They're also devilishly difficult to make. The 45 degree angles need to be cut dead-on to avoid gaps, and the gluing-up process is a horrorshow- the moment you clamp a miter-joint-to-be, the pieces slide around.
Various companies sell a host of jigs and tools for easing the problem. But here's a method that has worked quite well for me, using nothing beyond normal shop tools.
What you'll need:
- 4 mitered pieces to glue up. You may be able to adapt this process to work on individual joints, but here I'll only talk about gluing up a rectangle all at once.
- A table saw, with a sliding rip fence.
- a decently straight board, long enough to clamp at both ends of the table.
This Instructable was made at Xerocraft, the Tucson Hackerspace, and is intended to count towards our sponsorship from Instructables.
Step 1: Set Up Your Fences
Slide the fence to one edge of the tablesaw table, and lock it in place. Place the straight board square against the outside edge of the fence, and clamp it down.
Now, you can move the fence anywhere else, and it will be parallel to the wood straightedge.
Slide the fence away, and lock it down the exact width (or slightly closer) than the width of one side of your rectangle. With a piece of soft pine as a fence like what I've got here, I can set the gap slightly smaller, and the miter pieces will just stick when they're perpendicular.
Step 2: Glue Them Up!
It's always a good idea to do a test fit before applying glue- it's hard to go back later!
If everything looks right, apply glue to the miter areas, and place a clamp at each joint, as shown. Get close to the ends- clamping in the middle of the horizontal members could bend them, messing up the joint. Use a decent square to confirm that the angles are square- you can easily make adjustments.
A few notes: beware that some bar clamps tend to twist the pieces up. Fancier furniture clamps like the two I'm using here have less of an issue with this. You can also minimize this by accurately aligning the center of the clamp pads to the center of the board (if necessary, you can use a spacer to lift the glue-up off the table surface). Lastly, light presure will reduce this flexing (clamping tightly shouldnt be necessary, anyways).
The glue should neither rust nor stick (badly) to the cast iron of the table surface, if you've treated it as you should- with simple furniture or bees wax. This simple step eliminates the danger of damp air, and generally makes the saw easier to use, since wood slips over it very easily.