How is the sight line on a sheet metal flat layout for bending measured?

The sight line on a sheet metal flat layout for bending is measured as one radius from the bend tangent line. This is because when a bend is made in a sheet metal section, the actual bending occurs at a point that is tangential to the bend radius. Establishing the sight line at one radius from this point allows for proper alignment and ensures that the material is bent accurately and consistently. This measurement takes into consideration the radius of the bend, which affects how the metal will form and the final dimensions after bending. Utilizing the radius at the tangent line reflects the actual physical characteristics of the bend in practice, which is crucial for achieving precision in the layout and the finished product.

In contrast, the other options do not reflect the accurate measurement related to the design and physical properties of bending in sheet metal. The reference to one diameter from the bend line or two radii does not correspond with the established methods used in the industry for creating accurate flat layouts prior to bending. Similarly, measuring at the edge of the sheet disregards the necessary allowances needed to account for bending and the resultant deformations that occur during the process. Thus, the correct choice accurately captures the professional practice for measuring sight lines in sheet metal layouts.