Schneider instruments measure error factors

When measuring, because various elements will constitute a small number of errors, these elements need to be understood and effectively solved, so that the errors in the whole measurement process can be reduced to the smallest. When measuring, the main errors that constitute errors are system errors and random errors, and system errors have the following conditions: misreading, miscalculation, parallax, scale error, wear error, contact force error, deflection error, cosine error, Abbe error, thermal deformation error, etc. The size of the system error is constant in the measurement process, and can be calculated by calculation or experimental methods, that is, it can be predicted, and it can be corrected or adjusted to reduce it. These elements are grouped into five broad categories, which are described below:

Schneider instruments measure error factors

Schneider instruments measure error factors

1. The human element

Because of the errors caused by human elements, including misreading, miscalculation and parallax. Misreading often occurs in measuring tools such as vernier rulers and centimeter cards. The vernier scale is likely to misread a small reading, such as 10.02 mm or 9.98 mm at 10.00 mm. The centimeter card scale is easy to misread the size of a pitch, such as 10.20 mm is often misread as 10.70 mm or 9.70 mm. Miscalculations often occur when calculating faults or entering faulty data. Parallax often occurs when the direction of reading the measurement value is different or the scale plane is not in the same plane, and the difference between the two scale planes is about 0.3-0.4 mm, and if the reading scale is not perpendicular to the scale plane, the error amount will occur. In order to eliminate this error, the manufacturer of the measuring tool plans the carving of the vernier to be equal to or close to the same height as the engraving of the ruler, (the vernier is engraved with an arc-shaped composition and the ruler is almost the same height, the vernier is concave V-shaped and the ruler is convex V-shaped, thus constituting two carved equals.)

2. Gage elements

Because the errors caused by the elements of the measuring tool include scale errors, wear errors and uncorrected elements before use. Whether the scale division is accurate or not, it is necessary to calibrate and trace it through more sophisticated instruments. After a period of use, a considerable degree of wear and tear will occur in the measuring tool, so it is necessary to be calibrated or repaired before it can be used again.

3. Strength Factor

Because of the contact force used in the measurement or the deflection error caused by the contact. According to Hooke's law, when measuring the scale, if the measuring shaft is in contact with the machine with a certain measuring force, the shaft and the machine will be partially or fully elastically deformed, in order to prevent this elastic deformation, the shaft and the machine should be made of the same material. Secondly, according to Hertz's law, if both the axle and the machine are made of steel, the amount of error caused by its elastic deformation

When the workpiece is measured with a gauge, the gauge is fixed on the support, and the bracket will constitute elastic deformation because of the measured force, as shown in Figure 2-4-3, the cross-sectional secondary moment of the length is , the long pillar is , and the longitudinal elastic coefficients are respectively , so when the measured force is P, the deflection is . To prevent such errors, the pillars can be enlarged and the length of the axis extension can be minimized. In addition, larger gages such as centimeter cards, vernier rulers, straight gauges and long gauge blocks have twists and turns due to their weight and load. In general, when the fulcrum azimuth of the two end faces parallel to the vertical line is 0.577 full length, the two end faces can remain parallel, and this fulcrum is called Airey Points. The fulcrum of the line scale gauger is 0.5594 in the direction of its full length, and its full-length zigzag error is *small, which is called Bessel Points here

4. Measure the elements

When measuring, errors caused by poor instrument planning or placement include cosine error, Abbe error, etc. The cosine error occurs when the measurement axis and the surface to be measured are skewed at the viewpoint, as shown in Figure 2-4-5, and the error amount is , which is the actual measurement length. Usually, cosine errors occur in both measurement directions, and it is necessary to be especially careful. For example, when measuring the inner hole, the radial measurement scale needs to be *large scale, and the axial measurement needs to take *small scale. In the same way, when measuring the outside, it is also necessary to pay attention to its correct orientation. It is necessary to choose the anvil and the surface of the workpiece to be measured carefully, such as the spherical anvil when the surface of the workpiece to be measured is flat, and the planar anvil should be selected when the workpiece is cylindrical or spherical. Abbe' Law states that the axis of the measuring instrument and the axis of the workpiece to be measured should be in constant line. Otherwise, an error occurs, and this error is called Abbe's error. In general, if the axis of the measuring instrument and the axis of the workpiece to be measured cannot be together, the interval should be shortened as much as possible to reduce the error value. If the vernier ruler measures the workpiece as an example, as shown in Figure 2-4-6, the error is, so if you want to reduce the measurement error of the vernier ruler, you need to reduce the angle formed by the gap between the ruler and the vernier ruler and should be as close to the scale mark as possible when measuring. If the scale measures the workpiece as an example, as shown in Figure 2-4-7, the probe of the gauge is spherical, the workpiece is cylindrical, and the two axes have an offset amount, and the amount of contact error is . If the probe and workpiece of the gauge are flat, if the two planes are skewed, the amount of contact error is as shown in Figure 2-4-8, and this error is called sinusoidal error. Figure 2-4-9 shows the error anatomy diagram of the cam in the mechanism planning, in order to reduce wear, the end of the follower is often planned into a ball or cylinder with a radius of , and the pressure angle between the two is , thus causing the error to .

5. Environmental elements

When measuring, depending on the environment or place, the errors that may be constituted include thermal deformation error and random error are *significant. Thermal deformation errors usually occur due to room temperature, human contact and the temperature of the workpiece after processing, so it is necessary to control the temperature and humidity, not to touch the workpiece and measuring tools by hand, and the workpiece is measured after it is cooled after processing. However, in order to shorten the processing time in the processing in real time measurement, it is necessary to consider the thermal expansion coefficient of various materials as compensation, in order to cope with the temperature of the thermal expansion coefficient of the material caused by the error. The coefficients of thermal expansion of various commonly used materials are shown in Table 2-4-2. In general, it is necessary to use the following formula: CMR: The length of the workpiece at 20°C.

Schneider instruments measure error factors

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