What are the forms of quality inspection?

[Wu Feiyu]

The methods of quality inspection can be classified according to different characteristics.

Divided by inspection quantity
1. Total inspection
Full inspection refers to the inspection of a batch of products to be inspected. This method is generally more reliable. At the same time, it can provide more comprehensive quality information. If you want to get 100% qualified products, the only feasible way is to conduct a full inspection, or even more than one full inspection. However, we should also consider the possibility of missing inspection and wrong inspection. Total inspection has its inherent disadvantages:

First, the workload of inspection is large;
Second, the inspection cycle is long;
Third, the inspection cost is high;
Fourth, more inspectors and inspection equipment are required;
Fifth, a large rate of missed detection and false detection;
Due to the long-term repeated inspection fatigue and boring work of inspectors, the limitation of technical inspection level of inspectors and the rapid wear of inspection tools, it may lead to a large missed inspection rate and false inspection rate. According to foreign statistics, the missed detection rate and false detection rate may sometimes reach 10% to 15%.
Sixth, full inspection is not suitable for destructive inspection items.

Generally, the full inspection is applicable to the following occasions:
First, products and parts with high precision requirements;
Second, the size parts that have a great impact on the next or subsequent process;
Third, manual operation is more important and the quality is unstable;
Fourth, some products (including parts) and processes with small batch and no reliable quality assurance
Fifth, when the selective sampling scheme is adopted, 100% re inspection and screening shall be carried out for the unqualified batches.

Sampling inspection
Sampling inspection refers to the sampling plan formulated in advance according to the principle of mathematical statistics. Randomly select some samples from a batch of products for inspection, judge whether the whole batch of products are qualified according to the inspection results and the specified judgment criteria, and decide whether to accept or reject the batch of products, or take other laborious methods.

The main advantage of sampling inspection is that it obviously saves the inspection workload and inspection cost, shortens the inspection cycle, and reduces the inspection personnel and equipment. In particular, only destructive sampling inspection can be adopted. The main disadvantage of sampling inspection is that there is a certain risk of misjudgment. For example, qualified is judged as unqualified, or unqualified is misjudged as qualified. Although the use of mathematical statistics theory reduces the risk and improves the reliability to a certain extent, as long as the sampling inspection method is used, this risk can not be absolutely avoided.

Sampling inspection is applicable to the following occasions:
First, produce products or processes with large batch, high degree of automation and relatively stable product quality;
Second, products and processes with destructive inspection;
Third, the acceptance inspection of outsourced parts and purchased parts in batches;
Fourth, some products and processes with high production efficiency and long inspection time;
Fifth, inspect products and processes with too high cost;
Sixth, a small number of products that fail to pass the inspection will not cause heavy losses.

Divided by quality characteristic value
1. Counting test
Counting inspection includes inspection and counting inspection. It only records the unqualified number (or points) and does not record the specific measured value after inspection. In particular, some quality characteristics are difficult to be expressed by values, such as whether the appearance of the product is beautiful, whether the food tastes delicious, etc. they can only judge whether they are qualified through the senses. There are also some quality characteristics, For example, although the size of products can be expressed by numerical value or measured, in mass production, in order to improve efficiency and save manpower and cost, it is often only used to check whether the “passing” and “not passing” gauges are within the upper and lower tolerance range, that is, only distinguish qualified and unqualified products, and do not measure the actual size.

2. Metrological Inspection
Metrological Inspection is to measure and record the value of quality characteristics, and judge whether it is qualified according to the comparison between the value and the standard. This kind of inspection exists in a large number and widely in industrial production.

Classification by inspection nature
1. Physical and chemical test
Physical and chemical inspection is carried out by using some measuring tools or instruments and equipment with the help of physical and chemical methods, such as micrometer, vernier caliper, microscope, etc. the characteristics of physical and chemical inspection are usually that specific values can be obtained with small human error. Therefore, when conditions permit, physical and chemical inspection should be adopted as far as possible.

2. Functional test
The quality of human organs can be evaluated by the inspection of human organs. For example, the shape, color, taste, smell, scar, aging degree, etc. of the product usually rely on human visual, auditory, tactile, olfactory and other sensory organs for inspection, and judge whether the quality is good or bad or whether it is qualified. Functional tests can be divided into two categories:

The first category is the test of hobby form and function, such as whether it is beautiful or fragrant. This kind of test, which is judged by people’s feeling itself, often varies from person to person, because everyone’s hobbies may be different. For example, everyone has different aesthetic views, and the judgment results of the same thing may be different from person to person. In other words, such tests are often subject to strong subjective will.

The second type, analytical functional test, analyzes and judges the characteristics of the tested object through human sensory organs. If you want to detect the heating degree of the main shaft after the operation of a certain equipment, if there is no applicable thermometer, you should judge the approximate temperature through the touch of the inspector’s hand. This is not based on people’s hobbies, but on people’s experience.

Divided by the integrity of the inspected object after inspection
1. Destructive inspection
The inspection of some products is destructive, that is, the products no longer exist or damaged after inspection and can no longer be used. For example, the performance of military products such as shells, parts after heat treatment, the life test of electronic tubes or other components, and the strength test of cloth materials are all destructive tests. Destructive inspection can only take the form of sampling inspection. The main contradiction is how to realize the unity of reliability and economy, that is, to seek a certain reliable sampling inspection scheme with the least number of inspections.

2. Non-destructive inspection
As the name suggests, non-destructive inspection means that the inspection object is still intact after being inspected, which will not affect its service performance. For example, most inspections, such as the size of mechanical parts, belong to non-destructive inspection. Now, due to the development of nondestructive testing, the scope of non-destructive testing is expanding.

Divided by inspection location
1. Fixed inspection
The so-called fixed inspection is to set up a fixed inspection station in the production workshop. This kind of inspection station can be a public inspection station in the workshop. After processing, the products in a section, group or work site are sent to the inspection station for inspection in turn. It can also set up between processes of mobile or automatic lines or “line” or line terminals. This kind of inspection station is specialized and constitutes an organic part of the production line. Only some special inspection is fixed.

There are some disadvantages in setting up a fixed inspection station in the workshop. Fixed inspection is applicable to some inconvenient or precise instruments, which is conducive to the establishment of a better working environment and the use and management of inspection tools or equipment. However, from the psychological point of view, fixed inspection stations are easy to cause antagonism between inspectors and production workers, and production workers regard products sent for inspection as “pass”. At the same time, in the inspection station, it is easy to cause confusion in the storage of parts to be inspected and to be inspected, to be inspected and finished inspection, finished inspection and finished inspection, and occupy a large space. Therefore, the fixed inspection should be handled according to the specific situation.

Flow inspection
Mobile examination is also a clinical examination, that is, the inspectors go to the work area for examination. Flow inspection has the following advantages:
(1): it is beneficial to improve the relationship between inspectors and production workers. Because when inspectors go to the work area for inspection, if they have a good attitude, point out the problems in the work operation and reduce the occurrence of nonconformities, the production workers realize that inspectors do not just check their work, but serve themselves, which reflects the gay cooperative relationship and reduces their own economic losses caused by waste products;
(2): it has a preventive effect. The inspectors go to the working area according to the processing time sequence, so it is easy to find the changes in the production process in time and prevent the emergence of batch waste products;
(3): it can save the handling and delivery of inspected parts and prevent the occurrence of bumps, scratches and other damages;
(4): it can improve the production efficiency and save the waiting time of operators in the inspection station;
(5): when inspectors inspect in front of production workers, operators are easy to understand the quality problems, easily believe and accept the inspection results of inspectors, and reduce mutual contradictions and mistrust.

Purpose division of time
1. Inspection of acceptance nature
The inspection of acceptance nature is to judge whether the products are qualified, so as to decide whether to accept the batch or the product. Acceptance inspection is a widespread form, such as incoming inspection of raw materials, outsourced parts and purchased parts, inspection of semi-finished products before warehousing, and inspection of finished products before delivery.

2. Inspection of supervision nature
The purpose of supervision inspection is not to determine whether the products are qualified, but to accept or reject the batch of products. But to control the state of the production process, that is, to verify whether the production process is in a stable state. Therefore, this inspection is also called process inspection to prevent a large number of nonconformities. For example, the patrol inspection in the production process and the regular inspection when using the control chart belong to this kind of inspection. The result of random inspection is only used as a signal to monitor and reflect the status of the production process, so as to decide whether to continue production or take corrective and adjustment measures for the production process.

• This topic was modified 2 years, 8 months ago by Wu Feiyu.

[la ling]

Full inspection, sampling inspection, counting inspection, physical and chemical inspection, destructive inspection, etc.

1. Total inspection
Full inspection is generally applied to: important, critical and valuable products; Items that have a decisive impact on subsequent process processing; Processes and products with seriously uneven quality; Non interchangeable assembly parts; Products with small batch and without sampling inspection. For example, it is suitable for the refrigeration effect of freezer, not suitable for the life test of TV, the strength test of steel pipe and the thread of a large number of nuts.

2. Sampling inspection
Sampling inspection, also known as sampling inspection, is a statistical method and theory to randomly select a small number of products (samples) from a batch of products for inspection, so as to judge whether the batch of products are qualified or not. It is different from comprehensive inspection in that the latter needs to inspect the whole batch of products one by one and pick out the unqualified products, while sampling inspection infers the quality of the whole batch of products according to the inspection results of the products in the sample.

If the inference result shows that the batch of products meets the pre-defined qualification standards, they will be accepted, otherwise they will be rejected. Therefore, a batch of products that are considered qualified through sampling inspection may also contain some unqualified products. The main sampling methods include simple random sampling, systematic sampling and stratified sampling.

3. Counting test
For each unit product in the sampling group, only qualified products or unqualified products are determined by measuring the inspection items, so as to infer the unqualified product rate of the whole batch of products. This kind of inspection is called counting inspection. The count value and quality data of counting inspection cannot be taken continuously, such as the number of nonconformities, the number of defects, the number of defects, etc.

4. Physical and chemical test
The methods of quality inspection can be divided into physical and chemical inspection and functional inspection according to the nature of inspection. Physical and chemical inspection, also known as “instrument inspection”, is the inspection carried out by using some measuring tools or instruments and equipment, such as micrometer, dial indicator, inspection gauge, microscope, etc. with the help of physical and chemical methods.

5. Destructive inspection
A destructive inspection is an inspection form or method that changes the form of the inspected product and destroys the use function or performance of the product to a certain extent.
Non-destructive inspection refers to the inspection form or method in which the form of the inspected product has not changed and the performance and use function of the product have not been affected after the product has been inspected.

[Li wu]

Quality inspection methods can be classified according to different marks.
(1) It is divided into full inspection according to the inspection quantity; Sampling inspection.
(2) It is divided into counting inspection according to quality characteristic value; Metrological Inspection.
(3) It is divided into physical and chemical inspection according to inspection techniques and methods; Sensory examination; Biological examination.
(4) It is classified as destructive inspection according to the integrity of the inspected object after inspection; Non destructive inspection.
(5) Classified as fixed inspection according to the inspection location; Flow inspection.
(6) According to the inspection purpose: production inspection, acceptance inspection, supervision inspection, verification inspection and arbitration inspection
(7) According to the supply-demand relationship: first-party inspection, second-party inspection and third-party inspection.

[wy zw]

Several forms of quality inspection:
1. Check the original quality certificate;
Under the conditions of large purchase batch, stable quality of the supplier and sufficient reputation, quality inspection is often closely combined with product verification. Is to check the original quality certificate. Specific original quality certificates such as quality certificate, certificate of conformity, inspection (test) report, etc. are used to identify the quality.

Check the original quality certificate to determine its quality status. Generally:
A. It can confirm that the original documents of the supplier are complete;
B. The certificate quality characteristic data are complete and meet the requirements of technical performance and relevant standards;
C. Signature and seal procedures shall be complete;
D. The physical quantity is consistent. After confirmation, perform the signing procedures of the specified procedures, and distribute the vouchers to the relevant functional management departments. If necessary, physical quality inspection shall be carried out for the final performance of the product and the decisive material and quality characteristics.

2. Physical inspection
The full-time inspector of the unit or the external inspection unit entrusted by the unit shall issue inspection records after observation, test and measurement according to the specified procedures and requirements as evidence of product conformity.

3. Send personnel to the factory (resident) for acceptance
The purchaser shall send personnel to the supplier for on-site inspection and acceptance of its products and product components. Supervise the operation process and quality control of product formation and identify the quality of finished products, verify that the supplier’s quality is under control, and provide relevant inspection reports to confirm that the inspection results meet the specified requirements. The original vouchers for release and delivery are complete and the products are qualified, and they are accepted.

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