Method

Factors Affecting Ultra-Precision Machining

Factors Affecting Ultra-Precision MachiningFactors Affecting Ultra-Precision Machining

Requirements for temperature, humidity, purification and anti-vibration.

Constant Temperature

The ambient temperature can be controlled at 20 ℃ ± (1 ~ 0.02) ℃, and even reach (20 ± 0.0005) ℃ according to the machining requirements. The method of achieving constant temperature can adopt the overall constant temperature of the special constant temperature room (room) and the partial constant temperature of the constant temperature cover. The overall constant temperature can adopt multi-layer suite structures such as large constant temperature room, middle constant temperature room, small constant room, underground constant temperature room, etc., and gradually get room temperature with higher and higher control accuracy; meanwhile, the temperature distribution on the floor is used to make the temperature distribution even.

Due to the temperature difference in seasons, in order to save energy, the standard room temperature can be set at 23 ℃ in summer and 17 ℃ in winter. Facts have proved that such a regulation is feasible.

Constant Humidity

In the constant temperature room, the general humidity is maintained at 55% to 60% to prevent the corrosion of the machine, the expansion of the water absorption of the stone, and the zero drift of some instruments such as the laser interferometer.

Purification

In ultra-precision machining, the dust in the air may scratch the processed surface, and sometimes the size of the dust may be larger than the abrasive particles, which will damage the processed surface and prevent the abrasive machining from achieving the desired effect. Therefore, the air Clean treatment.

The main method of air purification is filtration, and the room to be cleaned is called a clean room or ultra clean room. Purification can be divided into overall purification and local purification. Local purification such as purification workbenches, purification chambers, etc. The method is to pass positive pressure clean air into the purification zone to prevent outside air from entering to maintain the purification effect and is more economical. Since people are going to work in the clean room, the clothes, hair, and skin of the staff will be brought into the dust. Therefore, before entering the clean room, you should change special clothes, even special dust-free clothes. Work in the room to control the influence of dust generated during personnel activities and maintain cleanliness.

Since the dust with a diameter greater than 0.5μm has the greatest influence on the surface quality of ultra-precision machining, the number of dusts with a diameter greater than 0.5μm per cubic foot is usually used to represent the air purification level. The general purification requirements are 100 to 10000.

Anti-Vibration

In precision machining and ultra-precision machining, the impact of vibration on the machining quality is relatively large. The source of vibration comes from two aspects. One is the vibration generated by machining equipment such as machine tools, such as the unbalance of rotating parts, and the insufficient rigidity of parts or components.

From Outside The Machining Equipment

The vibration introduced by the foundation, such as the vibration generated when working adjacent to the machine tool, requires the machining equipment to be placed on an anti-vibration foundation with anti-vibration grooves and vibration isolators, and an air spring (pad) can be used to isolate low-frequency vibration , Flexible and convenient, with good results.

Ultra-Precision Machining Sometimes Require Some Special Working Environment

Such as anti-magnetic, anti-static, anti-electron radiation, anti-acoustic wave, anti-X-ray, anti-atomic radiation, etc., according to the needs of the overall environment or local environment treatment.

Process Design

Process design often has a critical global impact on machining quality, productivity, and cost, especially in precision machining and ultra-precision machining. In addition to the general machining principles and laws, the following issues should also be considered.

  1. Step by step, strict requirements. Workpieces for precision machining and ultra-precision machining must have a good basis for rough, semi-finished, and refined machining. The raw materials and the machining quality of each process should be strictly required. The workpieces must not be damaged during handling and storage. The working environment should be clean and tidy.
  2. Correctly select the positioning reference and control the machining quality of the positioning reference. The machining surface itself is often used as the positioning reference to ensure the uniformity of the machining allowance; the principle of mutual reference is used to ensure the surface with the position accuracy requirements; to ensure the machining quality and cleanness of the positioning reference, pay attention to the positioning before each process Whether the datum is damaged and clean, when the coarse and fine processes are switched, the grinding and positioning datum process should be arranged.
  3. Pay attention to the clamping deformation of the workpiece and the deformation during machining. In ultra-precision machining, due to the small amount of removal, there may be situations where the installation deformation is greater than the machining allowance. It is best to achieve deformation-free clamping. For some parts that are easily deformed during machining, thin-shaped parts can be adsorbed by vacuum suction cups; irregular-shaped parts can be liquid rubber, asphalt, and industrial wax. Current variants (a paste-like liquid, which can solidify when a current is applied, and recover the liquid state after power-off) and other materials adhere (melt) the workpiece to the fixture for machining; of course, the positioning structure can also be used to improve the workpiece itself Of stiffness.
  4. Pay attention to the arrangement of heat treatment process. The internal stress and deformation in precision machining and ultra-precision machining have a serious impact on the machining accuracy of parts, and it has always been a problem. Therefore, during the conversion of rough, semi-finish, and finishing stages, artificial aging or natural aging should be arranged to eliminate internal Stress, heat treatment process should also be arranged before the key process.

Fixture design

The fixture is an important part of the mechanical machining technology system. Whether the design is reasonable and the manufacturing quality can ensure that it has a great impact on the machining of the workpiece. Fixtures and cutters (grinding tools), auxiliary tools, inspection tools, etc. constitute the process equipment.

In precision machining and ultra-precision machining, precision general fixtures, such as turntables, chucks, etc. can be selected, but in most cases, precision special fixtures are designed and often become the key to parts machining. Therefore, the following points should be noted when designing and manufacturing .

  1. Design and manufacture according to the requirements of precision fixture design. The positioning element of the fixture should have high precision and be resistant to wear, and the clamping part of the fixture and the machine tool should also have high positioning accuracy. The entire fixture should have high rigidity and accuracy retention.
  2. Pay attention to the clamping deformation of the workpiece. The clamping force should be sufficient, but the workpiece must not be deformed, especially for those parts with relatively low rigidity, some special clamping methods or clamping device structures can be used. If multiple auxiliary support structures are used, a fixed torque wrench is used for manual clamping, and over-positioning structures are used. Finite element analysis can be used for some necessary calculations.
  3. The positioning reference surface on the jig is “processed in situ”. In order to ensure the quality of the fixture, after the fixture is clamped on the machine tool, the machine tool can be used for “in-situ machining” to ensure the high-precision clamping of the workpiece. At this time, a sufficient amount of machining allowance should be left on the surface to be processed by the fixture. For example, when turning the surface of a magnetic disk with ultra-precision, a vacuum chuck is used for clamping. After the vacuum chuck is clamped on the spindle of the machine tool, the positioning surface is ultra-precisely turned to ensure the plane of the positioning surface of the vacuum chuck itself Degree and its perpendicularity to the axis of the machine tool spindle, which not only ensures the tightness of the disk adsorption and the required clamping force, but also avoids the deformation of the disk due to the uneven positioning surface of the vacuum suction cup during clamping , To ensure the uniformity of the flatness and thickness of the disc. In addition, it also makes the machining margin of the disc even during machining, reducing the error reflection.

Skills

At present, the machining quality and level of precision machining and ultra-precision machining are guaranteed to a certain extent by the skills of technicians and operators. The accuracy of the machining equipment, the accuracy of the testing instruments, the experience and skill level of the technicians and operators determine the machining accuracy of the workpiece. Therefore, people’s technical level, knowledge, experience and operation proficiency are often important factors that affect the quality and efficiency of precision machining and more precise machining. Special emphasis should be placed on the operator here. He must not only have superb skills, but also have a broad knowledge, and understand mechanical, electronic, physical, chemical, and computer technologies to be competent for this job. E.g,

Precision grinding is the key technology in the sharpening of diamond tools and the manufacture of precision hydrostatic bearings, which are closely related to human skills.

In addition to the above-mentioned influencing factors, there are still issues such as market demand and decision-making, organizational management and system, standardization and standardization, data and development tools, etc., all of which are included in “other” factors.

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