So I’ve been trying to make a simple wooden keepsake box for my niece, and I keep messing up the lid because the wood warps a little after I cut it. I’m just using basic pine from the home center. Is that normal? I feel like I must be skipping some basic step in the process.
Totally normal with pine especially from a home center. Wood moves as it dries and equalizes moisture so a lid can warp a bit after cutting. Are you letting the piece acclimate to your shop and keeping it sealed from humidity before you cut again?
From a more analytical angle pine is relatively unstable dimensionally. Warping shows up when moisture changes between stock cut faces and the environment. If the box or lid finish at different temperatures or if one side is exposed to more moisture you may see a bend. The general fix is to control moisture seal surfaces and let the wood reach equilibrium before assembly.
A lid that shows warping could be due to the grain running differently in the lid piece than the body causing uneven shrinkage
Seems like you are chasing a magic trick. Pine is cheap but warping is part of the game could be you are chasing perfect flat instead of accepting a little natural bend
Reframe instead of fighting the warp design the lid to accommodate movement like a lift off with a tiny gap and a simple stop or a wrap around lip that hides the mismatch
Craft wise this could be a good chance to test different finishes or wood choices maybe a thicker lid or a more stable species would behave better
Another angle is moisture equilibrium and seasoning even kiln dried pine can move if you go from bright sunshine to damp garage