Checking for Square

A large portion of woodworking is spent making sure things are square. That is, checking that one edge or surface is at 90 degrees to another. For example, you might want to verify that the table saw blade is square to the table. Perhaps you want to make sure or that the face of a board is square to it's edge. Is that case you just glued together square? Is your jointer fence square to the table? In this post, I'll discuss the various ways to check that your measuring tools are calibrated and that your assemblies are square.

Check your squares

Most woodworkers have a variety of squares that are used for measuring, marking and tool setup. These tools are used as the basis for nearly everything you do in woodworking. They're used for machine setup as well as checking that joints are cut and cases are assembled square. If you can't trust that your squares are square, it will be hard to build anything with a right angle.

To check that your square is at exactly 90 degrees, place one edge against the straight edge of a scrap board and draw a line along the other leg of your square. Now, flip the square so that the portion against the edge of the board is pointed the other way. That is, if the two arms of the square were initially pointed to the left and away from you, flip the square so that the arms of the square are pointed to the right and away from you. Line up the square with the first line you drew and draw another line. If the square is accurate, the lines will overlap. Otherwise, the square is off by half the angle between the two lines.

Sometimes, any problems with your square can be corrected. Combination and sliding squares can fixed by carefully filing the flat portion the ruler slides along. A framing square is fixed by puching a dimple in the inside or outside corner with a nail set. Otherwise, you'll need to go shopping for a new square.

Check your miter gauge

To check that your table saw miter gauge is at 90 degrees to the blade, there are a couple of different methods you can use. These methods also work to check the setup of your compound miter saw as well, with only slight variations.

Cut-n-Flip
Take a pice of scrap 6 to 8 inches wide, joint one edge and rip the other edge parallel. The wider the piece, the easier it will be to see any error in your setup. Place one edge against your miter gauge (or miter saw fence) and make a crosscut. Place the two pieces against a straight edge and flip one half of the board away from you so that the two freshly sawn ends are still facing each other. If the miter gauge (miter saw) is accurately set to 90 degrees, the two cut ends will meet without any gap. Otherwise, you will need to adjust your miter gauge by half the angle between the two ends.
5 Cuts
If you need a really accurate setting of your miter gauge, the five cuts method multiplies any error in your setup by five. If your miter gauge is off even a little bit, it will be quite obvious by the end of the process.
Take a piece of scrap about 6 to 12 inches square that has at least one straight edge. Starting with the straight edge number each edge counter-clockwise (clockwise if your miter gauge is to the right of the blade or the workpiece is to the left of your miter saw blade). Leave some room between the number and the edge, as you're going to be cutting a bit off of each edge.
Place the edge numbered "1" against the miter gauge and make a cut along the edge numbered "2". Now place the edge numbered "2" against your miter gauge and make a cut along the edge numbered "3". Repeat the process two more times. The fourth cut should be along edge number "1" with edge number "4" against the miter gauge. Finally, make one more cut along edge number "2" (the same as the very first cut you made).
Using a reliable square, check the corner between edge "1" and "2". If your miter gauge is not square, it should be quite obvious. If you don't trust your square, you can measure the diagonals of your board to see if they are equal. Make any adjustments to your miter gauge (or miter saw setup) and repeat the process until your test board ends up with a square corner after the last cut.

Check your cabinet assembly

Having accurate tools is all very well and good, but what really matters is how square your cabinet assemblies end up. You could have the most accurate squares and jigs made and still end up with a crooked cabinet if you don't check it after you glue it together.

The easiest way to check that your cabinet is square is to measure the diagonals. On a square cabinet, the measurement from one corner to the diagonally opposite corner should be the same in either direction. If not, you have a parallelogram. Adjust your clamping pressure or use corner blocks to pull the assembly square. Sometimes adding a clamp diagonally along the long direction can help as well.

These are just some of the ways to check that things are square. What's your favorite technique to check for square corners? What do you do to correct any problems?

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