How do I fix excessive flex in the Y direction… as in, when my router is moving towards the front or back of the machine, the whole X/Z unit in leaning away from the cut. Is that the eccentric nuts on the v-wheels, and if so, the X carriage ones or the ones that ride on the Y rails?
Yes, tightening up the vwheels via the eccentrics can help that, but only to a degree. If you’re still getting flex after tightening everything up, you may need to look at taking smaller/slower cuts. :-/
Also, shimming up your work stock as high as you can reasonably go will also help that effect, as it essentially shortens the lever and thus lessons the tourqe on the router.
The Shapeoko 2 used a pair of plates on the carriage and V rails on both sides of the gantry extrusion — folks have discussed that for the SO3 — you’d need a second plate and through bolts to connect the twain and some bolt-on V-rails and the patience to get things lined up and adjusted.
One low-hanging fruit option might be to add a 3rd pair of V wheels on the carriage plate if you could find room — might help.
ISTR someone adding a linear rail to the underside of the extrusion and using an L-shaped carriage plate — that might work well, but would be pricey.
Gave my top v-wheels a brushing with an old toothbrush, got lots of black powder, then checked my v-wheels as per the guide http://docs.carbide3d.com/support/tensioning-eccentrics/ and at least one wheel on each rail was spinning freely. Tightened the eccentric nuts and no more flex.