CBN abrasive grains have high hardness and thermal conductivity, and also have excellent thermal stability and chemical inertness, especially at high speed, which has more unique. The electroplated CBN grinding wheel is a typical superhard abrasive grinding wheel, which has the simple molding process and no truing and dressing during the grinding process.
Description:
High-speed grinding can achieve a higher surface quality of processing efficiency than that of the conventional grinding. In the actual grinding process of the workpiece, combining the high speed grinding technology with the traditional grinding technology to optimize the machining process can achieve better processing performance, and the material of the workpiece to be processed has machinability to some certain extent.
Since the electroplated CBN grinding wheel has only one abrasive layer, when the abrasive grains are worn, there is no supplement of the subsequent abrasive grains. As the grinding wheel wears, the geomorphology of the grinding wheel will also change to some extent, thereby affecting the surface quality of the workpiece to be ground.
The grinding wear of the electroplated CBN grinding wheel for hardened bearing steel by Shi Z and Malkin S shows that the main wear form of the electroplated CBN grinding wheel is crushed and peeling off the abrasive grains.
When the radial wear of the grinding wheel reaches 70%-80% of the size of the abrasive grains, there is peeling of the abrasive layer, which causes the failure of the grinding wheel. Upadhyaya R P has studied the effect of grinding wheel wear on the heat distribution coefficient.
It is found that the dynamic effective grain number of the grinding wheel surface will increase and become the main cause of the increase of the wear surface area of the grinding wheel with the increase of radial wear of the grinding wheel.
Frank C G improved the cooling effect of the electroplated CBN grinding wheel in grinding superalloy by improving the nozzle design, effectively avoiding the failure of the grinding wheel caused by the adhesion of the grinding wheel.
At present, the research on the wear of electroplated CBN grinding wheels mainly focuses on the condition of 30-40m/slow speed, which has achieved remarkable results.
80/100 mesh electroplated CBN grinding wheel is used for studying the nickel-based superalloy grinding wheel at the leaner velocity of 120m/s, and seek to increase the law of change and the main form of wear of the wheel with the increase the removal volume of the workpiece grinding force and grinding topography at high speed.
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Experiment
- Experiment conditions
The machine used in the experiment is BLOHM PROFIMAT MT-408 made in Germany, the spindle power is 45KW and the maximum RPM is 8000r/min. The working surface width of the electroplated CBN grinding wheel is 10mm, and the outer diameter of the grinding wheel is 400mm. The force measuring device is KISTLER 9272 made in Switzerland three-phase piezo citric type force transducer. Hirox KH-7700 three-dimensional stereo microscope is used for observing the wheel surface.
- Experimental methods
Nickel-based superalloys are widely used in the field of aircraft engine manufacturing.
In this experiment, GH41169 nickel-based superalloy is used as the workpiece material, and the workpiece width is 5mm, 30mm. The parameters used in the grinding experiment are shown in Table 1. The workpiece material removal volume is 3mm³/(mm.s). In order to observe the durability of the grinding wheel under the removal rate conditions, the grinding experiment was done according to the parameters of Table 1.
Table 1 Grinding Parameters
| Grinding Parameters | Value |
| Linear Velocity of Wheel Vx/(m/s) | 120 |
| Cutting Depth ap/mm | 0.06 |
| Feed Speed of Workpiece Vw(m/min) | 3 |
During the grinding experiment, the change of the grinding force is monitored in real-time, and the changing trend of the grinding force is taken as the main criterion for the degree of wear of the grinding wheel.
In order to observe the change of the geomorphology of the grinding wheel with the increase of the workpiece material removal volume of the, analyze the main wear form of
the grinding wheel, as well as the change of the number of dynamic effective abrasive grains actually participating in the grinding of the working surface of the grinding wheel, and the change of the wear area ratio of the abrasive grains, and the wear amount of the abrasive grains, and etc. When the workpiece material removal volume increase 3500mm³, observe the morphology of the surface of the grinding wheel.
Along the circumference of the grinding wheel, select a fixed point and mark it every
120°. When observing the morphology of the grinding wheel, use the additive silicone rubber impression material to make the grinding wheel replica at the marking place. The length of the replica is about 30mm.
When the workpiece material removal volume reaches 35000mm³, the vibration of the grinding machine has increased significantly, indicating that the grinding wheel has experienced significant wear at this time. Therefore, stop the experiment in order to protect the spindle of the grinding machine, not allow the grinding wheel to reach the limit of its durability.
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Results and analysis
- Grinding force analysis
The grinding force is directly related to the wear state of the grinding wheel, the surface roughness of the workpiece, the grinding force specific energy, and etc. And also the grinding force will be obtained and analyzed easily, so the grinding force can be used as an important index for evaluating the grinding state.
The changing relationship between the grinding force and the workpiece material removal volume is shown in Figure 1.
Whether it is up grinding or down grinding, the normal grinding force increase rapidly from 30N to 80N with the increase of the workpiece material removal volume, and the tangential grinding force increases rapidly from 12N to 24N.
When the workpiece material removal volume is more than 4800mm³, the normal force and the tangential force increase steadily as the workpiece material removal volume increases. This is because in the initial wear stage of the grinding wheel, the abrasive grains on the surface of the new grinding wheel are relatively sharp, and the number of the dynamic effective abrasive grains actually involved in the grinding is few, the workpiece material is more prone to chipping, scratching, and the actual contact area of the grinding wheel and the workpiece is small, and then the grinding force is small.
As the workpiece material removal volume increases, the original sharp abrasive grains may be blunt or crushed due to the weak strength of the sharpening edge. The part of the abrasive grains initially involved in the grinding may also drop rapidly due to the weak of bonding strength between the abrasive grains and the bonding agent layer, make the dynamic effective abrasive grain number increase, the contact area between the grinding wheel and the workpiece, so the grinding force has a process of first increasing rapidly and then increasing steadily.
It is found that when the workpiece material removal volume is more than 10000mm³,
the normal grinding force increases when down grinding is significantly higher than that of up grinding as the workpiece material removal volume increases. Also, the difference between the two will become larger and larger as the workpiece material removal volume increases.
When the workpiece material removal volume is 35000mm³, the normal force difference between up grinding and down grinding is 24N. Because the workpiece material removal is accompanied by the wear of the grinding wheel, the difference of the normal grinding force of the up grinding and down grinding can be used as one of the indicators for evaluating the wear of the grinding wheel.
The force signals measured during the experiment are shown in Figure 2, the left force signal is the normal grinding force and the tangential grinding force signal waveform when up grinding. The right is the normal grinding force and the tangential grinding force signal waveform when down grinding.
Observe the grinding force signal, it is found that the tangential grinding force of single stroke is unchanged, and the normal grinding force will change to a certain extent. As shown in Figure 2, the normal force gradually increases when up grinding, the normal force gradually reduces when down grinding.
As shown in Figure 3, the normal grinding force shows the climbing shown in Figure 2., because there is a converging wedge-shaped space near the contact arc between the grinding wheel and the workpiece. Due to the wedge effect of hydrodynamic lubrication, there form a dynamic pressure caused by the coolant near the contact arc of the grinding wheel and the workpiece, especially at high-speed grinding, which is particularly obvious. Since the direction of the dynamic pressure is perpendicular to the surface of the grinding wheel, it mainly affects the normal force and has little effect on the tangential force.
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Surface morphology analysis of the electroplated CBN grinding wheel
After finishing the experiment, observe the surface of the grinding wheel under the three-dimensional stereo microscope. It was found that there was almost no adhesion on the surface of the grinding wheel, and the abrasive grains falling off or the large area was crushed. The main wear form was abrasive grain wear and micro crushed of the abrasive grains.
Figure 4 shows the wear of the same abrasive grain on the surface of the grinding wheel when removing the different workpiece materials volume by replica technique track. It can be found that the abrasive grain mainly shows wear, and as the workpiece material removal volume increases, the area of the wear platform gradually increases.
As shown in Fig. 5, as the workpiece material removal volume increases, the wear ratio of the working surface of the grinding wheel first increases slowly and then increases rapidly. When the workpiece material removal volume is 10500mm³, the wear area ratio of the grinding wheel is only 0.13%. When the workpiece material removal volume is 35000mm³, the wear area is 1.75%.
In the initial wear stage of the grinding wheel, the number of effective abrasive grains involved in the work is few, so even if the abrasive grains involved in the initial grinding wear are faster, there is little effect on the increase of the wear area ratio of the surface of grinding wheel. When the workpiece material removal volume is more than 10000mm³, the grinding wheel enters the stable wear stage.
At this time, the number of dynamic effective abrasive grains actually involved in the grinding has increased greatly relative to the initial wear. So the wear rate of the grinding wheel surface will change rapidly and steadily as the wear of abrasive grains.
Conclusions:
1. With the increase of the workpiece material removal volume, the normal force and the tangential force both increase rapidly and then increase steadily. In the stable grinding stage, the normal grinding force during down grinding is gradually higher than that during up grinding, and the difference between the two reflects the state of wear of grinding wheel.
- When the linear velocity of the grinding wheel is 120m/s and the workpiece removal rate is
3mm³（mm.s）, the main wear form is abrasion wear when the superalloy is ground by electroplated CBN grinding wheel. With the increase the workpiece material removal volume, the wear area ratio shows the trend of increasing slowly and then increasing rapidly, and the wear area ratio has good correspondence with the grinding force.