It's not as easy as it sounds.
Q. I often see small boxes with near invisible joints and would love to know how to make them. I have an excellent mitre saw, a great table saw, and a wonderful planer, among other tools. My parts seem perfect -- squared and sanded and smoothed -- yet I still end up with gaps in my mitres. Being a self taught woodworker, have I missed some basic, simple steps in joining angles, mitering corners, etc?
A.1 "Three things come immediately to mind. First, check your tools. Cut a set of 45 degree angles, put the two boards together seamlessly, and check the outer and inner angles with a good square. Often, tools cut close to 45 degrees, but for invisible joints, they must be dead on. Tweak the tool if need be. Second, cut your mitres after the wood has stabilised to the relative humidity of the assembly room. If a board expands by absorbing moisture, the mitre will be open on the outside. If it contracts, the mitre will open on the inside. Finally, be aware of your fit before and after glue up. Water based glue can expand a mitre enough to throw it off, and too much glue will result in a visible glue line. Use the glue sparingly, and get the piece in clamps as rapidly as possible."
A.2 "Is your material flat? The longer the mitre you're making, the harder it is to make sure your work piece lies flat when you cut without cupping - especially when you're using a wider board. It's also possible when you're using solid lumber that your two pieces are from different logs, or even from different parts of the same board, and are not exactly the same thickness. Then, when you flip your mitres together, you get gaps.
"You say you have good equipment, but have you checked things like how sharp your saw blades are? To get your mitres right, all of your tools have got to be right on."
We'd like to add that if you are cutting mitres on a table saw and they aren't coming out, the problem may be your mitre gauge. Are you using the mitre gauge that came with your table saw? The typical stock mitre gauge that comes with a table saw is inaccurate and awkward because of its small body and single guide bar. If that's the trouble, a precision miter gauge might be a wise investment. At least you could rule out bad angle settings as a possible cause.