Cody's Disc Chunker

I am finally making a chunker. Looking around at what I have and what I can buy, I am going for a Disc style chunker.

Bought a 12"x12"x1/4" plate at Lowes. Holy cow they lied about the listed price. It said 36 dollars, when I scanned it it said 49 dollars :scream:. Oh well.

I also bought a W Weasler Hub. I have a length of 1" keyed shaft I can use.

I have pillow block bearings out the wazoo, no problems there.

I am going to rob one of my wheels from the mini truck build, it will act as my flywheel and final gear reduction. First reduction will be chain driven. Essentially I have a 20" pulley as the final drive. I’m looking at a 10:1 alone on the final leg. Do you guys think a 10:1 will be enough? I don’t know if I should run the engine at 1800 rpm or 3600 rpm.

Also for the blade profile, I’ve seen many use the “shark fin” style, I have seen a few that use an entire circle but it is rotating oblong. CamKutter - YouTube

Of course Kristijan has built one before, Wood chunker, first run - YouTube

And Jonathan Collins, Wood chunker wood slicer - YouTube

And Bobmac has done one with his square baler gearbox if I recall correctly.

I’m not sure which is ideal. Any tips for drawing out the shark fin style? I’m bad with swirls, the Golden Ratio scares me :joy:


It looks like about 60-90 rpm from the video. I would have to actually time it.

inkscape is free. :slight_smile: Then, blow it up to the size you want, print it and use it as a pattern.


Any design will work but in my experiances, the shark fin style is inferior because the tip is the weak point. If you just round it up a bit and give it a bit more meat, then its better.

As for rpm, if the blade is big enaugh that you can cut out a big enaugh window to be able to push the wood in in time, l guess 180rpm will do, but you will need to be quite fast. 60 to 100rpm is probably on the money


Sounds good, I can add more gear reduction before the final drive, probably a 20:1 ratio just from looking around at what I have to work with. I’d like to be able to use a 6hp engine so I should aim for the highest ratio I can build.

You say the shark fin is the least desirable, do you think a plain disc rotating at an askew axis like this voided area being the rotational axis?

I was thinking along these lines but also just cutting out an egg shape, like a proper Cam Lobe. Have a tip with a rounded edge like you suggested. At least with an egg shaped blade I can sharpen both sides and turn the blade around before I have to take it to the shop and re-profile.


A hybrid. Instead of this

Do this

Eggs and round blades will ofcorse work but you want a gap to open as soon as the chunk is cut, to give you time to position your wood in for a next chop. Specialaly if having higher rpm.


Aha so more of a Bean shape, gotcha.


Bingo. Best of both worlds


Hello, I just started catching up on these posts.

First of all, DO NOT BUY ANY MATERIAL FROM A BOX STORE!!! They will rape you. Go to a salvage yard or industrial metal supplier. Sometimes they have a scrap pile of excellent material for us small scale people.

I built a shark fin and mounted it to the back of my little 20hp tractor (that I also picked up from a salvage yard for 600 bucks).

Where I live we have circle irrigation. They are always dumping off the drive units at the salvage yard. I think the gear ratio is about 40:1.

I simply built a STURDY frame and put the shark fin blade on the wheel side and hooked the drive side right into the PTO on the tractor. I only had one cutting edge and it was a bit slow. I modified it into a butterfly shark fin if you can imagine. Two blades, like a lawn mower I guess.

I would upload the video but I don’t know how.

So, with all that said, I would encourage you to try a different style which is my next project. The shark fin worked great but was pretty slow. I was pushing through 3" dry cherry wood with no problem, but like I said, it was slow.

Consider an auger style. This is my next version. The nice thing about this is I don’t have to worry about limbs or anything. It just sucks it all in. There are some much smaller ones too that are pretty impressive but this should give you some ideas.

This gives you an idea of how the industrial ones are made:

Best of luck!



I don’t have the means to make an auger style

Continuing my chunker experiments on this topic. I’m going to try and make a disc style first since I’ve already bought materials for that. The platform will be relatively easy to change over to a drum style cutting blade if the disc doesn’t perform well enough.

My large sprockets have come in, 60 tooth #40 Industrial chain size. I’m going to do a double reduction. I’m waiting for the smaller sprockets to come in for that, 11 tooth.

Two shafts of 1" keyed shaft, one being a jackshaft to begin the double reduction.

Just with the 4 sprockets I will have a 10.9:1 ratio.

My input will be one of the trailer wheels and a serpentine belt clutch, so with the 20" outside diameter of that with an alternator sheave I recovered that’s another 8:1 give or take. I guess the real test will be seeing if I can slice wood using just hand rotation. I have a 2HP dual voltage motor that will run on 120 or 240vac. I’d be thrilled if I could use that with a pedal to turn it on as a safety.

If I want to turn it to a drum blade all I need to do is get another trailer rim that uses 4x4" lug pattern and weld my pipe section to that.


Make a little loop at the end of a string. Wrap the string around a cylinder, like a can of tomato paste. Tape down the tail end to the can OD so it won’t slip. Put it all on a piece of paper. Put a pencil in the loop and trace while unwinding the string. This gives you an involute curve. I just made a curve with a 2 inch diameter tomato paste can, and the curve looks about right.



The torque and horsepower curves cross at 5252 RPM.
(Only in English units)
In other words, each Lb-foot of torque gives you 1 HP at 5252 RPM.
You and I know that engine torque curves are not quite flat, but to make rough estimates we can act as though they are. Good enough for back-yard work.
HP is proportional to RPM, in other words half the RPM gives you half the HP.
So an engine that is factory rated to develop 6 HP at 3600 RPM is, by definition, developing (5252/3600) more torque than the engine next to it that develops the same 6 HP at 5252 RPM. And side-note : you probably noticed that a well-tuned engine (no blower, no nitrous, just naturally aspirated) develops about 1 Lb-Foot of torque per cubic inch. Just an accident of James Watt’s choice of strong horses for his horse power measurements. A happy accident that makes a part of this easy to remember. Ball bearings and chain drives are very efficient, so you can call it 100% for rough figures or 95% if you want to get technical. So an engine rated at 6 HP at 3600 RPM is developing 6X(5252/3600) Lb-Ft of torque at 3600 RPM. That is 8.75 LbFT of torque. Now, 61 cubic inches = 1000cc and torque is about equal to cubic inches in the best engines, but this is not a crotch rocket engine, so figure 80% of that scenario, which means divide 8.75 by 0.80 which gives you 11 cubic inches for an engine displacement estimate, and dividing that by 61 cubic inches works out to .179 part of 1000cc, which is .179 liter or 179cc. So your engine is about 180cc. Its torque at max HP is 8.75 LbFt, and you know that max HP occurs to the right of max torque on the power curve, so assuming a flat torque curve pinned at the max power point is not bad for rough figures. It understates peak torque and it is correct one more time in the mid-range. Taking 95% of 8.75 LbFt you get 8.3 LbFt from the little engine that could. Now you just multiply by gear ratios as you know to get your steady-state output shaft torque. This works out great if you are turning a boat prop. But the chunker has energy stored in a flywheel. The amount of force the shark fin will apply to the wood is the output shaft torque divided by the lever-arm in feet. It is from the center of the shaft to the center of pressure where the shark fin is crushing wood fibers. Your double reduction 11:60 gear train is better than you thought. It’s 29.75:1 because the ratios multiply on each other; they are not additive. Think about a double set of 3:1 pulleys. If you turn the input shaft 9 times, the jack-shaft will turn 3 times, and the output sprocket will turn 1 time, so it works out as 3 X 3 = 9:1 overall gear ratio. So with your serpentine belt primary drive reduction of 8:1 multiplied by your double reduction chain drive, the output torque from the little 6 HP engine will be (29.75 x 8) x 8.3 LbFt = 1975 LbFt so round it to 2000 LbFt average. And the output speed will be 3600 / 238 = 15 RPM or 1 rev every 4 seconds. OK, maybe a tad slow. Now, let’s use backyard math which is good enough for our purposes. We are committed to increasing the flywheel mass until the chunker does not get stuck, right? It’s fair to estimate that 1/2 of the revolution of the output shaft is the portion in which serious force is being exerted by the shark’s tooth. So 4000 LbFt will be exerted on the keyway in the 1 inch shaft. I’ll assume the shaft is made of SAE 1045 which has a yield strength of 75,000 Lbs per square inch, and the key is equally strong. Eyeballing that hub, it looks like it’s 1 3/4 inches long. A standard 1/4" key engages 1/8" so the area of the side of the keyway which is taking all the force is .125x1.75=.219 inches squared. The key’s effective lever arm is 7/16" or .03646 foot. 4000 LbFt / .03646 foot = 109,710 pounds exerted on .219 square inches of area, so the stress is 510000 PSI, which is 6.5 times more stress than the keyway can handle. The keyway can belt out .219 InSq X 75,000Lb/InSq = 16,425 Lbs of tangential force at a lever arm of .03646 Ft. = 600 LbFt of torque. The shaft itself is good for 680 LbFt. 600 LbFt / .42 Ft = 1429 Lbs distributed over 1 InSq of shark tooth edge smashing into 4 inch diameter wood = 1429 PSI available to crush the wood which is adequate only for soft wood. Sharpening the shark’s tooth will help some, but after some penetration, the full width of the tooth is bearing upon the wood. This chunker will be adequate for 4 inch softwood and 2 inch hardwood. Just don’t let some jerk stuff a 4 inch oak tree branch into it, calling on the 4,000 LbFt momentarily available, bending your ouput shaft or shearing its key. I suggest you design it with two of the keyed hubs and two bearings carrying the shaft, one on each side of the shark tooth, so the shaft will not be cantilevered. Now, the electric motor option. I like the foot-pedal switch idea for safety. It’s probably a 3600 RPM motor (3450 RPM actual speed, but close enough). So the 15 RPM output speed will be the same (14.4 RPM for the typical know-it-all uninvited neighbor, haha) The torque will be 1/3 nominally, but electric motors increase their torque in response to load, so the loss is not as bad as the numbers look. But expect to have to wait for it to recover RPMs or put smaller pieces of wood in it. To some extent the motor armature is a flywheel, as long as the primary drive belt does not slip. Overall, it will work but it won’t be mega-strong like Wayne’s, eating slab sandwiches! I agree with your idea to try it with minimal flywheel mass at first. It will be better to have it stall on oversize wood than to keep shearing the keys. I’ve tried hard to be kind to your idea here, but the output shaft really needs to have a larger diameter. -Mark the RedOak guy


I should have a larger output shaft, but my main issue is just what I have on hand. Maybe later on down the road I can revisit it with a 1.5" shaft or something that uses a 1/2" or 3/8" key, or just hard welded in place. I have an old truck inner axle I could use, just need to measure it and find a bearing that could fit it.

The gear ratio will be a lot higher in number when I add the 20" diameter wheel and alternator pulley. I think right now if I make it a double reduction with only one 11 tooth and one 60 tooth I’d have a 36:1 or higher using the trailer wheel. I haven’t measured the alternator serpentine pulley in a while but I want to say it’s either 3" or 2.5". If I made it Triple reduction using all sprockets and the wheel, it would be a very silly 197:1, but I have other smaller diameter wheels and serpentine pulleys I can use for a big pulley to even out the ratio.

I think for the most part my largest wood diameters will be 3", just looking at how my coppicing is going. I was able to process most of it this go round with my Fiskars geared lopper bolted to a sawhorse, and anything bigger than 2" I burned for charcoal.

Thanks for reminding me I should multiply each gear ratio against the other and not just x2 when they’re identical.


Just back from running cold water over my head, Mark the red oak guy. My brain was overheating from reading your post. Most excellent. I may have to print it out and post it in my workshop. :star_struck:


Cody, Did you ever finish this chunker?

I’ve been trying to design the disc to use a saw blade.

Chunker Blade2

The red circles on the right drawing is my third attempt after making these GIF’s.

Rotated chunker

rotating Chunker Blade3

I think either one should still work but the last one SHOULD give a little more open time. HINT: Put your finger or a pencil on the screen to simulate the wood needing chunked.


The speed of the rotating blade you have here is good. This is about the speed of rotation of my blade on my hay baler chunker. 4" dry round branches is about the limit, 5" green branches of hard cherry wood.
My gear box and fly wheel is a old hay square baler modified with the blade mounted in the stuffing box area where the ram arm has been cut off and is connected to the blade.