Steel rule

How it works

The user lays the rule against the work, aligns a graduation or the squared end with one feature, and reads the position of the second feature against the scale. Because the rule is thin and its graduations lie on its edges, the divisions can be brought close to the surface being measured.

Starrett advises that, for very close work, the eye reads better by measuring between two lines than by measuring from the end of the rule to a line. Its precision rules are ground square on both ends so that every edge can be read from either end.

The rule is also a transfer tool. In the Starrett 1965 handbook it is used to count the threads per inch of a bolt or tap by counting the threads between inch graduations, and Starrett’s hook and tapered rules are made to set inside calipers and to enter small holes and slots.

Anatomy

Blade: Steel bar; Starrett lists full-flexible (1/64 to 1/50 inch thick), semi-flexible (1/50 to 1/40 inch), spring-tempered (3/64 inch) and heavy spring-tempered (1/10 inch) rules.
Graduated edges (corners): Brown & Sharpe describes four graduated corners on a rule, each divided in a different fraction of the inch according to a numbered graduation style.
Squared ends: Ground square so that the end of the rule can be butted against a surface. Starrett notes that older “D” style rules had one square and one rounded end.
End graduations: Divisions cut across the ends for measuring depths, shoulders, recesses and grooves.
Hook or pocket clip (some rules): A hardened hook, fixed or reversible, to seat the rule on an edge; or a clip for carrying.

Variants

Standard (soft) steel rule: Brown & Sharpe’s “Standard” rules, 2 to 24 inches in its 1905 catalog, contrasted with its tempered rules.
Spring-tempered rule: Marked “Tempered” in the Brown & Sharpe catalog and about 1/20 inch thick; Starrett’s spring-tempered rules are 3/64 inch thick.
Flexible rule: A thin tempered rule, graduated on one side only in the 1905 Brown & Sharpe catalog, that will conform to the circumference of a circle.
Rule with end graduations: Brown & Sharpe’s “Patent End Graduations” for measuring grooves, countersinks and recesses; Starrett offers end graduations in thirty-seconds of an inch or millimeters.
Hook rule: Starrett’s single or double hook rules, with reversible hooks held by an eccentric stud, permit measuring from round corners or through hubs.
Narrow and tapered rules: Starrett’s narrow hook rule is 3/16 inch wide and enters holes as small as 7/32 inch; its tapered-end rule narrows from 1/2 inch to 1/8 inch for small holes and slots.
Shrink rule: Brown & Sharpe’s 1902 catalog lists tempered steel shrink rules, 12 inch; Starrett lists shrink graduations.
Metric and dual-unit rules: Starrett lists inch, millimeter, and inch-and-millimeter graduation styles; Brown & Sharpe lists “Metric and English” rules.

Specifications

Length · in or mm: 6 inBrown & Sharpe’s 1905 catalog lists standard rules from 2 to 24 inches and tempered rules from 2 to 12 inches; Starrett lists lengths from 1 inch through 48 inches.
Thickness · in or mm: Starrett: full-flexible 1/64 to 1/50 inch (0.4 to 0.5 mm); semi-flexible 1/50 to 1/40 inch; spring-tempered 3/64 inch (1.2 mm); heavy spring-tempered 1/10 inch (2.5 mm).
Graduation · fraction of an inch or mm: 1/64 inA common 64ths style combines 8ths, 16ths, 32nds and 64ths on the four edges; decimal styles divide the inch in 10ths, 50ths and 100ths.

Made to

DIN 866
JIS B.7516
GGG R.791
BS 4372

Tips

Measure between two lines, not from the end
Starrett advises that for very close work the eye reads better by measuring between two graduation lines than from the end of the rule to a line. The end of a rule can be worn or burred, and a line-to-line reading does not depend on it.1
Count threads per inch with the rule
Starrett’s 1965 handbook says to count the threads between inch graduations with the end of the rule aligned with the root of a thread. If the count is not a whole number, count the threads between the end of the rule and the first inch graduation that coincides exactly with a thread, and divide the threads by the inches.1
Use the end graduations for depths and shoulders
Rules with end graduations measure depths, shoulder widths, recesses and grooves. Starrett cuts them in thirty-seconds of an inch or millimeters on both ends of one side, and Brown & Sharpe says its patent end graduations suit grooves, countersinks and recesses.1 2
Read the staggered lines by their length
Starrett staggers its graduation lengths so that lines can be counted by height instead of read as a comb. It adds an “Aircraft Quick-Reading” pattern for 50ths and 100ths of an inch, and the suffix R in a catalog number marks a quick-reading style.1
Use a hook rule when the edge cannot be seen
A hooked rule seats against the work without the user needing to see the alignment. Starrett says this suits measuring from round corners and through hubs, and for setting inside calipers to a graduation.1
Choose tempered or flexible by the job
Brown & Sharpe contrasted its standard (soft) rules with tempered rules about 1/20 inch thick, and made a thin flexible rule that bends to a circle. A flexible rule follows a curved surface; a stiffer one holds a straight line for scribing.1

History

Earliest documented

1850 (Brown & Sharpe’s own account)

Providence, Rhode Island

Brown & Sharpe’s catalogs of 1902 and 1905 state that the manufacture of steel rules and other tools of precision was begun by Joseph R. Brown in 1850 and that Samuel Darling began a similar line of work in 1852. This is the company’s own history; an earlier steel rule or an independent record of the 1850 date has not been located.1 2

The steel rule is a machine-shop tool, and its history in the sources found here is the history of the firms that made precision tools for machinists. Brown & Sharpe places the start of its steel rules in 1850, within a line of precision tools that grew with its own work in dividing and gauging. The company also reports that it prepared standard yard and meter bars for its shop, had them compared by government officials with the standards in Washington, and found the mean errors to be 0.00002 inch for the yard and 0.000005 meter for the meter; it says its rules are as nearly exact copies of those standards as skill and special dividing machines can make them. This is the company’s account, not an independent test.

The material and temper of the rule were the early distinction. Brown & Sharpe’s 1905 catalog sets its standard, soft rules against tempered rules marked “Tempered,” which it says are as accurately graduated as the soft rules and have advantages that every machinist will see. Starrett’s own history makes the same contrast from the other side: its founder says he made a full line of spring-tempered rules, including a very thin watch-spring “flexible” rule, which he judged far better than the thick, heavy, soft rules of other makers. This is a company statement about its own product.

Graduation was standardized by numbered styles. Brown & Sharpe’s 1905 catalog assigns each style a number, 1 to 14, and states how the four corners of the rule are divided: No. 4, for example, uses 8ths, 16ths, 32nds and 64ths, and other styles use tenths, twentieths, fiftieths and hundredths. Starrett’s current numbering carries the same idea, with 4R marking quick-reading 8ths through 64ths. The same two firms added end graduations for measuring into recesses, which Brown & Sharpe calls patent end graduations.

Making the divisions changed with the means of making them. The Science Museum Group holds four steel rules described as machine divided. Starrett’s current rules are photo-engraved and inspected against master standards that it says are traceable to NIST, and it describes a satin chrome finish for reading and rust resistance. These are Starrett’s statements about its own product.

National standards followed. The Mitutoyo table of length-measuring-instrument standards lists a standard for metal rules in the United Kingdom (BS 4372, 1968), the Soviet Union (GOST 427, 1975), the United States federal specification (GGG-R-791H, 1994), Japan (JIS B 7516, 2005) and Germany (DIN 866, 2006, listed under rules for manufacturing purposes). A supplier of DIN 866 rules states an accuracy of ±0.04 mm per meter for its form A rules. The standards themselves have not been read for this entry.

Timeline

1850
Joseph R. Brown begins making steel rules and other precision tools, according to Brown & Sharpe.1 2
1852
Samuel Darling begins a similar line of work, according to Brown & Sharpe.1 2
1866
The partnership Darling, Brown & Sharpe is formed. In the same year, Brown & Sharpe says, Congress made the metric system the first legalized measure of length in the United States.1 2
1880
The L. S. Starrett Company is founded in Athol, Massachusetts; Starrett’s history says the founder took up the making of a full line of spring-tempered rules, including a thin “flexible” rule, after making square blades.1
1902
The Brown & Sharpe catalog lists tempered steel rules, metric and English, and tempered steel shrink rules.1
1905
The Brown & Sharpe catalog describes standard (soft) rules from 2 to 24 inches, tempered rules about 1/20 inch thick, flexible rules, narrow rules, patent end graduations and fourteen numbered graduation styles.1
1965
Starrett’s handbook The Tools and Rules for Precision Measuring describes using an ordinary steel rule to count threads per inch.1
1968–2006
National standards for metal rules appear in the Mitutoyo table: BS 4372 (1968), GOST 427 (1975), GGG-R-791H (1994), JIS B 7516 (2005) and DIN 866 (2006).1

Gaps in the record (6)

No steel rule earlier than Brown & Sharpe’s 1850 claim has been located; the earlier history of metal and steel scales (for example nineteenth-century English makers) is not sourced.
Brown & Sharpe’s 1850 and 1852 dates come from the company’s own catalogs.
No independent test of rule accuracy was located; accuracy statements are by the manufacturers.
The text of DIN 866, JIS B 7516, GGG-R-791 and BS 4372 has not been read; only their existence and editions are sourced, from a table by a measuring-instrument maker.
Care and safety have no sourced entries.
The date at which Starrett began making rules is not given in the sources found.