Rules and Straight Edges

A rigid rule holds its shape and provides a stable reference edge, useful for measuring against a flat surface or as a scribing guide. A flexible (spring tempered) rule can bend to follow a curved surface or fit into tight spaces while still returning to true when released, making it useful for measuring around curved stock or in confined areas a rigid rule can't reach.

Place the straight edge's precision-ground edge against the surface being checked and look for light passing beneath it — an even, unbroken line indicates a flat surface, while visible light gaps reveal low spots and rocking indicates a high spot. For higher precision, use feeler gauges to quantify any gap detected.

Grade A straight edges are manufactured and certified to a tighter flatness tolerance than Grade B, and are specified for toolroom, inspection, and calibration work where accuracy is critical. Grade B is adequate for general shop layout and fabrication checks where the tighter tolerance isn't needed.

Ground or etched graduations are permanently cut into the metal surface and won't wear off, fade, or shift with handling the way printed or stamped markings can over time. This matters most for rules used daily in a shop environment, where a worn graduation line directly degrades measurement accuracy.

The straight edge should span the entire length of the feature being checked in a single pass — checking a long surface in overlapping short sections can miss a gradual, low-amplitude bow that only becomes visible when the full length is checked at once. When a single piece isn't practical, use a straight edge at least as long as the largest feature increment being verified.