With the traditional mechanical polishing method for large-area PCD products, the grinding wheel will first contact the raised part of the product due to the deformation of the stress, resulting in a long polishing time and localized thinning of the thickness and other drawbacks. In this regard, the author designed and utilized a double rocker oscillating fixture, so that the polishing surface in the polishing process can be adapted to contact with the end face of the grinding wheel, this paper mainly describes the characteristics of this new technology and equipment and the use of the effect.
I.PCD Products
PCD products since the 1970s since the introduction of the 20th century, with its excellent performance, in aerospace, defense, energy, automotive, geological drilling and cable and other high-tech fields to get more and more widely used. Especially the application of large-area PCD products, so that the mechanical processing capacity and level of a big step forward, processing accuracy, processing surface quality continues to improve, the processing efficiency of dozens of times or even hundreds of times to improve. Large-area PCD products are mostly used in the manufacture of cutting tools for various materials, in order to good chipbreaking, improve the precision of the processed workpiece and surface quality, most of the PCD products need to be polished PCD surface, so that it reaches the mirror surface (surface roughness Ra ≦ 0.05μm). Although a lot of information about PCD surface electrochemical polishing, ultrasonic polishing and other new technologies, but in the industrialized mass production applications, PCD surface mechanical polishing still dominates.
II.Parameter selection for PCD surface polishing
PCD surface mechanical polishing process is a polycrystalline diamond abrasion and carbonization process, due to the high hardness of polycrystalline diamonds, generally use diamond polishing powder (paste) plus cast iron disk or grinding wheel for polishing. Practice has proved that the efficiency of polishing with diamond polishing powder (paste) and cast iron disk is too low, and most of them use grinding wheel for polishing (the contact area between grinding wheel and workpiece to be polished surface is large, and it is widely used).
PCD surface polishing quality requirements:
(1)Surface roughness Ra≦0.05μm
(2)Consistent surface gloss, no refractive surfaces
(3)No unpolished edges exist.
(4)No glossy uneven rings
(5)No scratches or stains.
In order to achieve the quality requirements of PCD surface polishing, when implementing mechanical polishing with grinding wheels, it is necessary to reasonably select the width, concentration and grain size of the grinding wheels, the rotational speed of the grinding wheels and workpieces, the pressure of polishing, and the time of dressing of the grinding wheels.
First of all, the grain size and concentration of the grinding wheel should be selected appropriately, the grain size is too coarse to meet the requirements of roughness of the polished surface of the workpiece; the grain size is too fine, the processing efficiency is low, the time for the abrasive grains to remain sharp is short, the friction when polishing is large, and the temperature rises. The width of the grinding wheel should be selected appropriately, if it is too narrow, the service life will be short, the frequency of grinding wheel dressing will be high, the contact surface between the workpiece being polished and the grinding wheel will be small, and the polishing efficiency will be low; if it is too wide, it will be difficult to flatten the end face of the grinding wheel, and the difference in the linear speed of the inner and outer rings of the grinding wheel will be large, and the difference in the internal and external consumptions of the grinding wheel will be many, and the friction contact area will be large, and the workpiece will become poor in heat dissipation conditions; if the concentration of the emery contained in the grinding wheel’s work layer is high, it is advantageous to shorten the time of contacting between the wheel and the surface being polished, but the cost is too low. High concentration of emery in the working layer of the grinding wheel is favorable to shorten the contact time between the grinding wheel and the surface to be polished, but the cost is high, and it will make the grinding wheel degranulate too fast, sometimes causing scratches on the surface to be polished
The workpiece is generally rotated at a low speed during the polishing process, which is favorable for the workpiece to maintain stable operation. The grinding wheel usually rotates at a high speed to realize the friction between PCD surface and the surface of the grinding wheel, which generates thermal effect. If the grinding wheel rotates too fast, the friction heat is too high, which can’t meet the requirement of polishing surface quality.
In the polishing process, the polished surface and the grinding wheel surface contact at the same time must be applied to the appropriate pressure, pressure is too small easy to produce jitter, the polished surface appears ripples; pressure is too large, the grinding wheel consumes fast, not only the friction temperature rises, but also may lead to the drive motor overload.
The grinding wheel surface dressing time should be appropriate, long time without dressing the grinding wheel end face, the grinding wheel is not sharp, the polished surface and the grinding wheel end face of the coincidence of a long time, processing efficiency is low; too often dressing the grinding wheel end face, the grinding wheel consumption is fast. If the grinding wheel end face is not smooth and the transition is not round and smooth, the polished surface will not have luster and sometimes there are scratches.
III.Problems with conventional polishing methods and equipment
The structure of traditional polishing equipment is that the grinding wheel rotates at a high speed, the fixed polishing fixture holds the workpiece and rotates at a low speed, the polished surface is in contact with the end face of the grinding wheel and a certain contact pressure is exerted, and the rotating center of the workpiece is fixed to the contact line of the grinding wheel, so that the surface of the PCD surface is polished by the friction, heat and carbonization among each other.
In the past, when the thickness of PCD products was large and the area of the polished surface was small (less than or close to the width of the grinding wheel’s end ring), it was appropriate to use traditional polishing equipment to carry out polishing. With the development of technology, the emergence of large-area PCD products, so that the polished surface is more than double the width of the endface grinding wheel, and the thickness of the workpiece is much thinner than in the past, when the polished surface area of the PCD products is more than 26cm2 and the thickness is not less than 2mm, the thin film deformation due to the stress makes the flatness of the polished surface worse, and the difficulty in polishing is increased. The following problems exist in the processing of large-area PCD sheets with conventional polishing equipment:
(1) The rotation center of the fixture (and its clamped workpiece) of the conventional polishing equipment has no relative motion with the grinding wheel end face. When the polished surface is in contact with the grinding wheel, the distribution of the initial contact points (or surfaces) on the entire polished surface has a decisive significance, because the polishing process will be extended along these points (or surfaces) to the periphery later on, and if there are fewer initial contact points (or surfaces) (which is known as the poor fit) and the initial contact points (or surfaces) are concentrated locally, then only these points or surfaces can be removed later on, and the other points (or surfaces) can contact with the grinding wheel end face. If the initial contact points (or surfaces) are few (called poor coincidence) and relatively concentrated in the local area, then only when these points or surfaces are removed, the other points (or surfaces) can be contacted with the end face of the grinding wheel, which makes the surface polishing become “de-measurement polishing”, and due to the hardness of the polycrystalline diamond and the poor cutting ability of the grinding wheel used for polishing, the expansion of the contact points (or surfaces) to the surrounding area is quite slow, which results in the excessively long polishing time and decreased machining efficiency. The processing efficiency is reduced.
(2) Since the contact line between the rotating center of the workpiece and the end face of the grinding wheel is fixed, even if all parts of the polished surface are in contact with the grinding wheel (perfect match), when the width of the polished surface of the workpiece is larger than the width of the ring of the grinding wheel, the contact geometry is different in different places, and the contact geometry of the outside of the workpiece is obviously less than that of the middle, which makes it easy for the polished surface to have refracted rings with different shades and fails to meet the quality requirements.
(3) Due to the polishing process, the middle part of the polished surface never leaves the end surface of the grinding wheel, the middle part is subjected to frictional heat more than the periphery, and the conditions of heat dissipation are poor, for large-area, thin-thickness PCD products, the localized heat leads to the deformation of the workpiece is aggravated.
(4) Stress deformation causes irregularities in the plane shape of the polished surface, accompanied by distortions. The only way to make the end face of the grinding wheel as close as possible to the polished surface (more contact points on the polished surface and a uniform distribution) is to trim the end face of the grinding wheel or to abrade the polished surface and the end face of the grinding wheel against each other. However, the stress deformation is random, the surface distortion makes the surface condition of each workpiece very different, when the grinding wheel end face is trimmed or abraded to match with the PCD surface of a workpiece, and then change to another workpiece, there is a mismatch, and it must be trimmed or abraded again by the grinding wheel to reach a new match, which is not only inefficient, but also increases the labor intensity of the operator, which is inappropriate for the large volume production.
(5) Due to the surface shape difference of each PCD product, it is very unlikely that the end face of the same grinding wheel can be trimmed to coincide with the PCD faces of two workpieces. Even if they can be matched at the same time, the friction temperature generated by a grinding wheel polishing two large-area PCD products at the same time is even higher, and the middle of the workpiece can not be detached from the grinding wheel (poor heat dissipation conditions), in the traditional polishing equipment (with the same grinding wheel) polishing two large-area PCD products at the same time, not only is there a big difference in the machining time required by each piece, but also too high a temperature, which is easy to burn the surface to be polished.
IV.Determination of Improvement Programs
From the above analysis, it can be seen that during the polishing process, it is important to increase the degree of contact between the polished surface and the end face of the grinding wheel in order to improve the polishing efficiency. During the polishing process, allowing the center of rotation of the polished surface to move along the radial direction of the grinding wheel, together with the adaptive contact function, is advantageous to improve the degree of contact (especially for the mid-convex deformation of the polished surface). When the center of rotation of the workpiece leaves the original contact line on the end face of the grinding wheel, some of the original contact points (surfaces) on the polished surface with the end face of the grinding wheel will be detached (microscopically, the end face of the grinding wheel is staggered along the radial direction, and it is not a flat surface), and the originally formed stable contact state is broken, and some points (surfaces) which are not in contact with the end face of the grinding wheel will be in contact with the end face of the grinding wheel with the cooperation of the adaptive contact function. With the adaptive contact function, some points (surfaces) which are not in contact with the end face of the grinding wheel are now in contact with the end face of the grinding wheel, thus increasing the new contact points (surfaces) and improving the mutual contact and matching state, thus shortening the polishing time.
The displacement of the center of rotation of the workpiece on the grinding wheel face also has the following advantages:
(1) The polishing process is a mutual abrasion process between the grinding wheel and the surface to be polished, the workpiece displacement at the same time, the high point of the grinding wheel end face will be trimmed flat, which not only eliminates the ring-shaped refractive ring that may appear on the surface to be polished, but also reduces the difficulty of the grinding wheel end face flatness trimming.
(2) The contact ratio between the middle and edge of the polished surface and the end face of the grinding wheel is balanced, so that the workpiece is heated evenly everywhere; furthermore, most of the polished surface is moved out of the end face of the grinding wheel, which improves the heat dissipation of the workpiece and reduces the thermal deformation of the workpiece during the polishing process.
(3) Reducing the time difference between workpieces with different surface deformation conditions to be polished at the same time. Since the polished surface is self-adapted to the grinding wheel end face, there is no need to dress the grinding wheel end face to adapt to the polished surface. In addition, the improvement of heat dissipation conditions of the workpiece enables the same polishing equipment (with the same grinding wheel) to polish two large-area PCD products at the same time.
There are several ways to realize the relative displacement of the contact line between the center of rotation of the polished surface and the end face of the grinding wheel: one is to rotate the spindle (grinding wheel) at high speed and carry out eccentric oscillation at the same time; the other is to shift the center of the workpiece along the radial direction of the grinding wheel or to oscillate at a certain angle (the center of the pressure point is not detached from the end face of the grinding wheel), and according to the introduction of the data, the polishing equipments produced in overseas countries adopt the high speed rotation of the grinding wheel and carry out eccentric oscillation at the same time.
Using the principle of double rocker oscillating mechanism, we have designed an adaptive fixture for rotating, pressurizing, oscillating and contacting the laminating plate, which allows the workpiece to oscillate within a certain angle (the center pressure point is not away from the end face of the grinding wheel) to realize the advantages of the polished surface’s center of rotation being displaced on the end face of the grinding wheel:
(1) Low cost, relatively simple mechanical structure, can maintain the basic structure of the traditional polishing equipment remains unchanged (only change the fixture part).
(2) Since the polishing environment is worse (with dust), adopting the traditional screw and guide rail transmission mechanism, the cost is higher, and the dust is easy to enter into the moving parts, which reduces the service life (the protection is more difficult); adopting the cam or pneumatic or hydraulic mechanism, the structure is complicated, and there are many accessories in the periphery; and the four-link mechanism is not only simple in structure, but also has strong reliability of movement, and the protection is simple, and strong adaptability to environment, so it is suitable to be applied in the polishing equipments.
(3) The Double Rocker Oscillating Mechanism of the 4-Link Mechanism can be used to realize the spinning of the workpiece and the oscillation of the center of rotation within a certain angle at the same time with one drive motor.
(4) Many parts of the original fixed structure fixture can be utilized without increasing the complexity of operation.
V.Comparative Experiments and Analysis
We conducted a comparative machining experiment with a new structure (equipped with a double-rocker oscillating polishing fixture) and a traditional polishing machine (equipped with a fixed polishing fixture).
First of all, choose the workpiece whose polished surface and grinding wheel’s end face shape basically matches to test, and choose the new grinding wheel. On the traditional polishing machine, even if the new grinding wheel basically matches the polished surface, the polishing effect is poor because the grinding wheel’s end face is not round enough (the polished surface is not bright and there are scratches), and on the new structure of polishing machine, the quality of the polished surface meets the requirements, which proves that in the displacement of the workpiece, the polished surface has a dressing effect on the end face of the grinding wheel.
After that, we choose the workpiece which is convex in the polished surface to test, in the traditional polishing machine, after processing for a period of time to observe, the polished surface of the polished surface and the uncontacted place has clear boundaries, if we don’t trim the end face of the grinding wheel, and continue to polish, not only the time is long, but also the middle of the PCD layer may be thinned out. In the new structure of the polishing machine, processing the same time to observe, the surface of the polished place and the non-contact place boundary blurred, and the edge of the contact marks, after polishing to check the surface flatness of the polished surface and the basic consistency of the pre-polishing. After polishing, the flatness of the polished surface is basically the same as that before polishing. We choose the workpiece with concave polished surface to test, the situation is basically the same, only the polished surface expands from the edge to the center.
We carry out processing efficiency comparison experiments, each taking 10 sets of equipment, respectively measuring processing time, the results show that: in the traditional polishing machine even if the grinding wheel end face basically matches with the polished surface, the average processing efficiency of the new structure of the polishing machine is still 20% higher than it.
We processed two pieces of workpieces (with the same amount of deformation of the polished surface) on a polishing machine with two fixed polishing fixtures, and found that the temperature of the workpieces was fast and high, and the processing had to be interrupted from time to time (otherwise the surface of the workpieces was burned) if the grinding wheels were not kept sharp, and that when one of the polished surfaces was in full contact with the end face of the grinding wheels, the other one had only 50% of the contact traces. Two pieces of workpieces are processed simultaneously on the polishing machine installed with two double-rocker oscillating polishing fixtures, and the cycle of dressing the grinding wheel is the same as that of single-piece processing with the traditional polishing machine, and the result shows that: not only the quality of the polished surface meets the requirements, but also the processing time of each piece increases by only 10% or so compared with that of single-piece polishing, and the difference in the time spent on polishing the two pieces of workpieces is very small, which proves that the processing efficiency of the polishing machine with two double-rocker oscillating polishing fixtures is 80% more than that of the traditional machine. This proves that the processing efficiency of polishing machine installed with two double swinging polishing fixtures is 80% higher than that of traditional polishing machine.
VI.Conclusion
(1) If the surface to be polished does not match with the end face of the grinding wheel in polishing, the processing efficiency can be improved significantly (at least 20%) compared with the conventional polishing equipment and the heat dissipation condition of the workpiece can be improved obviously.
(2) The polished surface of the workpieces processed by the improved polishing equipment will not have a ring-type refractive halo, and the surface quality will be significantly improved.
(3) For the improved polishing equipment, the grinding wheel end face dressing is less difficult than the traditional polishing equipment, where the grinding wheel end face not only needs to be dressed smoothly, but also needs to coincide with the polished surface as much as possible. In the new structure of polishing equipment, the dressing of grinding wheel end face only needs to make the abrasive grains on the grinding wheel sharp at the stage when the surface to be polished and the end face of the grinding wheel coincide with each other.
(4) The improved polycrystalline diamond surface polishing equipment realizes the simultaneous polishing of two large-area polycrystalline diamond products by one piece of equipment, thus increasing the equipment utilization rate by 80%.
