Weekly Tips Sept 08

SEPTEMBER 16th, 2008

Making Perfect Square Joints

We get a lot of email from hobby builders who have difficulty making that perfectly square joint in wood. It may be a frame for a mirror, a cabinet a chest or some other classic project.

There are two simple secrets you need to follow to make a perfect joint. The first thing to understand is that unless you own a $3,000 professional table saw with a micro adjust miter system, setting the saw to make perfect 90 degree joints with the existing mechanism will introduce inaccuracies and error in the joints.

In fact, a small variance in setting the miter guide to 45 degrees will compound itself when you cut two pieces to make the joint. If the miter is set below 45 degrees, your joint when set to 90 degrees will have a gap at the inside edge. If your saw is set a fraction higher than 45 degrees, you'll create a gap at the outer edge when you set the joint to a right angle.

Regardless of the saw you have, you can set the miter guide to exactly 45 degrees by ignoring the calibrations and manually setting it with a speed square. Use a metal one, they are dimensionally more stable than cheaper plastic ones.

Loosen the miter guide and calibrate it to the blade using the speed square, and firmly tighten it. Don't be surprised if the graduations are out a fraction of a degree or more!

Now, regardless of your saw's accuracy level, you're now set at a perfect 45 degrees, regardless of what your saw is telling you.

If you're making one right angle joint, your saw is adjusted to make two perfect 45 degree cuts now.

Here's the next secret. If you're making a four piece frame with four joints, cut the 45 degree miter cuts on one piece at both ends and use it as a template. Don't use a tape measure to measure out and cut the subsequent joints. The tape measure and variants in cutting to the line will also introduce errors and bad joints.

Use the template to cut the other three pieces:

1) Cut the template at both ends and mark it with a small "T".

2) Cut the remaining three pieces on one end with a 45 degree cut.

3) With a sharp pencil, use the template to mark the cutting mark for the other 45 degree cut on the remaining three boards.

4) Cut the remaining 45 degree angle on the remaining boards precisely on the line every time.

Now you'll have four virtually identical boards because you used a template and did not introduce errors into your project. You also started with a perfectly adjusted saw regardless of what the calibrations indicate.

If you need to make a cross cut miter with a tilted blade, calibrate the blade using the speed square as shown in the picture and follow the same process.