Question: I would like to get some help regarding mitered picture frames. I cut my frame pieces on my miter saw and they look great, until I try to put them together. There is always a small gap on one of the corners. When I adjust or tighten that corner it affects one of the others. How can I get a better fit or straighter miters on my frame pieces?
Answer: Those pesky miter joints seem like an easy joint, but can be very frustrating. In fact, some woodworking experts claim they are the hardest joint to make perfect, so give yourself a break if yours have yet to be perfect! Even if your miter saw cuts dead at 45°, if the length of one of your four pieces of the frame varies in length, things don't go together so well. As you fiddle with the getting the miter dead on, you may inadverdently alter the length of one or more of the frame parts, thus making the two 90° corners you're gluing up very difficult to join together as a whole and square frame.
To begin with, I will assume the frame stock you are using is flat, straight and square. (You can have a milled profile, but the stock cannot be twisted.) If not, you can have trouble getting things perfect.
Some miter saws are problematic because the detent 45° setting is off. To make matters worse, some saws do not allow you to adjust this setting. Besides making test cuts and checking for a true 45° angle, you can cut two pieces at 45° and put them together and check for a true 90° angle (with your most accurate square). I sometimes put my test pieces together using a little CA glue since it's quick and will hold the joint together for me to check it for 90° accuracy.
You can cheat your miter saw to tweak small fractions of a degree by using some masking tape stuck to the fence of your saw. You can do this near the blade to shim out the stock or towards the other end of the piece you're cutting to "toe in" the angle. This tape shim method lets you make very minor adjustments not possible by swiveling the bulky turret of the miter saw. Just build up pieces of tape to shim as needed.
Once you get the exact 45° angle on your test cuts you can cut the 45° on one end of the four pieces. Then you can stack two pieces up to make the second 45° cut, so two pieces will be exactly the same length. If you are making a rectangular frame, the longer pieces will be the same length and the two shorter lengths will be the same length. This will assure you of a perfect rectangle when you assemble the pieces.
If you have exact 45° angles but the length is different for a piece or two and you can't cut off any more to even things up for length, you still can fudge things a bit. When you glue up, line up the inside corners of the miters the best you can. This will leave you with an overhang on some of the outside corners of the assembled frame. In order to make the outside of the corner look acceptable, use a sharp block plane to shave with the grain on the side that is too long. This effectively reduces the width of the piece but is not noticeable. You don't just shave on the last inch or so to thin down the overhang, but start back a few inches or more depending on the amount of material to be block planed away. This fairing back keeps things looking straight. People have a tendency to cut off (or sand off) the overhanging end, thus putting a blunt end on the outside corner of the miter. The block plane method is a superior way to deal with this overhang.
When making frames, professional picture framers will cut close to the line with their miter saws then use another tool, the miter trimmer, to trim to the final length. The miter trimmer has a large, razor-sharp blade that slices an onion-thin shaving off the end of the miter and leaves a glass-smooth surface on the end grain. This tool works well when making frames with pre-finished stock (which most frame shops happen to use), as it does not cause chip out like a miter saw would.