The head of ingot rod is ground by vitrified CBN wheel and brown corundum grinding wheel respectively. Test, analyze and contrast the dimensional accuracy, surface roughness, surface metallographic structure and microhardness of 2 grinding wheels after grinding. Both the test results and the production show that after the head of the ingot rod is ground by CBN grinding wheel, the surface dimensional accuracy and roughness is obviously improved, and the workpiece no burns, greatly improving the production efficiency.
The spindle is an important part of the ring spinning frame, and the ingot rod is a key component of the spindle. Due to the high-speed rotation of the spindle, the head of ingot rod and the radius of the ball at the bottom of the spindle form a pair of friction, makes the head of ingot rod long term and strong friction and wear, which requires enough hardness, high manufacturing precision, and low roughness.
Brown corundum grinding wheel is used for grinding the head of ingot rod with the disadvantages of low surface roughness, easy burn, and low production efficiency, which restricts the processing precision, production efficiency and service life of the ingot rod.
In this paper, do the test of grinding ingot rod by vitrified CBN grinding wheel.
Ingot rod and its grinding process:
The ingot rods are mass production part, which is made of GCr15 steel. The shape and size are shown in Fig.1. The hardness of the head of the ingot rod is required to be above
HRC62, the roughness is Ra0.4μm, the dimensional accuracy is IT8, and the final step of the traditional ingot rod is on the special grinding machine for ingot rod. The brown corundum grinding wheels of 100#, 170# and 40mm width is used for rough and finish grinding.
Disadvantages of the brown corundum grinding:
- Owing to the brown corundum grinding wheel with poor sharpness and large wear, the dimensional dispersion of the workpiece in the mass production is large, which makes it extremely difficult to further improve the shape accuracy of the head of ingot rod.
At the same time, because the workpiece size is small, the grinding temperature is high, and the grinding surface is prone to large residual stress, reducing the fatigue strength of the workpiece, and even causing metamorphic layers and surface burns.
- The brown corundum grinding wheel wears fast, and it not only dress the grinding wheel but also need to frequently be replaced. It causes high labor intensity, high tool cost, more auxiliary working time, low production efficiency.
- It is not good for the working environment for the dust and odor of in the grinding process of the brown corundum grinding wheel.
Selection and dressing of vitrified CBN wheel for grinding ingot rod
Vitrified bond CBN grinding wheel is easy dressing and has many pores which are good for chip removal and reducing the blockage of the grinding wheel to avoid burns. Vitrified bond has good heat resistance, strong abrasive holding force, wear resistance and good shape
retention which is good for improving the surface precision. Therefore, vitrified bond CBN grinding wheel is used, and add some SiC, copper powder, aluminum powder, and etc into the bonding agent.
The diamond dressing roller method is used for dressing the grinding wheel. The dressing device is installed on the M61329 high precision cylindrical grinding machine work table. The diamond dressing roller will finish the dressing of vitrified CBN wheel with the longitudinal movement of the work table.
Test process and results analysis
Do the experiment of vitrified CBN grinding wheel and brown corundum grinding wheel for the head of the ingot rod. The grinding linear velocity is 40m/s, the workpiece rotation speed is 80r/min, the workpiece rotary feed is 0.16m/min, and the grinding coolant is water-based cutting fluid.
In the view of the visual inspection, the surface quality ground by CBN grinding wheel is better that of brown corundum grinding wheel. The surface of the head of ingot rod randomly selected from 20 pcs ground by vitrified CBN grinding wheel is not burned. In the head of ingot rod ground by 2 kinds of grinding wheels, 5pcs randomly selected, using British Taylors of 5M electric profiler, MM6 metallographic microscope, do the test for the surface quality of the head of ingot rod, surface metallographic structure, and surface microhardness. The test results are as follows:
Test results of surface roughness and accuracy
1. The dimensional accuracy of the head of ingot rod ground by CBN grinding wheel is IT6, and the surface roughness is Ra0.2μm, and the dimensional accuracy of the head of ingot rod ground by brown corundum wheel is IT8, and the surface roughness is 0.4μm.
2. Randomly select 5pcs samples respectively, cut into 2 symmetric small samples along the axial direction of the head of ingot rod. Take one of each of them to prepare a metallographic sample, using 4% nitric acid alcohol solution for etching. Observe the metallographic structure of the surface of the head of ingot rod as shown in Fig.2.
In Fig. 2 the white bright layer is grinding deformation layer, which is formed by the effect of grinding force and the grinding heat. Fig.2b is the metallographic figure after CBN wheel grinding, and the surface deformation layer is only 0.015mm, and almost no metamorphic layer is visible. The surface deformation layer is ground by the brown corundum grinding wheel as shown in Fig.2a, which is about 0.035mm.
There is a layer of the uneven metamorphic layer under the deformed layer. From the color point of view, the structure is whitish. However, due to the small size of the tissue in the metamorphic layer, the change is difficult to observe with the metallographic microscope, and further analysis to be observed by electron microscopy.
3.Surface microhardness distribution
From the surface of ingot rod along the vertical direction of its normal line, towards to the center point, test the microhardness as shown in Fig.3.
It can be seen from Fig.3 that the ingot rod will harden after grinding by 2 kinds of grinding wheels, and the surface microhardness is improved.
When grinding with Vitrified CBN wheel, the grinding heat has little effect on the surface of the workpiece due to the low grinding temperature. The hardening layer is not tempered softening, and the surface can maintain the high hardness by hardening. When the brown corundum grinding wheel grinding, the grinding force is large, and the grinding temperature is high, and the hardening layer may be tempered to soften or even burn the surface.
In the production comparison test, when grinding with the brown corundum grinding wheel, it will be dressing after grinding 200-300 pcs heads of the ingot rods. When grinding with vitrified CBN wheel, it will be dressing after grinding 1400-1500pcs heads of the ingot rods. The durability of the grinding wheel is increased by more than 5 times, which also greatly shortens the auxiliary working hours. With the vitrified CBN wheel, the final forming grinding can meet the technical requirements and the process is reduced in only one time.
In the process of the head of ingot rod, Vitrified CBN wheel can improve the grinding surface quality, dimensional accuracy, reducing the surface roughness, tool cost, and labor cost, improving the production efficiency.