If the perpendicularity between the fixed jaw and the workbench is incorrect, the workpiece will tilt or deviate from the ideal position after clamping, which will affect the machining accuracy and quality of the workpiece. How to correct the steps are as follows:

1. Check the levelness of the workbench with a level. If it is not horizontal, adjust the leveling screw of the workbench until the workbench is completely horizontal.

2. Clamp the high-precision workpiece on the fixed jaw, and measure the height of the workpiece on the fixed jaw with a micrometer or other high-precision measuring tools.

3. Turn the workpiece over 180 degrees, clamp it on the fixed jaw again, and measure the height of the workpiece on the fixed jaw again.

4. If the heights measured twice are equal, the fixed jaw has been perpendicular to the workbench. If the heights of the two measurements are not equal, it is necessary to adjust the position of the fixed jaw until the heights of the two measurements are equal.