Lines and Miters and Errors Oh My

In our June issue, our I Can Do That feature is a mitered CD/DVD rack. Our goal for these columns is to show that attractive, well made projects can be made with a minimal amount of tools and time. Most people want something to show for their effort while they are learning and you won’t need to apologize for any of these projects. If you want to go further in your developing skills you’ll have a good start as well as evidence that you can make some nice stuff. The idea for this column is to minimize the frustration level and hit the ground running. Miters can be especially frustrating, so we focused the technique in the article on gluing them together without having them slide out of place. That is miter frustration number 1.

One of my earliest projects was a clock face in a mitered frame. I was about 16 at the time and it was my dad’s idea to make this clock as a present for his aunt. Dad was an engineer, not a woodworker and the two of us really struggled to get the corners to close and look good. One of our problems was we didn’t know when to be really, really fussy and when we could say “close enough”. With the miters on the book rack above, you can ignore slight errors in the angles or in the length and concentrate on assembling the joints. If you make a four-sided frame however, you need to be a perfectionist.

I played around with SketchUp to demonstrate what can happen if you’re close, but not quite there. These four pieces are 3/4″ x 3/4″, and the angles on the ends are only off by one-half of a degree, a tolerance many people would consider “good enough”. As you can see, each of these small errors joins the others, and the final corner has a gap that is huge. You can try to close it up with various clamps and fasteners, but the chances of success are slim. So what if you get closer? What if you can come within 1/10 of a degree?

Here the gap is much smaller, but still significant. The other bad thing that can happen here is if the pieces aren’t perfectly straight, or all the same length. A bow in the length will change the effective angle and the joints won’t close. A variation in the length will cause similar problems. A lot of joints that seem more complicated than miters are actually less risky to make because there are ways to make small adjustments during fitting and assembly. Miters appear simple and easy. And they are, with one big “IF”. If the pieces are straight, the same length and the correct angle they go right together.

So make sure your saw or shooting board or whatever you use is dead on accurate. Get a reliable machinist’s square (I recommend a Starrett) to check your work. Stick the pointy end of the miter between the blade and stock of the square and hold it up to a source of light. If you can see any gaps your angle is off. Resist the temptation to say “close enough” and take the time to make it right. When you think you’re right, put two pieces together, and use the other end of the square to make sure the joint is really and truly at 90 degrees. Then you have defeated miter frustration number 2.

–Bob Lang