When installing a Topcon 2D or 3D grade control system on all motor graders (with the exception of the John Deere GP grader which utilizes the factory OEM rotation sensor), you will need to add calibration marks to the rotation circle.
It is very important to accurately make your measurements.
This process will require two people, a tape measure, and a marking pen. Once the calibration marks are made and confirmed to be correct, it is recommended to use a grinding wheel or some method to permanently scribe the circle.
Manually center the machine's A-frame, and rotate the blade so it is perpendicular to the main frame.
Measure and mark a point (A) 1-2 feet on the left and right sides of the blade equal distances from the side of the A-frame. Note the blade does not need to be centered; you just cannot side shift the blade after making your measurements.
Pick a location where the A-frame attaches to the front of the grader frame, and measure to the A point on the left and right sides of the blade.
Do not side shift the blade, but rotate the blade until the measurement B is equal on the left and right side.
Using the marking pen, mark the circle where the circle lines up with the top of the A-frame.
Double check that all your measurements are correct before using a grinding wheel or chisel to make a permanent set of marks that will be used to align the circle during the rotation sensor calibration.