How to inspect mechanical parts


Parts inspection is an important link in the process of mechanical processing, but also to ensure that the product meets the quality requirements of the necessary steps, machinery and parts through the inspection, can have enough understanding of the overall quality of the product, so as to decide whether to approve the shipment or use.
A.   Main contents of parts inspection
For the parts that are relatively precise or used for key parts, they are generally inspected one by one. The main contents can be divided into the following aspects:
1) Check the geometric accuracy of parts
Geometric accuracy includes dimensional accuracy and shape position accuracy, but the maintenance work is sometimes not the pursuit of the geometric size of a single part, but the relative matching accuracy, which is often an important part of the inspection work.
Shape and position accuracy in the maintenance of common roundness, cylindricity, concentricity, coaxiality, parallelism and verticality, etc.
2) Surface quality inspection
The inspection of the surface quality of the parts in the maintenance work is not limited to the inspection of the surface finish, but the inspection of the surface of the used parts whether there is scratch, burn and hair drawing and other defects.
3) Check the mechanical properties
According to the characteristics of mechanical maintenance, in addition to the mechanical properties of parts material in the hardness of a check, other indicators are generally not checked, but the manufacturing and maintenance of parts in the process of certain performance, such as balance, spring stiffness, can not be ignored.
4) Inspection of hidden defects
In the manufacturing process of parts, there may be original defects such as slag inclusion and cavity, and micro-cracks may occur in the process of use. These defects cannot be found directly from the general observation and measurement, but it may have serious consequences for the machinery, therefore, in the mechanical maintenance, it is necessary to carry out a comprehensive inspection of some parts purposefully.
B. Methods of parts inspection
There are many ways to inspect parts, and new inspection techniques are developing with each passing day. But from the actual point of view of mechanical maintenance work, it can be summarized as follows:
1) Sensory examination
This is a method to identify the technical status of the parts by the intuitive feeling of the inspectors, without checking the equipment. This method is simple, and in the mechanical maintenance, it is easy to distinguish the defects of a large number of disassembled parts. However, this method can not be used to inspect the parts requiring high accuracy, and the inspectors are required to have rich experience.
2) Instrument and tool inspection method
A great deal of inspection work is carried out with instruments and tools. Due to the different principles and types of instruments and tools, it can be divided into general measuring tools, special measuring tools, mechanical instruments and meters, optical instruments, electronic instruments and so on.
3) Physical examination method
This is a method of detecting the technical status of parts by using physical quantities such as electricity, magnetism, light, sound and heat. The realization of this method is also combined with instrument and tool inspection method. This method is usually used to inspect the hidden defects in the parts without damaging the parts themselves, now commonly known as nondestructive testing. In recent years, nondestructive testing technology is developing day by day. Magnetic particle method, penetration method, ultrasonic method and ray method are widely used in production.
C. Sensory inspection of parts
1) Visual examination
The main contents of visual inspection, such as parts fracture and macro crack, obvious bending and distortion, parts surface burn and abrasion, serious wear and tear, can usually be directly identified with the naked eye. In order to improve the accuracy of visual examination, magnifying glass can be used in some cases. In order to make up for the lack of visual inspection of the interior of some walls, the optical fiber can also be used as a light conduction endoscope for inspection.
2) Auditory examination
It is a long - standing inspection method to judge the defect of mechanical parts by the auditory ability of human ear. Routine inspection of rolling stock has followed this method for decades. The inspection of the workpiece is knocked, when the parts without defects, the sound is clear, and when there is an internal shrinkage hole, the sound is low, if the internal crack, the voice hoarse, therefore, according to the different sound, you can judge the parts without defects.
3) Tactile examination
You can feel the surface condition of a part by touching it with your hand. Relative shaking of the matching parts can feel its matching condition. Running machinery, through the touch of its parts, can feel its heating condition, so as to judge its mechanism condition.
D. Measures to ensure the quality of parts inspection
1) Strictly grasp the technical standards
Most of the mechanical parts and mating parts have stipulated technical standards, which is the main basis of the inspection work, in the absence of reliable according to the need to modify the standard, to strictly comply with these standards, never allowed to lower the standard, and the unqualified parts into use.
2) Select inspection equipment according to the requirements of inspection objects
In addition to the inspection equipment should be selected according to the nature and scope of the inspection items, special attention should also be paid to the requirements of precision. For example, when measuring the length, when the accuracy is required in the range of 1mm, the steel ruler can be used; When the accuracy is required in the range of 0.1mm, vernier calipers are generally used; When the accuracy of 0.01mm is required, micrometer and dial indicator are required. When the accuracy of 0.001mm is required, the comparator is required. If the accuracy of the inspection equipment is lower than the accuracy required by the tested object, it is unable to meet the quality inspection requirements, we must pay attention to prevent.
3) Improve the technical level of inspection operation
The technical level of inspection operation directly affects the inspection accuracy. Whether it is the self-inspection of the maintenance personnel or the inspection of the full-time inspection personnel, the operator is required to master the inspection equipment used and the inspection requirements of the inspection objects. Therefore, attention should be paid to the improvement of inspection technology, special training should be carried out for the use of special and important inspection equipment, and all kinds of inspectors should be relatively stable.
4) Reduce inspection errors
Any inspection results will inevitably have errors, which are influenced by many aspects. Therefore, it should be prevented and eliminated from the following aspects:
▶ The inspection equipment has its own accuracy level, it should be corrected regularly, and pay attention to maintenance and storage, so that it can keep the due accuracy.
▶ Error caused by temperature. Among the environmental factors that affect the inspection accuracy, the influence of temperature is the biggest. For example, the effect of line expansion in the length check, the variability caused by the temperature change in the electronic device, etc., should be corrected in the more precise measurement.
▶ Most of the errors caused by improper operation or inaccurate reading are accidental errors, so you can take the average of multiple measurements, which can greatly reduce its degree of error.
5) Establish and improve reasonable inspection rules and regulations
Reasonable inspection system is to do well in the inspection work of the organization guarantee, to establish the post responsibility system, clear responsibility, everyone check, and suggest a certain acceptance and handover system and measurement calibration system.

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