oil rubber parts such as barrels, belts or rollers.
If the motor won't restart after cooling, or the smell returns, replace it with the maker's part. Rocktumbler.com also suggests plugging tumblers into GFCI receptacles.
Noise
If a tumbler gets louder, check for overloading, loose screws on the motor or covers, and dry bearings. Rocktumbler.com says overloading can make the motor growl. Our quiet rock tumbler guide has more noise fixes.
Rock Tumbler Parts Worth Keeping on Hand
- A spare drive belt for your exact model.
- A spare lid gasket, or a spare lid for barrels with integral seals.
- A second barrel, which also lets you keep another batch going. Our double barrel rock tumbler guide covers twin machines.
- Light machine oil for the bearings, if your manual calls for it.
- A soft toothbrush for rims and lids.
- Ceramic media for topping up and cushioning.
Buy parts by exact model number. Lortone and Thumler's sell replacement belts, barrels and lids, and kit makers such as KomeStone sell motors, barrels and lid kits for their machines.
Prevent Problems Before They Start
- Sort rough by hardness and cull cracked, porous or crumbly pieces.
- Fill correctly, about half to two-thirds full.
- Don't rush coarse.
- Clean thoroughly between every stage.
- Clean the rim and lid every time you close the barrel; Rocktumbler.com says it takes about 30 seconds.
- Check the tumbler daily for bulging, seeping or odd noises.
- Keep slurry out of drains. Lortone and Kingsley North both warn that it settles and hardens in pipes.
Our step-by-step guide to rock tumbling walks through the full process.
FAQ
Why are my tumbled rocks not shiny?
Usually one of four things: the coarse stage was too short, coarse grit contaminated the polish, the barrel was underfilled so stones chipped, or the rock itself is too soft, porous or fractured to polish. Kingsley North's test is that a finished stone looks the same wet and dry. If it only shines wet, go back to pre-polish.
Why is my rock tumbler leaking?
Most leaks come from grit on the barrel rim or lid, a lid that isn't seated straight, a worn or cracked lid or gasket, or gas pressure. Rocktumbler.com lists grit on the sealing surfaces as the most common cause. Scrub the rim and lid with a toothbrush every time you close the barrel.
Is it normal for a rock tumbler motor to get hot?
Yes. Lortone says it's normal for its motor to run hot, and Rocktumbler.com says most tumbler motors are hot to the touch. It becomes a problem if you smell burning plastic or oil, see smoke, or the outlet is hot. Unplug it immediately in that case.
Why does my rock tumbler barrel bulge?
Gas. Some materials, including glass, obsidian, Apache tears, metallic ores and slag, release gas as they tumble. Rocktumbler.com says to check gas-prone loads at least twice a day and vent them by slowly opening the lid, pointed away from you, wearing eye protection.
Why do my rocks have pits after tumbling?
Pits that survive to the polish stage mean the coarse stage was cut short, or the stone has natural cavities, inclusions or soft spots. Return pitted stones to coarse grit with fresh grit. Rocktumbler.com notes that garnet often pits because of its inclusions, and rocks with voids shouldn't be tumbled at all.