Resin bond CBN grinding wheel is suitable for grinding workpiece such as high-speed steel, cast iron, stainless steel, and nitriding stainless steel. It has the advantages of good self-sharpness, small grinding force, no blocking, and high processing efficiency.

However, the resin bond CBN grinding wheel is not a panacea. When choosing different CBN abrasives and processing different materials, the grinding performance is different, so what is the result? Let us wait and see, speak with data.

Description of resin bond CBN grinding wheel:

The bonding agent of the CBN grinding wheel in this experiment is polyimide (PI) resin. At present, one of the development trends of the resin bond CBN grinding wheel manufacturing technology is high universality.

For an example of manufacturing key parts of automobiles, engine, gearbox, steering pump, air conditioner, and bearing refers to 11 different parts of 9 materials. It should design and manufacture grinding wheels with different performance according to the traditional idea due to the big difference in hardness of parts of different materials.

With the downward pressure on economic development at home and abroad, auto parts manufacturers are eager to reduce costs and increase efficiency and reduce the frequency of replacing grinding wheels, which need resin bond CBN wheel can meet the grinding requirements of more than 3 different material parts.

Dr. Chen studied the preparation process and grinding performance of polyimide (PI) resin cubic nitride grinding wheel, and obtained the double exothermic peak curing reaction mechanism of polyimide(PI) resin, and chosen the grinding parameters of 45# steel.

Dr. Cui studied the effect of the dressing process parameters of the resin CBN grinding wheel on the surface morphology and grinding effect, obtained the conclusion that the grinding effect of the grinding wheel after the curved plate method sharpens is good.

Dr. Liu obtained the parameters of strength, high-temperature resistance, and corrosion resistance of different CBN abrasives by studying the comparison of the abrasive performance of vitrified bond CBN grinding wheels. It is the quantitative reference data for the abrasive selection of vitrified bond CBN grinding wheel.

The above research provides new ideas and methods for the development and application of resin CBN wheel, but the above paper does not mention the CBN abrasive characteristics of the resin bond grinding wheel and its effect on the grinding performance of the grinding wheel.

Study the static crushing load, impact toughness TI and TTI of different CBN abrasives and test the grinding ratio of resin bond CBN grinding wheel made by different abrasives grinding cast iron, high-speed steel, stainless steel, and nitriding stainless steel.

Analyze the causes for the difference from the perspective of the microstructure of the abrasive and the combination of the bonding agent, which lays a foundation for the development of high universal grinding wheels.

1. Experimental conditions and processes

The experimental conditions and methods are as follows:

1.1Abrasives: 6 kinds of CBN abrasives, which are CBN-B1500, CBN-B2000, CBN810, CBN850, CBN115, and CBN120.

1.2Measure the static impact toughness(TI) of the abrasive by Diatest S1 intensity meter.

1.3 Put the abrasives in a CYCJ-B type TTI furnace, heating to 870 ° C, held for 120 mins, and measure the thermal impact toughness TTI by CYCJ-91 measuring instrument.

Observe the surface morphology of CBN abrasives by Inspect S50 scanning electron microscope.

1.4 The experimental grinding machine is WTG CNC universal grinding machine. Face grinding by resin bond CBN grinding. The specification is 6A2T 125×6×110×10×3 CBN 80/100 B100. The grinding ratio is the ratio of the workpiece removal rate to the wear amount of grinding wheel during the grinding process.

1.5Workpiece material: gray cast iron (HT250, HB240), high-speed steel (W6Mo5Cr4V2, HRC62-67), stainless steel （1Cr17, HB80）, nitrided stainless steel（1Cr17, liquid nitriding, HV900-1100）, workpiece size is 100mm × 18mm × 18mm.

1.6 The dressing tool is WA80(vitrified bond, hardness K) honing stone, the size is 80mm×25mm×25mm, the dressing method is in-process dressing, and the grinding fluid is water-based coolant.

1.7 Experimental grinding wheel: face grinding wheel made of CBN abrasives and resin bond by forming and precision machining. The formula of the grinding wheel is shown in Table 1. In addition to the different types of abrasives, the conditions of the hot-pressed forming technology, precision machining, and precision testing requirements of the grinding wheel are the same. The main parameters of the grinding experiment are shown in Table 2. The experimental data are the average of 3 times of experimental data.

 

Table 1 Formula of grinding wheel

 

Table 2 Grinding process parameters

 

Experiment steps:

First install the experimental grinding wheel, workpiece and dressing tool, and turn on the cooling water, dress the grinding wheel, close the coolant after installation, and install the workpiece, and start the toolset. Then set the grinding process program, and turn on the coolant, and do the grinding experiment according to the setting technology.

The program setting grinding process parameters mainly include machining allowance, feed amount, grinding wheel and workpiece RPM and feed rate. After finishing the program, turn off the coolant. Stop the rotation of the grinding wheel， and remove the workpiece and the grinding wheel.

After cleaning and drying, test the wear amount and calculate the grinding ratio. Measure the size and surface quality of the workpiece, and finally record and analyze the experimental results.

2 Analysis and discussion of results

2.1 The values of static pressure crushing load, impact toughness (TI) and thermal impact toughness (TTi) of different CBN abrasives are shown in Fig. 1. The surface topography of different CBN abrasives is shown in Fig. 2.

 

Fig.1 Static pressure crushing load, impact toughness(TI) and thermal impact toughness(TTI) of different CBN grains

 

 

Fig. 2 Surface morphology of different CBN grains

 

It can be seen from Fig.1a that the static crushing load of different CBN grains differs greatly. The Max static pressure crushing load of CBN120 grain is 34.2N, and the Min static pressure crushing load of CBN810 grain is 26.1N. Fig.1b shows that TTI of different grains is lower than that of TI.

It shows CBN grains cause the oxidation of impurities inside the abrasive and the rearrangement of atoms in the abrasive crystal during the calcination of the high-temperature furnace, resulting in a decrease of the impact toughness of the CBN abrasive. The two largest decreases of abrasives are CBN115 and CBN-B1500, TTI and TI of CBN115 are 458.8% and 57.2% respectively, the decrease of 14.7%. TTI and TI of CBN-B1500 are 54.0% and 61.2% respectively, the decrease of 11.8%.

The CBN static pressure crushing load is related to the crystallization state of the abrasive grains and is greatly affected by the inclusions, impurities and crystal defects. It can be seen from Fig.2 that the surface of CBN120 is smooth and the crystal shape is good. While the crystal form of CBN810 is irregular and there are many surface defects. The test results also show CBN120 grain has a larger static pressure crushing load than that of CBN810 grain.

2.2 Grinding ratio of CBN grains for the workpiece of different materials

According to the experimental parameters set in Table 2, do the grinding experiments of different CBN grinding wheels for different workpieces. The grinding ratio of different CBN grinding wheel on grinding different material workpieces is shown in Fig.3.

 

Fig.3 Grinding ratio of different CBN grinding wheel on grinding different materials

3 As can be seen from Fig.3a:

3.1 The grinding ratio of 6 kinds of CBN grains for cast iron HT250 materials is high, the grinding ratio is 147.9-213.9, of which CBN-B1500 is the highest and CBN850 is the second. For the same abrasive, the grinding ratio of high-speed material is lower. The reason for the analysis is that the hardness of cast iron HT250 is low.

There is a slender strip of graphite in its microstructure, which acts as cutting action on the cast iron material so that the grinding wheel can easily remove the cast iron material. The cast iron HT250 material with low grinding strength has low requirements on the strength of CBN grains. The previous static pressure crushing load test shows that the value of CBN-B1500 is lower.

The reason why the grinding of the cast iron material is relatively high is: CBN-B1500 grain has both rough surface morphology and suitable strength, and the low strength abrasive is matched with the low holding power of the resin bond.

Because the resin bond and the abrasive grains are mainly mechanically fitted, it is difficult to form the chemical bond for the interface between the two. The bond has a low holding force on the abrasive, and the abrasive with the high static pressure crushing load may not be firmly held during the grinding process, and it will not drop off.

3.2 The grinding ratio of 6 kinds of CBN grains grinding high-speed steel material is 76.2-95.0, and the grinding ratio of CBN850 grains is the highest. Analysis: the hardness of high-speed steel material is high, and it is HRC62-67, workpiece materials contain more tungsten, molybdenum, vanadium and other carbides, so CBN grains for high-speed steel is relatively low.

Because the hardness of these carbides is close to that of CBN grains, the low strength CBN-B1500 grains have a lower grinding speed on high-speed steel, which is in sharp contrast to the results of grinding cast iron HT250.

It can be seen from Fig.3b.

（1）The grinding ratio of 6 kinds of CBN grains for stainless steel and nitriding stainless steel material is relatively low. The grinding ratio of stainless steel material is 75.0-83.9, of which CBN-B1500 is the highest, and CBN115 grains is the second. The main reason for the analysis is that the stainless steel material is the soft-adhesive material.

During the grinding process of soft materials with soft-adhesive and toughness, the grinding debris easily adheres to the surface of the grinding wheel, forming the block and burn the workpiece. When the stainless steel is ground, there is wire-like wear debris, which enhances the drawing force of the workpiece on the abrasive, so that the grains fall off before being passivated, resulting in a low grinding ratio.

In addition, after the grains fall off, the grinding wheel cannot grind the workpiece due to there is not abrasive on the surface, which is likely to cause heat in the grinding zone, and burn ablate the resin bond while burning the workpiece. Therefore, when processing stainless steel soft materials, CBN grains are required to have low impact toughness and high micro-crushing ability, and the results show that the grinding effect of CBN-B1500 is good.

(2)  The grinding ratio of 6 kinds of CBN grains for nitriding stainless steel is 25.2-32.0.

The reason is as follows: nitriding stainless steel is a kind of hard and tough material, and the scaly skin produced on the surface of the workpiece after nitriding tends to block the surface of the grinding wheel and reduce the height of the edge of CBN grains on the surface of the grinding wheel.

The hardness of the white hard layer under the nitride stainless steel fish scale layer is as high as HV1100, which is one material that is extremely difficult to grind. It is also required that the resin bond has a high holding strength for the CBN grains.

Although the grinding ratio of CBN-B1500 grains is relatively high, the absolute value is still very low, and the grinding ratio of several CBN grains is not much different. Further research will be made to solve the grinding problem of nitriding stainless steel.

Compared Fig.1 and Fig.3, it can be found that when the resin bond is weakly held by the abrasive, the static pressure crushing load of the CBN abrasive, the impact toughness TI and the thermal impact toughness TTI and the grinding ratio data for different materials are not forming a linear correspondence.

Therefore, when making the resin bond CBN wheel, it is not easy to judge the quality of the CBN grains based on the strength data, but the CBN grains should be optimized according to the grinding features of different workpiece materials and the mechanical strength of the resin bond.

The features of the abrasive and the bond in the resin bond CBN wheel by mechanical fitting force determine that the CBN grains with both rough surface and static pressure crushing load is an excellent choice.

Conclusion:

Under the conditions of the same technology and bonding agent, the grinding ratio difference of the grinding wheel made by different CBN grains is big. Resin CBN grinding wheels have a higher grinding ratio for cast iron and high-speed steel materials。

but the lower grinding ratio of stainless steel and nitriding stainless steel material. CBN-B1500 and CBN850 grains have the relatively good grinding performance for several workpiece materials.