Key component technology for ultra-precision machining machines (4)

Table 2 Table of various feed mechanism characteristics

kind advantage Disadvantage positioning accuracy
Feed screw Sliding screw Easy to manufacture, but requires grinding technology and good attenuation Need to pay attention to crawling After careful grinding, the positioning accuracy is 0.01μm
Pre-processing needs to reach 0.1μm
Ball screw Has been standardized, easy to get (C0) level Bad attenuation,
Need to pay attention to crawling,
Pay attention to small vibration
Up to 0.01 μm
Pre-processing needs to reach 0.1μm
Hydrostatic screw High precision and low attenuation Large equipment, many auxiliary equipment and maintenance, oil pollution Quite good positioning accuracy is 0.01μm, usually
0.03 μm
Gas static screw High precision and easy maintenance Difficult to process 0.01μm
Friction drive High precision and simple structure Need appropriate preloading and management The current target is 0.01μm
Piezoelectric element Ultra-fine resolution (sub-nanometer, nm) Small stroke (several micrometers to ten micrometers) Nm,

5 Environmental conditions

There are three environmental conditions for ultra-precision machining. One is pollution. Ultra-precision machine tools must be placed in a clean, clean room to fully exploit their advantages. The cleanliness of the room is expressed in terms of the amount of dust above 0.5 μm in one cubic foot. The working environment of an ultra-precision machining machine should be 20,000 to 3000 or less.

The second is vibration. The disturbance of environmental vibration not only causes the vibration of the machine body, but also causes the relative vibration displacement between the cutting tool and the workpiece. The latter will directly reflect the accuracy and surface quality of the machined part. Therefore, ultra-precision machining machines must be equipped with excellent vibration isolation devices. At present, most of the foreign ultra-precision machining machines use vibration isolation systems with air springs as vibration isolation components, and have achieved good vibration isolation effects. This is mainly because the air spring has a lower bearing capacity while having a higher load carrying capacity. The low stiffness of the spring allows the vibration isolation system to achieve a lower natural frequency, away from environmental interference frequencies, and improve vibration isolation. After theoretical analysis and calculation, the HCM-I ultra-precision machining machine adopts a straight-contracted membrane structure with an internal and external angle of change of 0°. In this way, not only the linearity of the spring stiffness is good, but also the structure is simple, which facilitates the manufacture of the mold and the installation and adjustment of the device.

Table 3 Plan Objectives to Improve Ultra-Precision Machining Accuracy

Cause of error Japanese precision (μm) POMA plan value (μm)
Position detection accuracy Positioning accuracy yaw, pitch, tilt straightness axial runout radial runout spindle extension spindle drive heat influence workpiece clamping shape accuracy (comprehensive precision) 0.005
0.005
(0.05")
0.02
0.005
0.005
0.025
0.01
0.025
0.025
0.05
0.05
0.01
0.02
0.02
0.02
0.02
0.05
0.01
0.05
0.05
0.1
Note: POMA is based on the premise that the shape accuracy of a large aspherical mirror with a diameter of 800 mm is increased to 0.1 μm.

The third is temperature. The machining of ultra-precision machining machines must be carried out in a constant temperature room. The temperature change during the machining process will cause the machine tool movement accuracy to drop and the specified machining accuracy cannot be obtained. In order to solve this problem, we usually start from two aspects. One is to choose the right part material. The materials used in the ultra-precision machining machine and the candidate materials are alumina ceramic, cast iron, steel, invar, granite, resin concrete and zero. Expanded glass. From the practical point of view, almost all of the HCM-I ultra-precision machine tools use granite. The second is to maintain constant temperature control. After summing up domestic and foreign experience, Harbin Institute of Technology put forward the concept of “effective cold flow rate”. On this basis, the temperature control accuracy of the ultra-precision constant temperature oil supply system has reached the world advanced level.

6 Conclusion

The birth of sub-micron ultra-precision machine tool HCM-I marks that China's ultra-precision machining research has entered the international ranks. However, after all, it has not gone out of the laboratory, there is no commodification, and it is necessary to double the efforts to catch up with the international advanced level. Table 3 lists the accuracy target values ​​of the US POMA and the future accuracy target values ​​considered by Japanese scholars.

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NSK Bearing

NSK manufacture a full range of deep groove ball bearings. These bearings are the most common type and are used in a wide variety of applications.

Single-Row Deep Groove Ball Bearings
Pressed Steel Cages
Single-row deep groove ball bearings are the most common type of rolling bearings. Their use is very widespread.
In addition to open type bearings, these bearings often have steel shields or rubber seals installed on one or both sides and are prelubricated with grease. Also, snap rings are sometimes used on the periphery. As to cages, pressed steel ones are the most common.
For big deep groove ball bearings, machined brass cages are used.
Machined cages are also used for high speed applications.

Extra Small Ball Bearings and Miniature Ball Bearings
Miniature and instrument ball bearings can be divided into two basic types, deep groove and angular contact. The first (deep groove) can be further divided into the following five classes depending on their design details:

Standard type
Flanged outer ring
Extended inner ring
Expanded type in which one ring has a radial thickness that is larger than normal compared with the bearing width.
Thin section type in which both rings are extra thin in the radial direction. They can also be classified as: open, shielded, or sealed.
Maximum-Type Ball Bearings
Maximum-Type ball Bearings contain a larger number of balls than normal deep groove ball bearings because of filling slots in the inner and outer rings. Because of their filling slots, they are not suitable for applications with high axial loads. types of bearings have boundary dimensions equal to those of single-row deep groove ball bearings of Series 6200 and 6300 respectively. Besides the open type, ZZ type shielded bearings are also available. When using these bearings, it is important for the filling slot in the outer ring to be outside of the loaded zone as much as possible. Their cages are pressed steel.

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