1.Data analysis of metal bond diamond grinding wheel for optical glass

The optical glass features high hardness and brittleness. Optical glass requires special diamond tools for cutting and polishing. When conventional diamond grinding wheel for grinding with fine grain size, it can easily block the wheels, which can cause the increase of grinding force and cracks in the optical glass. ELID grinding technology can keep the sharpness of ultra-fine grain size metal bond diamond grinding wheel, and do the stable ultra-precision grinding. When using ELID grinding technology, the grain size and grinding depth of metal bond diamond wheel for optical glass have an important influence on the surface quality of optical glass.

2. Test equipment and methods:

Modified M7120 surface grinder, W10 metal bond diamond grinding wheel, the metal bond is cast iron matrix, ELID mirror grinding high-frequency pulse power supply, HDM Y2200 grinding fluid. Working table speed 0.04m/s, electrode gap 0.5mm, pulse spacing 24μs, pulse length 24μs, duty cycle 50%, voltage 90V. Without changing other parameters, grinding experiments with grinding depths of 5μm, 4μm, 3μm, 2μm were carried out on optical glass respectively. Each grinding depth was fed 3 times to ensure that the corresponding feed depth of surface quality was obtained, and the state of grinding wheel and optical glass was the same before grinding and measure the relationship between grinding depth and grinding force.

3. Test results and discussions:

3.1 Effect of cutting depth on the grinding force

When the feed speed of worktable is constant, only changing the grinding depth, measure the grinding force shown in figure 1. The grinding force increases with the increase of grinding depth. This is because the contact arc length increases with the increase of ground temperature. At the same time, the total number of effective abrasive grains participating in grinding increases, and the cutting thickness of single abrasive grain increases. Therefore, the grinding force increases with the increase of grinding depth, and the effect is significant. The grinding force varies with the grain size, structure density, hardness and strength of the material at the same grinding depth.

3.2 Effect of grinding depth on surface quality

The grinding depth has a great effect on the grinding quality. When the grinding depth is 2μm, the surface roughness of optical glass is 0.09μm. When the grinding depth is 5μm, the surface roughness of optical glass is 0.18μm. This is because the grinding mechanism of optical glass shows that the grinding depth determines the grinding mode of the glass surface. There are 3 grinding modes on the surface of optical glass: fracture mode, fracture and plastic mode, and plastic model. The machined surface of a plastic model is best, followed by that of fracture and plastic mode, and that of fracture mode is worst.

The cutting depth of abrasive grains is less than the critical value of brittle plastic transition. Optical glass is in the plastic mode for grinding. When that is more than the critical value, it will transit from fracture and plastic mode to fracture mode. The critical depth of LHG8 optical glass is 0.170 um. When the W10 metal bond diamond grinding wheel with an average grain size of 10 um is used for grinding, because the edge radius of grain size of the grinding wheel is much smaller than the average grain size, the edge radius of abrasive grains can reach the sub-micron level or even lower.

The cutting depth of abrasive grains is 1/10-1/3 of the cutting edge radius. Therefore, when the cutting depth of the grain size of the wheel and grinding depth is small enough, optical glass grinding can approach the critical value of brittle-plastic transition.

When W10 diamond grinding wheel is used, its cutting edge height should be 3-5μm, and the grinding depth is 2μm, and it is an ideal state. With the increase of grinding depth, the grinding state becomes complex. There are grinding effect of grinding edge, the rolling friction of oxide film, and the scraping effect of passive abrasive grains, which make the surface quality of the workpiece worse.

3.3 Selection of grain size of metal bond diamond grinding wheel for optical glass

Fine grain diamond grinding wheels are used under the condition of ensuring the grinding accuracy. Traditional fine grain size diamond grinding wheel can be blocked by debris easily, which causes the increase of ground force, thus affecting the surface quality of optical glass. For ELID grinding, the surface roughness of optical glass decreases with the decreases of grinding wheel grain size.

This is mainly determined by the features of electrolytic in-process dressing. ELID is unlike other mechanical dressing methods, and it causes abrasive grains to break up and form the finer edges. Electrolytic dressing can only expose the abrasive grains buried in the bond agent before. Therefore, there is a close relationship between cutting edge of electrolytic dressing grinding wheel and the wheel grain size. The finer the grinding wheel grain size used in the experiment, the lower the surface roughness is.

4. Conclusions

1) The grinding force increases with the increase of grinding depth, and it does not change with the time at the same feed depth in ELID grinding.

2) If choose smaller grinding wheel grain size and grinding depth, it can obtain higher surface quality. Under the experimental conditions, the grinding depth is 2μm, and the surface quality of the workpiece is the highest, and realize the plastic mode grinding of LHG8 optical glass.