If you have a table saw, here's a simple jig you can make for it that will turn out perfect 45-degree miter cuts time after time. Sure, I have all kinds of factory-made miter tools, including a motorized miter box, but I made this jig for two reasons:
1--It will cut stock wider than any of my other systems can handle.
2--It is dead accurate.
Its only drawback is that you have to own a table saw to use it. If you don't have a table saw, perhaps you should think about getting one. Next to an electric drill, I think it's the most important and versatile tool you can own. And small bench-top models sell for under $150.
OK, back to the jig. You can make the whole thing from scraps of plywood you may have lying around the garage. I like to use half-inch plywood for the base, and three-quarter-inch plywood for the triangular section on top.
For the two guide rails that run beneath the base, a dense hardwood like maple is best, but you can get by with pine in a pinch.
The dimensions shown in the sketch are for a rather large version I made to fit my big 10-inch table saw. If you have a small saw, you might want to scale things down a bit. None of the dimensions are really critical.
Start by cutting the two rails that run under the jig. Cut these on your table saw, sizing them to fit perfectly in the saw's miter gauge slots. You don't want them so snug that they won't slide smoothly in the slots, but you don't want them too loose or they will allow the finished jig to wobble and produce sloppy cuts.
Next, cut out the half-inch plywood base. When it's ready, lower the blade on the table saw. Place the two rails in the miter gauge slots, run a thin bead of glue down the top of each and carefully set the base in position over them.
If you set your saw's rip fence 13 inches from the blade (for a base 26 inches wide) you can use it as a guide to position the base. Put a few heavy objects on top of the base to weight it down, and let the glue dry for an hour.
When the glue is dry, check to see that the jig will slide smoothly in the table saw slots. You may have to sand the rails very lightly to loosen things up. I like to coat the rails and the bottom of the jib with paste wax to help assure smooth operation.
Set aside the base and cut out the plywood triangle that will sit on top of it. This is the only critical step in the whole process. You must make sure that the triangle has a perfect 90-degree angle as shown in the sketch. Check this with a square, and if it's not perfect, trim it slightly and check it again.
Happy with the 90-degree angle? Drill a pair of 1 1/2-inch holes through the triangle as shown in the sketch. The position of these isn't critical. Just put them about an inch from the edges of the triangle, and out of the line of cut.
You'll stitch your thumbs in these holes when you use the jig, so you certainly don't want them positioned in your lane of cut.
OK, raise the blade on your saw back up, place the jig in the slots on the saw, turn on the saw and cut a kerf to roughly the center of the jig. Then stop and turn off the saw.
Your next step is to fasten the triangle in position on top of the base. You can do this with glue, or you can use screws. I prefer screws because they let me replace the triangle when it gets worn out with use.
Position the triangle with its point aligned directly with the kerf you have just cut in the base. The saw kerf should bisect the 90-degree angle. Check this with a protractor.
There should be 135 degrees from the saw kerf around to both faces of the triangle. Screw the triangle down with two or three 1 1/4-inch flat head screws and the jig is ready to use.
To use, place a piece of stock against one face of the triangle. Place your thumb in the hole on that side, and use your fingers to hold the stock firmly in place. Turn on the saw and slide the jig into the blade until the cut is complete.
Make a couple of test cuts--both on the same side of the triangle. Place them together and check to see how they fit with a square. If you are off by more than a bit, you may want to realign the triangle, but for a slight inaccuracy, this isn't necessary. Why? Because the jig is designed to be self-compensating.
Whenever you use it, you should cut mating parts on opposite faces of the triangle. Cut the right side of the joint on the right side of the triangle, and the left side of your joint on the left side of the triangle. Then if the triangle is slightly out of line, any inaccuracy will cancel out. One miter may be 46 degrees, but the other will be 44 and the assembled joint will come out to 90. Tips for use:
--Always use a good, sharp blade, preferably a quality carbide-tipped crosscut blade. If you don't want to spend the money for one of these, try a steel hollow-ground planner.
--Always use the same blade and keep the kerfs in the base and triangle clean. If they fit the blade snugly, they'll help prevent chipping, and you can use them as visual guides to help align your cut.
--If your stock tends to slip against the triangle as you make a cut, face the edges of the triangle with strips of 80-grit sandpaper.
At this point, you may be wondering why I call this a manta ray miter jig. Simple. The triangle, with its two eye-like holes, reminds me of a manta ray.