Super smooth and high flatness polishing process f

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Super smooth and high flatness polishing process of hard and brittle materials

1 preface

hard and brittle materials such as sapphire single crystal and glass ceramics are widely used for their excellent characteristics. Microcrystalline glass is used as structural materials of astronomical telescopes, optical lenses, rockets and satellites, and can also be used as a standard meter ruler; With its good light transmission and wear resistance, sapphire is used in mirrors and windows of lasers, substrates of semiconductor materials or metal materials grown by heteroepitaxy, etc. The research on ultra precision machining method of hard and brittle materials will further expand its application scope and improve its performance

due to the existence of countless tiny particles in glass ceramics and the high hardness of sapphire, it is considered difficult to obtain a super smooth and high flatness surface. The usual optical polishing machine is a movable pendulum type, that is, the workpiece rotates relative to the grinding disc and swings along a certain arc: the workpiece is polished and the polishing die is constantly trimmed at the same time. However, when the polishing parameters are set, the surface shape of the workpiece and the polishing die is always in a non convergent change, that is, the surface shape changes monotonously in the direction of concave or convex. Constantly checking the surface shape and modifying the polishing parameters require high technical level of the operator. We used the ultra precision grinder cjy-500 developed by China Institute of Aeronautical precision machinery to carry out the experiment. Its upper and lower spindles are hydrostatic spindles, which can also realize ultra precision turning of grinding disc, and the flatness is less than 1 μ m/φ 500. Use high-precision grinding disc to ensure high-precision workpiece, and it is not necessary to trim the workpiece in polishing. When the workpiece and the tin grinding plate are eccentric, moving in the same direction and at the same speed, the material removal on the workpiece surface is the same, and all points of the workpiece lay the foundation on the smooth and high flatness surface around the grinding plate

2 mathematical model

polishing experimental device is shown in Figure 1. Polishing is the process of relative movement of the upper plate (workpiece) and the lower plate (tin grinding plate). Firstly, the mathematical model of plane removal and motion trajectory is established

2.1 mathematical model of plane removal

there are many factors affecting grinding and polishing, such as pressure, time, speed, light wave, temperature, etc. So far, the widely accepted mathematical model of surface material removal is Preston equation:

DR/dt=kpv... (2-1)

where K: the coefficient related to the processed material and process parameters:

P: the pressure between a point on the surface and the grinding tool at t instant

v: the relative movement speed between this point and the tool at the instant of T

dr/dt: material removal per unit time

in order to predict the relationship between the amount of material removed and the form of motion in the lapping and polishing test, the following assumptions are made for (2-1): (1) the amount of material removed is only caused by the interaction between the workpiece and the lapping tool. (2) When the lapping tool is in good agreement with the workpiece surface, the removal rate is small, and the edge is not exposed in the whole processing process, the pressure P can be considered unchanged. The amount of material removed in the time is expressed by R (Z, y), then

when the workpiece and the tin grinding plate are eccentric, in the same direction and above, respectively, the operation method of the bellows ring stiffness tester is related to the normal protection and maintenance. When the rotating speed is rotated, the moving speed V of any point on the workpiece relative to the tin grinding plate is the same, and the amount of material removed in the grinding time t is the same, so the surface shape of the workpiece may be improved very well Set the speed as ω, The eccentricity is e, and the material removal amount in time t is r=kp ω eT。

2.2 mathematical model of motion trajectory

grinding and polishing motion trajectory is an important factor affecting the surface roughness after machining. When the workpiece and the tin grinding plate rotate eccentrically, in the same direction and at the same speed respectively, the motion trajectory equation of the point on any radius r of the workpiece relative to the tin grinding plate:

(x x x RCOs φ o) 2+ (y + rsin φ o) 2 = E2... (2-3)

it can be seen that the trajectory of any point a on the workpiece on the grinding plate is (-rcos) φ o,rsin φ o) Is a circle with center and radius E. The moving distance of any point on the workpiece relative to the grinding disc within the grinding time t is the same, which may improve the roughness of the workpiece surface

from the above two models, it is known that the motion mode of constant eccentricity in the same direction and at the same speed is conducive to obtaining an ultra smooth high flatness surface

3 auxiliary process and tooling

3.1 fixture

fixture is the hub connecting the upper shaft and the workpiece, which drives the flat die to drive the workpiece to rotate. Attention should be paid to when designing the fixture: ensure that the movement is as stable as possible, otherwise the workpiece surface may collapse, warp, and astigmatism; Try to avoid the transmission of machine vibration to the workpiece; The part of the fixture driving the flat die shall be as smooth as possible to reduce friction: it is convenient for loading and unloading

3.2 polishing wave

polishing wave is a colloidal SiO2 solution with nano particle size. It appears in the form of sol, which will not disperse in polishing and is always in a stable state. It has a high melting point of 1710 ℃ and a Mohr hardness of 7. It is a good polishing material. The polishing solution required for the experiment is diluted with distilled water, and the pH value is adjusted with < 10% acetic acid solution or < 10% NaOH solution

3.3 tin grinding plate

asphalt is often used as the grinding tool material in traditional polishing. The most suitable formula of asphalt pan must be tested according to different process conditions to ensure appropriate hardness. Its flatness is guaranteed by the research of two planes, which has strong manufacturability. We choose tin as the tool material. Tin not only has good sanding performance, but also has good wear resistance. Its rigidity is higher than the former. Its recrystallization temperature of 1 ℃ ensures the long-term use of tin grinding disc. In polishing, the surface shape of the tin grinding plate will be directly reflected on the surface shape of the workpiece. The experimental grinder can be used for turning tin plates, and the accuracy can reach L μ M/500mm, on which there is a periodic concentric groove with a certain width, and there is a fine spiral groove on the groove ridge, as shown in Figure 1. The wide groove is convenient for storing excess abrasive, preventing the accumulation of abrasive and avoiding the edge collapse of workpiece: excess abrasive can participate in cutting at any time to meet this environmental challenge, which increases the cutting force; It can also remove the chips produced in the polishing process in time to avoid scratching the workpiece: it is conducive to heat dissipation. The fine thread of the box can make the surface of the workpiece better coincide with the surface of the tin grinding plate, and make the polishing wave better contact with the workpiece

3.4 bonding

the bonding material used in the processing of traditional optical parts is basically hot-melt bonding material. First heat the workpiece and the flat mold to an appropriate temperature, and then apply the hot glue evenly on the joint of the two. Of course, it is indispensable to apply a certain pressure in the subsequent cooling process. Because this can compensate the uneven bonding caused by the poor surface accuracy of the flat die and the high consistency of the workpiece, so as to avoid unstable movement in the movement process. In the process of practical application, due to the inconsistent cooling rate between the workpiece and the glue, the thermal deformation of the workpiece often cannot be recovered in time when the adhesive solidifies, resulting in the following results: Although the perfect surface is obtained at the end of grinding and polishing, the surface shape of the workpiece changes due to the release of stress after unloading the workpiece, thus the previous work is wasted. Figure 2 (a) shows the shape change curve of a certain diameter of the workpiece before and after bonding. It can be seen that for ultra precision grinding and polishing as the last process, the bonding method between the workpiece and the flat mold is also very important, and the above bonding method is extremely inappropriate

try to use the method of cold sticking in the experiment. The workpiece and the flat die are bonded together with the surface pressing glue, and then appropriate pressure is applied to make the surface pressing glue be compressed, so that the flat die and the workpiece are bonded firmly. Figure 2 (b) shows the shape change curve of a certain diameter of the workpiece before and after twisting. From the experiment, it is known that after the surface pressing glue is pressed, there will be unrecoverable deformation that will damage the operator, and there will be no elastic deformation that will affect the grinding and polishing quality due to friction or normal force during grinding and polishing

4 process parameters

4.1 polishing wave concentration and pH value

a certain concentration of polishing fluid is often used for polishing. When the concentration is too high, although the removal rate is increased and the efficiency is improved, it is easy to cause edge collapse. The pH value of the polishing solution will also affect the surface accuracy. PH value has an effect on the surface shape and roughness of a certain precision of glass-ceramic, but has little effect on sapphire: the combined effect of concentration and can not be ignored in polishing. During polishing, the pH value of polishing wave should be adjusted according to the chemical characteristics of different material surfaces

4.2 speed

when polishing, the speed range of the upper and lower spindle is 30-100rpm, which has little effect on the polishing accuracy. It should be selected according to the actual situation of the machine tool. When the rotating speed is high, due to the effect of centrifugal force, the polishing liquid is thrown towards the liquid cover. It is necessary to add a baffle to make the polishing liquid flow to the workpiece to make the polishing more sufficient

4.3 polishing time

under the condition that the concentration, speed and pH value of the polishing solution remain unchanged, the surface roughness value and flatness decrease with the extension of time, which is the process of the tin plate modifying the workpiece surface through the abrasive. At the beginning of polishing, the contact area between the workpiece and the tin plate is small, the pressure is high, and the removal rate is high. As time goes on, the accuracy of surface shape and roughness will eventually tend to a certain stable value, and at this time, it has become very sensitive to various factors. It can be seen that it is necessary to appropriately increase the polishing time and find the best combination point parameters of efficiency and accuracy

in addition, clean environment and constant temperature are necessary. Clean the environment to prevent dust from falling into the scratch surface; Constant temperature ensures stable accuracy of the machine

5 conclusion

(1) tin has suitable hardness, strength, good sand embedding performance and low recrystallization temperature, which is suitable for ultra precision polishing disc:

(2) when the workpiece and the tin grinding disc operate in the same direction and at the same speed, it is conducive to obtain an ultra smooth surface with high flatness

(the selection of Rockwell hardness scale is shown in table 2-2. 3) the concentration and pH of colloidal SiO2 solution with high melting point, high hardness and good stability have an impact on the surface accuracy of hard and brittle materials: it is also necessary to appropriately increase the grinding and polishing time

through experiments, we can obtain λ/10~ λ/15/φ 30、 λ/8/φ 100 flatness, 0.23nmrms for sapphire and 0.37 ~ 0.52nmrms for glass ceramics. The flatness is shown in Figure 3. The roughness of sapphire and glass ceramics is shown in Figure 4 (a) and (b)

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