Build a Dirt-Slinging, Rock-Spitting Trommel in One Weekend
Tired of picking lava rocks out of your garden one miserable handful at a time?
Meet your new best friend.
Spoiler: it's not a spaceship.
Put dirt in the HIGH end. Turn crank. Fine soil falls through. Rocks walk out the LOW end. That's literally it.
Shovel soil into the open mouth at the high end. Don't overload — half full max.
Turn the handle. The drum spins. Spokes tumble the soil and break up clumps.
Fine soil rains down through the mesh into your wheelbarrow below.
Rocks ride the slope down and tumble out the low end into a catch bucket.
Dirt goes in. Satisfaction comes out. Rocks get evicted. Everybody wins. If a complete beginner opens this page and says "Oh, I can build that" — mission accomplished.
Take this list to the store. Don't overthink it. Everything is in the plumbing and fastener aisles.
Less if you already have buckets or a wheelbarrow. Buy the exact sizes listed — there are only two PVC sizes in this whole build.
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Cut all the pipe first. Assembly goes fast once the parts are ready.
The 4-1/2" spoke length is a starting point. Dry-fit one hub inside your actual bucket and trim to fit. Different bucket brands taper differently.
| Part | Pipe Size | Length | How Many |
|---|---|---|---|
| Side rails | 1-1/4" | 44" | 2 |
| End-rail halves | 1-1/4" | 8" | 4 |
| Corner extensions (into elbows) | 1-1/4" | 3" | 4 |
| Front legs (infeed — TALL) | 1-1/4" | 36" | 2 |
| Rear legs (discharge — SHORT) | 1-1/4" | 30" | 2 |
| Main axle | 1" | 65" | 1 |
| Drum spokes (trim to your bucket) | 1" | ~4-1/2" (start long) | 8 |
| Crank arm (the throw) | 1" | 5" | 1 |
| Crank handle (grip) | 1" | 4" | 1 |
1-1/4" is the frame. 1" is everything that spins. The 1" axle rides inside the 1-1/4" tees — that's your bearing. No extra parts. Nature is clever.
This is the scariest-sounding step. It's not scary. You're just removing two bucket bottoms.
On each bucket, measure about 6" down from the top rim. Mark a line all the way around with a marker. You're keeping the top (the rim is the structural part). You're removing the flat disc at the bottom.
Use a jigsaw, rotary tool, or a sharp utility knife. Cut along the line. Keep the ~1-1/2" structural rim at the base intact. You just want the flat disc gone.
Cutting plastic throws chips. Safety glasses on, please.
Run a file or sandpaper over the cut edge. Each bucket is now an open tube. Ugly cuts are totally acceptable — you're wrapping mesh over this anyway.
Two bucket rings: check. Time for the actual exciting part.
This is the hub that connects the bucket to the axle. It also doubles as the tumbling paddle. Two for one!
Two hubs — one per bucket end. Each hub = 1 cross + 4 spokes + 4 capped bolts. The bolts punch through the bucket wall and hold the whole drum together.
Glue a ~4-1/2" spoke into each of the four openings of each 4-way cross. You now have two plus-shaped hubs. Let them set.
Slide both hubs onto the 65" axle. Space them 24–28" apart. Don't glue anything yet — you need to align them first.
Slide one bucket onto the axle and seat its hub about halfway down. Mark where the 4 spoke-ends touch the bucket wall. Drill 1/4" holes at each mark.
Pass a 1/4"-20 bolt with a fender washer through the OUTSIDE of the bucket wall into the spoke. Tighten a nut on the inside. Repeat for all 4 spokes, then repeat for the second bucket.
Look down the axle. Both buckets should be square and in line. Once you're happy, apply PVC cement where each hub's center cross meets the axle. This locks the drum so turning the axle spins the whole drum.
Those 4 spokes aren't just structural — they're paddles. As the drum spins they lift and tumble the soil, breaking up clumps automatically.
Wrap the hardware cloth around the buckets and zip-tie everything together. It's exactly as glamorous as it sounds.
Cut hardware cloth edges are razor sharp on every single side. This is how people get hurt. Gloves. Now.
Wrap the hardware cloth around one bucket to feel the circumference. Add 2" overlap for the seam. Cut the mesh to length with tin snips or aviation shears.
Wrap the hardware cloth tightly around the OUTSIDE of both buckets, spanning the gap between them. Aim for a firm, snug wrap — not loose.
Overlap the seam by at least 2". Stitch the overlap together with zip ties every few inches. Pull them tight and snip the tails.
Zip-tie the mesh to each bucket's ridge/lip around the circumference — every few inches. This is what keeps the screen from spinning on the bucket.
You now have a metal burrito that will be the bane of every lava rock in your garden. Time to build it a home.
The frame is a tilted PVC rectangle with two bearing tees that the axle spins through. Five steps and you're done.
Tall legs (36") at the feed end, short legs (30") at the discharge end. The slope is what moves rocks out. The bearing tees (orange) are where the 1" axle spins through the 1-1/4" frame.
Each end: push an 8" pipe into each side of a 1-1/4" tee. That center tee IS the bearing. Cap each outer end with a 90° elbow.
Each side: 3" extension → leg tee → 44" rail → leg tee → 3" extension. The leg tee branches point downward for the legs.
Connect side rails to end-rail elbows to form the rectangle. Check it sits square. Then glue the frame corners — but DO NOT glue the bearing tees (where the axle passes through).
Keep it away from the bearings and the axle. The frame can even be left unglued if you want to break it down for storage.
Push the 36" legs into the infeed tees. Push the 30" legs into the discharge tees. Slide the drum's axle through both bearing tees — it should spin freely. Add the two 1" coupling "collars" on the axle just outside each bearing to stop the drum from sliding sideways.
On the infeed (tall) end of the axle: 90° elbow → 5" crank arm → 90° elbow (facing out) → 4" handle → end cap. Test that it clears the frame and your knuckles. Then glue every joint heavily — the crank takes all the torque. 💪
Put a wheelbarrow under the drum. Put a bucket at the low end. Add dirt. Turn crank. Smile.
Don't fill it. Keep it under half full or it'll pack instead of tumble. One shovel. That's it. Test first.
Steady, moderate speed. You don't need to spin it fast. Gravity does the hard work. The slope walks material toward the discharge end automatically.
Fine soil rains down through the mesh into the wheelbarrow. Rocks ride the slope to the low end and tumble into the catch bucket. Lava rocks: defeated. You: victorious.
Damp soil clumps and blinds the mesh. Let a load dry in the sun first and it'll fly through. Wet clay is the enemy of every trommel ever built by anyone, ever.
Something's wrong. Don't panic. It's almost always one of these things.
Check these in order:
A rub of bar soap or candle wax on the axle where it rides in the bearing tees can make a big difference.
A bucket is misaligned or a hub bolt is loose.
Not enough slope. Push the discharge legs deeper, or trim them shorter to steepen the tilt. Even an inch more makes a noticeable difference.
Nature is mocking you. Here's why:
Vibration loosens plain nuts. Replace with nylon-insert lock nuts — they grip the thread and won't back off. Alternatively, add thread-locker (e.g. Loctite Blue). Snug everything after the first 20 minutes of use on a new build.
The thrust collars (1" couplings) are missing or loose. Add a 1" coupling pinned to the axle just outside each bearing tee. Use a screw through coupling + axle, or a dab of cement on the coupling only (not the axle).
Got the basics working? Here's how to level up.
Chuck the axle stub into a cordless drill. Run it slow. Instant motorized trommel — no extra parts needed.
Cut a third bucket lengthwise into a scoop. Screw it above the infeed end. Dump bigger shovelfuls without spilling.
Cut the zip ties and re-wrap with 1/4" mesh for fine compost, or 3/4" for coarser rock sorting.
Space two buckets farther apart (or swap to a barrel) for a much larger drum. Same design, bigger beast.
PVC in direct sun gets brittle over time. A coat of outdoor spray paint made for plastic extends the life significantly. Store under cover when not in use.
Take the left column to the store, the right column to the workbench.
Track your victory. Enter how many buckets of lava rock you've evicted from your garden.
Run through this after your first test. Check 'em off.
High-five yourself! You just built a garden superhero. Take a picture of your trommel in action vanquishing lava rock and celebrate with a cold drink.
Happy sifting, rock-slaying friend. 🌱🪨🚀