Yes, the knives pose a problem to me, the hardened steel is very difficult to machine to hide the heads of screws
I do not know yet how to solve this problem
I would make some pictures as soon as it is finished
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I can’t see why that would be nessessary as long as the cutting part of the knife is flat.
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Then make them out of softer steal but designed to be sharpened a few times and replaced. If you design for an easy replacement it shouldn’t be too bad.
You can also use heat to harden the steal from google.
Heat the entire piece of steel slowly at first. Then, concentrate the heat on the area that is to be hardened, such as a chisel point or screwdriver blade tip, until that area glows red hot. Quench the steel in a fluid. Dipping the hot steel into a liquid or gas rapidly cools it, hardening the metal.
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I did not know this technique!
Want to tell you that I can build my soft steel knives, heat them to red and cool them in water. They will be harder
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I like to use used oil to cool them I figure if the metal soaks up some oil it might take longer to rust as well. I did alot of this once when I has hydraulic oil that had water mixed in. It was a great way to get the water out of the oil so it would burn in a waste oil furnace. I can’t remember what I was making at the time but I needed it hard probably a ware bar for something.
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Ordinary mild steel doesn’t have enough carbon to respond much to hardening. Case hardening could be undertaken, but it’s not really practical. After there’s the issue of tempering, if not properly tempered, hardened steel won’t have the toughness to withstand the use, instead it will tend to shatter like file steel or drill bits tend to.
A potentially much more durable steel could be gotten from automobile leaf springs. It also has good thickness, promising robust service. If a person has access to some kind of forge, a better steel could be welded into the edge of mild steel cutting blades, allowing a heavy flexible back. For that approach I would suggest welding in grade 8 straightened chain link, then forging it to a rough point before grinding, hardening and tempering. Chain link steel has outstanding toughness.
Leaf spring is likely the easiest approach, cut to size, anneal, then drill holes, maybe forge the edge, then harden, and variably temper to make a tough cutting edge, and a flexible back.
I believe the chunking application calls for toughness, more than a very sharp edge. The steel shouldn’t have flex, and the edge shouldn’t be able to dent or deform. With the right grade of steel, wear resistance and toughness can be had.
It might be practical to look into commercial chipper knives, or tool steel blanks.
Edit: different steels call for different quenching methods. Some need oil quenching, others water, some brine, some tool steels just need to be raised to a certain temperature for a specific period of time. The internet, experimentation and testing will reveal the best approach for a particular steel and application. Some steels even respond to cryogenic treatment.
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There are a few ways to go when wanting blades for the job your wanting to do .
Look for granulator blades for small grinders for plastics, they have slots and or holes already and are hardened properly for the job , small machines like Cumberland , Blackfriars ,Rapid , ect will have all the sizes and you can build the Chunker around the blade sizes then .
Or if you get some non hardened steel made into blades of the size you want and use hard face welding rods for the cutting edge , that was the cheapest way we used to make our large blades .
Dave
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Hi
The blades in my rebak was water jetted out of hardox 12mm plate. Hardox does not need to be heat treated. In my machine they have stood the test of dry wood.
Werner
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You could just go to your local steel store and get some 1045 or 41/4340 in hot rolled annealed condition (ie. soft), make your blades, then flame harden just the cutting edge. Just heat the cutting edge red hot and quench in water. None of the above steels will thru harden enough to be brittle, especially water quenched. Spring steel is 70-95 carbon, I’d be a little more careful with that stuff as it could go max hardness all the way through somewhat like tool steel does.
Really, any alloy machinery, or medium carbon steel in the 40-45 range will quench out at 50RC or more, but still have a softer centre. An option to avoid heat treating is get some 4140/4340 in the heat treated condition. It will be 28-32 RC, so you can still machine it, but also have some hardness.
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Hardox is a QT abrasion resistant steel sold from 400-600 Brinell. 400 brinell is 43 RC so getting tough to machine. 600 brinell is near 60 RC and that’s tool steel territory. Also, Hardox is made in Sweden 
.
Here in North America there are a slew of similar steels all under ASTM A514, 100ksi yield, with various harnesses and properties sold under trade names. Stuff like AR plates, QT, HS, T1, CHT steels up here in Canada, etc… These are probably the least expensive quenched and tempered steels on the market, generally it is almost always sold as plate. This stuff is the same basic stuff as hardox.
Used a ton in mining and concrete heavy industry. Steel store on the corner that sells offcuts probably won’t have this stuff though…
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since you brought up steel… I have a project, a 3d printer, that normally uses hardened and chrome plated steel rod for the guides. How close is 316 stainless rod(round bar)? I assume it won’t flex as much as cold rolled mild steel or is there a really cheap easy way to harden the cold rolled mild steel? (I can buy a kit with the right stuff for 60 dollars but I can get the stainless rod and 3 bearings for like 12 dollars.)
Hard Chromed rod is usually 1045 and is nothing like 316. Surface hardness on chromed rod is north of 65 RC, surface finish is excellent at 9 rms, it is precision ground to stay .0005-.001" undersize so your bearings will always fit. If you are running linear bearings on them, you’ll need the precision ground and chrome plated stuff. 316 rod is hot rolled or cold rolled and has a wide tolerance on the diameter, it’s not straight, not hard, and the finish is nowhere near what you need if you’re running bearings on it.
What diameter and length do you need?
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Thanks perfect! I just started poking around at this. The bearings are radial not axial, and it was 6 rods the longest being 496mm of 8mm rod. I am not exactly sure how this all fits together at this point. It just seemed like 60 dollars for 3 bearings and a little over 8 ft of rod was a bit to expensive, but maybe it isn’t and maybe it is worth it.
It isn’t the first project on my todo list. I have like 2 things I wouldn’t mind printing, but it is mainly for kid toys like legos and stuff from recycled plastic (the filament is expensive) I might actually be able to find rods from an old printer/scanner as well as the motors. New stock is usually just a lot easier to work with. 
If you are just putting regular bearings on there, there is a product in stainless steel: PGS (precision ground shafting) or TG+P (turned ground and polished), some guys call it “pump shaft”. It is precision ground to a +0 tolerance, the minus side varies based on diameter, but basically it is -.001"-.003" and is made specifically to allow install of bearings. You can get the same product in C1045, and C1045 flash or hard chromed (cylinder rod).
You could look thru the online McMaster Carr catalogue, or try your local bearing store and get a price on “Thompson Rod” (Class L) or equal. Thompson rod is linear bearing rail, but I’ve always thought it was pretty well priced for the whole enchilada - Hard, straight, round, and tight tolerance.
Good luck!
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Today is Sunday. Hatchet and I spent an hour in the church of our own.
Later on wife and grandson sneaked up at me by the chunker.
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I love the Creator’s sanctuary you have there. It’s the best place to be on a Beautiful Sunday. You really can’t call it work can you, when you are there in that kind of place.
Bob
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I agree, you can’t call it work. Sometimes you feel the need to be alone for a while. Something has to be seriously wrong but this is the most relaxing activity I can think of.
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And all the time I thought something was wrong with me because I enjoyed splitting and chunking wood 
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I am NOT a member of the darker side but I did make, clean,crush, and grade some charcoal a while back just for giggles. Now with my burned down shop, I had about 12000 board feet of finished lumber stored in there. It burned but not totally. We pulled the partial burned boards out and shoveled the char that was on the ground into 55 gal drums (suppose to have been for a future WK gasifier). The remaining pieces that were partially burned I stacked into TeePee’s about 8 ft high and 12 ft around, and burned them. While burning them I couldn’t help but “work” the pile until I had nothing but hot glowing embers with no flame. It would have been excellent soft wood charcoal, but I didn’t have anything left to store it in.
TomC
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So since you tar treats the belt the motor actualy stalls on a big limb?
Amazing how much biting force you have with that “weak” motor
It will be no time when Walter will help his granddad chunking!
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