The most common and overlooked measuring tool is the steel rule or ruler. Steel rules can be in inches or metric with sub-markings as precise as 1/64 of an inch or one millimeter. Steel Rules come in 4 types: spring tempered, flexible, narrow, hook, and short-length rules.
Spring tempered steel rules are the most commonly used in aerospace shops. These rules are usually 6 inches in length and made for quick reading. Usually they are broken into 4 scales, two on each side. For English measurements, one side of the rule will have graduations in eighths and sixteenths, and the back is graduated in thirty-seconds and sixty-fourths. Some spring tempered rules will have English measurements on one side and metric on the other.
Flexible steel rules are more commonly found in construction and in the home. They are also commonly called tape measures.
Narrow steel rules are used to reach tight and hard to access places to measure.
Hook steel rules have a “hook” on the end that you can butt up flat against a corner or protrusion while measuring to ensure the rule doesn’t move.
Short-length rules are small rules that come in sets of four and range in size from 1/4 inch to 1 inch in length. These rules are placed in a holder and used to measure small parts or openings.
There are two major rules to follow when using steel rules to ensure accuracy:
1. Due to rules being worn on the edge over time and use, it is best to start your measurement from the 1 inch or 1 centimeter mark. Once you have your measurement, subtract the 1 unit you started at to get the final measurement.
2. Make sure the graduated markings are as close to the area of the part you are measuring to ensure accuracy. It is best if the graduated markings are actually touching the area your measuring to make it easier to obtain the correct measurement.
Steel rules can also be used to determine the flatness of a material. Lay the steel rule on it’s edge on a part, hold the rule and part up to a light. If light shows through between the part and the rule, then the part is not flat. If there is no light showing, then the part is flat.
Don’t underestimate the usefulness of the steel rule. It will be the most often used measuring tool in your tool box.
The Quality Technician’s Handbook (Griffith, 2003)
Technology of Machine Tools (Krar & Check, 1997)