This article studies the effect of grinding wheel linear velocity, traverse feed speed, grinding depth, and grinding stroke on the surface roughness of tungsten carbide ground which grinding by vitrified diamond wheel
Introduction of tungsten carbide:
Tungsten carbide features high hardness, wear resistance, high strength and corrosion resistance, especially its high hardness and wear resistance. With the development of science and technology, tungsten carbide as tool material, instead of tradition high-speed steel, is widely used for turning tools, milling tools, bits and etc. Some scholars put forward the concept of cutting instead of grinding, which is higher required for surface processing quality of tungsten carbide. Owing to its high brittleness and low thermal conductivity, sensitive to grinding force and heat, it is difficult to machine material.
If the machining method is not correct, it will cause brittle failure and thermal stress of tungsten carbide easily. Some domestic researchers have studied the machining technology of tungsten carbide. In addition to the requirement of the diamond grinding wheel, the grinding parameters such as the linear velocity of grinding wheel, transverse feed speed, grinding depth, grinding stroke, machine tool stiffness, and dressing conditions of tungsten carbide also affect the surface processing quality of tungsten carbide. So, it is very important to choose suitable grinding parameters for improving the surface quality of tungsten carbide.
1 Experimental procedure
In this experiment, 4 grinding parameters, i.e. grinding wheel linear velocity, transverse feed speed, grinding depth, and grinding stroke were studied by orthogonal experiment. And obtained the effect of 4 grinding parameters on the surface roughness of tungsten carbide, and put forward to the optimized grinding parameters.
This experiment design is orthogonal experiment with 4 factors and 3 levels. The total is 9 groups of grinding experiments. Did grinding test on 3MZK208A fully automatic CNC grinding machine, and it was applied to a vitrified bond diamond grinding wheel of D45.3mm x T5mm x H5mm. Diamond concentration was 150%, the diamond grain size was 230/270 mesh, the workpiece was tungsten carbide tool, the item was YT15, and coolant was a water-based emulsion.
4 grinding parameters studied in the experiment were: grinding wheel linear velocity Vs, transverse feed speed Vw, grinding depth αp, grinding stroke d. Tested the surface roughness of tungsten carbide sample after grinding by MarsurfPS1 surface roughness tester, and chosen 2#, 10# tungsten carbide samples did the microscopic analysis for ground tungsten carbide by automatic diamond micro image measuring instrument.
2.Experimental results and analysis
2.1 The results of orthogonal experiment analysis are shown in Table 1.
Table 1 Results of the orthogonal experiment
|Item No.||A||B||C||D||Experiment result|
In Table1, 4 factors A, B, C, and D were grinding wheel linear velocity, transverse feed speed, grinding depth and grinding stroke. 1, 2, and 3 horizontal parameters wereA:20m/s, 30m/s, 40m/s, B 300 mm/min, 500 mm/min, 700 mm/min, C:10μm, 20μm, 30μm, D:20000μm, 30 000μm and 40 000μm, respectively.
From Table 1, according to the range R data, column 2 was the largest and column 3 was the smallest. This reflected that when the level of factor B changed, the index fluctuated the most, and when the level of factor C changed, the index fluctuated very little. So, the order that affected the surface roughness of tungsten carbide is B>A>D>C according to the range data.
The orthogonal experimental effect analysis chart is shown in Fig. 1 directly described these relationships. From Fig. 1, it can be seen that the fluctuation of the index when the level of the selected factors changed. Obviously, factor B fluctuated big and factor C fluctuated least.
From average 1,2 and 3, it can be seen that when the level of each factor changed, the fluctuation of the index is actually not affected by the change of other factors. So, the simple combination of the better levels of each factor is the optimal combination. The experiment requires that the smaller surface roughness of tungsten carbide ground by the vitrified bond diamond grinding wheel, the better is. It should choose the factor level with small index, that is, the lowest level in the average.
So, choose the smallest level of the average of each factor 1, 2 and 3, i.e. the horizontal feed speed and grinding wheel stroke level 1, grinding wheel speed level 3 and grinding depth level 2. From Fig. 1, it is can be seen that the optimal linear velocity Vs = 40m/s, horizontal optimal feed speed Vw=300mm/min, optimal grinding depth αp=20μm, optimal grinding stroke d=20000μm.
The experiment proves that the surface roughness of grinding tungsten carbide (No.10 sample) is 0.041 when choosing the best combination of orthogonal experiment analysis, which is in accordance with the analysis results of the orthogonal experiment.
2.2 Effect of grinding parameters on surface roughness of tungsten carbide
Fig.2 is a microstructure picture of the surface of tungsten carbide ground 1600 times larger under the microscope. Fig.2a is the surface microstructure of No.2 tungsten carbide in the orthogonal experiment. Fig.2b is the surface microstructure of No.10 tungsten carbide under the best grinding parameters condition from the orthogonal experiment analysis. From Fig.2a, it can be seen that the scratches on the surface of tungsten carbide are obvious, the width of scratches is 1-6μm. From Fig. 2b, it can be seen that there are uniform and shallow scratches on the surface of tungsten carbide, and the width of scratches is less than 1 μm.
From Table 1, it can be seen that the effect of transverse feed speed and grinding wheel linear velocity on surface roughness of tungsten carbide is bigger than that of grinding depth and grinding stroke. At the same time, from the fluctuation of each parameter in Fig. 1, it can be seen that the surface roughness of tungsten carbide increases with the increase of the transverse feed speed of grinding wheel. The surface roughness of tungsten carbide decreases with the increase of the vitrified diamond wheel linear velocity. The surface roughness of tungsten carbide increases with the increase of grinding stroke. The surface roughness of tungsten carbide decreases first and then increases with the increase of grinding depth.
When the transverse feed speed is larger, the instantaneous friction temperature of the vitrified diamond wheel and the workpiece increases. When the grinding stroke of the grinding wheel is larger, the effective distance of the grinding workpiece is longer and the contact time between the grinding wheel and the workpiece is longer. When the grinding depth is larger, the plastic deformation of the workpiece is increased, and the friction between the grinding grains and workpiece is aggravated, resulting in grinding heat. These 3 factors cause the instantaneous friction temperature of vitrified diamond grinding wheel and tungsten carbide to rise, thus the grinding temperature rises.
With the increase of grinding times, the grinding heat accumulates gradually, which makes the grinding wheel have certain thermal stress, resulting in some abrasive grains falling off on the workpiece surface. In the grinding process, these abrasive grains scratch the workpiece surface under the drive of the grinding wheel, making the surface roughness of the workpiece increase. In the actual production, grinding fluid plays the role of cooling and cleaning at the same time. Adequate grinding fluid greatly affects the grinding effect of the grinding wheel.
When the linear velocity of vitrified diamond wheel increases, the effective abrasive grains in the vitrified diamond wheel that participates in the grinding process in unit time increase, and the grinding thickness and load of the single abrasive grain decrease, resulting in the reduction of the total grinding force, thus reducing the instantaneous friction between the grinding wheel and the workpiece, reducing the grinding heat in the grinding process, and thereby reducing the plastic deformation of the workpiece and plastic uplift on both sides of the plough groove. So, it can effectively reduce the surface roughness when increasing the vitrified diamond wheel linear velocity.
- The sequence of the effect of 4 grinding parameters on surface roughness of tungsten carbide is transverse feed speed > grinding wheel linear velocity> grinding wheel stroke > grinding depth.
- According to the effect of each factor and the orthogonal experimental effect curve, the optimum linear velocity of the vitrified diamond wheel is vs=40m/s, optimum transverse feed speed is Vw=300mm/min, optimum grinding depth is αp=20μm, optimum grinding stroke is d=20000μm, the surface roughness of tungsten carbide is 0.04μm under the optimum technology parameters.
- When the transverse feed speed, grinding stroke and grinding depth of grinding wheel is larger, the surface roughness of the workpiece is larger. When the vitrified diamond wheel linear velocity increases, it can effectively reduce the surface roughness of tungsten carbide.