Polycrystalline diamond PCD surface polishing technology

Large area PCD products are polished by traditional mechanical polishing. The grindstone first comes into contact with the protrusions that appear due to stress deformation, resulting in defects such as long polishing times and local loss of thickness.

Since its inception in the 1970s, PCD products have been widely used in high-tech fields such as aerospace, defense, energy, automobiles, geodrilling and cables due to their superior performance. In particular, the application of large area PCD products has greatly improved machining capacity and level. The machining accuracy and quality of the machined surface is continuously improved, and the machining efficiency is increased several times or hundreds of times. Large area PCD products are often used to create tools for cutting a variety of materials. To improve chip control and improve the accuracy and surface quality of the workpieces to be machined, the PCD surface of most PCD products must be polished to a mirror surface (surface roughness Ra ≤ 0.05 μm). Although many materials have introduced new technologies such as electrochemical polishing and ultrasonic polishing of PCDs, mechanical polishing of PCD surfaces remains dominant in industrial mass production applications.

Selection of polishing parameters for the PCD surface

The mechanical polishing process of the PCD surface is the abrasion and carbonization process of polycrystalline diamond. Due to the high hardness of polycrystalline diamond, it is usually polished to diamond polishing powder (paste) with a cast iron plate or grindstone. Practical evidence: Diamond polishing powder (paste) and cast iron plates are too inefficient to polish, most of them are ground by a grinding wheel (grinding wheels and workpieces have a large contact area that is widely used. It will be polished to hold).
Quality requirements for PCD face polishing:

  • 1. Surface roughness Ra ≤ 0.05 μm;
  • 2. The surface gloss is consistent and there is no refracting surface.
  • 3. There are no unpolished edges.
  • 4, matte non-uniform ring;
  • 5, no scratches or contamination.

When mechanically polishing with a grindstone to meet the quality requirements of PCD surface polishing, it is necessary to select the width, concentration, grain size of the grindstone, the rotation speed of the grindstone and workpiece, the polishing pressure, and the timing of dressing of the grindstone. I have.

First, you need to choose the right grain size and concentration of the grindstone. If the particle size is too coarse, it will not be able to meet the roughness of the polished surface of the workpiece. The particle size is too fine, the processing efficiency is low, the abrasive grains remain sharp for a short time, the friction during polishing is large, and the temperature rises. The width of the grinding wheel must be selected appropriately. If the grindstone is too narrow, it will have a shorter service life, dressing more frequently, the workpiece will be a polished surface, the contact surface of the grindstone will be smaller, and the polishing efficiency will be lower. If the width of the grindstone is too wide, it becomes difficult to level the end face of the grindstone, the linear speed difference between the inner ring and the outer ring of the grindstone becomes large, the internal consumption and the external consumption of the grindstone are different, and frictional contact in polishing. The area will increase and the heat dissipation of the workpiece will deteriorate. A high concentration of diamond in the working layer of the grinding wheel reduces the contact time between the grinding wheel and the surface to be polished, but due to the high cost, the grinding wheel falls off too quickly and is polished. The surface may be scratched.

Workpieces generally rotate at low speeds during the polishing process. This is advantageous for the smooth running of the workpiece. Grinding wheels typically rotate at high speeds, creating friction between the PCD surface and the grinding wheel surface to generate heat. The grindstone rotates too fast and the frictional heat is too high to meet the quality requirements of the polished surface.

During the polishing process, the surface to be polished must be in contact with the surface of the grinding wheel under appropriate pressure. If the pressure is too low, it will cause jitter and the polished surface will be corrugated. If the pressure is too high, the grindstone will speed up, which will not only increase the friction temperature, but may also overload the drive motor.

The timing of dressing the surface of the grindstone must be properly controlled, the edges of the grindstone are not trimmed for a long time and the grindstone is not sharp. The polished surface has a long bonding time with the end face of the grindstone, and the processing efficiency is low. The end face of the grinding wheel is frequently trimmed and the grinding wheel wears out quickly. The end face of the grinding wheel is uneven, the transition is not smooth, the polished surface is dull and sometimes scratched.

Problems with traditional polishing methods and equipment

The structure of a conventional polishing device is that the grindstone rotates at high speed and the fixed polishing jig holds the workpiece at low speed. The polished surface is in contact with the end face of the grindstone, and a constant contact pressure is applied. The center of rotation of the workpiece is fixed to the contact line of the grindstone, and the PCD surface is polished by friction, heat, and carbonization.

Conventionally, PCD products are thick and the area of ​​the polished surface is small (less than the width of the end face of the grindstone), so polishing with a conventional polishing device was suitable. With the advent of large area PCD products due to technological advances, the surface to be polished is more than twice the width of the end face grinding wheel, and the workpiece is much thinner than before. When the polished surface area of ​​the PCD article is larger than 26 cm 2 and the thickness is 2 mm or less, the flatness of the surface to be polished is lowered due to the 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 center of rotation of the fixture of a conventional polishing device (and clamped workpiece) has no relative movement with the end face of the grinding wheel. When the surface being polished is in contact with the grindstone, the distribution of points (or surfaces) of first contact across the surface to be polished is decisive. After that, the polishing process spreads around these points (or faces). If the initial contact points (or faces) are small (called poor fit) and are locally concentrated, then only those points or faces are subsequently removed. Other points (or surfaces) can come into contact with the end face of the grinding wheel, which makes surface polishing “unpolished”. Due to the low cutting capacity of hard diamonds and the grindstone used for polishing, the spread around the contact point (or surface) is considerably slower, the polishing time is longer, and the processing efficiency is reduced.
  • 2. Since the center of rotation of the workpiece and the center of contact of the end face of the grinding wheel are fixed, even if they are in contact with the grinding wheel everywhere (perfect match). If the width (length or diameter) of the workpiece to be ground is greater than the width of the grindstone, the contact probability will vary from place to place. Since the contact probability of the outer workpiece is significantly lower than the contact probability of the central part, the polished surface is prone to different refracting rings with different brightness and darkness that cannot meet the quality requirements.
  • 3. Due to the polishing process, the central part of the polishing surface does not separate from the end surface of the grindstone at all, the frictional heat in the middle is larger than the surroundings, and the heat dissipation condition becomes worse. For large area and thin PCD products, local heating will increase the deformation of the workpiece.
  • 4. Due to stress deformation, the planar shape of the surface to be polished becomes irregular and distortion occurs. Only by trimming the end face of the grinding wheel or the surface of the grinding wheel and the end face of the grinding wheel will the end face of the grinding wheel be as close as possible to the surface to be polished (the contact surface will be more contact points and even. Is distributed in). However, the stress deformation is random, and surface strain causes the surface condition of each workpiece to change significantly. When the end face of the grinding wheel is trimmed or worn to match the surface of the PCD of a particular workpiece. If another workpiece is modified, it will not match and will need to be trimmed or ground again with a grinding wheel to achieve a new match. Not only is this less efficient, but it also increases the labor intensity of the workers, making it unsuitable for mass production.
  • 5. Due to the different surface shapes of PCD products, it is not possible to trim the end faces of the same grinding wheel to match the PCD surfaces of two workpieces. Even if they can be matched at the same time, the increase in friction temperature due to the simultaneous polishing of two large area PCD products by one grindstone cannot be removed in the center of the workpiece (heat dissipation conditions are poor). Simultaneously polishing two large area PCD articles with a conventional polishing device (same grindstone) not only requires a large difference in the processing time of each sheet, but also has an excessively high temperature that tends to burn the polished surface.

Decision on improvement plan

From the above analysis, we can see that: In the polishing process, it is important to improve the polishing efficiency by increasing the degree of contact between the polishing surface and the end surface of the grindstone. During the polishing process, the self-rotating center of the surface to be polished moves radially along the grindstone at the end face of the grindstone and uses adaptive contact features to improve the degree of contact fit (especially convex deformation). Polished surface). When the center of rotation of the work leaves the contact line of the end face of the grindstone, a part (face) of the contact point of the polishing surface and the end face of the grindstone come off. (Microscopically, the end faces of the grinding wheel are radially offset and not flat). The stable contact state that was originally formed has been destroyed. With the cooperation of the adaptive contact function, some points (faces) that are not in contact with the end face of the grindstone at this point are in contact with the end face of the grindstone. This adds new contact points (faces), improves mutual contact and anastomotic conditions, and reduces polishing time.

Polycrystalline diamond wire drawing die produced in China

The displacement of the center of rotation of the workpiece on the end face of the grindstone also has the following advantages.

  • 1. The polishing process is a mutual wear process between the grindstone and the surface to be polished. While the workpiece is moving, the top edge of the grindstone edge is flattened, eliminating the ring-shaped refracting ring that appears on the polished surface and reducing the difficulty of dressing the flat edge of the grindstone.
  • 2. The contact between the center and edges of the polished surface and the end face of the grindstone is balanced so that the workpiece is heated evenly throughout the workpiece. Further, when most of the polished surface is removed from the end face of the grindstone, the heat dissipation state of the workpiece is improved and the thermal deformation generated by the workpiece polishing process is reduced.
  • 3. Reduces the time difference required for simultaneous polishing of workpieces with different surface deformations. The surface to be polished matches the end face of the grindstone adaptively, so there is no need to adjust the surface of the grindstone to adapt to the surface to be ground. In addition, the improved heat dissipation conditions of the workpiece allow two large area PCD products to be ground simultaneously with the same grinder (using the same grindstone).

There are several ways to achieve the relative displacement of the contact line between the center of rotation of the polishing surface and the end face of the grinding wheel. One is to perform eccentric vibrations while the spindle (grinding wheel) is rotating at high speed. The other is to vibrate along the radial direction of the grinding wheel or in the radial direction of the workpiece within a certain angle (the central pressure point does not deviate from the end face of the grinding wheel). According to the data, polishing equipment produced overseas uses a grindstone to rotate at high speed while performing eccentric vibration.

Using the principle of the double rocker focal mechanism, we designed a composite clamp for rotation, pressure, swing and contact. The workpiece vibrates within a certain angle to achieve a displacement of the center of rotation of the grindstone end face (the center pressure point does not deviate from the grindstone end face). Its advantages:

  • 1. Low cost and relatively simple mechanical structure. The basic structure of a conventional polishing machine can be maintained unchanged (only the clamp part is changed).
  • 2. Due to harsh polishing environment (dust). The utility model uses a conventional screw rod and guide rail transmission mechanism. It is expensive, dust can easily get into moving parts, and its life is shortened (difficult to protect). Adopting a cam or pneumatic and hydraulic mechanism complicates the structure and has many peripheral accessories. If the 4-section linkage has a simple structure and is highly reliable in operation, as well as simple protection and strong environmental adaptability, it is suitable for application to polishing equipment.
  • 3.4 Select the double rocker swing mechanism with the bar link mechanism. A drive motor can be used to rotate the workpiece at a specific angle and swing it from the center of rotation at the same time.
  • 4. Many parts of the original fixed structure fixture can be used without increasing the complexity of operation.

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