Why don't my corners read square?

Question

If my table saw fence is set at 15 1/2 inches and I cut a 3/4 inches piece of plywood first one way then the other, why won't the corners register as square? The measurements are right down both sides. Any help would be greatly appreciated.

Answer 1

Because your fence is referencing off of whatever nonsquare side it is up against. The correct solution is to clamp or nail a strip of known straight wood onto your stock (square to your intended cut), then rip with this straight piece of wood up against the fence.

Once you do this you can remove the scrap block, flip the board and rip it again. This will yield two parallel sides. To square the ends just draw a square line from both sides (which should intersect if both sides are parallel) and crosscut this line on each end.

Answer 2

Start with the miter gauge. If one side of the plywood is reasonably straight and won't rock against the miter gauge's fence, you can use this procedure.

1. Place the reasonably straight face against the miter gauge, make a cut. Now you know the cut you just made is straight. But don't trust the mostly-straight face that was against the miter gauge; nothing is necessarily square yet.

2. Rotate the work piece 90° so the known-straight face you just cut is against the miter gauge. Make another cut. Now you know the face you just cut is straight, and it's also square to the first face.

3. Now ditch the miter gauge and bring back the rip fence. You can cut to width using the rip fence against either of the two faces that are known straight and square to each other.

Note that I'm assuming your miter gauge is square to the blade and your rip fence is parallel to the blade. If they're not, you'll have to fix that first. (And note that a rip fence that isn't parallel to the blade can be dangerous if it causes the work piece to pinch against the blade as the cut proceeds.)

Some tips:

• Get your plywood stock roughly to the size you want it with a circular saw first. Clamp a guide to make the circular saw cuts straight enough to use against the miter gauge. Make your rough cuts so that you have enough stock to comfortable trim to square with the table saw.
• A cross cut sled helps make the cuts easier and more accurately. It eliminates friction between the top of the table saw and the bottom of the work piece, so the work piece doesn't slide across the miter gauge fence/cross cut sled fence as easily.