To achieve quality in any organization, the collective work of all the areas and functions that are performed in them is required, being INSPECTION AND TESTING a vital aspect to achieve it, especially in order to prevent the occurrence of defects in the processes.
In our economic entities the inspection is established, but based on production experience and not taking into account each and every one of the aspects to be considered in order to establish it.
Precisely the objective of this work has been to collect all the criteria and considerations that exist around this topic in the literature and to elaborate a procedure that allows specialists dedicated to this task, to make designs with a scientific argument, the inspection system in processes and also diagnose and improve the existing inspection systems, detecting the deficiencies they present and proposing solutions.
technology-for-the-development-of-an-inspection-system-1This procedure has had practical application in several entities, as an example we can name the accumulator factory in Manzanillo, the Guantánamo bicycle factory, achieving with the implantation to reduce the percentage of defective production in different operations of the process and with it the losses that this entailed, thus increasing the final quality of the finished product.
In an industrial company, inspection is the procedure by which the specifications of raw materials and finished products are checked, as well as the operating regime, process parameters, etc.
Therefore, both:
- The characteristics of the product: For acceptance purposes (in-process, in-process and final inspection). The quality of the process for the purposes of regulation or process control (preventive).
In the early stages of development, quality control was based on the inspection of the finished product, but in this way the inspection was faced with a fait accompli, separating good and defective products.
At present, due to the massiveness of production and the complexity of the manufacturing processes, the losses that would entail separating good productions from defective ones would be high, so the inspection is organized based on the principle of prevention.
The main effort for quality in some countries and entities that are at the forefront in achieving quality is basically directed at the stages of research and development of new high-quality products and technologies, all of which is summarized in the criterion that it is more beneficial from the economic, social and other points of view, to make the product right from the beginning.
In this sense, they have paid special attention to the automation of the projection and manufacturing activities of the products, including those related to the process control, which causes a displacement of the quality control personnel and the inspection of the finished product and the process towards previous stages (development and perfection of the product).
In relation to the entry inspection, this tends to be considerably reduced as the suppliers' agreements and quality commitments are consolidated, to the extent that the materials received are guaranteed, the entry inspection has evolved since the 100% inspection until no inspection when you have absolute confidence in what you receive.
Regarding the process inspection:
- Process control is ensured through the use of statistical methods Emphasis is placed on the control of manufacturing process parameters The worker is trained in technical and statistical control skills, placing him in self-control Inspection personnel is reduced moving on to carry out verification activities and audits. Searches for low-cost automated means of control.
Regarding the final inspection:
- Audits are carried out on the final product just before its delivery, which is reduced as good control is achieved during the process.
So the trend is to reduce inspection, as automation levels increase:
- That they establish agreements and stable relationships and mutual recognition supplier-producer. The state of self-control of the operators is guaranteed.
But our production processes do not have the real conditions to adapt to these trends, all of which justifies that our country is not downplaying the inspection function, which will reduce its role to the extent that we are able to counteract the limitations that we have and the implementation of a quality management system is achieved.
RESULT OF WORK.
The concept of quality according to ISO 8402/94 refers to all the characteristics of an entity (activity, process, product, organization, people system, or some combination of them), which influence its ability to satisfy expressed or implicit needs.
To achieve quality in every entity, a set of activities must be carried out, independent of each other! and in a logical projection. These characteristics are collected in the spiral of quality and is known as the quality function throughout the company, which conceives the work of all departments considering the department that delivers the product as its client.
The activities included in the quality progress spiral are:
- Marketing and market research. Product design and development. Process planning and development. Purchasing. Production and service offers. INSPECTION AND TESTING. Packaging and storage. Sales and distribution. Installation and commissioning. Technical assistance and after-sales services. After sales Disposal and recycling at the end of the useful life.
In the context of interacting activities in an organization, marketing and design should be emphasized as especially important aspects in determining and defining customer needs and expectations as well! as other product requirements.
After the product is designed, the products that will later be offered to customers must be planned and developed, taking action progressively to verify if the planned is fulfilled, taking here! inspection or testing is of great importance.
Inspection according to ISO 8402/94 are activities such as measuring, examining, testing or verifying with a standard one or more characteristics of an entity and comparing the results with the specified requirements to establish whether conformity has been achieved in every feature.
Based on this concept, we can say that the operations to be executed in the inspection process are:
1.- Interpretation of the specification.
2.- Sampling.
3.- Measurement of the characteristic.
4.- Comparison of what is interpreted with what is measured.
5.- Judgment of conformity.
6.- Registration of the data obtained.
In order to carry out these operations, there must be an inspection system whose design consists of the following elements:
I-) Select the quality characteristics to inspect.
It is impossible to submit all the characteristics of an operation to be inspected since it would imply a gigantic supervision apparatus, which is uneconomical, so the following two criteria are taken into account:
1.- The importance or impact on the quality of the product.
2.- Its impact on costs.
Those fundamental characteristics based on the economic losses that would result from their abnormal behavior and the effect on the quality of the product would be selected.
II-) Determination of the places where the inspection points will be established.
For this it is important to study the organization of the technological process (through the OTIDA diagrams), which will help us to determine in which phases or operations of the process it is necessary to establish inspection points.
It is important that the inspection be carried out in the place closest to where the characteristic is generated.
According to Jurán, inspection points should be established:
- Before terminating a costly and irreversible operation During machine setup operations During high-quality or high-cost operations that require high accuracy In some cases at natural points of observation in the process In operations requiring adjustments frequent.
At INC 22:80 it is stated:
- When the quality characteristics to be inspected are masked in the following operations When the defects that may appear in a given operation can cause critical defects in the finished production When, regardless of the aforementioned economic balance, the average percentage of defectives in a given operation point is very high.
Whenever possible, an estimate of the average defective percentage should be made in the operation or phase of the process and subsequently make a balance between the cost of establishing the quality inspection at that point and the cost that would entail letting the defective units pass.
It is also necessary to assess whether inspection points should be established in operations of: handling, storage, packaging, and dispatch of materials.
III-) Determination of the form of inspection.
The form of inspection can be:
* by attributes.
* by defect count.
* by variables.
It is important to know the characteristics of the inspection object, as well as the characteristics of each of these forms to select the most appropriate one.
By attributes: the units are considered defective or not observing one or more characteristics.
By counting defects: the number of defects found in each unit is recorded (it is used mainly in continuous materials.)
By variable: for quantitative characteristics that can take any value on a scale of continuous values recorded using some means of measurement.
The advantages and disadvantages offered by one or another form of inspection must be taken into account in order to select the most convenient one.
IV-) Determination of the type of inspection to be carried out at the inspection point according to the number of units of products to be inspected.
It must be defined whether the total units are selected (100% inspection) or only a representative part of the products in process (inspection by sampling.)
There are different criteria from different authors on (What is the most convenient type of inspection to be executed at each inspection point?)
The 100% inspection is used for the final testing of special or complex products since it allows the consumer to be delivered products free from defects.
Be reserved to check quality characteristics that may have a great impact on the products that are manufactured and whose failure to obtain may lead to the occurrence of failures or defects with great risks or serious consequences for consumers or significant economic losses for the company. It is also used when the capacity of the process does not allow meeting the product specifications.
100% inspection is often impractical or downright uneconomic when testing is excessively costly, destructive on a large scale. Sampling inspection has a number of psychological advantages over 100% inspection. Inspector fatigue caused by repetitive operations can be a serious obstacle to a good 100% inspection, it is more economical and requires less time to perform.
That is why investigations were carried out in the field of probability theories and statistics, reaching the conclusion that to make decisions about the quality of the production in process and finished, there is no need to carry out a 100% inspection on all items, but it is enough to inspect only a part of the lot, that is, a sample. Thus came the inspection by sampling.
According to Kaouru Ishikawa in the text Quality Control Guide indicates the situations in which inspection by sampling is necessary:
1.- Destructive tests.
2.- Inspection of long products.
3.- Inspection of large quantities.
Other criteria are:
a.- When you want to lower inspection costs.
b.- When you want to encourage the manufacturer or consumer.
c.- When there are many items or inspection areas.
d.- In erratic processes where the batches are fully compliant or not.
Another criterion for deciding whether to use 100% inspection or sampling is the breakeven point, which consists of determining the breakeven point from the cost of inspecting an item or quality characteristic (QI) and the cost of leaving pass that defective item (CD) and then compare it to the average defective percentage (100p).
CI _ add Pi
BEP = - * 100% 100p = --- * 100%
CD K
K: number of lots (10 or more)
So yes: It is recommended:
_
Yes 100p approx BEP Inspection by sampling.
_
If 100p >>>>> BEP Inspection 100%.
_
Yes 100p <<<<< BEP But the process is erratic, inspection by sampling. For the purpose of protection.
_
If 100p <<<<< BEP But the process is stable, no inspection is performed.
V-) Determination of the number of units that will compose the sample.
In the case of process inspection for preventive purposes, when the inspection method is variable, the sizes of our most employees are between 1 and 25 units. Samples of 2 or 3 units are little used due to their low sensitivity, being used only when the cost of measurements is very high.
Samples of size 5 facilitate calculations of means, compared to 4 or 6.
Sample sizes 10 through 25 are used when high graph sensitivity is desired and sample sizes greater than 25 units are used exceptionally.
When the form of inspection is by attribute, the size of the samples and the interval between them should be such that approximately 5% of the production is inspected.
In very massive processes that do not present frequent difficulties or the percentage of defective production is not serious, this percentage can be reduced to less than 5% where it is recommended that there should be at least 25 defectives in each sample to establish an adequate behavior of the process.
n> = 25 P: historical or normed average.
p
In the process that often suffers variations in the quality of its production, or when the cost increase caused by the presence of defective units is very high, it may be considered economical to select up to 10%.
Hansen argues that there must be at least one defective item in each sample taken (like NC 92 08:80) therefore
n> = 1
p
This criterion is recommended for destructive testing as it offers the smallest sample size.
Jurán raises n> = 9- 9p; used in any case
p
Another criterion starts from:
If the equilibrium point has been calculated in a process, there is a table recommended by Dr. Nelson Espinosa in his book, Direction of Quality, p. 333 that allows, starting from the BEP, to determine the AQL and with it, to determine the sample size.
VI-) Determine the frequency with which the sample will be extracted.
There are several factors to consider when establishing the inspection frequency.
- The sample size (n) The characteristics of the technological operation: knowing the production volume (number of articles produced in one hour) and the process conditions or its behavior (erratic, stable and controlled), the inspection frequency is determined according to the following table:
Frequency (hours)
Process condition.
Production rate / h Erratic Stable Controlled
Less than 10 8 8 8
10 - 19 4 8 8
20 - 49 2 4 8
50 - 99 1 2 4
100 or more 0.5 1 2
- The consequences of deviations in quality characteristics: if the consequences are very serious, it is necessary to reduce the inspection intervals The nature of the product, taking into account its importance The result of previous inspections The nature of the production process: includes the result of analysis of the production process from the points of view:
1.- The capacity of the process.
2.- The dominant factor: it is the one that fundamentally dominates in a process and can be:
Predominant system Frequency
Machine setup Every time the machine is setup.
Periodic Time.
Components According to their arrival.
Random workmanship.
Information According to arrival.
VII-) Establishment of measurement, test or analysis methods to verify the quality characteristics selected at each inspection point.
It is important to analyze the factors that determine the uniformity of measurements:
- Observer.
- The object of measurement.
- the means of measurement.
- The measurement methods.
- The environmental conditions.
- Calculation methods.
Hence, the variability of an observation can be expressed as the sum of the variation introduced by each of these aspects.
E 2 = E 2 + E 2 + E 2 + E 2 + E 2 + E 2
obs prod oper mm method cond. ambi.
But taking into account, according to their physical nature, the errors that can be made in the measurement can be random or systematic, and that the latter, since they can be predicted, can be eliminated by taking corrective measures, so work must be done to reduce random errors. that cannot be predicted and for this, experiments can be carried out that allow defining:
- The most accurate measurement.
- The most precise operator.
- The most accurate MM.
In this element, the application of methods to detect gross or gross errors in the set of measurements should also be considered.
VIII-) Determination of the forms of registration and processing of the information resulting from the quality inspection at each inspection point.
A set of specific models will be designed and implemented in correspondence with the purpose of the inspection; that is, if the purpose is preventive, control charts will be established so that the model will respond to their requirements according to the type of chart to be used, and if the purpose is of acceptance, the model will be established in correspondence with the sampling plan. to use.
When designing these models, the forms of information processing to be used will be taken into account: manual, mechanized or automated.
The flow to be followed by said information, those in charge of filling out the models, their review and approval, and the analysis of the information, as well as decision-making with a view to control, must also be defined. All of which must have the corresponding methodological basis.
CONCLUSIONS.
As a trend in countries that are at the forefront in achieving quality, inspection tends to be reduced, however in our country where there are no high levels of automation, nor the necessary resources to guarantee the state of self-control of the operators. inspection continues to be a very important aspect.
For the design or improvement of the inspection of the development processes, the procedure explained in the context of the work which allows specialists in the entities to carry out these tasks with scientific argument and diagnose the current behavior of said activities in their companies, detecting the deficiencies that it presents and providing solutions.
BIBLIOGRAPHY.
- Deming, Edwards W. Quality, Productivity and Competitiveness: The way out of the crisis / Edwards W. Deming. _ _ Mexico: Ed. Díaz de Santos, 1990. _ _ 532 P. Espinosa Pena, Nelson. Quality Management / Nelson Espinosa Pena…. _ _ Havana: Ed. ISPJAE, 1986. _ _ 593 p. Fariñas Molina, Jorge. Quality Control in the Sugar Industry / Jorge Fariñas Molina. _ _ La Habana: Ed. ISPJAE, 1986. _ _ 319 p.García Padrón, R. Standardization, Metrology and Quality Control in the Light Industry / R. García Padrón…. _ _ Havana: Ed. People and Education, 1991. _ _ 947 p.Hansen, Bertran. Quality Control: Theory and Practice / Bertran _ _ Barcelona: Ed. Hispano-Europa, 1980. _ _ 570 p.Jurán, JM Quality Control Manual / J. M Jurán. _ _ 2.ed. _ _ Barcelona: Ed. Reverte, 1987. _ _ 1509 p.Jurán, JM Manual de Control de la Calidad / J. M Jurán._ _ 4.ed. _ _ Spain: Ed. Mc Graw-Hill, 1993. _ _ 1794 p.Jurán, JM Planning and analysis of Quality / J. M Jurán. _ _ Barcelona: Ed. Reverte, 1981. _ _ 736 P. Pyzdez, Thomas. What Every Engineer Should know about Quality Control? / Thomas Pyzdez. _ _ New York: Ed. Marcel Dekker, 1989. _ _ 221 p. Rodríguez Inzua, Armando. Quality Control / Armando Rodríguez Inzua…. _ _ Havana: Ed. ISPJAE, 1984. _ _ 219 p.INC 22:80. NNMCC. Methodological Indications for the Organization and Development of Quality Inspection in Industrial Companies. _ _ Havana: Ed. CEN, 1980. _ _ 35 p.NC 92-04: 1979. Quality Control. Inspection by attributes and by counting defects. Acceptance sampling plans. _ _ Havana: Ed. CEN, 1979. _ _ 124 p.NC 92-09-1: 1984. Quality Control. Uniform distribution of random numbers.Boards. _ _ Havana: Ed.CEN, 1984. _ _ 10 p.NC 92-09-2: 1984. Quality Control. Random sample selection methods. _ _ Havana: Ed. CEN, 1984. _ _ 16 p.
