Do you know how to use all these tools, even if you’re a mechanic?

　　Steel ruler

　　A steel ruler is a simple length-measuring instrument available in four standard lengths: 150 mm, 300 mm, 500 mm, and 1000 mm. The figure below shows the commonly used 150-mm steel ruler.

　　A steel ruler is used to measure the length dimensions of parts, but its measurement results are not very accurate. This is because the spacing between the markings on a steel ruler is 1 mm, while the width of the markings themselves is already 0.1 to 0.2 mm. As a result, the reading error during measurement is relatively large, and only millimeter values can be accurately read—meaning the smallest readable unit is 1 mm. Values smaller than 1 mm can only be estimated.

　　If you use a steel ruler directly to measure the diameter dimension of a part (shaft diameter or hole diameter), the measurement accuracy will be poorer. The reason is that, in addition to the relatively large reading error inherent in the steel ruler itself, the ruler cannot be precisely positioned at the correct location along the part’s diameter. Therefore, to measure the diameter dimension of a part, you can also use a combination of a steel ruler and inside/outside calipers.

　　Two

　　Internal and external calipers

　　The figure below shows two common types of calipers—external and internal calipers. External and internal calipers are simple comparative measuring tools. External calipers are used to measure external diameters and flat surfaces, while internal calipers are used to measure internal diameters and grooves. Neither type of caliper can directly read measurement results on its own; instead, the measured dimensions (including diameters, which are also length measurements) must be read off a steel ruler, or first marked off on the steel ruler before being used to check whether the part’s diameter meets the required specifications.

　　1. To adjust the opening of the caliper, first check the shape of the jaws. The shape of the jaws significantly affects measurement accuracy; therefore, it’s important to regularly maintain and refine the jaw shape. The figure below shows a caliper.

　　A comparison of good and bad jaw shapes.

　　When adjusting the opening of the caliper, gently tap the two side surfaces of the caliper’s jaws. First, use both hands to adjust the caliper to an opening that closely matches the workpiece’s dimensions. Then, lightly tap the outer side of the caliper to reduce its opening, and tap the inner side to increase its opening, as shown in Figure 1. However, do not directly strike the jaws themselves, as illustrated in Figure 2. Doing so could damage the jaws and introduce measurement errors. Furthermore, never strike the caliper against the machine tool’s guideways, as shown in Figure 3.

　　2. Using an external caliper: When taking measurements with an external caliper on a steel ruler—as shown in the figure below—one of the caliper’s measuring jaws should be placed against the end face of the steel ruler, while the other measuring jaw should be aligned with the middle of the scale marking corresponding to the desired dimension. The line connecting the two measuring surfaces must be parallel to the steel ruler, and the observer’s line of sight must be perpendicular to the steel ruler.

　　When using an outside caliper whose dimensions have already been set on a steel ruler to measure the outer diameter, ensure that the line connecting the two measuring surfaces is perpendicular to the axis of the part. As the caliper slides along the outer circumference of the part under its own weight, the sensation in your hand should be that the caliper’s measuring surfaces are making point contact with the outer surface of the part. At this moment, the distance between the two measuring surfaces of the caliper precisely represents the outer diameter of the part being measured. Therefore, when using an outside caliper to measure the outer diameter, you’re essentially comparing the snugness of the caliper’s contact with the outer circumference of the part—as shown in the figure below, it’s ideal when the caliper just barely slides down under its own weight. If, while sliding the caliper over the outer circumference, you feel no contact at all, it means the caliper’s dimensions are larger than the part’s outer diameter. Conversely, if the caliper fails to slide over the outer circumference even under its own weight, it indicates that the caliper’s dimensions are smaller than the part’s outer diameter. Never place the caliper at an angle when taking measurements on the workpiece—this will introduce errors, as illustrated in the figure below. Since the caliper is elastic, it’s incorrect to forcefully press the outside caliper against the outer circumference; even more so, you must never try to clamp the caliper horizontally onto the part, as shown in the figure below. For large-sized outside calipers, the pressure exerted by their own weight as they slide over the outer circumference can become excessive. In such cases, you should support the caliper with your hand while taking the measurement, as illustrated in the figure below.

　　3. When using an internal caliper to measure the inner diameter, the line connecting the measuring surfaces of the two jaws should intersect perpendicularly at the axis of the inner hole—meaning that the two measuring surfaces of the jaws should correspond to the two end points of the inner hole’s diameter. Therefore, during measurement, the measuring surface of the lower jaw should be held against the hole wall as a fulcrum.

　　The jaws of the caliper above are gradually probed outward from slightly inside the hole’s opening, while oscillating in a circular motion along the hole’s wall. When the range of oscillation along the circumference of the hole’s wall becomes minimal, it indicates that the two measuring surfaces of the internal caliper jaws have reached the opposite ends of the inner hole diameter. By slowly moving the caliper from outside to inside thereafter, you can verify the hole’s roundness tolerance.

　　Use the inside calipers, which have already been set to the correct dimension on a steel ruler or outside calipers, to measure the inner diameter.

　　The key is to compare the tightness of the inside caliper within the part’s hole. If the inside caliper can swing freely within the hole, it means the caliper’s dimensions are smaller than the hole’s diameter. On the other hand, if the inside caliper cannot be inserted into the hole at all, or if it fits snugly and cannot swing freely once inserted, it indicates that the caliper’s dimensions exceed the hole’s diameter. However, if the inside caliper can be inserted into the hole and, when measured according to the method described above, exhibits a free swinging clearance of about 1 to 2 millimeters, then the hole’s diameter is precisely equal to the caliper’s dimensions. When taking measurements, do not hold the caliper directly with your hands.

　　This eliminates the tactile feel, making it difficult to compare the tightness of the internal caliper within the part’s hole and causing the caliper to deform, thus introducing measurement errors.

　　The 4-inch caliper has a wide range of applications. As a simple measuring tool, it is widely used for measuring and inspecting the dimensions of parts that do not require high precision, thanks to its simple structure, ease of manufacture, low cost, and convenient maintenance and use. In particular, the caliper is an ideal measuring instrument for assessing and inspecting the dimensions of rough forgings and castings. Although the caliper is a simple measuring tool, as long as...

　　If we master it well, we can also achieve higher measurement accuracy. For example, we can use an external caliper to compare two...

　　When it comes to the diameter of the root shaft—even if the shaft diameters differ by just 0.01 mm—experienced master craftsmen...

　　It can also be distinguished clearly. For example, when using an internal caliper in conjunction with an outside micrometer to measure the dimensions of an internal bore, an experienced master craftsman can confidently rely on this method to measure internal bores with high precision. This internal-diameter measurement technique, known as the “internal caliper-plus-micrometer” method, involves using the internal caliper to read accurate dimensions directly from the outside micrometer.

　　Next, measure the inner diameter of the part. Alternatively, insert the internal caliper into the hole, adjust its fit snugly against the hole’s surface, and then read the exact dimension from the outside micrometer. This measurement method is not only an excellent approach for measuring inner diameters when precise inner-diameter measuring tools are unavailable; moreover, for certain parts—such as the one shown in Figure 1-9—where the hole contains a shaft making it difficult to use precision inner-diameter gauges, employing the internal caliper combined with an outside micrometer to measure the inner diameter can effectively solve the problem.