Selecting a Straight Edge on Amazon.com
A straight edge, or straightedge, is a rigid bar with a curve-free edge for checking and marking straight lines and inspecting the flatness of a surface. It looks similar to a ruler, but a straight edge generally does not have graduations. Some straight edges have graduations along one edge, usually in inch or half-inch increments, to serve as a measuring guide and point of reference. A straight edge may be precision ground to a particular accuracy and may have a tolerance as tight as + or - 0.0002" per foot. It is commonly used in carpentry, metalworking, drafting, and engineering, and is often paired with another tool such as a compass, layout square, or feeler gauge.
Straight edges are available in lengths from 4" to 8', and in various widths and thicknesses. To retain their straightness, they are usually made of metal such as hardened steel or carbon steel, and are thicker than typical rulers. Some longer length straight edges are made of aluminum for light weight. A straight edge may have a bevel or knife edge to make the line of contact with the work surface more distinct and aid in the drawing or scribing of lines.
While a straight edge is most often placed flat upon a work surface, it can be positioned on its edge. Some straight edges, particularly those 36" long or longer, have arrows to mark the points at which the tool should be suspended to minimize deflection when used in the edge position.
For jobs that require a master gauge to inspect the straightness of the axis movements of high-precision machinery such as a coordinate measuring machine (CMM) or semiconductor manufacturing equipment, a straight master may be selected. These instruments are usually made of alumina ceramic and have precision-lapped measuring surfaces. They are available in different lengths and accuracy ratings.