I started building a rebak chunker today for gasifier wood probably max 2-3” thick and thought I’d do a build thread in case someone else wants to build one and learn from my inevitable mistakes
I had some axles with sprockets and bearings I intend to use, I think they are 112mm (4 3/8”) diameter without the teeth and a 20mm (3/4”) axle
I will not use the inserts in the sprockets since they are only held there by friction so I will make inserts and weld those in the sprocket to use woodruff keys in them.
That’s all I had time for today.
The plan is to have the sprockets and bearings divided by a 10mm (3/8”) metal plate on each side from the knives, ideally there should be flanged bearingholders but I didn’t have any at home and I think this is sufficient if I turn them so the foot takes each blow.
The sprockets and the knifesupport/fastener could also have been welded on to the axle but I do like serviceability even though it takes longer to make and needs more planning and time to build.
Thanks J-O. I hope that it will turn out as I imagine it but knowing myself this will become a heavy beast.
I’m planning to build it with wheels and the feeding slot at elbow height for easy feeding plus not having to lift it at all since it will be higher than a trailer so the chunks can fall directly on it.
This will be interesting to follow
I was into building a rebak myself once, but ended up with a bought mechanism (got an offer i couldn’t refuse)
I belive you are going the right way with the bearing holders, placing them so the foot is located “away” from the knives, it was the way i was in to build myself, the holders are much sturdier that way.
Are you going to put separate knifes on? Or use the blades as is?
As for re- sharpening fixed knives, you could maybe use shims under the foot of the bearing holder, to get them closer? The sprockets should leave a little space available maybe?
By the way, Johan, a VERY good idea to start with the fuel processing equipment, i believe not many woodgassers has gone that route, most starts with a gasifier and then realize they need chunks, and a lot of them
I have not yet figured out how to make the knives adjustable because of wear, oval holes is not good, I believe the knives could slide in those from all the impacts unless I weld on something on the opposite side of the knife towards the axle each time I grind them, and putting on a square bar on the knife with two bolts towards the center for adjusting possibilities takes too much room from where the wood will be.
So I think I’ll cross that bridge if ever needed, I happened to come across maybe ten years ago some scrapped flat bar, most likely tool steel grade, in roughly good size to make knives out of just cut to length. Haven’t measured yet.
Perhaps it is not even needed to sharpen the knives in my lifetime either, I don’t think @JO_Olsson has sharpened his yet.
Oh yes, I don’t think the sprockets leave any room for adjustability and for a chain drive, that would probably been the way to go if I didn’t have sprockets.
I did not have a chain drive lying around unfortunately.
Got out some material for the sides, it is 10mm (3/8”) thick and originally used to hold wooden beams in place on railway wagons for transporting massive 30-40 tonnes flat steel ’bars’ to steel mills on to keep them from sliding, a little side note
I saw that I have to make a holder for a 6mm (1/4”) mill bit (not sure if that is what it is called) as there were only an 8mm in there, I should add that I have never needed used the key mill before so we’ll see if I have to change my plans completly
Also visible on the pic in the red box is three throttlebody butterfly valves, wire controlled and with a return spring, I ordered after @Jan posted them on another thread for Volvo S40/V40, they were 4,5$ each plus shipping so I bought three to save time on my gasifier to use as choke valve and air intake+gas delivery shut-off valves.
It was a gamble if they can be used, we’ll see.
I also took out one bar of the tool steel for measuring, it is 6x70mm (1/4”x2 3/4”) so I have to cut a bit of the width also to fit it on the supports.
I have done some sharpening over the years. Needed or not - I don’t know, but the sharp edges do take some abuse.
I would very much recommend to have the blades individually adjustable though. They have to be perfectly point to point at all times, or chunks will still be attached to eachother, like a string of sausages.
At first I was worried too, about bolts not holding the blades in place, but it seems they hold on to the blades just fine.
I was under the impression the “stars” you welded were only supports to where actual blades will be attached.
I have been sawing and making some wood for a couple of days, and thinking about your chunks. Can you JO, run 30x50mm dry birch in yours without it being destroyed? I’m not good with small trees, so I have to cut ribs.
Jan, 30×50 would be a perfect size. But dry wood - I don’t know. I never chunk dry material. Spruce will get terribly hard when dry. Also, I guess there will be a lot of unwanted splinters and the blades will take a lot of abuse.
Edit: I have chunked birch “young growth” that’s been laying on the ground over winter - semi dried. That didn’t affect the chunker though.
Edit2: I see now you wrote birch - not spruce. In any case - the wetter the better.
Wet wood has the cells walls water held separated. So you are mostly only first putting the wall fibers under tension and then the sharp edge is Nick-Cutting them.
Dry wood the cell walls collapse down onto each other, compressing stacking up, making for immense strength and resistance.
When toothed hooked blade cutting you are first lifting a group of cells walls into tension, the blade sharp edges then Nick-Cutting them. Cell walls cannot collapse down stacking up. Why dry sawing works well.
The axle chunker guys learn well if not shearing green wood to wet their wood down too. Chunking after rained on. They show water-sap oozing out on their shear cutting.
With proper steel for the blades and enough power, one could probably chunk anything - green or dry. I use ordinary stainless and only 2,2kW (3hp). Works for what I use it for.
Inspirational video Limbs only.