How Constraints work?

[[Ra...]]

I am looking in my advanced book under the measuring small arcs and reporting diameters from them, the book says to constrain its location to report diameter and I noticed a substantial size difference, how does constraining where an arc is at change its diameter?

[[Ky...]]

Hello Ramon,

As a brief description, constraints tell the software to not evaluate a particular element of the feature (usually in this case either the radius or the x,y,z position). For instance, if you tell a circle to constrain the radius of the circle, and then you create a characteristic for the radius or diameter of that circle, it will return the exact radius/diameter set in the nominals.

Because of the math involved, circles with small angle segments can be unrepeatable, because when the points cover so little of a feature, tiny changes in the points can have significant effects on the evaluation. To combat this, we recommend constraining one of the elements to increase the repeatability. For instance, if you constrain the position of the circle, then ask for the radius, the result is basically an average distance of the measured points from the nominal center. Likewise, if you constrain the radius and ask for a position, you are effectively fitting a circle of a given radius to that data and asking for the location of the center.

I hope this helps explain it a bit.