Inspection is an important tool to achieve quality concept. It is necessary to assure confidence to manufacturer and aims satisfaction to customer. Inspection is an indispensable tool of modern manufacturing process. It helps to control quality, reduces manufacturing costs, eliminate scrap losses and assignable causes of defective work.
The inspection and test unit is responsible for appraising the quality of incoming raw materials and components as well as the quality of the manufactured product or service. It checks the components at various stages with reference to certain predetermined factors and detecting and sorting out the faulty or defective items. It also specified the types of inspection devices to use and the procedures to follow to measure the quality characteristics.
Inspection only measures the degree of conformance to a standard in the case of variables. In the case of attributes inspection merely separates the nonconforming from the conforming. Inspection does not show why the nonconforming units are being produced.
Inspection is the most common method of attaining standardization, uniformity and quality of workmanship. It is the cost art of controlling the production quality after comparison with the established standards and specifications. It is the function of quality control. If the said item does not fall within the zone of acceptability it will be rejected and corrective measure will be applied to see that the items in future conform to specified standards.
1. Objectives of Inspection
- To detect and remove the faulty raw materials before it undergoes production.
- To detect the faulty products in production whenever it is detected.
- To bring facts to the notice of managers before they become serous to enable them discover weaknesses and over the problem.
- To prevent the substandard reaching the customer and reducing complaints.
- To promote reputation for quality and reliability of product.
2. Purpose of Inspection
- To distinguish good lots from bad lots.
- To distinguish good pieces from bad pieces.
- To determine if the process is changing.
- To determine if the process is approaching the specification limits.
- To rate quality of product.
- To rate accuracy of inspectors.
- To measure the precision of the measuring instrument.
- To secure products-design information.
- To measure process capability.
3. Types of Inspection
- Floor inspection
- Centralized inspection
- Combined inspection
- Functional inspection
- First piece inspection
- Pilot piece inspection
- Final inspection
3.1. Floor Inspection
In this system, the inspection is performed at the place of production. It suggests the checking of materials in process at the machine or in the production time by patrolling inspectors. These inspectors move from machine to machine and from one to the other work centres. Inspectors have to be highly skilled. This method of inspection minimize the material handling, does not disrupt the line layout of machinery and quickly locate the defect and readily offers field and correction.
- Detection of errors of the source reduces scrap and rework.
- Correction is done before it affects further production, resulting in saving cost of unnecessary work on defective parts.
- Material handling time is reduced.
- Job satisfaction to worker as he can’t be held responsible for bad work at a later date.
- Greater number of pieces can be checked than a sample size.
- Does not delay in production.
- Delicate instruments can be employed.
- Measuring or inspection equipment have to be recalibrated often as they are subjected to wear or dust.
- High cost of inspection because of numerous sets of inspections and skilled inspectors.
- Supervision of inspectors is difficult due to vibration.
- Pressure on inspector.
- Possibility of biased inspection because of worker.
- Heavy products are produced.
- Different work centres are integrated in continuous line layout.
3.2. Centralised Inspection
Inspection is carried in a central place with all testing equipment, sensitive equipment is housed in air-conditioned area. Samples are brought to the inspection floor for checking. Centralised inspection may locate in one or more places in the manufacturing industry.
- Greater degree of inspection due to sensitive equipment.
- Less number of inspectors and tools.
- Equipment needs less frequency of recalibration.
- Cost of inspection is reduced.
- Unbiased inspection.
- Supervision of inspectors made possible.
- No distraction to the inspector.
- Defects of job are not revealed quickly for prevention.
- Greater material handling.
- High cost as products are subjected to production before they are prevented.
- Greater delay in production.
- Inspection of heavy work not possible.
- Production control work is more complicated.
- Greater scrap.
3.3. Combined Inspection
Combination of two methods whatever may be the method of inspection, whether floor or central. The main objective is to locate and prevent defect which may not repeat itself in subsequent operation to see whether any corrective measure is required and finally to maintain quality economically.
3.4. Functional Inspection
This system only checks for the main function, the product is expected to perform. Thus an electrical motor can be checked for the specified speed and load characteristics. It does not reveal the variation of individual parts but can assure combined satisfactory performance of all parts put together. Both manufacturers and purchasers can do this, if large number of articles are needed at regular intervals. This is also called assembly inspection.
3.5. First Piece or First-off Inspections
First piece of the shift or lot is inspected. This is particularly used where automatic machines are employed. Any discrepancy from the operator as machine tool can be checked to see that the product is within in control limits. Excepting for need for precautions for tool we are check and disturbance in machine set up, this yields good result if the operator is careful.
3.6. Pilot Piece Inspection
This is done immediately after new design or product is developed. Manufacturer of product is done either on regular shop floor if production is not disturbed. If production is affected to a large extent, the product is manufactured in a pilot plant. This is suitable for mass production and products involving large number of components such as automobiles aeroplanes etc., and modification are design or manufacturing process is done until satisfactory performance is assured or established.
3.7. Final Inspection
This is also similar to functional or assembly inspection. This inspection is done only after completion of work. This is widely employed in process industries where there is not possible such as, electroplating or anodizing products. This is done in conjunction with incoming material inspection.
4. Methods of Inspection
There are two methods of inspection. They are: 100% inspection and sampling inspection.
4.1. 100% Inspection
This type will involve careful inspection in detail of quality at each strategic point or stage of manufacture where the test is involved is non-destructive and every piece is separately inspected. It requires more number of inspectors and hence it is a costly method. There is no sampling error. This is subjected to inspection error arising out of fatigue, negligence, difficulty of supervision etc. Hence, completer accuracy of influence is seldom attained. It is suitable only when a small number of pieces are there or a very high degree of quality is required. Example: Jet engines, aircraft, medical and scientific equipment.
4.2. Sampling Inspection
In this method randomly selected samples are inspected. Samples taken from different patches of products are representatives. If the sample proves defective, the entire concerned is to be rejected or recovered. Sampling inspection is cheaper and quicker. It requires less number of Inspectors. It is subjected to sampling errors but the magnitude of sampling error can be estimated. In the case of destructive test, random or sampling inspection is desirable. This type of inspection governs wide currency due to the introduction of automatic machines or equipments which are less susceptible to chance variable and hence require less inspection, suitable for inspection of products which have less precision importance and are less costly. Example: Electrical bulbs, radio bulbs, washing machine etc.
5. Drawbacks of Inspection
Following are the disadvantages of inspection:
- Inspection adds to the cost of the product but not for its value.
- It is partially subjective, often the inspector has to judge whether a products passes or not.
- Fatigue and Monotony may affect any inspection judgment.
- Inspection merely separates good and bad items. It is no way to prevent the production of bad items.
Source: KumarAnil, Suresh N. (2009), Production and operations management, New Age International Pvt Ltd; 2nd Ed. edition.