The questioner said: Wooden bench with a hole in the middle. The bolts go all the way through the legs and it is very heavy. Found in the basement of an old farm house
And send this images:

Some of the people answers were:
1. The rounded corners on the ‘legs’ make me think this was/is an improvised agricultural tool. Trammell? Furrower? Thought was put into this fabrication, I doubt it was just knocked together.

2. That’s a lot of bolt going on. This suggests that this device had to be reliable under some considerable strain. I notice that the four bolts near the hold are countersunk while the bolts at the outside edges are not. I wonder if whatever went into the hole also needed a flat surface around it.

3. I think an attachment for a diy wooden vise. Looks like it might slip onto a 4×4 with some sort of cotter pin. Check how easily 2x4s etc slip into those up rights could have been a jig for holding lumber to cut with a handsaw. I vaguely remember my grandpa having something like that for cutting lumber with a handsaw.

4. Whatever it was for it was meant to be under a lot of strain, and looks like it was maybe meant to rotate. The horizontal countersunk bolts on the end only go through the top board, indicating they’re meant to prevent it from splitting. The square hole in the middle has a horizontal hole bored through, indicating that a tight fitting square shaft fitted into it and was locked in position as well as the horizontal board being reinforced there too. The center bolts being countersunk but not the outer ones suggests a concern for them catching on something with a low clearance, but not a large diameter. The rounded corners suggest that force was hitting the upright mainly on the ends rather than the faces, which would also fit with how the top board is reinforced. My guess would be something beating, separating, or pulping something, in essence a large slow moving blender/beater. The lack of stain suggests that wine and such was not the use. Possibly some sort of grain separator, perhaps for removing grain from the chaff, or for knocking kennels loose from cobs or the rachis?
5. Looks like a carpentry jig. Heavily braced, with a square slot for placement on a larger work bench.

6. Looks like the bench of a wood shaving horse