Cutting tight, clean miters requires that two conditions be met: The angle cut on each end of the joint is the same and adds up to the desired corner angle (a 90 degree corner requires two 45 degree miter cuts) and, for square or rectangular assemblies, that the length of opposing pairs of parts of the assembly be exactly the same. Each member in a rectilinear construction equals the length of the outside dimensions of the assembly. For example, a 10 in. by 8 in. mitered frame requires one pair of members 10 inches long and the other 8 inches long.
There are lots of different ways to cut parts for a basic miter-joined frame: With a handsaw and miter box (Photo 1), with a table saw and miter gauge (Photo 2) or special miter jig, or using a dedicated crosscut saw, such as a radial-arm saw, compound-miter saw or sliding compound miter saw (Photo 3). When cutting miters for boxes and carcase parts, a table saw fitted with a fine-tooth crosscut blade (tilted for bevel cutting) is the tool of choice. You can miter solid-wood parts either with or across the grain, depending on the construction. You can also miter plywood carcase parts, but there's a caveat: the tip of a mitered plywood corner is very fragile and prone to damage.
Regardless of the material or method you use to cut mitered components, accuracy is a must: very small discrepancies in the angle of mitered parts results in "open miters:" joints with gaps either at the tip or the base (Photo 4). To assure tight fitting corners, set your saw (or miter box) to the desired angle and cut a pair of sample pieces. Put the pieces together and check it with the largest, most accurate square or angle gauge you have. If your cuts are off by only a degree or two, the resulting corner will show a visible gap.
Mitered parts can be butted together and glued, as both solid and plywood members will have partial side grain contact. Gluing produces enough strength in plain butted miters that don't have to endure heavy use, such as picture frames, small boxes and trays, etc. But miter joined assemblies that lack reinforcements (dowels, splines, etc.) are a bit tricky to glue up, as parts tend to slip out of position during glue up. There are several ways of keeping parts in place as clamps are fitted and tightened. One way is to use a special frame clamp (Photo 5) or web clamp that applies pressure on all four corners of the assembly as the clamp is tightened. If you use regular bar or pipe clamps, try driving a small nail into the miter at each corner, to keep parts from sliding around (Photo 6).
A useful method for gluing up small boxes, trays and drawers is to use masking tape to hold parts together. Start by laying all four parts inside-face down on a flat bench with mating corners paired together. Align the parts' edges flat against a yardstick or straight board. Apply a strip wide masking tape across each joint seam (Photo 7). Next, carefully flip the taped-up assembly inside-face up and spread a thin layer of glue to each miter surface, then set the assembly on edge and "roll it up" so that all the miter joints close (Photo 8). Add tape to draw the open ends of the assembly together, and apply clamps as necessary to draw each corner tight.
Mitered polygons
Although miters are most commonly used to create square-cornered, rectilinear forms, miters can be cut at other angles to create other shapes: Hexagons, Octagons and other polygonal forms, both regular (all miters cut to the same angle, like to make a stop-sign-shaped frame) and non-regular (not all miters are the same, such an isosceles-triangular box to hold a flag). For regular shapes, you can calculate the miter angles by first dividing 360 by the number of sides. Then, subtract that number from 180, and divide the result by 2. For example, for a pentagonal (5-sided) frame: 360 divided by 5 = 72. Then, 180 - 72 = 108. Finally, 108 divided by 2 = 54. Hence, you'd cut each miter for the pentagon at a 54 deg. angle. (Photo 9) As with all regular-shaped structures, you must make all the members of a polygonal structure the same length in order to end up with tight-fitting joints with no gaps.
Compound miters
Instead of making a flat frame or a straight-sided box, you can use miter joints to build a slant-sided form, such as a planter box or tapered sculpture base. Parts for either rectilinear or polygonal shaped projects can be cut on a table saw or compound miter saws, as long as you set both the right miter angle and bevel angle correctly. Calculating the required saw settings for any particular combination of side slope and number of sides is tedious, but fortunately, the web comes to the rescue. This site has an excellent, easy-to-use compound miter calculator:
Just enter number of sides of the desired form and the slope angle of sides (how much they slant) and the calculator automatically generates the miter and bevel settings you need to use. For accurate, tight-fitting joints, use an accurate protractor or bevel gauge to set your saw (Photo 10) and make trail cuts to test your settings before cutting up good stock.
Reinforcing Miter Joints
Just as with butt joints, you'll achieve a stronger miter joint by reinforcing the connection between parts. Nails are easy to drive, but don't add much strength, and you always run the risk of having an errant nail poke through the face of the joint or split the stock. A few of the more effective ways to add reinforcement are glue blocks, plate-joinery biscuits, splines and dovetail keys.
Glue blocks
Glue blocks are a very easy way to add strength to a mitered assembly. They're especially appropriate for carcases, such as kitchen, utility room or entertainment center cabinets as blocks can be located on inside corners where they won't be seen. If these cabinets are built from plywood, you can use the triangular-cross-section cutoff scraps left after cutting the mitered panels as glue blocks. Once the cabinet has been assembled and the glue is dry, simply apply glue to each glue block and press it into place (Photo 11). By sliding it back and forth a little, the block will adhere with impressive grip, so no clamping is required.
Plate joinery biscuits
Plate-joinery biscuits add strength to basic miter joints and are completely hidden. Simply plunge-cut slots in both mating parts using a plate joiner machine (aka biscuit joiner). Plate joinery works with both frame or carcase miters and you can even add biscuits to shaped moldings and trim-any stock that has a flat side to reference the base of the biscuit joiner.
First, mark the position of the slots by dry assembling each miter and marking across the joint, roughly in the center of the mitered edge. (Photo 12) On really wide frames, you may want to put two smaller size (#10 or #0 biscuits instead of just one #20 biscuit. Just make sure that the slots won't show at either end of the frame members. With the stock clamped to the benchtop, line up the centerline of the biscuit joiner with your mark and plunge cut the slot in the face of the miter. (Photo 13) then repeat the process on each joint member. The glued biscuits not only provides a very strong joint, but they help keep each frame corner flat during clamping. Biscuits are also great for plywood and solid wood carcase construction. Most biscuit joiner machines have an angle-adjustable front fence that allows plunge cut slots on the mitered edges of sheet stock.
Splines
Another relatively easy way to beef up a miter connection is to add a spline across the joint. This involves cutting two identical grooves in the matching faces of the miter joint, then gluing a spline, cut from either plywood or solid wood, into place. The spline offers lots of side-grain surface area, so it adds lots of strength to the joint. However, unlike biscuits which are invisible in the final joint, splines show at both the outside and inside corners of the assembled miter.
An easy way to cut spline grooves in a mitered frame is with a tablesaw fitted with a dado blade. A tenoning jig, either commercial (Photo 14) or shop made firmly supports the stock and guides it during the cut. Set up the dado blade to cut a groove that's about 1/3 of the thickness of the frame member. To make the strongest splines, cut them from hardwood, with the grain running across the length of the spline. At glue up, (Photo 15) the splines serve to keep the frame members flat and reduce the tendency of the miters to slip as clamps are applied. After the glue dries, trim and sand the protruding ends of each spline.
Dovetail Keys
A cool way to add both strength and decoration to any miter joined assembly is with dovetail keys (Photo 16). Added across the corners of a thick frame, box or chest, dovetail keys are relatively easy to cut and fit, and are very attractive. You cut the slots for the keys on a glued-up assembly using a dovetail bit fitted in a router table. A shop-made jig (Photo 17) supports the workpiece with it's sides angled at 45 degrees relative to the router table. One side of the jig slides along the router table fence, guiding the workpiece straight over the raised dovetail bit as the slot is cut. By repositioning the work side to side in the jig, key slots may be cut at various positions along the corner, spaced as desired. By spacing multiple slots close together on a box's edges, the keys create a look similar to dovetail joinery.
Once slots are cut in all four corners of your project, you're ready to cut out the key stock on the table saw. Tilt the saw blade to match the angle of the dovetail bit, and cut the key stock just wide enough to fit the routed slots snuggly. Spread a bit of glue in each slot, slide the key stock into place, and then trim it off. Repeat on each slot, then plane and/or sand the corners flat and smooth after the glue dries.
Lock miters (cut w. router/shaper)
All forms of reinforcement add strength to miter joints by providing more surface area and side-to-side grain contact when the joints are glued. A lock miter achieves the same goal, but does so by altering the contact surfaces of the miter itself. Cut with a special bit in a router table or cutter in a shaper, a lock miter creates a sort of zigzagging joint surface that interlocks the ends of each joint. (Photo 18) The bit can shape the edges of solid wood or plywood carcase parts, to form 90-degree mitered corners for a box, chest or other rectangular form. Locking miter corner are good looking as well as impressively strong.
Interestingly, a single bit/cutter is used to cut both halves of the lock miter joint: The first half is cut with the stock laying flat on the router/shaper table. (Photo 19) The mating part is cut in the vertical position, guided by the router/shaper fence. (Photo 20) The bit cuts the joint directly on square-edged workpieces-no other stock preparation is necessary. Various lock miter bits/cutters are designed to work with a specific range of stock thicknesses, so make sure your bit and stock match.
Cutting lock miters requires careful setup work and a certain amount of trial and error, and requires that you to take numerous test cuts on scrap stock that's the same thickness as your workpieces. Some lock miter bit manufacturers offer set up blocks made to work with their bits, which makes the setup process much easier.
The goal is to set the bit's height and fence position so that the bit is centered relative to the thickness of your stock (Photo 21) . It's best to set bit height first, and then adjust fence position until the two halves of your test joint fit together perfectly. For a detailed explanation of the setup process, see this website:
www.woodshopdemos.com/cmt-lm5.htm
Cut the parts for your project all to the full length of the side, just like for a regular miter construction, and leave the ends square. When cutting plywood, thick or dense stock or woods prone to splintering, it's a good idea to take multiple passes for each lock miter cut.
Half-lap miters
An interesting way to make a stronger miter-joined frame is to cut a half-lap miter. This joint is really a hybrid of a lap joint and a miter. From one side, the joint looks like a regular miter; on the other, like a lap joint. (Photo 22). Although it requires more work to cut, a half-lap miter provides a good face-grain to face-grain glued surface, so that it's reasonably strong without needing any further reinforcement (Photo 23).
To cut a lap miter, start by cutting all frame parts to final length. For each joint corner, cut the end of one part at 45 degrees and leave the other one square. Now set up a tenoning jig to cut off half of the overlapping part of each joint. (Photo 24) First, raise the saw blade to the same height as the width of the frame members and set the jig so that the saw blade will leave one-half the thickness of the stock between the blade and the face of the jig. Set the jig's fence at 90 deg. and take a cut on the end of each 45-degree-cut member, orienting the miter face forward for the cut. Now reset the tenoning jig's fence to 45 degrees and set it's position so that the cut will leave one-half the thickness of the stock on the other side of the cut. Clamp on of the square-end frame members into the jig and reset the saw blade height so that the cut just reaches the forward top-facing corner of the member. Cut all the square-end members this way. Next, remove the tenon jig and set the table saw's blade to a height of one-half the stock thickness and use the miter gauge to trim off the triangular scrap piece from each frame end. As always, it's best to run through the entire joint cutting procedure with scrap stock before moving onto your good stock.
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