Reply To: How square should a square be?
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It should be square along the entire length of the ruler, otherwise it isn’t square.
First, check if the ruler is rocking in the head with the screw tightened. If it is, return it as defective and test another one until you find a good one.
From this point, you must decide if you want to attempt fixing it or to simply return it. Below are the various things I would try if I decided against returning it.
Check the face of the head for imperfections. Lap the face until they are gone and test the square again.
If there is no imperfection on the face, test the ruler at various extensions. If it is only inaccurate from a certain extension, check the groove of the ruler around the area where the nut is for that extension for any imperfection.
Lightly wax the ruler blade, oil the screw and nut. That should make the ruler move more easily, which can have some impact on the accuracy in my experience.