Experimental Study of Electroplated Diamond Abrasive Belt for Alumina Ceramic

The crystal lattice structure of alumina ceramics is composed of the covalent bond, ionic bond or the mixture of the two. It features high hardness, high compressive strength, and low fracture toughness. Alumina ceramic material is very difficult to process and it is a typical hard-to-process material. With the development of flexible grinding tools, electroplated diamond abrasive belt for brittle material has incomparable advantages compared with traditional diamond abrasive tools. such as diamond grinding wheels. In this article, the electroplated diamond abrasive belt for alumina ceramic is used for grinding. Study the influence of grinding process parameters on grinding efficiency and the influence of structural parameters of the electroplated diamond abrasive belt on surface roughness.

1 Testing Process

Materials:

High purity alumina engineering ceramic, hexagon structure, length of a side: 50mm,

thickness: 10mm, density: 3.9g/cm³, modulus of elasticity: 350GPa, Vickers hardness:26GPa, fracture toughness: 4.4(MPa·m)1/2

Electroplated diamond abrasive belt

Electroplated diamond abrasive belt for alumina ceramic is mainly composed of diamond abrasive grains, coated metal(nickel) and matrix materials. Diamond abrasive grains are attached to the matrix material by electroplating nickel, forming a metal-coated diamond abrasive grains layer. Diamond abrasive grains are fixed firmly on the matrix by the nickel coating layer. Electroplated diamond abrasive belt features high hardness, high strength, and long service life.

Grinding testing on the sanding machine, the power of the machine is 750W, stepless speed. When the diameter of the contact wheel is 20mm, the grinding speed of sanding abrasive belt can be adjusted within 0-45 m/s. Wet grinding method is used to reduce the temperature of grinding zone in the grinding process, avoid the damage of high temperature to diamond abrasive grains and affect its grinding performance.

Grinding form adopts the contact wheel structure shown in Figure 1, which is mainly composed of driving wheel, contact wheel, sanding abrasive belt, spring tensioning mechanism and workpiece. In the grinding process, the driving wheel drives the sanding abrasive belt to move. The contact wheel is usually made of soft rubber material and can support the sanding abrasive belt, ensure the contact force between the belt and the workpiece. The spring tensioning mechanism can adjust the force between the contact wheel and workpiece.

Figure 1Sanding Abrasive Belt Grinding Structure of Contact Wheel Type

Measure the quality of the workpiece before and after grinding with an accuracy of the 0.1g electronic scale. Record the time with the stopwatch, and measure the surface roughness with TRX300 roughness meter.

1.2 Test Design

In the grinding process, the grinding process parameters have an important impact on the grinding efficiency. Analyze the influence of belt speed, grinding pressure and workpiece feed speed on the grinding efficiency by the single factor testing, and the quality of the workpiece before and after grinding with an electronic scale, grinding time recorded by stopwatch, grinding efficiency for material removal rate under different conditions per unit time. Design orthogonal test of three factors and three levels, study factors on the influence of grinding efficiency by range analysis, find out the reasonable grinding process parameters.

The linear velocity of the sanding abrasive belt is too big or too small, which will affect the grinding efficiency. In the experiment, the linear velocity of the sanding abrasive belt is 20m/s, 30m/s, and 36m/s. The grinding pressure directly determines the grinding load and grinding depth. The grinding pressure is 32N, 44N and 55N in the experiment. The material removal rate is not high when the feed speed of workpiece is too small, and too high feed speed of workpiece will cause the sanding abrasive belt not be cut the materials in time, and then the abrasive grains of sanding abrasive belt can be crushed or fall off. The feed speed of workpiece is 2 mm/s, 2.5 mm/s and 3 mm/s. The orthogonal test shows the material removal rate per unit time measure grinding efficiency. The level of test factors is shown in Table I.

The abrasive grain size and density of coating grain of electroplated diamond abrasive belt for alumina ceramic have great influence on the grinding surface roughness. Electroplated diamond abrasive belt  for alumina ceramic with grain size of 80 #, 120 #, 150 #, 180 #, 240 # and density of coating grain of 30%, 45%, 60%, 75% and 90% respectively is chosen. The effects of grain size and density of coating grain on grinding surface roughness are studied by single factors experiments.

2 Test Results and Analysis

2.1 Effect of Grinding Process Parameters on Grinding Efficiency

2.1.1 Linear velocity of sanding abrasive belt on grinding efficiency

When grinding pressure is 55N and the feed speed of the workpiece is 2mm/s, the effect of the variation of the belt linear velocity on the grinding efficiency is shown in Figure 3. It can be seen that the grinding efficiency increases with the increase of the belt linear velocity when the belt linear velocity is lower than 30 m/s, and then decreases with the increase of the belt linear velocity. This is because with the increase of belt linear velocity, the number of abrasive grains in grinding per unit time increases, and the grinding stroke of diamond abrasive grains in unit time increases, which improves grinding efficiency.

With the grinding process going, the wear of abrasive grains leads to the decrease of belt sharpness and grinding efficiency. But too fast belt linear velocity will cause to the single abrasive grain stay too short in the grinding zone. It cannot cut into the workpiece surface, which easily scratches and affects the improvement of grinding efficiency. At the same time, it generates a lot of heat, causing diamond abrasive grains fall off and oxide and the decline of diamond proportion the wear acceleration of sanding abrasive belt. Therefore, when the belt linear velocity exceeds a certain value, further increasing the sanding abrasive belt linear velocity will not improve the grinding efficiency, even decrease it.

Figure 3 The Effect of Belt Linear Velocity On Grinding Efficiency

2.1.2 Effect of grinding pressure on grinding efficiency

When the linear velocity of diamond sanding abrasive belt is 30m/s and the feed speed of workpiece is 2mm/s, the effect of different grinding pressure on grinding efficiency is shown in the following figure. It can be seen that the material removal rate of diamond sanding abrasive belt increase with the increase of grinding pressure. Because the increase of grinding pressure makes the cutting load and grinding depth of diamond grains increase, and more materials can be removed per unit time, and the grinding efficiency increases. When grinding pressure reaches 55N, grinding efficiency decreases obviously. When grinding pressure is too high, the grinding depth of diamond grains increases, which cause the difficulty of alumina ceramic crushed.

Moreover, the bigger grinding depth makes the diamond grains bear a bigger grinding load, resulting in an increase in the proportion of crushing and shedding of diamond grains and a decrease in the number of diamond grains involved in grinding. In addition, owing to the limitation of machine power, too high grinding pressure causes the insufficient torque to the spindle, which reduces the linear velocity of diamond sanding abrasive belt and grinding efficiency. Therefore, when the grinding pressure of diamond sanding abrasive belt exceeds a certain value, the increase of grinding pressure causes a significant decrease in grinding efficiency.

Figure 4 The Effect of Grinding Pressure On Grinding Efficiency

2.1.3 Effect of Workpiece Feed Velocity on Grinding Efficiency

When the grinding pressure is 55N and the linear velocity of diamond sanding abrasive belt is 30m/s, the effect of workpiece feed speed on grinding efficiency is shown in the figure 5below.

Figure 5 The Effect of Work piece Feed Speed On Grinding Efficiency

It can be seen from the figure 5 that the material removal rate increases with the increase of feed rate, but when the feed rate increases to 2.2mm/s, the material removal rate will decrease significantly with the increase of feed rate. This is because the feed capacity of alumina ceramics is very small. When the feed speed is too fast, due to the limitation of diamond sanding abrasive belt grinding ability, the corresponding feed cannot be removed in a unit time, easily slips.

In this case, the grinding pressure is high, and the diamond grains fall off and crush easily, resulting in the material removal rate of diamond sanding abrasive belt decreased.

2.2 Orthogonal Test Analysis

Study the effect of various experimental factors on grinding efficiency and optimal level combination by orthogonal experiments. The results of orthogonal experiments are shown in table 2 below.

Table 2 Orthogonal Test Results

In order to accurately analyze the effect of the linear velocity of diamond sanding abrasive belt for alumina ceramic speed, grinding pressure and workpiece feed speed on grinding efficiency, do the range analysis of orthogonal test results. The calculation results are shown in Table 3. Ki (i = 1, 2, 3) is the sum of material removal rate test results in level 1, 2 and 3 of each factor; ki (i = 1, 2, 3) is the average value of material removal rate test results in level 1, 2 and 3 of each factor; R is range.

Table 3 Range Analysis

The results of range analysis show that RB > RC > RA, so the sequence of the effect of experimental factors on grinding efficiency of alumina ceramic is grinding pressure> workpiece feed speed >linear velocity of diamond sanding abrasive belt. When electroplated diamond sanding abrasive belt grinds alumina ceramic, the effect of grinding pressure on grinding efficiency is the biggest, and the effect of the linear velocity of diamond sanding abrasive belt on grinding efficiency is the smallest. Select the best level of material removal rate of each factor according to the average value of the same level of each factor.

For linear velocity of diamond sanding abrasive belt, kA2 > kA3 > kA1, the optimal level of the linear velocity of a belt is 30m/s. Similarly, the optimal level of grinding pressure is 55N, and the optimal level of workpiece feed speed is 2mm/s, so A2B3C1 is the optimal level combination of various test factors, namely, that means linear velocity of diamond sanding abrasive belt is 30m/s, grinding pressure is 55N, workpiece feed speed is 2mm/s. At this time, the grinding efficiency of the electroplated diamond abrasive belt on alumina ceramics is the highest.

2.3 Effect of Structural Parameters of Diamond Sanding Abrasive Belt on Grinding Surface Roughness

Test conditions: The optimum combination of an orthogonal test is adopted in the process parameters, i.e. linear velocity of diamond sanding abrasive belt is 30m/s, grinding pressure is 55N and workpiece feed speed is 2mm/s. The effect of grain size of diamond abrasive belt and density of coating grain on grinding surface roughness was studied by reducing grinding surface temperature by wet grinding. ( see in Figure 6)

2.3.1 Grain size of diamond sanding abrasive belt

The grain size of the diamond sanding abrasive belt has a great influence on the grinding surface roughness (see figure 6 above). When the grain size of the diamond abrasive belt for alumina ceramic increases from 80# to 120#, the grinding surface roughness larger decreases.  After 120#, the grinding surface roughness declines smaller. This is the diameter of diamond grains changes caused. When the grain size of the diamond abrasive belt is small, and the diameter of the diamond grain is big, and the grinding depth is big, and the grinding surface roughness is big. After 120#, the diameter of diamond grains change little, the abrasive is fine, and the grinding depth changes little, and the grinding surface roughness changes little.

The hardness of alumina ceramic is bigger, and the grain size of 80# diamond sanding abrasive belt is too rough, and the roughness value is too big, and the surface quality is too bad, while diamond sanding abrasive belt wears too fast. When the grain size of the diamond abrasive belt is more than 120#, the surface roughness value changes little, and the abrasive is too fine, which cause the grinding efficiency low and debris accumulates. Therefore, 120#-150# electroplated diamond abrasive belt is used for grinding alumina ceramic, which can meet the requirements of surface roughness and grinding efficiency.

2.3.2 Density of Coating Grain

Figure 6 The Effect of Structural Parameters of Diamond Sanding Abrasive Belt on Surface Roughness

With the increase of density of coating grains, the grinding surface roughness decreases ( see Figure 6b). When the density of coating grain is 30% – 45%, the grinding surface roughness is too large to meet the requirements. When the density of coating grain is 60% – 90%, the declining trend of the grinding surface roughness is not obvious. This is because the density of coating grain is smaller, and the abrasive grains on the surface of diamond sanding abrasive belt are less, the grinding pressure and grinding depth of single abrasive grain are big under the constant grinding pressure, which causes the surface roughness value higher.

When the density of coating grain reaches 60%, the abrasive grains passing through the workpiece surface in a unit time are much, and the grinding depth declines, and the surface roughness value of material declines.  It can achieve a better grinding effect.  However, when the density of coating grain is too high, too small debris clearance on the surface of diamond abrasive belt causes debris to be discharged in time, and debris accumulates and the grinding heat cannot be dispersed. Therefore, the density of the coating grain of diamond abrasive belt for alumina ceramic should not be too high. 60% density of coating grain can achieve better surface roughness and avoid the debris accumulation.

Conclusion:

The grinding efficiency increases with the increase of the linear velocity of diamond sanding abrasive belt, and too fast linear velocity causes the grinding efficiency decreases. The grinding efficiency increases with the increase of the grinding pressure, and when grinding pressure reaches a certain value, the grinding efficiency decreases significantly. The grinding efficiency increases with the increase of workpiece feed speed, when the feed speed reaches 2.2mm/s, the grinding efficiency decreases significantly.

The sequence of the effect of grinding process parameters on grinding efficiency is grinding pressure> workpiece feed speed >linear velocity of diamond sanding abrasive belt. When the linear velocity of diamond sanding belt is 30m/s, the grinding pressure is 55N and the workpiece feed speed is 2mm/s, the grinding efficiency is the highest.

The surface roughness of alumina ceramic grinding decreases with the increase of grain size and density of coating grain. When the grain size of diamond sanding abrasive belt is 120#-150#, and the density of coating grain is 60%, and the surface roughness of alumina ceramic grinding is better.