Superfinishing Technology Test for Ceramic Cylindrical Rollers

Abstract: It is difficult to obtain smooth and non - damage surface of ceramic cylindrical rollers by traditional abrasive tool, the polycrystalline nano - diamond abrasive tool is used for grinding cylindrical rollers. The polycrystalline nano - diamond abrasive tool is prepared by using polyacrylonitrile gel method to improve dispersibility of abrasive particles in abrasive tool. The influence of different abrasive grit ratio of abrasive tool, grinding pressure and grinding speed on surface roughness and material remove rate is studied, and contrast experiment is carried out for phenolic autoclave abrasive tool. The experiment result shows that the nano - diamond in gel abrasive tool is uniformly distributed and it has no obvious agglomerate phenomenon. The surface roughness Ｒa of ceramic cylindrical rollers decrease from 0.32 μm to 0.07 μm after grinding 30 min under the parameters of abrasive grit ratio 10, grinding speed 300 r /min, grinding pressure 0.5 MPa. 

Key words: cylindrical roller bearing; ceramic roller; polycrystalline nano - diamond; gel abrasive tool; grinding

The roller and raceway of cylindrical roller bearings are in line contact, with a large radial bearing capacity, suitable for high-speed and heavy-duty working conditions. With the increasing complexity of mechanical equipment, higher requirements have been put forward for the safe service, dynamic performance, and load-bearing capacity of cylindrical roller bearings. Ceramic cylindrical rollers have excellent characteristics such as light weight, electrical insulation, high temperature resistance, corrosion resistance, and wear resistance, and have been applied in special fields. A large number of experiments have shown that about 60% to 70% of high-speed bearing failures are caused by varying degrees of fatigue damage to the rolling elements. As an important component of precision ceramic bearings, the surface quality of ceramic cylindrical rollers directly affects the rotational accuracy and life of the bearings, especially defects such as surface unevenness and cracks that can reduce the high-precision operation of the bearings. At present, ceramic cylindrical rollers are processed using a centerless ultra precision machining method similar to steel cylindrical rollers. The oil stone oscillates back and forth on the rollers to improve surface quality. Due to the hard and brittle properties of ceramics, their machining process is prone to defects such as microcracks, scratches, and residual stresses on the surface of rollers, which limits the application of ceramic cylindrical roller bearings.

Polycrystalline diamond is composed of multiple sub crystals with embedded structures, which have irregular shapes and rough concave surfaces. The crystals are uniform and have good brittleness. During the grinding process, particles fracture along the interface between sub crystals. The worn sub crystals detach from the crystal particles and show new micro cutting edges. Its self sharpening ensures high precision and efficiency in processing, without scratches and burns, and the surface quality of the parts is better. Traditional resin bonded grinding tools use compression molding, but the dispersion of nanoscale diamond is poor. In the mixing process with micrometer sized bonding powder, due to the large difference in particle size, it is difficult to distribute uniformly in the binder. Especially during the hot pressing process, the surface area of nanoscale diamond is large, the activity is high, the oxidation resistance temperature is low, and the sensitivity to additives is high, which restricts the application of nanoscale polycrystalline diamond grinding tools.

In the following, polycrystalline diamond abrasive tools are made by using the new gel method to finish the ceramic cylindrical rollers, and the changes of surface roughness Ra and material removal rate of the cylindrical rollers are tested and analyzed under the technological conditions of different sanding ratio, grinding speed and grinding pressure of the abrasive tools.

1. Production and performance of polycrystalline diamond grinding tools

1.1 Polyacrylonitrile gel method

Polyacrylonitrile (PAN) is a high molecular weight polymer with a white powder shape and a density of 1 14-1 15 g/cm3, softened when heated to 220~230 ℃, and decomposed at the same time. It is composed of acrylonitrile monomer (AN) through arrangement, stacking and aggregation of several layers, and is mainly used for gel spinning. However, there are still few reports on the application of PAN in the production of grinding tools. When choosing PAN as the bonding agent for nano polycrystalline diamond grinding tools, the main considerations are:

1) PAN can be solidified by gel. As the bond of abrasive tools, its higher softening temperature can ensure that gel abrasive tools will not deform and soften as a whole during grinding;

2) When the grinding tool is worn unevenly for a long time, the local protrusion area has a large compressive load, high friction force, and high temperature rise effect. A softening temperature of around 220 ℃ can soften and smooth the local protrusion area, achieving the effect of self repairing the grinding disc.

1.2 Grinding tool manufacturing process

The nano polycrystalline diamond used in the experiment was obtained by directional blasting method, with a particle size range of 0.02-0.2 μ m; The average relative molecular weight of PAN is 75000; Dimethyl sulfoxide (DMSO), chemically pure. The basic process of preparing nanocrystalline polycrystalline diamond abrasive tools based on gel reaction consists of mixing gel making and gel curing. The preparation process is as follows: 1) mix deionized water and DMSO in a certain proportion, then evenly disperse a certain amount of PAN copolymer in the mixed solution containing DMSO/H2O, shake it on the vibrator for 30 minutes, put it in a 50 ℃ air drying oven for 2 hours, transfer it to a 90 ℃ air drying oven for high-temperature dissolution, and then stir and dissolve it with a mixer at 70 ℃ for 6 hours until it is completely transformed into a transparent solution without impurities. 2) Add nano diamond abrasive and silane coupling agent KH570 with mass fraction of 0.1% into the solution, use a homogenizing mixer or ultrasonic vibrator to fully disperse and evenly, defoaming the solution in a 70 ℃ vacuum oven, pouring it into the mold, cooling the gel at room temperature, and finally sintering in a heating furnace at 160 ℃.

1.3 Grinding tool performance

The properties of nano polycrystalline diamond abrasive tools prepared by gel method are related to the sand bond ratio (the ratio of nano diamond to PAN) and the ratio of water and solvent MDSO. The effect of deionized water on the strength of abrasive tools is not obvious. Its main role is to interact with solvent DMSO through hydrogen bond during the preparation of sol, weaken the interaction between PAN and DMSO, reduce the solubility of DMSO to PAN, and make PAN molecular chain prone to gel transformation. The ratio of water to DMSO was determined to be 1:39 in the experiment.

When the sand binder ratio is 12.5, the SEM photos of the interface between polycrystalline nano diamond abrasive and polyacrylonitrile gel binder in the prepared gel abrasives are shown in Figure 1. The abrasive particles are closely bound to the gel binder, and the nano diamond is evenly dispersed in the binder matrix, without obvious agglomeration; The size and distribution of pores are relatively uniform, and a higher porosity can play a cooling and chip holding role in ultra precision grinding. The bending strength, density, and porosity of the prepared grinding tool were measured by a universal mechanical testing machine and an electronic scale, and were 56.68 MPa, 2.15 g/cm3, and 10.75%, respectively.

Fig. 1 SEM Diagram of gel Abrasive Tool Surface

2. Grinding comparison test

2.1 Test method

To optimize the process parameters of grinding ceramic cylindrical rollers with a new type of grinding tool, grinding experiments were conducted on ceramic cylindrical rollers under different grinding tool sand ratio, grinding pressure, and grinding speed conditions, and the surface roughness Ra and material removal rate of the rollers were measured. The single factor test method is adopted, and the specific process test conditions are shown in Table 1. The experimental roller is a zirconia (ZrO2) cylindrical roller blank, with a size of 16 mm x 24 mm and a surface roughness Ra of (0.32 ± 0.03) μ m.

Table 1 Grinding process parameters for ceramic cylindrical rollers

In order to further analyze the abrasive particle dispersion of abrasive tools prepared by gel method, the cylindrical roller grinding contrast test was carried out by using gel abrasive tools and traditional hot pressing resin abrasive tools. The hot press grinding tool uses phenolic resin powder as the binder, and the abrasive particles are nano polycrystalline diamond abrasives.

The experiment used a self-made ceramic cylindrical roller grinding test platform, as shown in Figure 2. Two slightly inclined driving rollers drive the cylindrical rollers forward, and the upper pressure plate is adhered with grinding tool pellets, which are pressurized by the cylinder.

1- Cylinder; 2- Grinding tools; 3- Cylindrical rollers; 4- Drive roller

Figure 2 Self made cylindrical roller grinding test platform

2.2 Testing methods

Measure the surface roughness Ra of zirconia ceramic cylindrical rollers at different points three times using the Taylor Hobson surface roughness tester, and calculate the average value as the surface roughness Ra value under this grinding condition; Observing the surface morphology of the workpiece before and after grinding using a super depth of field microscope; The precision electronic scale measures the mass of cylindrical rollers and takes the average of three measurements, which is then converted into the material removal rate of cylindrical rollers, i.e

In the formula: MRR is the material removal rate, μ m/min; V is the volume of the cylindrical roller, mm3; Δ m is the change in mass of the cylindrical roller before and after grinding, g; S is the cylindrical surface area of the cylindrical roller, mm2; m0 is the initial mass, g; t is the grinding time, min.

3. Test results and analysis

3.1 The influence of sand to aggregate ratio on grinding tools

Choose a driving roller speed of 300 r/min and a grinding pressure of 0 The ceramic cylindrical roller grinding test results are shown in Figure 3 at 5 MPa and different sand ratios of 6.25, 7.14, 8.33, 10, and 12.5. As shown in the figure, when the sand to aggregate ratio is less than 10, as the sand to aggregate ratio increases, the material removal rate increases and the surface roughness Ra value decreases rapidly; When the sand ratio exceeds 10 and continues to increase, the material removal rate slightly decreases, and the surface roughness Ra value actually increases.

Figure 3: Effect of different sand ratio on material removal rate and surface roughness

This is because, when the sand content is relatively small, the effective abrasive density during the grinding process is low, which cannot effectively remove surface roughness peaks. Moreover, the pressure that a single abrasive particle experiences at the contact point is high, which intensifies the wear of the abrasive cutting edge. The relatively high content of the binder leads to the abrasive particle being firmly adhered by the binder and unable to fall off; As the sand ratio increases, the relative concentration of abrasives in the grinding tool increases, the opportunity for contact between abrasives and the workpiece surface increases, the material removal rate increases, and the surface quality improves; When the sand to aggregate ratio exceeds 10 and continues to increase, there is too much abrasive on the unit surface of the grinding tool, and the holding force of the binder on the diamond abrasive is weakened. During the grinding process, the abrasive particles fall off prematurely, the material removal rate decreases, and fine scratches are found on the surface of the workpiece. At the same time, it increases the wear of the grinding tool, shortens its service life, and reduces the accuracy of the disc shape.

3.2 Impact of grinding pressure

The ceramic cylindrical roller grinding test results are shown in Figure 4 under different grinding pressures of 0.1, 0.3, 0.5, 0.7, and 0.9 MPa, with a driving roller speed of 300 r/min and a sand to aggregate ratio of 10. As shown in the figure, when the pressure is low, the liquid flow layer between the workpiece and the grinding tool is thicker, the friction force at the contact point is relatively small, the material removal rate is low, and the surface quality of the workpiece is good; As the pressure increases, the friction force increases, the liquid flow layer becomes thinner, and the abrasive cutting edge exposes the liquid flow layer and performs micro cutting on the surface of the workpiece, improving the material removal rate. At the same time, the appropriate thickness of the liquid flow layer plays an effective lubrication role, which can achieve good grinding quality; When the pressure is too high, the cylindrical roller and the grinding tool are in solid solid contact, with almost no grinding fluid between the two. The friction force is high, and the depth of the abrasive particles pressing into the workpiece increases. The cutting amount of a single abrasive particle increases, and the material removal rate is high. At the same time, the abrasive particles have deep scratches on the surface of the roller, which is easy to cause surface scratches.

Figure 4: The effect of different grinding pressures on material removal rate and surface roughness

3.3 Impact of grinding speed

When the sand ratio is 10, the grinding pressure is 0.5 MPa, and different driving roller speeds are 100200300400500 r/min, the grinding detection results of ceramic cylindrical rollers are shown in Figure 5. As shown in the figure, the material removal rate of the ceramic cylindrical roller basically conforms to the Preston equation, and the material removal rate increases with the increase of the driving roller speed. The main reasons are: 1) the increase in the speed of the driving roller, the liquid dynamic pressure on the grinding area forces the speed of the grinding fluid passing through the contact point of the roller to increase, the liquid flow layer becomes thinner, and the cutting edge is exposed, which is conducive to removing the surface material of the ceramic roller; 2) As the speed of the driving roller increases, the speed of the roller relative to the grinding tool increases, the number of grinding times on the roller surface increases, and the material removal rate increases.

Figure 5: Effect of different grinding speeds on material removal rate and surface roughness

When the driving roller speed is less than 300 r/min, the surface roughness Ra of the roller improves with the increase of the driving roller speed. When the speed exceeds 300 r/min, the surface quality will have a deterioration process. This is because when diamond grinding tools grind ceramic rollers, the main reason is that the abrasive particles cause material fission and collapse after rolling and plowing on the roller surface. The material removal methods are plastic removal and brittle removal. With the increase of speed, the number of cutting times of the abrasive particles on the roller surface increases, the mechanical effect gradually strengthens, and the amount of brittle removal gradually increases. The roller surface is covered with pits formed by brittle removal, and the surface roughness Ra value gradually increases. This is beneficial for achieving ultra precision of the ceramic cylindrical roller to be achieved. Poor processing.

3.4 Comparative test of gel abrasives and phenolic hot pressing abrasives

Under the same grinding conditions with grinding pressure of 0.5 MPa, driving roller speed of 300 r/min and sand cement ratio of 10, the comparative test of gel abrasive tool and phenolic hot pressing abrasive tool grinding ceramic cylindrical roller is shown in Figure 6. It can be seen from the figure that the material removal rate of gel abrasives is higher, which can obtain better surface quality in a shorter time, and the corresponding processing efficiency is improved; At the same time, the material removal rate of gel abrasives did not decrease significantly with the increase of grinding time, while the material removal rate of hot pressing abrasives decreased rapidly after grinding for a period of time, indicating that gel abrasives have better self sharpening.

Figure 6 Comparison of material removal rates between two types of grinding tools

The surface morphology of ceramic cylindrical roller after rough machining on centerless grinder (before grinding) and grinding with gel abrasive tools and hot pressing abrasive tools is shown in Figure 7. It can be seen from the figure that the scratches on the roller surface are reduced after grinding with abrasive tools, and the surface quality of rollers processed with gel abrasive tools is better than that processed with hot pressing abrasive tools. After grinding with gel abrasives, the zirconia ceramic cylindrical roller is as shown in Figure 8. The cylindrical surface of the roller is smooth without obvious defects.

Figure 7 Microscopic photo of the surface of ceramic cylindrical rollers

Figure 8 Picture of Zirconia Ceramic Cylindrical Roller after Grinding with gel Abrasive Tool

The SEM diagram of the worn surface of gel abrasive tool is shown in Figure 9. As can be seen from the figure, there is a localized glazing phenomenon on the grinding tool, which can lead to a certain degree of decrease in grinding efficiency. However, as the machining process progresses, as the abrasive gradually wears out, abrasive detachment occurs, and the glazing phenomenon will be improved. Different from the traditional abrasive tools, the large abrasive particles after falling off will fall into the soft gel bond in the subsequent processing, without causing scratches or pits on the surface of the workpiece, which means that the abrasive tool has a "semi fixed" effect.

Fig. 9 SEM Diagram of gel Abrasive Wear Surface

4. Conclusion

1) The nano polycrystalline diamond abrasive tool prepared by the gel method is used to grind zirconia ceramic cylindrical rollers. When the sand binder ratio is 10, the driving roller speed is 300 r/min, and the grinding pressure is 0.5 MPa, a smooth and undamaged roller machining surface can be obtained. 2) The machining efficiency and precision of nano polycrystalline diamond abrasive tools prepared by gel method are higher than those of traditional hot pressing abrasive tools. 3) In the initial grinding and polishing stage of gel abrasives, the abrasive particles are mainly abrasive wear. With the progress of processing, the abrasive particles will gradually fall off. Due to the flexibility of PAN bond, the large abrasive particles falling off during the grinding process will fall into the abrasives, with a "semi fixed" effect, reducing the probability of scratches on the workpiece surface.

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