(1) Influence of Grinding Wheels on Surface Quality and Its Control
When the grinding wheel is uneven, under the action of the centrifugal force generated by its unevenness, in addition to generating radial vibration relative to the workpiece, the grinding wheel will also generate a wobble relative to the grinding machine headstock due to the limited rigidity of the front and rear supports of the spindle and the cantilever shape of the grinding wheel. In the forced vibration formed by the above centrifugal force, the radial vibration of the grinding wheel will cause waves, and the wobble will cause a roughened surface, affecting the surface roughness of the workpiece. When processing brittle materials like ferrite, due to the change in cutting amount and cutting force caused by the forced vibration formed by unevenness, self-excited vibration is likely to occur under certain circumstances. At this time, the deterioration of surface quality caused by this mixed vibration is more obvious. Therefore, before grinding ferrite, the grinding wheel must be dynamically balanced. Ensuring a certain dynamic balance accuracy is a prerequisite for ensuring the grinding surface quality of ferrite.
Similarly, as a factor causing vibration, the roundness and runout deviation of the outer circle of the grinding wheel will also result in a higher surface roughness of the workpiece. Experiments show that when the roundness of the grinding wheel exceeds 0.02 mm, the surface roughness of the ground parts will significantly increase. To ensure the roundness of the diamond grinding wheel, it is possible to install the diamond grinding wheel on a cylindrical grinder and use an ordinary grinding wheel for grinding against it; it is also possible to fix an ordinary waste grinding wheel piece in a vise on a surface grinder as the part to be ground and directly grind the diamond grinding wheel against it. Obviously, the latter online grinding method has higher accuracy because there is no problem of secondary installation of the grinding wheel, and it is more economical and convenient because ordinary waste grinding wheel pieces are used. The better the roundness of the grinding wheel, the lower the roughness of the surface formed by grinding. However, if the roundness exceeds its economic accuracy, the grinding and sharpening cost of the grinding wheel will significantly increase, while the improvement in the surface quality of the workpiece is not obvious. Referring to the radial runout value of the taper surface of the grinding wheel spindle of the surface grinder in the national machine tool design inspection standard, it is required to control the roundness of the grinding wheel within 0.012 mm.
Although the hardness of the diamond grinding wheel is quite high, due to the hard and brittle and difficult-to-machine characteristics of ferrite, the wear of the grinding wheel is still relatively serious and its life is limited. As the grinding process progresses, problems such as the shedding of diamond abrasive grains, the blunting of cutting edges, and the decline in sharpness will occur in the grinding wheel. The wear of the grinding wheel has a great impact on the surface quality of ferrite grinding. As the degree of wear of the grinding wheel deepens, phenomena such as scratches and chipping on the ferrite surface will significantly increase. Therefore, after grinding for a period of time, when the grinding wheel shows wear, it should be sharpened immediately to obtain sharp and equal-height micro-edges, which is beneficial for reducing the surface roughness of the workpiece.
Grinding particles (mainly including workpieces and abrasive particles) are very likely to be embedded between the grinding wheel particles, protrude from the original surface of the grinding wheel, or enter between the grinding wheel and ferrite parts during grinding, resulting in a decrease in the grinding ability of the grinding wheel and scratches on the surface of ferrite parts. Therefore, the surface of the grinding wheel should be cleaned at any time according to the situation. In practice, the outer circle surface of the grinding wheel can be brushed with a brush or a copper wire brush dipped in grinding head oil.
The particle size of the grinding wheel also has a great impact on the quality of the ferrite grinding surface. The finer the particle size, the lower the roughness of the grinding surface. However, if the particle size is too fine, the grinding efficiency is not high, and at the same time, the chips of ferrite are likely to clog the cracks between the grinding wheel particles, which will scratch the ferrite surface and affect the surface quality. It is more appropriate to choose a diamond grinding wheel with a particle size of 100 - 120 for grinding ferrite.
(2) Influence of Grinding Parameters on Surface Quality and Its Control
Regarding the influence of grinding parameters, according to cutting processing evaluation, under certain circumstances, the higher the grinding speed, the better the surface roughness. The impact of increasing the rotational speed on surface quality will not be discussed in detail for now. Attention will be focused on discussing improving surface quality by adjusting the cutting feed amount and feed method.
When roughly grinding ferrite, a more efficient downward straight cutting method is selected, and the feed amount is 0.01 - 0.02 mm per pass. The feed amount should be more at first and then less to prevent the occurrence of defects such as coarse scratches or cracks that are difficult to remove in the fine grinding stage, and a fine grinding allowance of 0.02 - 0.03 mm should be reserved. During fine grinding, the longitudinal feed method should be selected (as shown in Figure 3). The feed amount is reduced to 0.005 mm per pass. Since the rotational speed of 1500 r/min is not high, the feed speed should not be too fast and should be maintained between the 2nd and 3rd gears - approximately 7 - 8 m/min is more appropriate. If conditions permit the use of high-speed cutting above 3000 r/min, the effect may be better. In the case where the surface roughness requirement is less than Ra0.8, a very small polishing allowance (less than 0.01 mm) can be reserved, and a satisfactory workpiece can be obtained by using a polishing machine.
