A Speed-brand layout square is a handy guide for making straight crosscuts with a circular saw. Just hook the square's lip on the edge of the lumber nearest to you and run the saw's shoe along the square's edge as you make the cut. This also works on boards that are wider than the square, provided that the saw's shoe remains in contact with the square's edge through the entire cut.
Marking Ripping Cuts
When I need to rip lumber down to a narrower width, I can use a Speed square to quickly mark the line of my cut. Say I want to take 2 inches off the width of a board. I hook the square's lip over the edge of the board and place my pencil on the square's 2-inch mark. Then I just slide the square down the board's length, holding my pencil steady and keeping the lip of the square flush against the edge as I go (Slide 1, at left).
Checking for Square Ends
It takes two seconds to check whether the end of a board is cut square—much quicker than cutting a board to length only to find that it's flawed. I even do this check on factory-cut lumber ends, just to be sure. Simply place the square's lip against one edge and line up its 90-degree corner flush with the corner of the board. If the square's edge isn't perfectly flush with the board's end, trim it before you cut it to length (Slide 2)
A Speed square makes a handy protractor for marking any angle from 0 to 90 degrees. If you want to mark, say, a 30-degree line, push the square's lip against the edge of the workpiece, then pivot the tool on its 90-degree corner until the 30-degree mark on its hypotenuse aligns with that same workpiece edge. Make your mark along the pivot side of the square, as shown in Slide 3.
Remember that the square's degree markings are relative to a line perpendicular to the workpiece's edge. The complementary angle—60 degrees in this case—is relative to the edge of the workpiece (Slide 4).
To keep the square from moving as you draw the line, place your thumb against the pivot point, with your first two fingers on the square and your third and fourth fingers on the workpiece.
Is the Blade Square?
Here's how to check that your circular saw is making accurate 90-degree cuts. Set the blade for a full-depth cut at 0 degrees and flip the unplugged saw upside down. Pull back the blade guard and position the Speed square with one side against the saw's shoe and the other against its blade. Make sure the square is not touching any of the blade's carbide teeth.
If you see a gap between the blade and the square, adjust the saw's shoe until the square sits flush against both (Slide 5).