Metal bonded grinding wheels are superhard wheels made of CBN or diamond power mixed with metal bonded by sintering or electroplating.
1 Introduction of metal bonded grinding wheels:
At present, grinding processing with superhard fine abrasive wheel( diamond grinding wheel and CBN grinding wheel) is high precision and high efficient method for hard and brittle materials. The metal bond grinding wheel features strong abrasive holding force, high bonding strength, good wear resistance, long service life, and can withstand large grinding pressure, and is widely used superhard abrasive grinding wheel in precision and super precision grinding for hard and brittle materials.
However, the self-sharpness of the metal bond wheel is poor, and the surface is easily blocked, and the surface burns during the grinding process, which affects the quality of the workpiece and not easy dressing. It is difficult to get abrasive grain edge and the exposure is difficult to maintain.
In order to solve the defects of the metal bond wheel in the precision grinding process, the preparation of the metal grinding wheel, the formulation design of the bonding agent, and the dressing method have been continuously studied at home and abroad.
2 Types of metal bonded grinding wheels
2.1 Sintered metal bonded grinding wheels
A sintered metal bond wheel is mostly made of metal such as bronze and cast iron by high-temperature sintering method. It has high bonding strength, good formability, high-temperature resistance, good thermal conductivity, wear resistance, long service life and can withstand the big load.
There is a problem of uneven distribution of abrasive grains in the conventional sintered metal bond grinding wheel. Dr. Wang used direct mixing and wrapping to sinter and prepare SiC abrasive metal bond grinding tools, and SiC grains are more uniformly dispersed.
Dr. Xiao from Nanjing University of Aeronautics and Astronautics has studied metal bond diamond abrasives by mechanical mixing caused by the multi-layer abrasive uniform distribution technique for the current uneven distribution of abrasive grains, low efficiency, and burn of bond and diamond.
In order to avoid the oxidation of diamond or other damage to diamond during sintering and the reduction of the bonding performance of diamond grains and the bond, Ihara has developed agglomerated diamond wheel structure by meal coating diamond grains or multi-coating metal layers of abrasive grains bonded together.
2.2 Porous metal bonded grinding wheels
The bond of the traditional metal bond grinding wheel is relatively dense, and there are few or almost no pores in the grinding wheel, which greatly affects the grinding performance and the efficiency of the wheel, and dressing is difficult.
In order to improve the grinding and dressing ability of metal bonded wheels, Tanaka, a researcher of Ritsumeikan University in Japan, proposed the porous metal bonded wheel (PMBDW) in 1992 which introduces the pore structure into the wheel to obtain the advantages of easy dressing, convenient use, and etc.
Japanese scholars Onishi, Tomino, and Trong have developed the hot isostatic press, pulse electric current sintering and vacuum sintered method for a porous metal wheel.
2.3 Electroplated metal bonded grinding wheels
Electroplated diamond wheels are usually coated with nickel or nickel alloy. In order to simplify the manufacturing method of the electroplated diamond grinding wheel and adjust the concentration of the diamond abrasive grains on the working surface of grinding wheel, the invention indicates that the thickness of metal bond agent is less than 1/2 of the height of diamond abrasive grain, and the size of the filler is 1.5-5.0 times of that of diamond abrasive grain.
Dr. Yu has done the electroplated process testing of ultrasonic electroplated diamond wheels and studied the abrasive grain density testing to obtain the reasonable ultrasonic electroplated process and improve the performance of electroplated diamond tools.
2.4 Monolayer vacuum brazed wheel
In order to overcome the poor strength of the bond agent of electroplated wheel and falling off the abrasive grains, try to increase the holding force of the metal bond on the abrasive grains and improve the bonding strength of the grinding wheel. In the early 1990s, scholars at home and abroad began to study the monolayer high temperature brazed superhard grinding wheels by brazing instead of electroplating. Trenker achieved the strong bond between the diamond and the matrix by vacuum brazed method of active solder and nickel-based solder, improving the strength, performance and service life of diamond tools.
Cubic boron nitride(CBN) has high chemical stability, and it is more difficult to braze than diamond brazed. Dr. Xiao used Ni-Cr and Ag-Cu-Ti active brazing filler to sinter monolayer CBN grinding wheel, which improved the bonding strength of the abrasive and the life of the grinding wheel.
Dr. Yang used laser brazed method to analyze the diamond abrasive grains. It was pointed out that the elements in the brazing process are mutually diffused to form the chemical metallurgical bond, which is the main factor of high bonding strength of the Ni-Cr alloy layer and steel matrix.
2.5 Metal bond additive element
Add carbide to form the element, rare earth element, and other elements in the metal bond agent. The study has shown that adding the rare earth element La, Ce, which can improve the bonding strength of the diamond and the matrix, the mechanical performance of the carcass, the height of the diamond edge, and the self-sharpness of the diamond tools.
In order to meet the performance requirements of metal bond grinding wheels, Luciano proposed to add Si (<2wt.%) to improve the wear resistance of matrix and the holding force of the diamond abrasive grains in the matrix materials. Truong added Sn in the Ni-Cu bond and then vacuum sintered diamond wheel,
the bond bridge composed of crystal between abrasive grains is more significant. Add Co, Cr, etc. carbide to form the elements in the bronzed bon, which can improve the bonding state of the metal matrix and the diamond abrasive grains to some extent.
3 Metal bond grinding wheel dressing
3.1 Electrolytic in-process dressing (ELID)
In 1985, Murata first proposed the Electrolytic in-process dressing(ELID), and grind the hard and brittle materials such as ceramic by 400# metal bond diamond wheels. In 1990, OHMORI of the Institute of Physical and Chemical Research of Japan successfully solved the problem of dressing cast iron-based grinding wheel with ELID and made the ultra-fine diamond wheel( grain size of several micrometers to 5nm) for hard and brittle materials mirror grinding by ELID.
Islam did electrolytic in-process dressing experiment with 8000 mesh cast iron-based bond diamond wheel and studied the effect of current on the grinding performance.
3.2 Electron discharge machining (EDM)
Electron discharge machining refers to remove the excess metal material on the surface of the grinding wheel by thermal energy or electric discharge between the electrode and the rotating metal bond diamond grinding wheel, achieving the purpose of dressing the grinding wheel.
Dr. Wang from Tsinghua University has studied the electron discharge machining method of the bronze bond diamond grinding wheel, and discussed the pulse power voltage, pulse current limiting resistor, power pulse frequency, pulse duty cycle, grinding wheel leaner velocity, the initial eccentricity of the grinding wheel, etc effect factors on the speed of dressing.
Miller used EDM technology to trim the metal bond diamond grinding wheel to obtain the good diamond abrasive grain edge height. Japanese scholars did EDM on the diamond wheels and did the grinding experiment of BK7 glass to obtain the surface roughness of Rz=7 μm. Mr. Gao from Tianjin University used deionized water as the discharge medium to dress the cast iron-based precision diamond wheel with electron discharge machining technology. The roundness of the grinding wheel was 0.75μm and the taper error was less than 0.5μm.
3.3 In-process electro-discharge dressing (EDD)
In-process electro-discharge dressing (EDD) technology is a kind of dressing technology for the super-hard abrasive wheel, which is first proposed by Suzuki and Uematsu.
It is to etch the metal bond agent on the surface of the grinding wheel by pulse electron discharge between the diamond roller dresser(rotary dresser) and the tool electrode, making the diamond abrasive exposed to achieve the purpose of sharpening and dressing.
In order to eliminate the requirements of special brushes and insulating spindles, Suzuki and Tamaki proposed the “ two-electrode method” and rotary electrode method” respectively, so that the electron discharge dressing is gradually applied to the actual practice.
Spanish Sanchez used a single electrode electron discharge method to achieve the dressing of the metal bond CBN grinding wheel. Related research shows that good surface and grinding performance can be obtained by dressing the metal bond superhard abrasive grinding wheel with the mist jet electron discharge dressing technology.
3.4 Electro-contact discharge dressing (ECDD)
Electro-contact discharge dressing(ECDD) was first proposed by Tamaki and Kondoh in 1999. It is to form the current loop and generate the instantaneous discharge and partially remove the metal bond at high temperature by the metal bond of the grinding wheel to contact with the metal chips, achieve the purpose of dressing the grinding wheel.
XieJin from South China University of Technology and Tamaki from Kitami Institute of Technology in Japan used contact discharge dressing to dress 600# diamond wheels. Ra is 0.12 μm after grinding the optical glass(BK10). It is shown that the electrolyte largely determines the performance of the electro-contact discharge dressing by the study of electrolytes.
3.5 EDM in gas dielectric
The EDM is a gas dielectric method was first proposed by Kunieda and Yoshida of Japan in 1997. It is to remove the vaporized and melted workpiece material, making the discharge energy highly concentrated in the minimum zone by high-speed airflow jetted from the electrode of the tubular tool, achieving the purpose of removing the bond agent.
3.6 Laser dressing method
The laser dressing method utilizes the optical system to focus the laser beam into the very small spot to act on the surface of the grinding wheel, and the metal bond material on the surface is removed by evaporation vaporization and melting sputtering in the very short time.
The surface of the grinding wheel forms the chip space for the purpose of dressing. Scholars from many countries such as Japan, India, Germany, USA have begun research on laser dressing method very early. Among them, Japanese scholars have many studies.
Japanese scholar Kunidea used frequency multiplier Nd: YAG laser on cast iron-based diamond grinding wheels to do dressing experiment. Hosokawa of Kanazawa University in Japan used the YAG pulse laser to dress the bronze bond diamond wheel, which can effectively and reliably remove the diamond wheel.
Dr. Chen from Hunan University used the acousto-optic Q-switched Nd: YAG laser to perform the micro-abrasion and dressing test for the bronze bond diamond wheel. The effects of laser power density, average powder and pulse repetition frequency on the ablation pit depth were obtained.
3.7 Ultrasonic vibration dressing method
The ultrasonic vibration dressing method was proposed by the Bulgarian Institute of Technology scholars. It is to drive the end face of the tool for ultrasonic vibration by ultrasonic energy, forcing the abrasive grains in the mixed oil abrasive to continuously hit with large speed and acceleration, polishing the surface of the grinding wheel, and finishing the dressing of the grinding wheel.
Dr. Gao did the technical research on the rapid and precise dressing of metal bond diamond wheel by elliptical ultrasonic-assisted mechanical dressing technology. It can realize low-cost rapid dressing of fine-grained diamond grinding wheel by selecting the reasonable dressing parameters.
CBN /diamond metal bonded grinding wheels are the important superhard tool in the precision grinding. Therefore, it is still the main direction for improving the strength, grinding efficiency, precision retention, durability and research of new dressing technology of the grinding wheel.