How to Check the Accuracy of Square Tool

As there are different types of square tools you can choose, picking the right one is important to match the type of your work and accomplish your project swiftly. Unfortunately, having the right tool is not enough, you have to make sure that this square tool is accurate as well.

So, what to know that your square tool is accurate? What parameters to check? What tools or instruments or standards to implement to check the accuracy of the square?

Here we will discuss what parameters of the square to check and what tool/methods to check the accuracy.

Parameters to Check

Basically, the squares we are going to discuss here such as double square, T-square, combination square, try square, framing square, speed square, etc possess these parameters:

  1. graduation accuracy,
  2. blade straightness,
  3. and perpendicularity.

They can lose accuracy and disoriented if treated roughly.

If you drop a square on a corner, it may lose its perpendicularity or squareness. Poor manufacturing can impact the 90-degree corner accuracy and affect negatively the accuracy of graduation. Wear and tear can damage the blade and thus, the straightness can decrease.

Make sure you repeat the process, maybe at least three times so that the square is accurate and precise.

1. Graduation Accuracy

Some squares contain a ruler feature. And a ruler contains graduation. The graduation on your squares may be not accurately put on the surface. Maybe, it’s off 1 mm. In this case, you can use a precision ruler to check the square graduation accuracy. Even, the best practice is to use the calibrated ruler to inspect your square’s ruler.

The wear can also impair the graduation after some time. Just take a closer look and try to spot any dents, fading, and rust spots. Especially the painting or printing technic, it can easily fade off due to scratch and heavy use. That is why laser engraved, embossed, or etched gradings are superior and preferable.

In addition to scratch, rust is also another enemy of graduation marks on the square. Fortunately, squares made of stainless steel or anodized aluminum come to solve this issue.

2. Straightness of the Blade and Square Sides

The blade edge must be straight. Due to the straightness quality of the blade, it can also be considered a straightedge.

Its edge helps us make straight lines, check the straightness of an already drawn line, check the straightness of a surface or straight object, measure short distance, a guide for a saw, etc.

If the edge is not perfectly straight, all the markings will be inaccurate. In addition to its role, if the blade is not straight, the perpendicularity of the square is questionable.

The cause root may come from the manufacturing issue and the use as well. The blade can bend if not used correctly, dropped, used to support a weight, or hammer a nail.

The tools that can be used to assure a straight blade are dial indicator and straightedge.

3. The 90-Degree Angle

Aside from having the blade with good straightness, your square must have its corner shaped 90 degrees accurately.

If the corner of your square is not properly 90-degree angled, then it cannot establish perpendicularity, squareness, and even a 90-degree angle.

To check the corner, you can use a vernier protractor and a machinist square. You can use angle blocks as well. Mitutoyo 187-906 vernier protractor (affiliate link) is available to purchase. It serves you with a resolution of 5 minutes. If you don’t need that resolution, here we have the list of mechanical protractors and digital protractors that come with lower resolution.

Tools To Use

Before starting the procedure make sure that the tools you are using to calibrate other tools are accurate. That is why you should perform the check multiple times while using different tools so there is no chance of any mistake.

1. Vernier Protractor

Vernier protractor is an angle measuring micrometer that can measure angles ranging from 0 to 180 degrees. This tool is known for its accuracy, and it can be used to check 45, 90, or any angle in its range. You can check the 90-degree angle of your square by simply placing the square in the vernier protractor. Close the jaws and note the reading on the protractor, which will give you the measured angle which should be 90.

If you can’t afford the vernier protractor, the conventional single-arm or two-arm protractor can also be used to check the 90-degree corner of your square. Check our top protractors here.

2. Straightedge and Dial Indicator

Dial indicator, dial test indicator, and straightedge can be used to check the straightness of a square blade and its sides. A straightedge has good straightness, therefore, used in drawing straight lines and checking the straightness of a line or a tool.

To check if the blade is straight or not, hold the square against an edge using its stock and press the straight edge against it. If the straightedge sits properly on the blade and remains in contact along the length of the blade, then the blade is straight. A steel rule can also be used for this purpose if you do not have a straightedge as long as the straightness tolerance is within the standard.

The dial indicator is a little complex to use, you must fix the dial above the blade and move the blade throughout its length. If there was no fluctuation on the dial to either side that means the blade is straight. You can use the same procedure for square sides.

3. Machinist Square with Surface Surface

A machinist square is an accurate and precise square that consists of a blade and stock joined together at 90 degrees. Machinist square can be used to check the accuracy of a square. It can check the 90-degree angle of the corner and check straightness.

Put the machinist square on a surface plate, then put your square so that the blade edges of these two tools get in contact. If the gap appears, your square is not square or perpendicular.

Checking the try square using machinist square
Checking the try square using machinist square

4. Feeler Gauge

Feeler gauge is a simple tool that can measure gaps. It commonly comes in a set that consists of different leaves with different thicknesses. Their models are also different, allowing you to match your gap condition. This feeler gauge can be used to measure the gap between the machinist square and your square. Then, you know how much it’s off. Make sure your feeler gauge is calibrated with calibrated micrometer.

5. Vernier Caliper and Steel Rule

The simplest way to examine the issue of graduation accuracy is through a visual inspection. Still, you require some tools. You could match the graduation of your square with the graduation of a metal ruler by comparing them side by side through a magnifying glass.

The best practice is actually by comparing with the graduation of a precision ruler.

Here, you can directly see how they are the same. Make sure you bought the precision metal ruler for this case.

Any type of caliper can help check the graduation accuracy of a square as well. Simply open the jaws and measure the graduation.

Another expensive method is by using a gauge block. A gauge block set comes with different lengths of blocks. Compare them side by side, and see whether they read the same number. For example, if you compare with a 1-inch gauge block, but your square graduation shows a .9″ reading, then your square graduation is not accurate.