CBN grinding wheel in grinding

Overview of CBN grinding wheels in grinding

CBN grinding wheel has the characteristics of heat resistance, good thermal stability, chemical inertness with iron family, sharp sharpening edge, excellent grinding performance, low grinding tension, low temperature rise, no burns and cracks on the workpiece surface, grinding surface There is residual compressive stress, which makes the dimensional accuracy and geometric accuracy of the ground workpiece greatly improved compared with the ordinary grinding wheel, which improves the performance of all aspects of the workpiece, but there are some problems in the use of the CBN grinding wheel.


First of all, we can use the conventional modification and sharpening method to modify and sharpen the CBN grinding wheel. In theory, the ceramic bond is brittle and brittle and should be able to be trimmed using conventional finishing methods. However, tests have shown that the current dressing method is not suitable for the new CBN grinding wheel. In order to get the best shape and sharpening (collectively trimming) effect, we must determine a new set of working parameters and conditions. By studying better conditions of use, CBN wheel dressing and machine control technology have made greater progress.

The precise setting of the dressing process and the advanced adaptive control make the user's CBN grinding wheel work at its best. Without ceramic bonding agents, CBN grinding wheels are unlikely to enter mass production. It is this porous binder material that creates the necessary small voids to preserve the coolant and ensure the finishing of the CBN to grind the ferrous metal.

CBN grinding wheel dressing method

CBN abrasives are an expensive abrasive and the grinding wheel manufacturer must provide a dressing method that consumes as little abrasive as possible so that the user can grind the workpiece as much as possible before discarding the grinding wheel. In general, the CBN wheel can be re-sharpened by simply removing a few microns.

At the same time, manufacturers must find an effective way to expose the cutting edge of the abrasive particles. We use the Austrian Taylorly B64 CBN grinding wheel as a typical abrasive. According to reports, this level of abrasive grinding synthetic defects are controlled at sub-micron levels. Telelite is able to modify the rupture characteristics of abrasive particles and produce abrasives with good controlled burst strength. In the grinding test, we found that the ratio of dressing feed speed to dressing speed has a great influence on the particle rupture characteristics. The dressing speed ratio refers to the ratio of the dressing line speed to the grinding line speed. The speed ratio is positive in the dressing contact position, and the grinding wheel rotates in the same direction as the dressing wheel.

In a study of the B64 type ceramic bond CBN grinding wheel, the grinding wheel was trimmed by a rotating diamond wheel at a speed ratio of +0.2. When the dressing amount was 1 μm, the CBN abrasive grains showed micro cracks because it retained A large number of cutting edges and concentrated on the surface of the abrasive particles, micro cracks are beneficial to the sharpness of the grinding wheel.

When the trimming amount was increased to 2 to 3 μm, significant macroscopic cracks were found. When macroscopic cracks appear, important parts of the abrasive grains are lost. As a result, there are only a few sharp cutting edges on the surface of the grinding wheel, which results in many bad cutting surfaces. A large amount of expensive abrasive particles fall off and the surface abrasive density is reduced, which shortens the service life of the grinding wheel and reduces the cost performance. These results indicate that minor changes in trim feeding can easily affect the effects of the trimming process. In order to make the necessary fine-tuning, CBN users must have a high-resolution trim feed mechanism.

The result of changing the trim rate is similar to changing the trim depth. Because the trimming force caused by the dresser rotating in the same direction as the grinding wheel is greater than the normal grinding force, the trim removes more ceramic binder and less abrasive particles. When the dressing rate is increased from +0.2 to +0.8, more binder is removed and the macro-cracks of the abrasive particles are also exacerbated. Even when the feed depth is 1 μm, a large part of the abrasive grains have been broken. According to the above results, the user cannot think that the higher the ratio, the better the removal of the binder only because of the high dressing ratio. If the high speed ratio exacerbates the macro crack, the dressing of the grinding wheel leads to accelerated wear of the grinding wheel. During the dressing process of the large-sized grinding wheel for grinding the outer cylindrical surface, it was also found that the increase of the dressing speed ratio and the feed depth caused the grinding wheel to change from micro crack to macro crack, which should be paid special attention because the large abrasive grain is more likely With the defects that become the starting point of the crack later, it is easier to distinguish the difference between the two cracks when we observe the large crack.

Through other studies, we have recognized the importance of using a suitable dressing rate to ensure proper contact of the dressing wheel with each abrasive grain on the CBN wheel. Ideally, an abrasive grain can only be sharpened by a diamond on the dressing wheel. If the number of diamonds per abrasive particle is less than one, trimming will miss some unblind blunt abrasive particles. In general, the rotation of the dressing wheel causes the diamond to act more than once for each abrasive grain, i.e., the effective dressing of each abrasive grain more than once. Due to the surface of the abrasive particles, each sharpening action after the first time is carried out under micro-feeding. The multiple sharpening caused by the feed rate produces a relatively unsharp surface. Through years of research on the grinding effect of the grinding wheel, we have revised the design of the dressing device. We have found that it is relatively easy to obtain a good dressing effect and to further calculate the appropriate dressing speed for a given wheel speed and CBN grain size.

We carefully studied the surface of the ceramic bond CBN grinding wheel after trimming and use, and found that the dressing made the density of the surface layer CBN abrasive grains and ceramic bond of the grinding wheel lower than other parts of the grinding wheel. The ceramic bond ordinary grinding wheel also has such a surface layer, which is called a surface metamorphic layer. This surface layer can range from a few microns to 30 microns deep. As the wheel is used, the depth of the metamorphic layer will continue to increase and the effect on the surface of the wheel will continue to strengthen.

The dressing after the grinding wheel can eliminate the increasingly deeper and more harmful surface deterioration layer in use. Since the dressing depth of the ceramic bond ordinary grinding wheel is much larger than the ceramic bond CBN grinding wheel, the dressing condition of the ceramic bond ordinary grinding wheel is even more so. The trimming feed depth ensures that almost all the grinding and deterioration layers are removed, resulting in only one A new surface layer consisting of abrasive particles and binder determined by the trimming parameters. However, when the ceramic bond CBN wheel is trimmed, the dressing depth may be much smaller than that of the metamorphic layer, which profoundly affects the performance of the wheel after dressing.

To fully repair the CBN wheel, the user must have a dresser with a large feed depth. We also found that the trimming of four times of co-feeding of 2 microns compared to a trimming of 2 micrometers, the macro cracks are minimal, and the trimming eliminates the metamorphic layer.

Absorptive phase

By studying the properties of CBN and binders, manufacturers can redefine the finishing process. However, even if the dressing process is optimized according to the above theory, the grinding wheel needs to be combined with the loosening phase of the abrasive particles and the residual binder.

Under normal circumstances, after a certain number of parts are ground and trimmed, the grinding force, the wear of the grinding wheel and the surface change are shown in Figure 1. After dressing, the grinding force is relatively large, but the grinding force drops rapidly after grinding several parts. Typically, 2% to 10% of the parts of each trim cycle are affected by this change. At this stage of unstable wheel performance, the quality of the part is also affected by the eccentricity of the workpiece and the grinding wheel shaft, while the grinding wheel wears faster and the surface roughness increases rapidly. To solve these problems, the user must check the performance of the grinding wheel after each part is ground and make the necessary adjustments before the next part is ground.

The dressing and use of the grinding wheel and the grinding of the metamorphic layer directly determine the extent to which the grinding force is reduced after dressing. In general, the grinding force of the new grinding wheel is reduced faster than the grinding wheel that has been used for several finishing cycles.

After the initial shedding period, the grinding surface quality is gradually improved, the grinding force tends to be stable, the wear rate is slowed down, the grinding metamorphic layer is gradually deepened, and the grinding wheel is sharpened by grinding away the binder during grinding. Finally, there are too few grinding wheel binders and the grinding wheel begins to wear quickly. At this time, the wear reduces the diameter of the grinding wheel, the grinding wheel is no longer in contact with the grinding workpiece, and the grinding force drops rapidly.

Ensure precise contact between the dresser and the grinding wheel

CBN grinding wheel manufacturers have developed new technologies to meet the technical requirements for dressing per step, total feed dressing, dressing rate and lateral dressing speed. These measures are achieved by contact sensors and adaptive control.

When trimming the outer cylindrical grinding wheel, the manufacturer designed a system that would enable a shallower dressing wheel. The cylindrical grinding system is relatively rigid and the change in grinding force generally does not degrade the quality of the workpiece. The user should ensure that the surface of the grinding wheel is not too smooth to avoid burning the workpiece, so it can be roughened. To obtain a grinding wheel with satisfactory characteristics, the user should choose to produce only small micro-cracks and maintain the dressing parameters of the CBN abrasive grain density on the surface of the grinding wheel.

Figure 2 shows a typical wear curve of a grinding wheel using a variety of CBN grinding wheels to grind cylindrical surfaces, especially camshafts. As shown in Fig. 2, when the cylindrical surface grinding is started, the wear of the grinding wheel is generally 2 to 3 μm, and the special case is up to 10 μm. In order to grind more workpieces for each dressing, the user can trim the wheel when the wheel wears 15μm or more. In order to maintain the appropriate deterioration layer of the grinding wheel, the user has to trim the grinding wheel by a few microns. The general dressing parameters are: cutting speed ratio is +0.2~+0.5, the total dressing depth is 3μm~10μm, and the dressing depth is 0.25~3μm. .

According to the above theory, we use Telilai B64 CBN grinding wheel, Italy Jinfei (GAMFIER) electric spindle, Telilai diamond cup dresser, 3MZ203 automatic internal grinding machine, grinding rolling bearing inner ring 6203/02, the result of each grinding 200 pieces, dressing wheel 2μm, grinding workpiece size dispersion 4μm, ellipse 2μm, taper 2μ m, surface roughness Ra value 0.2μm, grinding ratio increased by 70 times. The most difficult part of maintaining these parameters is the heat transfer of the machine during dressing. During the dressing process, the change of machine temperature causes the positional deviation between the dresser and the grinding wheel to be even larger than the dressing amount. To detect and compensate for these errors, the user needs some method to accurately measure the relative position of the dresser and the grinding wheel.

Most of the time, the relative position of the dresser to the grinding wheel is detected when the dresser is in exact contact with the grinding wheel. Some methods of detecting grinding power or grinding force are used to detect contact. Methods for detecting grinding forces using piezoelectric effects or strain gauges are widely used for internal grinding. A new method uses a set of piezoelectric elements to detect the grinding force and translate it into the feed motion of the dresser.

Another method uses the principle of acoustic emission. The AE sensor is the most effective and simple method for external cylindrical grinding. The AE sensor operates in the frequency range of 50-600K, picking up the sound signal of the dresser diamond trimming CBN abrasive grain, and most of the devices can accurately pick up the sound frequency of the given grinding wheel speed and the grinding size wheel.

The location of hardware such as fixed sensors has a large impact on the accuracy of the sensor. If the AE sensor is not reliably attached to the machine, the noise of the spindle bearing will block the sound signal. If the sensor is mounted on the dresser spindle housing, the signal will also weaken after passing through the bearing. Users can estimate the signal-to-noise ratio with an analog signal meter. Most AE systems can provide a pattern of picked-up signals. On these graphs, the dresser trimming the wheel's signal is represented as a pulse peak on a low background noise signal. We tested the AE Acoustic Emission System installed on a CNC cam grinder with a high frequency rotary dresser, diamond dressing wheel and ceramic bond CBN grinding wheel. Before the dressing, the machine tool used to grind narrow parts and left a wear band in the middle of the surface of the wheel.

For the first time, the feed is 2.5μm per cycle, and the total length of the grinding wheel is 50μm. When the sensor detects a large amplitude, the last trimming cycle only feeds 1μm. When the dresser first touched the grinding wheel, the sensor detected two high points at both ends of the grinding wheel. This situation lasted for three cycles, and then the dresser was in contact with the entire grinding wheel surface, and the sensor detected a signal of 1 μm feed depth. Through experiments, the sensor system can detect the 0.25μm feed depth signal on the grinding machine without the influence of the spindle bearing noise.
However, the study found that there is a difference in the trimming results of the same feed dressing for one feed dressing and several feed dressings, generally 1 to 2 μm, and the coarse grinding grain outer grinding dressing feed amount is 8 to 10 μm. This difference is not enough. Seriously affecting the performance of the grinding wheel, reducing the amount of feed per trim will reduce this difference, but this will actually increase the dressing cycle time.

Another AE detection method that can eliminate bearing noise is being developed. In this method, the independent probe contacts the grinding wheel to obtain the acoustic signal to determine the position of the grinding wheel. When the grinding wheel is trimmed to the required amount, the probe contacts the position of the grinding wheel calibration probe and the dresser again, and compensates for the wear of the dresser. Since the probe is in close contact with the dresser, the heat transfer of the machine does not affect their relative position, but the probe wear will cause an error of 1 ~ 2μm. Another disadvantage of this method is that the system cannot analyze the signal of all the grinding wheels because the probe Only touch one position of the grinding wheel, and the contact process must be dry to avoid interference from the coolant error signal. Relatively speaking, these are secondary problems. Like the method of controlling the feed rate, the probe control method has been successfully used all over the world.

CBN grinding wheel internal grinding challenge

Internal bonding of ceramic bond CBN grinding wheels requires a new finishing condition. Modern internal grinding machines have ultra-precise feed accuracy, and the dressing time is usually much shorter than that of a cylindrical grinding machine because the ground workpiece is generally smaller and the grinding cycle is shorter. Therefore, when the grinding wheel grinds a certain amount of workpiece and trims it, the time for the inner grinding dressing should be shorter than the outer grinding. Generally, the dressing time is short enough that the heat transfer of the machine tool is insufficient to affect the relative position of the grinding wheel and the dresser. In fact, under stable operating conditions, the internal grinding machine can guarantee a total dressing amount of 1 μm. The internal grinding machine is generally stable, only High-precision systems use sensors to precisely guarantee ultra-high precision tolerances.

The biggest problem of internal grinding of CBN grinding wheel is the deformation caused by the change of grinding force during dressing. The inner grinding wheel of the grinding wheel is weak, and any change of grinding force will cause the deformation of the post. Affects the size, taper and roundness of the workpiece. In many cases, some measures can minimize this problem, one of which is to redesign the wheel post by reducing the length, increasing the diameter, and using a highly rigid material such as molybdenum steel or titanium carbide.

Optimizing the trimming parameters can also reduce the deformation of the post. The trimming parameters of the inner grinding are different from the outer grinding. The micro cracking of the inner grinding is not a problem, because the abrasive grains are much smaller than the outer grinding, so it can be added. The large cutting speed ratio increases the sharpness of the grinding wheel without worrying about the reduction of the abrasive grain density on the surface of the grinding wheel. The development of the rotary dresser is also to increase the sharpness of the grinding wheel and reduce the metamorphic layer. The total trimming feed rate is less than 2μm, the grinding wheel needs multiple dressings. The typical dressing parameters for internal grinding are: dressing speed ratio is +0.8, total dressing feed rate is ≤2μm, one dressing is in addition to high rigidity post and optimized dressing parameters, the grinding wheel manufacturer also A special ceramic bond has been developed which is beneficial for reducing the deformation of the grinding wheel in order to reduce the dressing grinding force. They have developed a porous construction wheel with a certain degree of durability. This type of grinding wheel has achieved great success in the automotive and bearing industries, especially in applications requiring dimensional tolerances of ≤ 2 μm and high efficiency. At the same time, because of the good lubrication effect, CBN grinding wheel grinding oil has better effect of cooling, and for other reasons, grinding tends to use emulsified oil or other synthetic materials, so the durability of CBN grinding wheel will be affected.

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