 Welcome to this module on the steel rule. With the technical advances being made, most people are inclined to think that complex electrical measuring equipment is needed to inspect parts made by the machinist or toolmaker. Although this is true in some cases, the majority of the parts that are made do not require this sophisticated equipment. One tool that is very common in the machine shop area that is not a high precision measuring tool is the steel rule. The rigid 6-inch rule is a common measuring tool found in the machine shop. The rule is a strip of metal, graduated in inches and fractions of an inch, to give actual measurements. When tolerances of fractional dimensions are required, the steel rule is used. The most commonly used steel rule is the 6-inch rule. Although rules come in 6-inch increments, for example, 6, 12, 18, 24, and 36-inch lengths, the 6-inch rule is the most popular, because it fits into the apron pocket easily. It also comes in various widths and thicknesses to meet varying requirements. The terms rules and scales are used in different ways by different workers, sometimes causing confusion. While some manufacturing workers do not differentiate between these terms, others use the term rule when referring to a measurement tool that provides actual linear measurements. In other words, 1-inch on the rule represents exactly 1-inch of linear distance. On the other hand, to these workers, a scale is a measurement tool in which an actual measured linear distance represents another imaginary, smaller or larger, linear distance. For example, on an architect's scale, 1-inch of measured distance may represent 1-foot of actual linear distance. When this principle is in operation, a key should indicate the ratio of measured distance to actual distance or scale that is being used. There are four basic divisions that are found on a fractional-inch rule. These are 1-64th, 1-32nd, 1-16th, and 1-8th of an inch. Let's look at each graduation a little closer. A 1-64th-inch graduate scale means that in a 1-inch length, there are 64 lines dividing that inch. These are the smallest graduations on the rule, therefore making the accuracy of a steel rule 1-64th inch. This is sometimes argued by some of the metal workers who say they can measure to within plus or minus three one-thousandths with the rule. They're right for they've worked with it a long time and have become masters at reading the graduations. However, the rule is only intended to measure to 1-64th inch accuracy, and other instruments are used to measure to closer tolerances. The next set of graduations is the 1-32nd-inch scale, which divides a 1-inch length into 32 divisions. A 1-32nd division is equal to 2-64th divisions. The 1-16th-inch scale divides a 1-inch length into 16 divisions and is a total of 4-64ths or 2-32nd graduations. The 1-8th-inch scale divides a 1-inch length into 8 equal divisions and is a total of 8-64ths, 4-32nds, or 2-16th graduations. This is a decimal equivalent chart that shows all the various fractions possible in 1-inch. As you can see, a combination of two or more of the various graduations will make up a larger graduation. The same holds true with graduations larger than 1-8th of an inch. Any combination of the four different sized graduations will give a reading up to the 1-inch length. As can be seen on the chart, 3-8th-inch is only given as 3-8th-inch. It is not 24-64ths, 6-16ths, or 12-32nds. Any of these fractions does equal 3-8th-inch, but it is reduced to its lowest terms. Another example would be 7-16th-inch. This could be given as 14-32nds or 28-64ths, but these are not in their lowest terms. Also, the half division, the 3-quarters division and 1-inch division are not read as 8ths, 64ths, 30 seconds, or 1-16th, even though they are made from these combinations. They are read as half-inch, 3-quarters-inch, or 1-inch. Now that you have this information under your belt, let's look at how a rule should be used. To get the full accuracy out of a rule, it is important to use it correctly. Never use the end of the rule to align with the edge of the work for a measurement. The end of a rule is often rounded off from misuse, and a true measurement will not be made. Even if the workpiece is held firmly against a reference surface, such as an angle plate, this will not assure an accurate measurement if the end of the rule is worn off. To offset this, use an inch graduation as a reference point on the rule. Precision and reliable measurements are possible this way. With the graduation directly on the edge of the work, and by not using the end of the rule, whereas inconsequential. When measuring a length, the rule must be kept in a straight line parallel to the centerline of the work. If it is tilted, the measurement will be longer than the actual part. One other important factor in using the rule is to be aware of parallax. This is an observation error from the person measuring or holding at the part in relation to the part being held. The drawing on the left shows an incorrect way of measuring, and parallax is greatly increased because of the thickness of the rule. The graduations do not come in direct contact with the work. The arrows pointing to the right and left will cause parallax, and even though the arrow pointing straight up is the correct way to view the rule, there is a chance for error in reading due to the thickness of the rule. The drawing on the right shows the rule on its edge. As can be seen, the graduation comes in contact with the work, which is the correct way of measuring. Although the arrows pointing to the right and left will cause an improper reading, it will not be as great an error as when the rule is used in the manner shown in the drawing on the left. The proper way is to view the graduations straight up as the center arrow illustrates. You have completed this learning activity, the steel rule.