- Detail

Basic knowledge of technical measurement

1. General concept of measurement

technical measurement is mainly a technology that studies the measurement and inspection of geometric parameters of parts

the so-called "measurement" is the process of comparing a physical quantity to be determined with a standard quantity as a measurement unit. It includes four factors: measurement object, measurement method, measurement unit and measurement accuracy

"inspection" has a broader meaning than measurement. For example, this report contains the "forward-looking statement" as defined in item 21e of the securities and exchange act of 1934. In these cases, the concept of measurement cannot be used

II. Length unit datum and size transmission system

in order to ensure the accuracy of measurement, it is necessary to establish a unified and reliable measurement unit first. The basic length unit of the metric system is meter (m), and the metric unit commonly used in mechanical manufacturing is millimeter (mm). In precision measurement, microns are often used（ μ m) Is the unit, and the conversion relationship between them is:

1m=1000mm 1mm=1000 μ M

using the speed of light as the length reference can achieve enough accuracy, but it is not convenient to be directly applied to the dimension measurement in production. In order to ensure that the length reference value can be accurately transmitted to the production, a system must be established in terms of organization and technology, which is called the size transmission system. For example, the table shows the size transfer chart in China, which reflects the whole process of size transfer in China

table size transmission system

III. classification of measurement tools

measurement tools can be divided into the following four categories according to their measurement principles, structural characteristics and uses:

1. Reference measuring tools: ① fixed value reference measuring tools; ② Variable value measuring tool

2. general measuring tools and meters: it can be used to measure any value within a certain range. According to the structural characteristics, it can be divided into the following:

(1) fixed marking measuring tool

(2) vernier measuring tool

(3) screw micrometer measuring tool

(4) mechanical measuring instrument

(5) optical measuring instrument

(6) air momentum instrument

(7) electric measuring instrument

3. Limit gauge: it is a special measuring tool without scale

4. Inspection measuring tools: it is a combination of measuring tools, measuring instruments and other positioning elements, which is used to improve measurement or inspection efficiency and measurement accuracy. It is widely used in mass production

IV. classification of measurement methods

1. Due to the different methods of obtaining the measured results, the measurement methods can be divided into:

direct measurement method

indirect measurement method

2. According to the different reading values of the measured results, the measurement methods can be divided into:

absolute measurement method (full value measurement method)

relative measurement method (differential or comparative measurement method)

3. According to whether the surface of the measured part is consistent with the measurement, the key of this technology is to reduce energy consumption The workers who reduce the production cost have mechanical contact, and the measurement methods can be divided into:

contact method

non contact method

4. According to the number of simultaneous measurement parameters, they can be divided into:

comprehensive measurement method

itemized measurement method

5. According to the different effects of measurement on the mechanical manufacturing process, the measurement methods are divided into:

passive measurement

active measurement

v Measurement indicators of measurement tools

measurement indicators: refers to the main performance of measurement tools that should be considered in measurement. It is the basis for selecting and using measurement tools

1. Scale interval C: referred to as scale, it is the actual distance between two adjacent scribes on the scale

2. Division value I: the measured value represented by each scale on the scale

3. indication range of the scale: the measurement value that all scales on the scale of the meter can represent

4. measurement range: ① the indication range of the scale ② the maximum and minimum size range of the whole measuring tool or meter

5. sensitivity: the minimum change of the measured dimension that can cause the change of the indicated value of the gauge. The sensitivity indicates the sensitivity of the meter to small changes in the measured value

6. indication error: the difference between the reading on the measuring tool or gauge and the actual value of the measured dimension

7. measuring force: the contact force between the measuring surface of the measuring tool or instrument and the measured workpiece during the measurement process

8. amplification ratio (transmission ratio): the meter refers to the ratio between the linear displacement (or angular displacement) of the needle and the change of the measured size. This ratio is equal to the ratio of the scale interval to the division value

VI. measurement error

1. Measurement error: the difference between the measured value and the real value

i.e δ= X – Q

where: δ— Measurement error

x - actually measured measured

q - the true size of the measured value

since X may be greater than or less than Q, δ It can be positive, negative, or zero. In this way, the above formula can be written as

q=x ± δ

2. Cause of measurement error (i.e. composition of measurement error)

(1) error of measuring instrument

(2) error of reference part

(3) deformation error caused by measuring force

(4) reading error

(5) error caused by temperature change

3. Classification of measurement error

(1) system error: error with certain change regularity

(2) random error: error with irregular change, The characteristics and treatment of random errors will be introduced later

(3) gross error: some large errors caused by carelessness in measurement (such as reading error can make all experimental results more accurate, firm error, calculation error, etc.) or sudden changes in environmental conditions (impact, vibration, etc.). (end)

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