Application of Counting Adjustment and Isometric Sampling Method in Steel Barrel Inspection

Application of Counting Adjustment and Isometric Sampling Method in Steel Barrel Inspection

First, the preface
The counting adjustment type sampling inspection scheme is a sampling method commonly used in the world. It is a kind of adjustment of the degree of lenient according to the actual changes in product quality, and minimizes two types of misjudgments (ie, misreceived: misqualification of failure is accepted as acceptable, and received. Mistaken: misqualified as unqualified , do not accept) probability sampling program, generally suitable for large-scale continuous production occasions, especially suitable for continuous mass production of continuous mass production.
The isometric sampling program is also called mechanical sampling or systematic sampling. It is a sampling method in which all units (or products) in the survey population are numbered in a certain order, and then samples are taken mechanically at regular intervals for investigation.
Combining these two schemes, it is scientific and effective to control, inspect and accept the quality of steel drums.
Second, the proposal of the program
There are more than 200 steel drum production plants in China, with an annual output of more than 30 million steel drums and an annual output value of more than 600 million yuan. Steel drum is a kind of packaging, which is widely used in industry, agriculture, national defense, foreign trade, and food sectors, and plays an important role in socialist construction. Steel drum manufacturing generally has to go through blanking, stamping, coiling, seam welding, ribbing, crimping, painting, pressure testing and other processes. Steel drum production is generally produced in a large batch with a high degree of mechanization. At present, due to raw materials, equipment, management and other reasons, the quality of steel drums in China is not very stable. The quality of products of most manufacturers has not yet met the requirements stipulated in the International Maritime Dangerous Goods Transportation Regulations. There is still a big gap between the barrel factories in developed countries and the world. Therefore, how to stabilize product quality and improve the quality of steel drums has become a problem that must be carefully studied and solved. We believe that the cause of unstable steel drum quality, in addition to raw materials, equipment, processes, etc., steel drum production process can not be controlled is an important reason. In recent years, according to the characteristics of steel drum production, we have designed a steel drum counting adjustment type and an isometric type sampling inspection program. Through the experiments in production, we have achieved good results in monitoring, testing and controlling the production process.
Third, the characteristics of the program
The counting adjustment type sampling plan has dynamic characteristics. For a certain product batch quality, instead of using a single solution, a different set of rigorous schemes (normal, rigorous, and relaxed) are used, and they are linked by a set of transformation rules. Under normal circumstances, the normal detection scheme is used. When the inspection result indicates that the product quality is obviously better, it is switched to the use of the relaxation inspection scheme. When the inspection result indicates that the product quality is significantly deteriorated or the production is unstable, the conversion is tightened to use. Inspection plan. These three schemes are converted according to the quality of the products, so as to achieve supervision and control of the steel drum production process.
The isometric sampling program is reliable and easy to use. In this method, since all the parts can be included in the sample to a certain extent, it is better to ensure that the extracted samples are evenly distributed in the whole, thereby increasing the representativeness of the sample; and, since the steel drum production is generally continuous Pipeline production, sample extraction is carried out at a certain production interval, therefore, the work of taking samples in the production process can also be carried out by the machine, which is convenient.
When the two schemes are combined to test the quality of the steel drum, the roles are different. The role of the adjusted sampling plan is to determine the number of samples, the number of samples received, the distance to the collection, and the conversion according to (Table 2) according to the output of the steel drum or the number of stacking codes and the quality level of the steel drum. Rules are converted. The role of the isometric sampling program is to provide a method for sample extraction. Combining the two together to form a complete inspection program has great significance for the quality inspection of steel drums. First of all, these two schemes are based on the sampling characteristic curve, which conforms to the binomial distribution and the Pwson distribution. The sampling error is small and reliable, and it can be used to estimate the overall quality level. Secondly, due to the adjustment sampling scheme It includes a set of solutions that can be converted to each other. Therefore, it can be used not only for the acceptance of steel drums, but also for the dynamic control and supervision of the steel drum production process. Finally, since the number of samples can be determined without complicated formulas (direct Check the table 1), easy to use and easy to implement.
Therefore, it is scientific and simple to test and control the quality of steel drums by counting adjustment type and isometric sampling inspection scheme.
For example, our factory previously conducted a total of 18 tests on the finished products according to the “Quality Inspection Rules for 200-liter closed steel drums” of the Food Department of the Ministry of Commerce. Since the sampling is carried out after all the products are produced, it is equivalent to "post-mortem examination", which does not control the quality of the production process, and it is extremely inconvenient to carry out manual number sampling after product stacking. Using the "counting adjustment and equidistant sampling method", the product is electronically or mechanically counted (for example, the counting device of the Shanxi Hydraulic Machinery Factory crimping machine), which is randomly intercepted, convenient for sampling, and the inspection is carried out simultaneously with the production, product quality information. Timely feedback has played a role in supervising, controlling and ensuring the quality of the production process. At the same time, the quality of the product in the process is manually controlled and controlled, and the quality of the process product is also guaranteed.
Fourth, the implementation of the program
(a), steps:
1. First determine the quality level of the steel drum and the quantity of the batch (parent). The general quality level can be based on the national standard of the product, the ministerial standard, the enterprise standard or the standard agreed by the user; the batch (maternal) quantity is given in Table 1 according to the principle of large batch size and large sample capacity. For example, the general quality level of our steel drum is 2.5% (non-conforming product rate), and the batch size is 151-280.
2. According to the batch (maternal) number and quality level, check the number of samples (n), the number of received {Ac), and the number of rejects (Re) for the normal, tightened, and relaxed plan of the fruit quality level.
3. Extract samples with an isometric sampling plan
(1) All the stacked and flat steel drums are numbered from left to right (or from top to bottom, or in order). Make the same number of signatures based on the quantity produced in the steel drum and number them sequentially. If you sign one, you can take one (do not put it back). If the number is 97, the first steel drum is 97, and one is taken (not put back). If the number is 88, then the second. The number of barrels is number 88... and so on, all steel drums are numbered. The steel drums being produced may not be numbered, and the sample buckets are randomly intercepted according to the sampling distance.
(2) Calculate the distance of the sample to be extracted according to the formula h=N/n (N in the formula indicates the number of the parent. n indicates the number of samples. h indicates the sampling distance), and then the sample is taken according to the sampling distance. In the production process, qualified manufacturers can also mechanically count and mechanically sample by sampling distance.
4. Test the sample taken according to the steel drum quality inspection project. Determine the quality status and direction of the steel drums.
(1) First, judge whether the batch of steel drums is qualified or not. If the non-conforming product is less than or equal to the number of receiving, the batch of products is judged to be qualified and accepted. If the non-conforming product is greater than or equal to the rejected number, the batch of steel drums is judged. Not qualified or undergo a full inspection.
(2) After several batches of steel drums are tested, they are converted according to the rules in Table 2 to control the quality of the products in the production process.
(II) Example: Set a factory to produce steel drums N=250 per shift, and the required quality level is AQL=2.5%, and try to develop a sampling plan for the batch of steel drums.
Steel drum adjustment type sampling plan (Table 1)

batch
(N)
range

sample
(n)

Acceptable quality level (failure rate)

1%

1.3%

positive
often

plus
strict

put
width

normal

Strict

Relaxation

normal

Strict

Relaxation

Ac

Re

Ac

Re

Ac

Re

Ac

Re

Ac

Re

Ac

Re

91-150

20

20

8

0

1

0

1

0

1

1

2

1

2

1

2

151-280

32

32

13

1

2

1

2

1

2

1

2

1

2

1

2

281-500

50

50

20

1

2

1

2

1

2

2

3

1

2

1

3

501-1200

80

80

32

2

3

1

2

1

3

3

4

2

3

2

4

(Continued from Table 1)

batch
(N)
range

sample
(n)

Acceptable quality level (failure rate)

2.5%

4%

positive
often

plus
strict

put
width

normal

Strict

Relaxation

normal

Strict

Relaxation

Ac

Re

Ac

Re

Ac

Re

Ac

Re

Ac

Re

Ac

Re

91-150

20

20

8

1

2

1

2

1

2

2

3

1

2

1

3

151-280

32

32

13

2

3

1

2

1

3

3

4

2

3

2

4

281-500

50

50

20

3

4

2

3

2

4

5

6

3

4

2

5

501-1200

80

80

32

3

6

3

4

2

5

7

8

5

6

3

6

1. According to N=250 and AQL=2.5%, look up the sample capacity (n), the number of received (Ac), and the number of rejects (Re) under various degrees of severity. See (Table 3)
Steel drum counting adjustment and isometric sampling plan (Table 3)
Known batch N = 250, quality level AQL = 2.5%

Degree of tolerance

Sample size, n

Receive number, Ac

Rejected number, Re

Sampling distance, h=N/n

Normal inspection

32

2

3

8

Strict inspection

32

1

2

8

Relaxation test

13

1

3

19

That is to say, from the 250 steel drums produced by the class, under the normal inspection conditions, the number of samples is n=32, the number of acceptances is Ac=2, the number of rejections is Re=3, and the sampling interval is h=8. One of the steel drums in production is pumped at intervals of eight. A total of 32 barrels were taken for testing. If the test result is within 2 or less, it is determined that the batch of barrels is qualified and accepted; if the test result is more than 3, the batch is judged to be unqualified, not accepted, and after being processed by the production workshop. Re-delivery inspection. Under the condition of relaxed inspection, when the number of non-conforming products sampled is 2, it exceeds the number of receptions 1 and the number of rejections 3, and it can be judged that the batch of steel drums is qualified, but in the next batch, it returns to the normal inspection, that is, Eligible for the condition.
2. After detecting a number of barrels, convert them according to the rules of the table, see Table 2.
Steel drum sampling conversion rules (Table 2)

Conversion direction

Conversion condition

Normal → tighten

Two of the six consecutive batches failed

Normal → relaxed

All of the following conditions are met:
(1) Qualified for six consecutive batches;
(2) Stable production;
(3) The competent department considers it necessary.

Relaxation → normal

One of the following conditions is met:
(1) A batch is unqualified;
(2) A batch of qualified conditions;
(3) Production is abnormal or stagnant;
(4) The competent department considers it necessary.

Strictly tightened → suspended

Ten consecutive batches stayed in strict inspection

V. Problems that need attention and research
(1) Practical conditions of the program:
1. The production of steel drums must be continuous production line production. The larger the batch size, the better the practical effect.
2. Production must be in a stable state. This program cannot be used when the test, trial production, and production are not in stable anger.
3. Quality management personnel must have certain mathematical statistics and quality management qualities; workers should strengthen overall quality management education and emphasize the three-level inspection system.
4. The key items of product quality should be tested. For example, the steel drum leakage test should be inspected by the production workshop and the inspectors should take random inspections.
(2) For the treatment of non-conforming products:
1. Disposal of unqualified batches: For the unqualified batches that have been tested and judged, in principle, they are returned to the production workshop, and after the unqualified products are picked out, the inspectors perform the sampling inspection according to the above methods. If the number of unqualified products is less than or equal to the number of receipts, the batch is considered to be qualified. If the number of unqualified products is still greater than or equal to the number of rejects, the batch is considered unqualified. And the production workshop is required to conduct a full inspection.
2. Disposal of non-conforming products: The non-conforming products found during the inspection process shall be returned to the production workshop for repair, replacement, repair, and replacement of the products, and shall also be accepted before being accepted.
(3) Simplification of inspection items:
The workload of steel drum quality inspection is largely determined by the number of inspection items and the difficulty of inspection items. Therefore, strengthening process quality control and simplifying inspections have become issues that should be carefully studied.
1. Modification of GB325: The inspection items are determined according to product standards. The implementation of GB325 has played an important role in guiding the production of barrels for several years, but many barrel mills have found some problems that need to be modified during implementation. For example, there are too many routine inspection items; the product grade is too strict, the inspection workload is large; the film thickness and the adhesion of the paint film are not easy to detect. The formulation of product standards must first meet the principle of improving product quality and facilitating implementation. Therefore, when modifying the standard, you should seriously consider the problem of simplifying the inspection items.
2. In the process of steel drum production, the quality inspection of each process and the inspection of finished products should be made of special work card measuring tools and tooling. For example, the test press, the tooling that guarantees the distance between the two holes, the blanking tooling, the threaded plug ring, and the like.
In general, the “counting adjustment and isometric sampling program” is a scientific and simple method based on total quality management. It will play a certain role in promoting the quality level of steel drums and improving the scientific management level of steel drum production.

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