Re: sod cutter attempt
You definitely need a more agressive angle at the point at which you initiate penetration. Then after you have penetration you need to retard the angle and have a means to maintain that and prevent deeper penetration. The outward horizontal element of your sides may do that for you naturally. Perhaps even a set of 6" wheels on the outside could serve as the depth limiter and create less friction during the cut.
Another thing to try is to get your assembly started manually and to see what results you get by pulling it. If it cuts well once started, then maybe no change to the vertical element of the horizontal cross member will be nessesary (but I think it will). Perhaps two rollers offset from one another a small distance, one that rolls on the lower earth, the other the rolls the sod up and over it could improve cut efficiency.
I do believe you will ultimately need the sharpened edge of the horizontal crossmember to shear the earth beneath the sod. I see you have that covered.
I would not think you would need to add weight to the assembly, if you have the ability to supply down force from your hitch. If not, then static weight may necessary.
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