Ok - here's my miter secret.

Ok everybody - I am going to reveal a secret technique for perfect miters, which I have devised.  I have searched around a while on line, and couldn't find any examples of this technique in use.  I also asked several professional cabinet makers about how they accomplish perfect miters, and most just said "I don't".  The others offered up ideas that looked good on paper, but had failed me in practice.

So, here it is, my big secret.  I am re-publishing a post I made on the Boston Reefers DIY Forum.

Recently, a customer asked me to make mitered doors for his custom aquarium stand/canopy. It's been a long time since I cut a 360-degree "circle" of miter joints, such as a miter-cornered door. The problem is, the 8 miter cuts' angles have to add up to 360 degrees, in 45 degree increments.

A lot of people - DIYers and professional cabinet makers alike - miss a simple trick for making mitered doors, frames, or any good miter joint. As a result they slave for hours trying to tune in a perfect 45 degree angle into their chop saw, miter gauge, or whatever. I'll admit, when I re-visited making a four-sided mitered frame last Saturday, I tried a few methods of setting up a perfect 45 degree angle cut.

Then, this morning, it hit me: there's no need to cut each board's end to a perfect 45 degrees. All that's needed is that any two sides of the miter joint add up to 90 degrees. It was an "AHA" moment. But a few hours later, while discovering the many complicated and "good on paper" schemes for perfect miters proposed by professional cabinet makers, I came across an ingenious product that uses my idea (sort of): the Rockler 45 Degree Miter Sled.

If you don't want to buy something, you can use my solution, which lets me use my existing crosscut sled and all the clamping I need. I suggest my solution instead of trying to clone Rockler's design because my solution eliminates the need to cut any perfect angles. To make Rockler's miter sled, you need to cut a PERFECT 90 degree angle in something, like MDF or plastic.

Since cutting a perfect 90 is just as hard as cutting a perfect 45 (I'm talking PERFECT now...perfect enough for perfect mitered doors, with perfect joints), my solution eliminates the need to cut any perfect angles at all. All you need to be able to do is cut two perfectly straight, parallel edges. That's it. I have never seen this solution anywhere; it is by far the best solution for a DIY'er who doesn't want to spend $20 on a Rockler Miter Sled. ;-)

Here's what to do:
1. Get out your basic 90 degree crosscut sled. Don't have one? It's worth it if you want to make cabinets.
2. Get a piece of MDF or plastic (the blank for the two miter fences), roughly 24 inches long by 12 inches wide. Adjust the size to fit your sled. These dimensions fit my Incra 5000 crosscut sled, and allow the work and the jig to be clamped down with the sled's clamps.
3. Just get this part right, and the rest is cake: Make at least one long edge of the piece straight: joint the edge with a jointer plane or a power jointer. Then, running that edge on your table saw fence, rip the opposite edge so it's parallel to the jointed edge. Now you have two straight, parallel edges.
4. The awesomeness begins: set your miter saw or radial arm saw or circular saw or table saw miter gauge or ANY straight-cutting saw,  to make a 45 degree cut across the board. you don't have to calibrate anything. Just make dead sure that you're referencing the cut off of the STRAIGHTEST EDGE YOU CAN MAKE.
5. The Coup de Grace - you are left with two 4-sided polygons, each with one "roughly" 45 degree angle, and one "roughly" 135 degree angle.The 45 degree angle will be slightly over or under true 45. And the other piece will be off by the same amount in the opposite direction.
6. Use these two "fences" to cut each side of all four of your miter joints. Each individual joint will add up to 90 degrees...no more, no less.

There is no other way to make perfect miters in a wood shop, without a computer, that I know of. Everything else is just tweaking, filling, patching, and sweating.

Of course, there's the $20 Rockler jig...but this is much cooler.

Even better, you can now use these two angled pieces to make better fences, which are fixed at the same angles as your pieces. This allows you to extend the length of each fence far beyond the length of the original MDF pieces you cut, which means bigger doors, and the use of stop blocks to ensure every door part is exactly the same length.

YES!