Cutting thick boards/posts square.

[fdengel]

I have a 10" Ryobi SCMS, overall quite happy with it -- sure makes a lot of dust though, and I'm not all that fussy about being super-square with the types of cuts I tend to use it for (I use it mostly for close-is-close-enough cuts).  Probably needs to be adjusted (at least) if I need very square cuts from it, but I'll get there if I get there.

8)

 

[CDM]

Another approach to try is marking your piece, cutting at least half way through from one side, then flipping it and cutting the rest of the way from the other side.

 

[fdengel]

Yes, I've seen that done already too.

Or maybe use the groove from the first cut as a guide to run a hand saw through?