Precision Rules & Edges

The collection carries eight Starrett precision layout and reference tools across three categories: precision rules (Starrett C404R-72, C416R-48, C416R-72, C635-1800MM, and CH416R-72 hook rule), a precision straight edge (385-72), and master precision squares (20-12 and 20-24). All are made by L.S. Starrett Company, a Massachusetts-based manufacturer with over 140 years of precision tool manufacturing heritage, and one of the most trusted names in metrology and layout instrumentation worldwide. Every product ships with manufacturer documentation and, where applicable, factory inspection certification.

A precision rule is a dimensionally stable, hardened and tempered steel instrument manufactured to tight tolerances on graduation accuracy, flatness, and edge straightness specifically designed to serve as a reference standard for layout and inspection work, not just a convenient measuring aid. Standard tapes and hardware store rulers are made from flexible, lower-grade materials with tolerances appropriate for general construction or casual use. Starrett precision rules in this collection are ground to close thickness tolerances, graduated by machine etching (not printing) for permanence and clarity, and finished with satin-chrome to minimize glare resulting in tools that hold their accuracy across decades of shop use and remain reliable references for machining, quality inspection, and precision layout.

Both are 72" hardened steel precision rules, but they differ in graduation pattern and intended use. The C404R-72 is a single-sided rule with a specific graduation layout suited to general precision measurement and layout work in machining and fabrication environments. The C416R-72 and C416R-48 are part of Starrett's 416 series is a heavier-duty rule format with a wider blade, stiffer cross-section, and graduation sets designed for longer-span layout on larger workpieces such as structural steel, plate fabrication, and large-format machining. The C416R-48 at 48" and C416R-72 at 72" offer two length options within the same platform, with the 48" suitable for table-scale work and the 72" for six-foot spans. Both are hardened, tempered, and satin-chrome finished for long-term shop durability.

The C635-1800MM is an 1800mm (approximately 71") hardened precision steel rule graduated in millimetre increments are the metric-system counterpart to the 72" inch rules in the collection. Metric graduation is the appropriate choice for any operation working in SI units: metric machining programs on CNC equipment calibrated in millimetres, international manufacturing environments, aerospace and automotive suppliers working to metric engineering drawings, or any shop floor where mixing inch and metric measurements would introduce conversion error risk. The C635-1800MM covers the same large-format layout span as the 72" rules while delivering a native millimetre reference without requiring mental or calculated unit conversion.

A hook rule has a fixed metal hook attached at one end that projects perpendicular to the rule face, with the zero graduation beginning at the inside edge of the hook rather than at the end of the rule. This allows the operator to hook the rule over the edge of a part, into a slot, or against a shoulder and measure from a defined, repeatable zero point without the rule slipping or requiring a second hand to hold it in position. On a standard rule, measuring from an edge or step requires carefully positioning the rule's end flush with the reference surface which is a task that introduces positioning error, especially on larger parts or when working alone. The CH416R-72's 72" hook rule format is specifically suited to long-span measurements on structural components, sheet metal, large castings, and machine frames where the hook provides a reliable anchor at one end while the full length of the rule is measured along.

A precision straight edge is a reference tool manufactured to a tight flatness tolerance along its length with its defining function is to reveal whether a surface is straight or flat by checking the gap between the edge and the surface in question, either visually or with feeler gauges. A rule has accurate graduation markings but is not designed or guaranteed to be straight along its edge to precision tolerances. The Starrett 385-72 is a 72", 40 lb hardened and ground steel straight edge with a flatness tolerance of approximately ±0.0005" over its full length and a beveled working edge for precise contact against a surface. It is the standard reference tool for verifying the flatness and straightness of machine tool ways, surface plates, welded fabrications, granite surface plates, and large machined surfaces any application where a long, flat reference edge is needed as the datum for assessment or corrective scraping.

A beveled edge is ground to a narrow, angled working edge rather than a flat face contact. This creates a thin line of contact between the straight edge and the surface being checked, rather than the full width contact a flat-face straight edge would produce. The practical advantage is precision: a thin beveled contact point makes it much easier to detect small gaps using the light gap method holding the straight edge against the surface with a backlight and observing where light passes through because the narrow contact eliminates the masking effect that a wide flat face would produce over irregularities in the surface. The "one edge" designation means the bevel is on one side of the instrument, while the opposite edge remains flat giving the user two different reference geometries in a single tool depending on the orientation they need.

A master precision square is a fixed, non-adjustable reference tool manufactured to the highest achievable squareness tolerance which is used as the ultimate standard against which other squares are checked and from which machine setups and part inspections are validated. The Starrett 20-12 and 20-24 are ground, lapped, and hardened steel squares with squareness accuracy of ±0.0001" per 6", shipping with a factory inspection certificate documenting that accuracy. A standard combination square uses an adjustable head on a graduated blade convenient for layout work but adjustable, which means its squareness is only as good as the last setting and is not an absolute reference. A master precision square has no adjustment mechanism: the blade and beam are a single rigid, immovable assembly permanently at 90°, which is why it can be trusted as a calibration reference for other squares, as a machine tool alignment standard, and as an inspection datum for tight-tolerance squareness verification.

Both are master precision squares with the same ±0.0001" per 6" squareness accuracy and the same hardened, ground, and lapped construction, the choice is purely a function of the blade length required for the work. The 20-12 with its 12" blade and approximately 8" beam is the right choice for most machining, inspection, and toolroom squareness checks: verifying the squareness of vise jaws, checking spindle alignment, calibrating other squares, and layout work on smaller to medium parts. The 20-24 with its 24" blade and correspondingly longer beam is required when the part or machine surface being checked is tall enough that a 12" blade would not reach the full height needed to assess squareness. Applicable to large vertical machining center column alignment, tall fixture verification, large press brake setup, and structural fabrication inspection where long reference spans are required.

All precision rules and edges ship free to the continental USA, backed by a best-price guarantee. A 30-day return policy and Affirm financing at checkout are available. The master precision squares and straight edge all ship with factory inspection or calibration certificates. Government agencies and inspection departments can submit an RFQ through the dedicated Government RFQ page (DUNS: 13-715-2209, CAGE: 15F67). For guidance on selecting the right rule graduation, length, or whether a straight edge or master square is the appropriate reference tool for a specific application, call or text Gustavo directly at 617-398-7852, Monday through Friday, 8am–5pm EST.