Dewalt 12" sliding miter saw bevel issues

[branimal]

I'm cutting a 1"x12" board with a 45 * bevel to make a box. After my first cut I figured out the saw isn't cutting square. It seems the miter is off on the bevel cut. It's taking off more material the further the blade is from the fence and less material closer to the fence. I checked the miter squareness with the saw in 0* bevel and everything seems ok. I squared up both 45* bevel angles on the saw with a combination square. And I checked the miter squareness with a framing square. The table and fence seem to be square as well.

Looks like the cut is also concave. The blue square is touching on the top and bottom of the cut, but not in the center. See the last picture. I'm totally confused.

What am I doing wrong? I ordered a digital angle gauge. Should be here tomorrow.

Saw: dewalt 779; 12" sliding miter saw
Blade: diablo fine finish blade.

 

[tarbellb]

Is your board (wood?) warped at all?

I imagine the board may be cupped, try cutting something you know is flat and straight.

These compound cuts reveal any deficiencies in your equipment and materials

 

[branimal]

Is your board (wood?) warped at all?

I imagine the board may be cupped, try cutting something you know is flat and straight.

These compound cuts reveal any deficiencies in your equipment and materials

Yeah it’s warped. Clamped it down to the table and square to the fence. I thought that would be adequate. Will try a square 12” piece of plywood tmr.

hopefully it’s just the board and not my saw or blade.

 

[KnurledNut]

I imagine the board may be cupped, try cutting something you know is flat and straight.

This.
Pretty common issue.