Steel ruler

The steel ruler is the simplest length measuring tool, which has four specifications of 150300500 and 1000mm. The following figure is a commonly used 150mm steel ruler

The steel ruler is used to measure the length of parts, and its measurement results are not very accurate. This is because the spacing between the scribed lines of the steel ruler is 1mm, and the width of the scribed line itself is 0.1~0.2mm, so the reading error is relatively large when measuring, and only millimeters can be read, that is, its minimum reading value is 1mm, and the value smaller than 1mm can only be estimated

If a steel ruler is used to directly measure the diameter of a part (shaft diameter or hole diameter), the measurement accuracy is worse. The reason is that, in addition to the large reading error of the steel ruler itself, the steel ruler cannot be placed at the correct position of the part diameter. Therefore, the measurement of part diameter can also be carried out by using a steel ruler and internal and external calipers

II

Internal and external calipers

The following figure shows two common types of internal and external calipers. The internal and external calipers are the simplest measuring tools for comparison. The outer caliper is used to measure the outer diameter and plane, and the inner caliper is used to measure the inner diameter and groove. They can not directly read the measurement results themselves, but read the measured length (diameter also belongs to the length dimension) on the steel ruler, or take down the required size on the steel ruler first, and then check whether the diameter of the part conforms to

1. Adjust the opening of the caliper. First check the shape of the jaw. The jaw shape has a great influence on the measurement accuracy. Pay attention to the shape of the jaw. The following figure shows the caliper

Comparison between good and bad jaw shape

When adjusting the opening of the caliper, gently tap the two sides of the caliper foot. Use both hands to adjust the caliper to the opening close to the size of the workpiece, then tap the outside of the caliper to reduce the opening of the caliper, and tap the inside of the caliper to increase the opening of the caliper. As shown in Figure 1 below. However, the jaw cannot be directly knocked, as shown in Figure 2 below. This will cause measurement error due to the jaw damage measuring surface of the caliper. Do not knock the caliper on the guide rail of the machine tool. As shown in Figure 3 below

2. Use of external calipers When taking down the size on the steel ruler with external calipers, as shown in the following figure, the measuring surface of one clamp foot is against the end face of the steel ruler, and the measuring surface of the other clamp foot is in the middle of the required size scale line. The connecting lines of the two measuring surfaces should be parallel to the steel ruler, and the line of sight of the person should be vertical to the steel ruler

When measuring the outer diameter with an external caliper whose size has been taken on a steel ruler, the connecting line of the two measuring surfaces should be perpendicular to the axis of the part. When the external caliper slides across the outer circle of the part by its own weight, the feeling in our hands should be that the external caliper and the outer circle of the part are just in point contact. At this time, the distance between the two measuring surfaces of the external caliper is the outer diameter of the part to be measured. Therefore, measuring the outer diameter with an external caliper is to compare the tightness of the contact between the external caliper and the outer circle of the part. As shown in the figure below, it is appropriate that the self weight of the caliper can just slide down. If the caliper slides over the excircle and we don't feel it in our hands, it means that the external diameter of the external caliper is larger than that of the part. If the weight of the external caliper cannot slide over the excircle of the part, it means that the external caliper is smaller than the external diameter of the part. Never place the caliper on the workpiece to measure, because there is error. As shown in the figure below. Since the caliper is elastic, it is wrong to press the outer caliper over the outer circle, and it is not allowed to clamp the caliper horizontally, as shown in the following figure. For large external calipers, the measuring pressure of sliding over the outer circle of the part by its own weight is too large. At this time, hold the calipers for measurement, as shown in the figure below

3. Use of internal calipers When measuring the inner diameter with internal calipers, make sure that the connecting lines of the measuring surfaces of the two clamp feet are perpendicular to the axis of the inner hole, that is, the two measuring surfaces of the clamp feet should be the two ends of the inner hole diameter. Therefore, the measuring surface of the lower clamp foot shall be stopped on the hole wall as a fulcrum during measurement

The upper clamp foot gradually probes outward from the orifice slightly to the inside, and swings along the circumference of the hole wall. When the distance that can swing along the circumference of the hole wall is the minimum, it means that the two measuring surfaces of the inner clamp foot are at the two ends of the inner hole diameter. Then slowly move the caliper from outside to inside to check the roundness tolerance of the hole

Measure the inner diameter with the internal caliper that has been taken from the steel ruler or the external caliper

It is to compare the tightness of the internal caliper in the part hole. If the inner caliper has a large free swing in the hole, it means that the size of the caliper is smaller than that in the hole; If the internal caliper cannot be put into the hole, or it is too tight to swing freely after being put into the hole, it means that the size of the internal caliper is larger than the hole diameter. If the internal caliper is put into the hole, there can be a free swing distance of 1~2mm according to the above measurement method. At this time, the hole diameter is exactly the same as the size of the internal caliper. Do not hold the caliper for measurement

It is difficult to compare the tightness of the internal caliper in the part hole, and the caliper will be deformed, resulting in measurement error

4 Scope of application Caliper is a simple measuring tool. Because of its simple structure, convenient manufacturing, low price, convenient maintenance and use, it is widely used in the measurement and inspection of parts with low requirements, especially for the measurement and inspection of forging and casting blank size. Caliper is the most appropriate measuring tool. Although the caliper is a simple measuring tool

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

When the diameter of the root shaft is large, the difference between the shaft diameters is only 0.01mm. An experienced teacher

It can also be distinguished. For example, when measuring the internal hole size with the internal caliper and the external diameter micrometer, the experienced teacher is fully confident of measuring the high-precision internal hole with this method. This internal diameter measurement method is called“ Internal snap percentage gauge”, Use the internal caliper to read the accurate size on the outside diameter micrometer

Then measure the inner diameter of the part; Or it can be clamped in the hole to adjust the tightness of the contact with the hole, and then the specific size can be read on the outside diameter micrometer. This measurement method is not only a good way to measure the inner diameter when there is a lack of precision inner diameter measuring tools, but also for the inner diameter of a part, as shown in Fig. 1-9, it is difficult to use precision inner diameter measurement because there is a shaft in its hole, so the problem can be solved by measuring the inner diameter with an internal caliper and an outer diameter micrometer