How can i fix miter joints that gap on a bookshelf?

[Grace.S]

I’ve been trying to get into woodworking and decided to make a simple bookshelf, but my miter joints keep ending up with tiny gaps no matter how carefully I measure and cut. I’m using a basic miter saw and I’ve checked that the angle is set to 45 degrees, but something’s off. Is there a trick to getting them perfectly flush, or is this just something that always needs filling and sanding?

[John56]

I had that same problem last month. I triple checked the 45 and even the blade height, but the gaps were still there. Turns out the ends of my boards weren't actually square. I took a small hand plane and trued the ends a touch, then did a fresh dry fit before gluing. After that the joint closed up pretty tight.

[Isabella_H]

I went with a dry fit, two clamps, and a little tape to hold the pieces together while I glued. I eased the corners with a rubber mallet and kept the joint skin-tight as the glue set. It helped to wipe away squeeze-out quickly and not rush the clamps.

[IsabellaYT]

Could the real problem be that the whole shelf isn't square, not just the corners? I once built a frame where the sides were out of plumb and the corners looked fine until I set the top on; then gaps appeared because the frame was skewed.

[DanielZT]

Honestly I still see tiny gaps sometimes, and I end up filling and sanding. It feels like a tease and I move on to another part of the project.