Polycrystalline diamond PCD surface polishing technology
Large area PCD articles are polished by conventional mechanical polishing. The grinding wheel will first contact the convex portion which appears due to stress deformation, resulting in defects such as long polishing time and local thickness thinning.
Since its inception in the 1970s, PCD products have been widely used in high-tech fields such as aerospace, defense, energy, automotive, geological drilling and cable with their excellent performance. In particular, the application of large-area PCD products has made machining capacity and level a big step forward. The processing precision and the quality of the machined surface are continuously improved, and the processing efficiency is increased by several times or even hundreds of times. Large area PCD products are often used to make tools for cutting a variety of materials. In order to improve chip breaking and improve the precision and surface quality of the workpiece to be processed, the PCD surface of most PCD products needs to be polished to achieve a mirror surface (surface roughness Ra ≦ 0.05 μm). Although many materials introduce new technologies such as electrochemical polishing and ultrasonic polishing on PCD, PCD surface mechanical polishing still dominates in industrial mass production applications.
I. Polishing parameter selection of PCD surface
The PCD surface mechanical polishing process is a polycrystalline diamond wear and carbonization process. Due to the high hardness of polycrystalline diamond, it is generally polished with a diamond polishing powder (paste) plus a cast iron plate or with a grinding wheel. Practice has proved: The polishing efficiency of the diamond polishing powder (paste) plus the cast iron plate is too low, and most of them are polished by the grinding wheel (the grinding wheel and the workpiece are polished to have a large contact area, which is widely used).
Quality requirements for PCD face polishing:
1. Surface roughness Ra ≦ 0.05 μm;
2, the surface gloss is consistent, no refractive surface;
3. There are no unpolished edges present;
4, matt uneven ring;
5, no scratches and pollution.
In order to achieve the quality requirements of PCD surface polishing, when grinding mechanically with a grinding wheel, it is necessary to select the grinding wheel width, concentration and particle size, grinding wheel and workpiece rotation speed, polishing pressure and grinding wheel dressing timing.
First, the particle size and concentration of the grinding wheel should be selected appropriately. If the particle size is too rough, the roughness of the polished surface of the workpiece cannot be met; The particle size is too fine, the processing efficiency is low, the time for the abrasive grains to remain sharp is short, the friction during polishing is large, and the temperature is raised. The width of the grinding wheel should be chosen appropriately. If the grinding wheel is too narrow, the service life is short, the dressing wheel has a high frequency of dressing, the workpiece is polished surface and the grinding wheel has a small contact surface, and the polishing efficiency is low. If the grinding wheel is too wide, the end face of the grinding wheel is difficult to level, the linear speed difference between the inner and outer rings of the grinding wheel is large, the internal and external consumption of the grinding wheel is different, the friction contact area in the polishing is large, and the heat dissipation condition of the workpiece is deteriorated. The high concentration of diamond in the working layer of the grinding wheel is beneficial to shorten the contact time between the grinding wheel and the surface to be polished, but the high cost will cause the grinding wheel to thresh too quickly, sometimes causing scratches on the polished surface.
The workpiece is generally rotated at a low speed during the polishing process, which is advantageous for the smooth running of the workpiece. The grinding wheel usually rotates at a high speed to achieve friction between the PCD surface and the grinding wheel surface to generate heat. The grinding wheel rotates too fast and the frictional heat is too high to meet the polishing surface quality requirements.
During the polishing process, the surface to be polished must be in contact with the surface of the grinding wheel while applying appropriate pressure. If the pressure is too small, it will cause jitter, and the polished surface will be corrugated; Excessive pressure, the grinding wheel is fast, not only the friction temperature rises, but also may cause the drive motor to be overloaded.
The timing of dressing the surface of the grinding wheel should be properly controlled. The end of the grinding wheel is not trimmed for a long time, and the grinding wheel is not sharp. The polished surface has a long bonding time with the end surface of the grinding wheel, and the processing efficiency is low; The end face of the grinding wheel is trimmed too frequently, and the grinding wheel is consumed quickly. The end face of the grinding wheel is uneven, the transition is not smooth, the polished surface is dull, and sometimes scratches occur.
II. Problems with traditional polishing methods and equipment
The structure of the conventional polishing equipment is that the grinding wheel rotates at a high speed, and the fixed polishing fixture holds the workpiece at a low speed. The polished surface is in contact with the end face of the grinding wheel and a certain contact pressure is applied. The center of rotation of the workpiece is fixed to the contact line of the grinding wheel, and the PCD surface is polished by friction, heat and carbonization.
In the past, PCD articles were large in thickness and the area of the polished surface was small (less than or close to the end face width of the grinding wheel), and polishing with a conventional polishing apparatus was suitable. With the development of technology, the appearance of large-area PCD products has made the surface to be polished more than double the width of the end face grinding wheel, and the thickness of the workpiece is much thinner than in the past. When the polishing surface area of the PCD article is greater than 26 cm 2 and the thickness is not more than 2 mm, the flatness of the surface to be polished is deteriorated due to stress deformation of the sheet, and the polishing difficulty is increased. Processing large area PCD sheets with conventional polishing equipment has the following problems:
1. The rotation center of the fixture of the conventional polishing equipment (and its clamped workpiece) has no relative motion with the end surface of the grinding wheel. When the surface being polished is in contact with the grinding wheel, the distribution of the points (or faces) of the initial contact over the entire surface to be polished is decisive, since thereafter the polishing process will spread around these points (or faces). If the initial contact points (or faces) are small (referred to as poor fit) and are locally concentrated, then only those points or faces will be removed thereafter. Other points (or faces) can be in contact with the end face of the grinding wheel, which makes the surface polishing become "de-polishing". Due to the poor cutting ability of the grinding wheel used for hard diamond and polishing, the contact point (or surface) spreads to the periphery rather slowly, resulting in a long polishing time and a decrease in processing efficiency.
2. Since the contact center of the rotating center of the workpiece and the end face of the grinding wheel is fixed, even if it is in contact with the grinding wheel everywhere (completely matched). When the width (length or diameter) of the workpiece to be polished is larger than the width of the grinding wheel, the contact probability varies from place to place. The contact probability of the workpiece outside is significantly less than that of the middle part, so that the polished surface is prone to different refractive rings with different brightness and darkness, which cannot meet the quality requirements.
3. Due to the polishing process, the middle part of the polished surface does not leave the end face of the grinding wheel at all, the middle frictional heat is greater than the surrounding, and the heat dissipation condition is poor. For large-area, thin-thick PCD products, localized heating leads to increased deformation of the workpiece.
4. Stress deformation causes the planar shape of the surface to be polished to be irregular, accompanied by distortion. Only by trimming the end face of the grinding wheel or the surface of the polished surface and the end face of the grinding wheel, the end face of the grinding wheel is as close as possible to the surface to be polished (the contact surface is more contact points and evenly distributed). However, the stress deformation is random, and the surface distortion makes the surface condition of each workpiece vary widely. When the end face of the grinding wheel is trimmed or worn to match the surface of the PCD of a certain workpiece. If another workpiece is changed, it will not match, and it must be trimmed or ground again by the grinding wheel to achieve a new match. This not only has low processing efficiency, but also increases the labor intensity of workers, which is not suitable for mass production.
From the above analysis, we can know: In the polishing process, it is important to improve the polishing efficiency by increasing the degree of contact between the polished surface and the end face of the grinding wheel. During the polishing process, the self-rotating center of the surface to be polished is radially moved along the grinding wheel on the end face of the grinding wheel, and the adaptive contact function is used to improve the degree of contact fit (especially for the convexly deformed polished surface). When the center of rotation of the workpiece leaves the contact line on the end face of the grinding wheel, a part of the contact point (face) on the surface to be polished and the end face of the grinding wheel will be disengaged. (Microscopically, the end face of the grinding wheel is staggered in the radial direction, not a flat surface). The stable contact state that was originally formed was broken. With the cooperation of the adaptive contact function, some points (faces) that are not in contact with the end face of the grinding wheel are in contact with the end face of the grinding wheel at this time. Thereby, new contact points (faces) are added, and the mutual contact and anastomosis states are improved, thereby shortening the polishing time.
China's production of polycrystalline diamond Drawing Die
The displacement of the workpiece rotation center on the end face of the grinding wheel also has the following advantages:
1. The polishing process is a mutual wear process between the grinding wheel and the surface to be polished. While the workpiece is displaced, the high end of the grinding wheel end is flattened, which not only eliminates the ring-shaped refracting ring that may appear on the polished surface, but also reduces the difficulty of dressing the flat end of the grinding wheel.
2. The contact between the middle and the edge of the polished surface and the end face of the grinding wheel is balanced, so that the workpiece is heated evenly throughout the workpiece;
Moreover, when most of the polished surface is removed from the end face of the grinding wheel, the heat dissipation condition of the workpiece is improved, and the thermal deformation generated by the polishing process of the workpiece is reduced.
3. Shorten the time difference required for simultaneous polishing of workpieces with different surface deformation states.
Since the surface to be polished is adaptively matched with the end face of the grinding wheel, it is not necessary to adjust the surface of the grinding wheel to adapt to the surface to be polished. In addition, the improvement of the heat dissipation condition of the workpiece enables the same polishing equipment (using the same grinding wheel) to simultaneously polish two large-area PCD products.
There are several ways to achieve the relative displacement of the rotation center of the polished surface and the contact line of the grinding wheel end face: One is to perform eccentric oscillation while the spindle (grinding wheel) rotates at a high speed; The other is to oscillate in the radial direction of the workpiece along the radial direction of the grinding wheel or within a certain angle (the central pressure point does not disengage the end face of the grinding wheel). According to the data, the polishing equipment produced in foreign countries uses the grinding wheel to rotate at high speed while performing eccentric oscillation.
Using the principle of the double rocker oscillating mechanism, we designed the composite clamp for rotation, pressure, swing and contact. The workpiece is oscillated within a certain angle (the center pressure point does not leave the end face of the grinding wheel) to achieve displacement of the center of rotation of the polished surface on the end face of the grinding wheel. Its advantages:
1. Low cost and relatively simple mechanical structure. The basic structure of the conventional polishing equipment can be kept unchanged (only the clamp part is changed).
5. Aufgrund der unterschiedlichen Oberflächenformen der PKD-Produkte ist es nicht möglich, die Endflächen derselben Schleifscheibe so zu trimmen, dass sie mit den PKD-Oberflächen der beiden Werkstücke zusammenfallen. Selbst wenn es zur gleichen Zeit angepasst werden kann, kann der Reibungstemperaturanstieg, der durch eine Schleifscheibe beim gleichzeitigen Polieren von zwei großflächigen PKD-Produkten verursacht wird, nicht in der Mitte des Werkstücks gelöst werden (der Wärmeabgabezustand ist schlecht). Das gleichzeitige Polieren von zwei großflächigen PKD-Gegenständen auf einer herkömmlichen Poliervorrichtung (mit derselben Schleifscheibe) erfordert nicht nur einen großen Unterschied in der Bearbeitungszeit für jedes Blech, sondern auch übermäßig hohe Temperaturen, die dazu neigen, die polierte Oberfläche zu verbrennen.
III. Festlegung des Verbesserungsplans
III. Festlegung des Verbesserungsplans
China's production of polycrystalline diamond Drawing Die
The displacement of the workpiece rotation center on the end face of the grinding wheel also has the following advantages:
1. The polishing process is a mutual wear process between the grinding wheel and the surface to be polished. While the workpiece is displaced, the high end of the grinding wheel end is flattened, which not only eliminates the ring-shaped refracting ring that may appear on the polished surface, but also reduces the difficulty of dressing the flat end of the grinding wheel.
2. The contact between the middle and the edge of the polished surface and the end face of the grinding wheel is balanced, so that the workpiece is heated evenly throughout the workpiece;
Moreover, when most of the polished surface is removed from the end face of the grinding wheel, the heat dissipation condition of the workpiece is improved, and the thermal deformation generated by the polishing process of the workpiece is reduced.
3. Shorten the time difference required for simultaneous polishing of workpieces with different surface deformation states.
Since the surface to be polished is adaptively matched with the end face of the grinding wheel, it is not necessary to adjust the surface of the grinding wheel to adapt to the surface to be polished. In addition, the improvement of the heat dissipation condition of the workpiece enables the same polishing equipment (using the same grinding wheel) to simultaneously polish two large-area PCD products.
There are several ways to achieve the relative displacement of the rotation center of the polished surface and the contact line of the grinding wheel end face: One is to perform eccentric oscillation while the spindle (grinding wheel) rotates at a high speed; The other is to oscillate in the radial direction of the workpiece along the radial direction of the grinding wheel or within a certain angle (the central pressure point does not disengage the end face of the grinding wheel). According to the data, the polishing equipment produced in foreign countries uses the grinding wheel to rotate at high speed while performing eccentric oscillation.
Using the principle of the double rocker oscillating mechanism, we designed the composite clamp for rotation, pressure, swing and contact. The workpiece is oscillated within a certain angle (the center pressure point does not leave the end face of the grinding wheel) to achieve displacement of the center of rotation of the polished surface on the end face of the grinding wheel. Its advantages:
1. Low cost and relatively simple mechanical structure. The basic structure of the conventional polishing equipment can be kept unchanged (only the clamp part is changed).
3. Select the double rocker swinging mechanism in the four-bar linkage mechanism. A drive motor can be used to simultaneously swing the workpiece from a rotation and a center of rotation at a certain angle.
4. Many parts on the original fixed structure fixture can be used without increasing the complexity of operation
