CBN (Cubic boron nitride) is an abrasive whose hardness is second only to diamond. CBN grinding wheel has many advantages such as high grinding efficiency, high machining accuracy and long service life. In particular, the application of ceramic bond CBN grinding wheel greatly promotes the grinding process, and it is also very suitable for the grinding of bearing parts. How to achieve high-quality grinding of CBN grinding wheels, equipment, tools, materials, grinding conditions, grinding fluid, processing environment and other factors cannot be ignored.
01 Comparison between CBN grinding wheel and ordinary grinding wheel
The grinding capacity (cross-sectional area ground per unit time) of an ordinary grinding wheel is generally 1 to 3 mm3/(mm•s). It uses alumina or silicon carbide abrasive grains with a hardness of 2100~2500 HK. The strength of the bonding agent that holds the abrasive grains together is low. Due to the grinding force and self-sharpening, the abrasive grains often break and fall off, and the surface condition of the grinding wheel Changes occur frequently, the service life of the grinding wheel is short and needs frequent dressing, and the processing efficiency is low.
The grinding capacity of CBN wheels is 1-15 mm3/(mm•s), the hardness of CBN abrasive grains is 4700HK, and the CBN abrasive grains are consolidated together with high-strength bond (ceramic bond), and the abrasive grains are broken or fall off The incidence is very low. Therefore, as the processing progresses, the surface state of the grinding wheel changes little, and the grinding accuracy can be maintained for a longer time.
02 Grinding surface texture
The key point of high-quality grinding is the surface texture of the grinding. Not only must the surface roughness, straightness, roundness and other grinding accuracy be guaranteed, but also the aesthetics when the light is reflected from the ground surface. For CBN grinding wheels, even if the surface roughness is the same as that of conventional grinding wheels, grinding streaks (vibration marks) may be more likely to appear in chattering in the vertical direction, and uniform scaly patterns may appear in the grinding direction.
03 How to achieve high-quality grinding of CBN grinding wheels
The grinding surface is affected by many factors such as equipment, tools, materials, grinding conditions, grinding fluid, processing environment, etc. It is impossible to obtain a high-quality grinding surface if these factors are ignored. In order to achieve high-quality grinding with CBN grinding wheels, measures need to be taken:
1) Establish visual measurement technology/analysis method of surface texture
The optical interference non-contact three-dimensional profiler is used to add a variety of analysis methods to the measurement data, so that the scaly pattern can be truly reproduced, measured and analyzed. Quantitatively associate the actual change of the ground surface with the factors causing the bad surface texture, and effectively improve the surface texture according to the contribution rate of the problem factors.
2) Analyze the factors that cause bad surface texture
Through the measurement of the grinding cross-section, the frequency analysis of the surface unevenness based on the equipment specifications and grinding conditions, etc., to determine the cause of the chattering and scaly patterns. The results show that it is mainly related to the frequency of the grinding wheel, and the influencing factors include : Vibration of the grinding wheel spindle rotation system, grinding wheel correction, grinding conditions and grinding force, etc.
04 Development of high-quality grinding technology for CBN wheels
1) Perfect damping countermeasures
In order to reduce the vibration caused by equipment-related factors during the micro-feed processing, measure the respective amplitudes of the axle, motor and pulley, and assemble these components after correcting the dynamic balance. Finally, a complete vibration countermeasure will be established near the grinding point. The vibration of the front part of the grinding wheel is controlled at a set level.
2) High-precision dressing technology
In the CBN grinding wheel cutting edge modification process, a rotary diamond dresser is used to obtain a sharp grinding wheel cutting edge, and the size of the cutting edge is adjusted to make it uniform to optimize the modification state.
3) Optimization algorithm for surface texture
When grinding a rotating workpiece, the relative vibration between the grinding wheel and the workpiece is minimized, but it cannot be completely eliminated. For forced vibration caused by disturbance and self-excited vibration caused by changes in cutting performance, the use of algorithms can reduce the unevenness of the grinding surface.
4) Development of low grinding force grinding wheel
In order to ensure that the final grinding surface is uniform at a small feed rate, a CBN grinding wheel with reduced grinding force is developed. In order to ensure the cutting performance of the grinding wheel, the focus is on the type of abrasive grain, the shape of the abrasive grain, the reduction of the binder content and the improvement of the binder.
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