When traditional mechanical polishing methods are used on large-area PCD products, the polishing wheel first contacts the raised areas caused by stress deformation. This leads to longer polishing times and localized thinning. To address this issue, the author designed and implemented a dual-rocker swinging fixture, allowing the polished surface to adaptively contact the polishing wheel’s end face during the process. This article primarily discusses the features and effectiveness of this new processing equipment.
1. PCD Products
Since the advent of PCD products in the 1970s, they have gained increasingly widespread applications in high-tech fields such as aerospace, defense, energy, automotive, geological drilling, and wire cables due to their excellent performance. In particular, the use of large-area PCD products has advanced mechanical processing capabilities, with improvements in precision, surface quality, and processing efficiency by dozens or even hundreds of times.
Large-area PCD products are mainly used to manufacture cutting tools for various materials. To achieve good chip breaking and improve the precision and surface quality of the workpieces being processed, most PCD products need to be polished to achieve a mirror-like surface (with a surface roughness of Ra ≤ 0.05μm). Although many sources describe new techniques such as electrochemical polishing and ultrasonic polishing for PCD surfaces, mechanical polishing of PCD surfaces remains dominant in industrial mass production applications.
2. PCD Surface Polishing Parameter Selection
The mechanical polishing process for PCD surfaces involves the wear and carbonization of polycrystalline diamonds. Due to the high hardness of polycrystalline diamonds, diamond polishing powder (paste) with a cast iron plate or grinding wheel is typically used. It has been proven that using diamond polishing powder (paste) with a cast iron plate has too low an efficiency, so grinding wheels are predominantly used because of their larger contact area with the workpiece surface.
PCD Surface Polishing Quality Requirements:
(1) Surface roughness Ra ≤ 0.05 μm
(2) Uniform surface gloss with no refraction points
(3) No unpolished edges
(4) No uneven gloss rings
(5) No scratches or contamination
To meet these quality requirements, the choice of grinding wheel width, concentration, and grit size, along with the rotational speed of both the grinding wheel and the workpiece, polishing pressure, and the timing for dressing the grinding wheel are crucial.
Grit size and concentration must be chosen carefully: coarse grit won’t meet surface roughness requirements, while too fine a grit reduces efficiency, shortens the sharpness retention of the abrasive grains, and increases friction and temperature rise.
Grinding wheel width must be appropriate: too narrow a wheel reduces lifespan and increases the frequency of dressing; too wide a wheel causes uneven wear, leading to poor heat dissipation and friction inconsistencies.
High abrasive concentration in the wheel can shorten the time required for optimal contact between the wheel and the polished surface, but it increases costs and can accelerate the wear of the grinding wheel, potentially causing scratches on the surface.
During polishing, the workpiece typically rotates at a low speed to maintain stability, while the grinding wheel rotates at high speed to generate friction and heat for polishing. If the wheel rotates too fast, excessive heat may affect surface quality.
Polishing pressure is also crucial: too little pressure can cause vibrations and surface ripples, while excessive pressure accelerates wheel wear, raises the temperature, and risks overloading the drive motor.
Finally, the timing of grinding wheel dressing is important. If the wheel is not dressed for too long, its surface becomes dull, prolonging the time required for proper contact with the polished surface, reducing efficiency. However, too frequent dressing leads to rapid wheel consumption. If the wheel surface is unevenly dressed, it can leave the polished surface without gloss and cause scratches.
3. Issues with Traditional Polishing Methods and Equipment
Traditional polishing equipment involves a high-speed rotating grinding wheel, while a fixed polishing fixture holds the workpiece in place, rotating it at a lower speed. The surface being polished comes into contact with the grinding wheel, applying a certain contact pressure. The center of rotation of the workpiece and the contact line with the grinding wheel are fixed, and the polishing process is achieved through friction, heat, and carbonization.
For earlier PCD products, which were thicker and had relatively small surfaces to polish, traditional equipment was sufficient. However, with the development of technology and the advent of large-area, thin PCD products, these surfaces often exceed the width of the grinding wheel, and their reduced thickness presents significant challenges. Polishing these large-area, thin PCD surfaces using traditional methods reveals several issues:
(1) Initial Contact Points: In traditional equipment, the fixture’s rotation center has no relative movement with the grinding wheel surface. When the workpiece first contacts the wheel, the distribution of these contact points (or surfaces) across the surface determines the polishing process. If there are few initial contact points (poor conformity), polishing will begin at these points and spread outward. Since polycrystalline diamond is hard and the grinding wheel’s cutting capacity is low, the contact points expand very slowly, leading to excessive polishing time and reduced efficiency.
(2) Uneven Contact Across the Surface: Even when the workpiece surface fully conforms to the grinding wheel, if the surface area exceeds the width of the grinding wheel, contact probabilities vary across the surface. The outer areas have less contact with the grinding wheel than the center, causing uneven light refraction patterns, resulting in substandard surface quality.
(3) Thermal Deformation: As the middle of the workpiece remains in constant contact with the grinding wheel, it experiences more friction and heat than the edges. For large, thin PCD products, this uneven heating can cause increased deformation, further complicating polishing.
(4) Stress-Induced Surface Irregularities: Stress-induced deformation leads to irregularities and twisting in the surface shape. These irregularities require frequent adjustments to the grinding wheel surface to improve conformity between the wheel and the workpiece. However, the random nature of the deformations makes it impossible to create a universal grinding wheel surface that fits multiple workpieces. Frequent adjustments significantly reduce processing efficiency and increase operator labor, making this method unsuitable for mass production.
(5) Surface Shape Variability Among Products: Due to differences in surface shapes across various PCD products, the likelihood of a single grinding wheel conforming to multiple workpieces simultaneously is low. Even if simultaneous conformity is achieved, the frictional heat generated by polishing two large-area PCD products at once increases, leading to excessive heat buildup and potential surface damage, especially in the workpiece’s middle area where heat dissipation is poor. This issue significantly increases the risk of surface burn and further prolongs processing time.
Determining the Improvement Plan
Based on the analysis, improving the contact conformity between the surface being polished and the grinding wheel is crucial to enhancing polishing efficiency. To achieve this, the workpiece’s rotational center must move radially on the surface of the grinding wheel during polishing, while incorporating adaptive contact mechanisms. This approach is particularly effective for surfaces with convex deformations. When the rotational center moves away from its original contact line with the grinding wheel, new points or areas of contact are introduced, thereby improving conformity and reducing polishing time.
The displacement of the workpiece’s rotational center relative to the grinding wheel has several advantages:
(1) Smoothing the Grinding Wheel Surface: As the workpiece moves, high points on the grinding wheel are gradually worn down, which eliminates potential light-reflecting rings on the polished surface and reduces the difficulty of leveling the grinding wheel.
(2) Balanced Contact and Heat Distribution: This movement ensures that the middle and edges of the workpiece have a more balanced chance of contact with the grinding wheel, leading to more even heat distribution. Additionally, parts of the surface frequently move off the grinding wheel, improving heat dissipation and reducing thermal deformation.
(3) Consistent Polishing Time: By allowing for adaptive contact between the workpiece and the grinding wheel, this method reduces the difference in polishing time for workpieces with varying surface deformations. Improved heat dissipation also allows for polishing two large-area PCD workpieces on the same equipment simultaneously.
There are several methods to achieve relative movement between the workpiece’s rotational center and the grinding wheel, such as eccentric oscillation of the spindle or radial displacement of the workpiece’s rotational center. Foreign polishing equipment typically employs a high-speed rotating grinding wheel combined with eccentric oscillation.
The proposed solution in this case uses the double-rocker oscillating mechanism. This mechanism allows for the rotation, pressure application, oscillation, and adaptive contact of the workpiece. The workpiece oscillates within a certain angle, ensuring the rotational center moves on the grinding wheel surface, with the following advantages:
(1) Low Cost: The mechanical structure is simple, maintaining most of the traditional polishing equipment structure (only the fixture part is modified).
(2) Durability: The polishing environment contains dust, which can affect screw and guide rail systems due to dust accumulation. In contrast, the four-link mechanism (including the double-rocker) is simpler, more reliable, easier to protect, and better suited for use in polishing equipment.
(3) Efficient Motion: The double-rocker mechanism allows a single drive motor to simultaneously achieve the workpiece’s self-rotation and oscillation within a certain angle.
(4) Ease of Integration: Many parts from the original fixed fixture structure can be reused, which does not increase the complexity of operations.
5. Comparative Experiment and Analysis
We conducted comparative experiments using both the new polishing machine (equipped with a double-rocker oscillating polishing fixture) and a traditional machine (equipped with a fixed polishing fixture).
Experiment 1: Polishing Surfaces Initially Matching the Wheel
When testing with workpieces whose surfaces already matched the grinding wheel, we observed the following:
Traditional Polishing Machine: Even with a new grinding wheel, the results were poor, as the surface lacked smoothness and showed scratches.
New Polishing Machine: The polished surface met quality requirements, demonstrating that the workpiece’s movement helped smooth the grinding wheel.
Experiment 2: Polishing Convex Surfaces
With convex workpieces:
Traditional Machine: After a period of polishing, distinct boundaries appeared between polished and unpolished areas. Without reshaping the grinding wheel, continuing polishing would require a long time, potentially thinning the PCD layer in the middle.
New Machine: The boundary between polished and unpolished areas was blurred, and edge contact marks were observed, showing that the surface remained flat after polishing.
Experiment 3: Polishing Concave Surfaces
The results were similar to the convex surface test, except that the polished areas expanded from the edges toward the center.
Efficiency Comparison
A test comparing 10 machines showed:
New Machine: On average, it was 20% more efficient than traditional machines, even when the grinding wheel matched the workpiece surface.
Dual Workpiece Polishing
Testing two workpieces with the same deformation on a machine with two fixed fixtures revealed:
Traditional Machine: Rapid and high temperature increases occurred. Without frequent interruptions to sharpen the grinding wheel, surface burn damage was likely. One workpiece achieved full contact while the other only had 50% contact.
New Machine with Double-Rocker Fixtures: Both workpieces were polished simultaneously with minimal time differences. Polishing quality met requirements, and processing time only increased by about 10% compared to polishing a single workpiece. This demonstrated an 80% improvement in efficiency over traditional machines.
6. Conclusion
(1) Efficiency Improvement: The new polishing equipment significantly improves efficiency (by at least 20%) and enhances heat dissipation.
(2) Surface Quality: The new equipment eliminates the formation of circular reflective rings, greatly improving surface quality.
(3) Easier Grinding Wheel Maintenance: The new system requires less frequent and simpler grinding wheel maintenance compared to traditional machines.
(4) Dual Workpiece Polishing: The new polishing equipment allows simultaneous polishing of two large PCD workpieces, increasing equipment utilization by 80%.
