In the manufacturing of PCD tools, choosing the suitable machining technology and grinding wheel can improve the efficiency for grinding PCD tools, ensure the quality of sharpening PCD tools and save the cost.
1 Description of PCD tools
With the rapid development of modern science and technology, the variety of cutting tools made of superhard materials such as polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) is becoming more and more abundant, and their performance is constantly developing and improving. Blade abrasive grain size ranges from ten microns, several microns to nanometers. Diamond is divided into low, medium and high content.
The bond has metal, non-metal and mixed materials. PCD layer thickness is from millimeter to micron. PCD layer and tungsten carbide substrate are bonded in plane and wave surface. PCD layer has different features of high wear resistance, high toughness, and high heat resistance.
At present, PCD and PCBN tools are widely used in automotive, aerospace, precision machinery, household electrical appliance, wood, electronic and electrical industries. They are used to making turning tools, boring tools, milling cutters, drill bits, reamers, countersinks, saws, carving tools, razors, insert, blank, etc.
Although PCD tools have developed fast, there is the difficulty of tool sharpening caused by their high hardness for most users. Re-sharpening of blades is mainly finished by the original tool manufacturers. It is not only high price and long delivery time but also occupies the current funds of enterprises. Therefore, it should study the grinding features of PCD and sharpening technology of PCD tools.
2.Manufacturing technology of PCD tools
The manufacturing technology of PCD tools usually includes polishing, cutting, bonding, sharpening, quality inspection and etc. The diameter of PCD superhard material blank is usually 1/2”, 1”, 2”, 3” and 4”, and its surface is usually rough(Ra2～10μm), which cannot be directly used to make tools. It needs grinding and polishing to make the surface of the PCD superhard material mirror (Ra≤0.01μm).
Then it is processed to a certain geometric shape and size by laser cutting or wire electrical discharge machining(WEDM), and then it further does the mechanical and chemical treatment for bonding surface of tool and matrix. And then it is applied to silver-based brazed technology for bonding the tool on the matrix, and finally sharpening by the diamond grinding wheel.
One of the key PCD cutting tool manufacturing technology is the sharpening quality of cutting edge. No ideal sharpening technology for high-quality tool materials will cause resource waste. Good sharpening technology will improve the tool quality and reduce the cost of the tool.
3.The features of grinding PCD tools
PCD is sintered by special treatment diamond and few agglomerant at high temperature and extra high pressure. The disorderly arrangement of diamond grains makes PCD have uniform, extremely high hardness and wear resistance. PCD can be used for cutting tools, grinding wheel dressing, geological drilling, measuring tool probe, wire drawing dies, sandblasting die, and etc. However, high hardness and wear resistance of PCD also brings great difficulties in its processing.
Scholars at home and abroad have done a lot of research and experiments on the problems caused by high hardness and wear resistance of PCD materials, including EDM, ultrasonic machining, electrochemical machining, laser machining, and etc. and get some results. However, comprehensive analysis shows these technologies are most suitable for rough machining of PCD material. Diamond grinding wheel for PCD tools is still an ideal choice for good PCD cutting edge quality.
The grinding process of PCD is mainly the result of mechanical and thermochemical effects. The mechanical effect is that diamond grains of diamond grinding wheel impact continuous impact on PCD materials and form diamond crushes, wear, falling off or cleavage. The thermochemical effect is that high temperature formed by diamond grinding wheel grinding PCD makes the oxidation or graphitization of diamond. The 2 actions are to remove PCD materials. The grinding features are mainly as follows:
Large grinding force
Diamond is the hardest material in minerals, and the wear amount of friction with various metal and non-materials is only 1/50-1/800 of tungsten carbide. The hardness of PCD(HV) is 80-120KN/mm2, which is the second only to monocrystalline diamond and much higher than that of tungsten carbide. When the diamond grinding wheel is used for grinding PCD, the initial cutting strength is very high, which is about 10 times higher than that of tungsten carbide (0.4MPa). Therefore, the grinding force of PCD is much higher than that of tungsten carbide.
Small grinding ratio
Owing to high hardness and wear resistance of PCD (16-199 times wear resistance of tungsten carbide), the grinding ratio of PCD is only 0.005-0.033, which is about 1/1000-1/100000 of tungsten carbide, and the grinding efficiency is only 0.4-4.8mm3/min. The grinding time is very long and processing efficiency is low in order to ensure the cutting edge quality and removal. In addition, when the hardness, content and grain grit of PCD is different, the grinding time of PCD varies greatly.
The great effect of grain grit
PCD tools are mainly divided into 3 types according to the grain grit: coarse(20～50μm), medium（10μm）,fine（5μm）.The difference between grinding force and ratio is several to tens of times. The grinding ratio of coarse grit PCD tool is the highest, and it is the most difficult for grinding. And the sawtooth shape of cutting edge is the most serious after grinding, and quality is the worse, but the wear resistance is the strongest. The grinding ratio of fine grit PCD tool is the lowest, and it is easy for grinding and the quality of cutting edge is the best after grinding.
4.Requirements of fixture for sharpening PCD tools
Based on the above grinding features of PCD, the requirement of sharpening equipment for PCD with a diamond grinding wheel is much higher than that of general tool grinder.
The machine tool has a good process system rigidity
Because the hardness of PCD material is high, so the grinder must have a high anti-deformation ability, especially the spindle system and tool clamping system. The grinding force of PCD tool is 100-500N when sharpening. Therefore, the machine tool is required to have a large diameter, high axial rigidity, and strength of bearings.
The machine tool has a short-range swing mechanism with adjustable stroke and speed
The grinding ratio of PCD is very low. The grinding mechanism of PCD is mainly that diamond grinding wheel continuously impacts PCD materials, and form crushes, wear, falling, cleavage and oxidation, graphitization and thermochemical action. Therefore, it is good for improving the grinding efficiency and cutting edge quality by short-range swing mechanism. Usually, the swing distance is 0-50mm and the swing speed is 20-60times/min.
The fixture of the machine tool has a high precision rotary function and on-line measuring device.
Owing to high hardness, brittleness and wear resistance of PCD materials, usually design the tooltip as arc-shaped and it is good for reducing the relative vibration amplitude of the tool and the workpiece. The tool clamping of a machine tool should have a high precision rotary function and on-line-measuring device for tooltip arc radius size and quality. In this way, it can avoid positioning error caused by multiple clamping, and double improve the machining efficiency.
5.1 Selection of sharpening PCD tool technology
One of the purposes of cutting tool grinding is to obtain the high-cost performance ratio of the cutting edge quality. And the key to the quality is the choice of grinding wheel grit size. The finer the grinding wheel grit size, the smaller the cutting edge break, and the lower the grinding efficiency is. So there are processing stages: rough, semi-finish, finish for PCD tool sharpening technology according to the precision and application of cutting edge( see Table 1) or invalid expired( see Table 2). Make reasonable sharpening technology can greatly improve the processing efficiency according to the detailed conditions.
Table 1 Classification term of the precision and application of cutting edge
Item
Cutting edge precision
Grinding wheel grain size
Application
a Rough
0.05mm
230/270#-320/400#
Rough machining
b Semi-finish
0.02mm
M20-M40
Semi-finish machining
c Finish
0.005mm
M5-M10
Finish machining
Table 2 Classification in term of invalid expired of cutting edge (re-sharpening PCD tools)
Item
Invalid expired
Grinding wheel grain size
Notes
a Rough
Damage of cutting edge 0.5mm
230/270#-320/400#
Electromachining
b Semi-finish
The collapse of cutting edge 0.3mm
M20-M40
c Finish
Wear of cutting edge 0.1mm
M5-M10
There is no high requirement for cutting edge when rough machining, and it can be electro machining or grinding. Electromachining has high efficiency and is suitable for processing complex cutting tools such as drills for printed circuit board, forming a milling tool for cutting hardened wood flooring, and etc. Coarse grit grinding wheels can be chosen for grinding, which has a large contact area and high grinding force(300-400N) and can quickly remove extra machining allowance. Fine grit grinding wheels are chosen for finish grinding, which has small contact area, low grinding force(100-200N) and low grinding heat but low material removal rate. It can further improve the quality of cutting edge by grinding and polishing.
5.2 The main points of sharpening technology
(1) The spindle accuracy is good, and the end run out of the grinding wheel should be less than 0.02mm. If the end runout is too big, and the grinding wheel intermittently impacts the cutting edge, which causes easily the cutting edge to collapse, and it is difficult to obtain a high precision cutting edge.
(2) The grinding wheel should have a dynamic balance. The unbalance of the grinding wheel will lead to the vibration of the machine tool, which will affect the cutting edge quality and machining accuracy of the tool.
(3) Vitrified bond diamond grinding wheels should be first chosen for sharpening wheels.
Because the vitrified bond cracks easily in the grinding process, and make the abrasive grains renew and self-sharpened, and the grinding process is stable, which is good for improving the surface accuracy and efficiency. Secondly, resin bond diamond grinding wheel with high heat resistance can be chosen.
(4) Pay attention to the suitable grinding wheel cutting edge and the appropriate grain size of stones. When diamond grinding wheel for grinding PCD tool, the wheel will block, passivated, high temperature and quick wear, resulting in the reduction of processing speed and it will have chatter marks, noise, and burns. Usually, choose soft silicon carbide stones with finer grain size 1-2# than the grinding wheel as the cutting edge stones.
(5) Because it is easy for diamond diffuse chemically with ferrous alloys to speed up the wear of wheels, so it should avoid grinding metal and PCD at the same time as far as possible.
(6) The turning direction of the grinding wheel must turn from the rake face to the flank. From the force of cutting edge of PCD tool, it can be seen that the grinding force( sum of tangential and normal force) acts on the cutting edge inward, that is , the tool is under the compressive stress and not collapse easily when the grinding wheel turn form the rake face to the flank face. Otherwise, the tool is under the tensile stress, and the cutting edge collapses easily. If reverse sharpening because of the structure of the tool, should choose resin bond diamond grinding wheel. It is better than the vitrified and metal bond diamond grinding wheel.
(7) The clearance angle is divided into large and small clearance angle in order to ensure the quality of cutting edge and improve the grinding efficiency. Because of large contact area, big grinding force and high sharpening efficiency, the large clearance angle is first used for grinding by coarse grit grinding wheel. And then small clearance angle is used for grinding by fine grit grinding wheel so that the width of the small clearance angle is controlled within 0.1-0.3mm, and the sharpening quality is good, and the contact surface is small.
(8) The cutting edge of the tool can be machined in one time of clamping as far as possible.
(9) It should choose water-based grinding fluid as the cooling fluid for sharpening PCD tools.
Owing to the high hardness and poor heat resistance of PCD materials, the cooling effect of water-based grinding fluid is better than that of oil-based grinding fluid, which can improve the processing efficiency and edge quality. In addition, it needs sufficient cooling, not interrupt the flow to avoid the great consumption of diamond (grinding wheel, cutting tool) and cutting edge damage caused by the small amount of grinding fluid or intermittent supply.
Conclusion: Vitrified bond diamond grinding wheel can improve the efficiency for grinding PCD tools, ensure the good surface quality for sharpening PCD tools, save the labor cost.