Garden Mill

A wind-powered compost tumbler and trommel sifter.

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This is a wind-powered compost tumbler with integrated sieve in Trommel fashion. Finished product falls through the bottom to a waiting cart while unfinished stays inside until broken down sufficiently. This can be just one component in a larger system but ultimately it will work as a standalone machine to be marketed to individuals, community gardens, or even municipalities, for the purpose of reducing the need for waste collection.

I may opt to add wireless sensors to monitor core temperature while in constant (very slow) motion and provide feedback to a watering system. I may also add a modular conveyor accessory to pivot underneath for collection of fines and feed another process, like a vermicompost system.

Waste to be processed is loaded into the drum where it will decompose slowly, over time.  Green and brown waste both are added as available, trying to maintain ratios.  The drum will rotate to churn the waste and as it does, fines which are processed enough to fall through the sieve will do so, collecting into a waiting cart.  Drum rotation is accomplished by a pair of axles on which the drum sits.  The axles are linked with a roller chain and can be turned by hand.  One axle is also connected to a gearbox which is driven by a vertical axis turbine.


LibreCAD sketch

image/vnd.dxf - 62.13 kB - 04/24/2023 at 20:08


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  • Drive

    load.nikon05/08/2023 at 17:04 0 comments

    The drum must be turned.  The easiest way to accomplish this with the current design is to rest the drum atop axles which turn in unison.  The axles are joined with a roller chain and matching sprockets.  The sprockets are capable of being rotationally fixed to the axle via set screws or a key and in our case, set screws were utilized.  The axle was marked and small flats were drilled into it for the set screws to seat in.

    This mess of metal and grease is not going to last years outside without some protection so a housing was made to protect from the elements.  It's well in the category of "can be mocked up in situ" rather than well thought out in advance.  Left brain or right brain, whatever gets the job done around here is good enough!  You can see it sits flush with the frame on the sides but atop the axles.  I will be notching out the space for the axles to pass next time I'm working on it.

    The next concern of note will be connecting the gearbox to the axles.  I have the first one I got still.  It's an old winch from Harbor Freight.  I still have the old pulley attached and I think we'll defo want to update that bad boy.  However the belt and pulley is a promising start for this as it has been planned all along to incorporate a clutch mechanism to disconnect the drum from the turbine.  The easiest way to accomplish this will be to use an idler pulley on a beltway to disengage the belt when necessary.

    The winch fits well enough on this frame but may be too close to be functional, considering more components are to be added to attach it.

    It's time to discuss the math going on here so we can determine where improvements will be made later on.  This drum is 48 inches in diameter, resting atop two axles of a 1 inch diameter.  Our current gearbox is a 40:1 ratio per the manufacturer's specifications.  So our current ratio is as follows.

    Axle to Drum
    Driving - 1" D x pi = 3.14" circumference
    Driven - 48"D x pi = 150.8 circumference
    150.8 / 3.14 = 48.02 so 48:1 gear ratio, eyeballed.
    40:1 (trust the spec sheet)

    Above math subject to criticism.  If we consider then that for the drum to rotate one time, the axles must rotate forty-eight times.  Assume a 1:1 ratio from the gearbox to the axles.  And calculate the 40:1 ratio of the gearbox, we will conclude that for the drum to rotate once, the turbine will have to make 1921 revolutions.  Almost a 2000:1 ratio.  We'll see if that's enough torque once we've got a load in this thing!

  • Potential

    load.nikon05/01/2023 at 11:50 0 comments

    I needed to solve for the way the fines escape by flinging everywhere and have a means to store them until ready for use since this is a continuous flow design.  The garden cart I had in place earlier is used for other purposes and I needed to commit something here that would withstand the elements.  In the spirit of sustainability, this would have been much better if I could source this or a similar poly cart secondhand or similarly.  But as it stands, this little 7 cu. ft. dump cart might just be the perfect thing as the materials can be transported directly to the point of use.

    Some more plywood here will help funnel the fines down into the cart.  From this angle, you can see we will need to capture the fines which fall out in both greater depth and width than the cart is capable of catching.  Since the hopper will need to extend above the frame and tangent to the sides of the drum, those longest pieces will be made first.  The panels which will cover spillage both front and rear, will only extend to the frame itself and will be shaped trapezoidal to fit within the other two lateral pieces.

    The cart couldn't be a better fit for this original mock up.  We will be able to determine the rate at which we process by logging every time we pull the little cart out to dump it.  Yay for science!  Once the enclosure is built, extending down to the deck blocks, this operation will be much better protected and the cart won't be subject to filling with rainwater.

    Having solved for finished product storage (and serendipitously including transport and delivery) makes this a win in my book.  Still a ways to go with this project but we're coming along nicely as I ramp up the gardening and landscaping operations here.

  • Inertia

    load.nikon04/26/2023 at 14:16 0 comments

    The frame and reactor having been roughly assembled, the next step is the drive.  Getting that big drum to rotate and be rotated by external forces necessitates a means to transfer that energy.  Axles are a fine choice in many cases and in ours will also support the drum.  The axles are 1" Cold Rolled 1018 carbon steel and are cut a smidge long so I've got room to play with various means of driving the thing.

    Those axles need something to hold them while they spin.  Modern ball, roller, and needle bearings are a very new invention on the time scale of all of civilization's mechanical feats.  Pillow block bearings can and historically have been made from hardwoods such as oak and beechwood.  An old partial pallet was on hand to donate its oak.  I made several bearings so to have spares on hand.

    The bearings were spaced arbitrarily at 12" medial to the frame's edges and mounted atop a span of 2x4 to distribute the weight more directly into the 4x4 uprights which have the most strength of all frame components to withstand compressive forces.  A 1" diameter hole was bored for the axle to pass through, sanded a bit to give a thou of clearance, a hole drilled for lubricant, and the whole bearing was toenailed to the frame.  Once the front bearings were fixed, the axles were checked for square in relation to the frame, double checked with the rear bearings' spacing to the lateral edges, and checked again by measuring the distance between the axles and ensuring they were equidistant along the length of them.  Once inserted and secured, they were verified to spin.  Presently, they're not able to spin freely but will do so with a modest torque.  The bearings may be iterated on if it's found that they impose too much friction to make turning the drum difficult once we get to that point.

    The next step will be tying the axles together and adding a means to input manual power.  Presently, my best idea is a roller-chain and sprockets to distribute power across axles, then I'll grind a hex shape out of the end of an axle to fix a socket to so this can be turned with a ratchet or driver for testing.  There's plenty of room to test all this up front but once concepts are solidified I will want to determine if that is still the best course or if it would be wisest to put power components in the back.

  • Momentum

    load.nikon04/23/2023 at 21:27 0 comments

    I couldn't leave the reactor on the wagon for concern that the neighbors may think I was building my own Conestoga wagon to go West and strike rich.  The black gold I seek will be made here, in this.  But the real compost will be the friends we made along the way.

    The frame was easily mocked up to fit the mock reactor.  Simple 4x4 uprights, 3' each are sat on four deck blocks.  These are supported by 2x6 cut 4' across the axis of the drum and just over 4' 6" in depth.  The joints are butted as you can see at the front bottom where it is left open for access.  Diagonal bracing will be needed but has not yet been added as I've yet to think through the hopper to be built in.

    The reactor is simply sat upon the 4x4s at present and has no means of rotation.  Axles will be added and pass through pillow block bearings.  Those will need to sit atop frame components which are not yet present.  A 2x4 may simply be cut to width and attached front and back, and secured flush with the front of the frame.

    The means of rotation will be next to solve for.  Two axles will support the reactor, on some kind of fixed wheels.  The axles must also be connected so power transfer is even across all points of contact with the drum.  Power input will be a crank to start.  It will need to be put on a clutch mechanism so it can operate independent of additional power transmission components to be added later on.  Presently, the first obstacle I may need to solve for is ensuring enough coefficient of friction that the drum does not slip and sit still as the axles beneath it rotate.  I may need to investigate options for both gears and belts as means of transferring power.

    This was good progress today.  It's situated well enough in the space that I have cleared.  I may attempt a few locations around this area.  But I feel good about finally making this amount of progress.

  • Perseverance

    load.nikon04/23/2023 at 01:33 0 comments

    It has been long time since the last update.  That's how it goes sometimes.  I have found some willpower and began roughing together the machine.  This time at scale.  The materials are cheap and the methods may not be the smartest.  But perfection is the enemy of done.

    Behold, the reactor.

    Precariously balanced on an old scrap frame from another project and then again on a wagon.  This is the intended size, 4' diameter x 4' length.  The purpose of this component being mocked up at scale is to begin understanding how to construct the frame and drive system.

    Of note, the Mantis compost tumbler seen to the right of the image is only four years old and has already begun to fail under the stresses of use.  Piece of junk and wildly overpriced.  An expensive lesson.

  • Impulse

    load.nikon05/19/2015 at 12:29 0 comments

    I started prototyping the Garden Mill today. I took my existing trommel sifter which normally mounts on top of my wheelbarrow and begun with it as the sieve in a new framework to test drive capabilities.

    The zip ties have not held up over the years.  Recommend anyone who wants to do this uses bailing wire to fasten the hardware cloth to the rims.

    I've had the gearbox for some time but needed to visualize how it was going to drive the rollers. I cut some wheels and pulleys out of scrap plywood (because shoestring budget) and ran them on to threaded rod for axles. I mounted the gearbox to an appropriate surface and had to kludge together a belt tensioner.

    All was for naught though as at the end of the day, the single drive axle did not have enough traction to drive the drum.

    (volume warning)

    Maybe I should have cut additional pulleys and put another belt on thus turning the idler axle into another drive axle. This still relies on the weight of the drum to provide the high coefficient of friction necessary to create traction between the plywood wheels and bicycle rims. Unless... damn. I just realized I could have cut some gears.

    I'm presenting this idea to my local hackerspace on Wednesday and I want to have something tangible to show rather than just the sketchup file. I think tomorrow I'll see if I can't make some 24" diameter gears to wrap around the bicycle rims.

    edit 20230505: Updated project log title from "Prototype Failday" to something more fitting as we never fail, we only learn.  Also fixed a broken image link.

  • Flow

    load.nikon05/07/2015 at 03:02 0 comments

    The garden-mill has found it's way into a flowchart. It's listed in green, in this project's main image gallery.

    edit 20230505: The main gallery is limited to six pictures so putting this here.

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Enjoy this project?


Discussions wrote 05/01/2023 at 22:22 point

I have found the an Instructable from forever ago that also turns a compost drum on a smaller scale and I had even commented on it back then with a pic of my idea!  Here was the original concept sketch of this project from late 2008.

edit: pic not posting in comment, will load to project gallery

Link to kdunner's project: 

  Are you sure? yes | no

Will F. wrote 07/10/2015 at 21:01 point

Either cutting gears like you mentioned or wrapping belt around the rims and the drive shaft would probably fix your issue there. looks like you've got plenty of torque.

  Are you sure? yes | no wrote 04/23/2023 at 01:37 point

That is brilliant.  I will likely test both.  Thank you for mentioning!

  Are you sure? yes | no

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