Introduction
CBN (Cubic boron nitride) is an abrasive second only to diamond in hardness. CBN grinding wheels have many advantages such as high grinding efficiency, high machining accuracy and long service life. In particular, the application of ceramic bond CBN grinding wheels has greatly promoted the process of grinding and is also very suitable for the grinding of bearing parts. How to achieve high-quality grinding of CBN grinding wheels, the factors such as equipment, tools, materials, grinding conditions, grinding fluid, and processing environment cannot be ignored.
01 Comparison of CBN grinding wheel and ordinary grinding wheel
The grinding capacity (cross-sectional area of grinding in a 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 combines the abrasive grains is low. Due to the grinding force and self-sharpness, the abrasive grains often break and fall off. The surface state of the grinding wheel Frequent changes, short service life of the grinding wheel and frequent dressing are required, resulting in low machining efficiency.
The grinding capacity of CBN grinding wheels is 1 ～ 15 mm3 / (mm · s), the hardness of CBN abrasive grains is 4700HK, the CBN abrasive grains are consolidated with high-strength binder (ceramic binder), and the abrasive grains are broken or fall off. The incidence is very low. Therefore, with the progress of the process, 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 to high-quality grinding is grinding the surface texture. Not only to ensure surface roughness, straightness, roundness and other grinding accuracy, but also to ensure the aesthetics when light is reflected from the grinding surface. For CBN wheels, even if the surface roughness is the same as using a conventional wheel, chattering in the vertical direction may be more prone to grinding fringes (chatter lines), and uniform scaly patterns may appear in the grinding direction.
03 How to achieve high-quality grinding of CBN grinding wheels
Grinding surface is affected by many factors such as equipment, tools, materials, grinding conditions, grinding fluid, processing environment and so on. Ignoring these factors cannot obtain a high-quality grinding surface. In order to achieve high-quality grinding with CBN wheels, measures need to be taken:
1) Establish visual measurement technology / analysis method of surface texture Use optical interference non-contact 3D profilometer to add a variety of analysis methods to the measurement data, so that the scaly pattern can be realistically reproduced, measured and analyzed. Quantitatively correlate the actual change of the ground surface with the factors that cause poor 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. By measuring the grinding cross section, frequency analysis of surface unevenness based on equipment specifications and grinding conditions is performed to determine the cause of chatter and scaly patterns. The results show that the main It is 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.
04 Development of CBN grinding wheel high-quality grinding technology
1) Perfect vibration reduction countermeasures In order to reduce the vibration caused by equipment-related factors during the micro-feed process, the respective amplitudes of the axles, motors and pulleys are measured, these components are assembled after correcting the dynamic balance, and finally perfect vibration countermeasures are established. , To control the vibration of the front of the grinding wheel near the grinding point at a set level.
2) High-precision dressing technology
During the dressing process of the CBN grinding wheel cutting edge, a rotary diamond dresser is used to obtain a sharp grinding wheel cutting edge, and the cutting edge size is adjusted to make it uniform to optimize the dressing state.
3) The surface texture optimization algorithm minimizes the relative vibration between the grinding wheel and the workpiece when grinding a rotating workpiece, but it cannot be completely eliminated. For forced vibration caused by disturbances and self-excited vibration caused by changes in cutting performance, algorithms can be used to reduce the unevenness of the ground surface.
4) Development of low grinding force grinding wheel In order to make the final grinding surface uniform at a small feed rate, a CBN grinding wheel with reduced grinding force was developed. In order to ensure the cutting performance of the grinding wheel, the focus is on the type of abrasive grains, the shape of the abrasive grains, the reduction of the binder content and the improvement of the binder.
Author: [date] H.MORITA
Source: JTEKT ENGINEERING JOURNAL
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