The technical key of the diamond lathe Compared with the mirror milling machine, the mechanical structure of the diamond lathe is more complicated and the technical requirements are more stringent. In addition to high motion stability, high positioning accuracy and repeatability must be achieved. When the mirror surface is milled, the spindle requires only high axial motion accuracy and low radial motion accuracy. Diamond lathes must have high axial and radial motion accuracy to reduce the impact on the shape accuracy and surface roughness of the workpiece.
Most of the spindles of diamond lathes currently available on the market use hydrostatic bearings. The axial and radial motion errors are below 50 nm, and the motion errors of individual spindles are less than 25 nm. Most of the slides of diamond lathes used gas static pressure support before the 1990s, and Hembrug of the Netherlands used hydrostatic bearings. Since the 1990s, Nanoform 600 and 250, the main products of Pneumo (now merged with Precitech) in the United States, have also adopted hydrostatic bearing slides with high rigidity, high damping and high stability.
The layout of the diamond lathe The layout of the diamond lathe initially follows the structure of a conventional lathe. The main shaft is fixed to the bed and mounted laterally along the table (X-axis) on the longitudinal slide (Z-axis). Because the guide rails of the vertical and horizontal slides are perpendicular to each other, they are also called a cross slide layout. The advantage is that the technology is mature and compact, and the Hembrug company's super-mikroturn has been using this structure (Fig. 1). The disadvantage of the cross slide layout is that the two slides interact with each other when moving, and this disadvantage is particularly acute when the dynamic accuracy is required.
The main reason for the mutual influence of the cross slide is that the weight of the X-direction slide is supported by the Z-direction slide. In order to solve this problem, the German Zeiss company has developed an improved cross slide (Fig. 2). The key is that the bed uses a large area of ​​granite, the Z-direction guide rail is directly processed on the bed, and the X-direction guide rail is still processed on the Z-direction slide table, but the weight of the X-direction slide table is no longer supported by the Z-direction along the table. Instead, it is supported directly by the bed through four static pressure pillars.
The Z-direction slide only acts to guide and guide the X-direction slide without an support function. Another disadvantage of the cross slide is that it is difficult to machine. To achieve high verticality between the vertical and horizontal slide rails, a large amount of manual scraping work is required. Today, with the increasing labor costs, the shortcomings of such time-consuming and laborious structures are becoming increasingly apparent. Thus, in the 1980s, a T-Base layout appeared.
The main shaft of the T-shaped layout lathe is mounted on the longitudinal or lateral slide table, and the tool holder is mounted on the other slide table (see Fig. 3), thereby completely solving the problem of mutual influence of the two slide tables. This layout helps to increase the closed-loop stiffness of the machine. In addition, the verticality of the two vertical and horizontal moving axes can be adjusted during assembly, and the production cost is low, which becomes the mainstream layout of the current diamond lathe.
When the diamond lathe of the above structure is processed into simple geometric shapes such as a plane, a cone and a cylindrical surface, the contact between the blade and the workpiece remains unchanged during the machining process, but when machining a complicated shape such as an ellipsoid, the contact point of the blade with the workpiece follows The position of the tool changes. If the geometry of the blade is not accurate, the error will be directly copied onto the workpiece, which limits the machining accuracy of the machine. There are usually two ways to solve this problem: one is to improve the shape accuracy of the tool, but whether it is to buy a new tool or re-grind the tool, it has to pay twice the cost of the ordinary tool; another way is to change the structure of the machine tool, Install a CNC precision turntable under the knife holder (see Figure 4). When the tool moves, the turntable rotates according to the curvature of the workpiece and the radius of the arc of the tool tip, so that the contact point between the workpiece and the blade remains unchanged. However, the cost of CNC precision turntables is very high, so economic analysis must be compared when choosing between the two options.
Application range and technical parameters of diamond turning Diamond cutting is mainly used to process non-ferrous metals such as uranium or aluminum alloy. The main products are mirrors in various optical systems, such as the main mirror of radio telescope, LiDA (laser detection). Each mirror in the system, as well as mirrors in laser cutting machines. During the arms race between the East and the West, the demand for various infrared optical components has soared. Diamond turning can process various infrared optical materials such as germanium, silicon, ZnS and ZnSe. The shape of the workpiece is mostly aspherical, which can greatly reduce The number of optical components, because of the low transmittance of the infrared material, the low component can improve the light transmission performance of the optical system, and also save expensive infrared materials.
In everyday consumer products, diamond turning is often used to process plexiglass and various plastics. Examples of applications include large projection TV screens, plastic lenses for cameras, and resin contact lens lenses.
In mass-produced products, optical components are often formed by extrusion or injection molding. The cavity used for forming is mostly done by diamond turning. In addition to ultra-high strength nickel steel, the cavity material also has tool steel and ceramics. Ultra-high-strength nickel steel is the most widely used material in press forming because it meets the hardness requirements of the mold and can be used to cut the best shape accuracy and surface quality. When machining tool steel with a diamond tool, the tool is prone to chemical wear because the carbon in the tool steel reacts with the diamond. Therefore, an ultrasonic vibration device should be attached to the tool holder at this time, or a cubic boron nitride tool should be used for processing.
When a workpiece having a diameter of 100 mm or less is turned by diamond, the shape error can be controlled to be 0.1 μm or less. In addition to the cutting parameters and machine characteristics, the surface roughness of the workpiece depends on the characteristics of the material. The surface roughness of most diamond-turnable materials can reach Rq1~5nm.
The parameters of diamond turning tools are similar to mirror milling. Metal materials are mostly processed with zero-degree rake angle tools, while infrared materials and brittle materials are mostly processed with negative rake angle tools.
The cutting parameters for diamond turning are based on the material of the workpiece and the characteristics of the machine. Usually the spindle speed is lower than 2000r/min, and the individual can reach 5000r/min. Contact lens lens lathe is special, its speed can reach 10000r/min.
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