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Inspection Methods - The Steel Rule
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Steel Rule Measurement

With the technical advances being made, most people are inclined to think of complex electrical measuring equipment needed to inspect parts made by the machinist or toolmaker. Although this is true in some cases, the majority of the parts made do not require this sophisticated equipment.

One tool that is very common in the machine shop area which is not of a high precision measuring tool is the steel rule (Figure 1).

Figure 1.

The rigid six-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" rule. Although rules come in 6-inch increments (Example: 6", 12", 18", 24", and 36" 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 as will be seen in the slide series.

Rules vs. Scales

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 which provides actual linear measurements. In other words one inch on the rule represents exactly one 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 (Figure 2), one inch of measured distance may represent one 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.

Figure 2. Architectural Scale

Increment Divisions

There are four basic divisions that are found on a fractional inch rule. These are 1/64, 1/32, 1/16 and 1/8 of an inch.

Let's look at each graduation a little closer. A 1/64" graduate scale mean that in a 1-inch length, there are 64 lines dividing the 1-inch length (Figure 3).

Figure 3

These are the smallest graduations on the rule, therefore making the accuracy of a steel rule 1/64". This is sometimes argued by some of the metal workers who say they can measure to within ±.003 with the rule. They are right for they have worked with it a long time and have become masters at reading the graduations. However, the rule is only intended to measure to 1/64" accuracy, and other instruments are used to measure to closer tolerances.

The next set of graduations is the 1/32" scale which divides a 1-inch length into 32 divisions. A 1/32 division is equal to 2/64 divisions (Figure 4).

Figure 4

The 1/16" scale divides a 1-inch length into 16 divisions and is a total of 4/64 or 2/32 graduations (Figure 5).

Figure 5

The 1/8" scale divides a 1-inch length into 8 equal divisions and is a total of 8/64, 4/32, or 2/16 graduations (Figure 6).

Figure 6

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 1/8". Any combination of the four different sized graduations will give a reading up to the 1-inch length. Figure 7 is a decimal equivalent chart that shows all the various fractions possible in one inch.

Figure 7

As can be seen on the chart, 3/8" is only given as 3/8". It is not 24/64", 6/16", or 12/32". Any of these fractions does equal 3/8", but it is reduced to its lowest terms. Another example would be 7/16". This could be given as 14/32" or 28/64", but these are not in their lowest terms.

Also, the 1/2 division, the 3/4 division and 1-inch division are not read as 8ths, 64ths, 32nds, or 1/16 even though they are made from these combinations. They are read as 1/2", 3/4", 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 (Figure 8). The end of a rule is often rounded off from misuse, and a true measurement will not be made.

Figure 8.

Figure 9.

Even if the workpiece is held firmly against a reference surface such as an angle plate (Figure 9), 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 (Figure 10). 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, wear is inconsequential.

Figure 10.

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. See Figure 10.

Correct

Incorrect

Figure 10.

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.


Incorrect (Rule Lying Flat)	Correct (Rule Standing on Edge)

Figure 11.

The figure on the left is 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 figure on the right is used with the rule on 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 used like the figure on the left. The proper way is to view the graduation straight up as the center arrow.

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