1. What is mirror finish grinding?
Mirror grinding is a grinding that the surface roughness of the workpiece is Ra< 0.01μm, and the light is like a mirror after grinding, so it can be clearly photographed. The flatness of the grinding surface is not greater than 3μm /1000mm. The surface machining of high precision and high added value parts requires mirror grinding.
2. Grinder for mirror grinding should meet the following conditions:
1. High precision, stiffness and adopts vibration reduction measures
2. The rotation accuracy of grinding wheel spindle is higher than 1 μm
3. The amplitude of the wheel head relative to the working table is less than 1 μm
4. The transverse feed mechanism can be precisely under inching control
5. The working table has no crawling phenomenon when moving at low speed
3. Methods for mirror grinding
Balance and dressing grinding wheel
1. Balance grinding wheel. After installing the flange on the grinding wheel, the coarse static balance is carried out first, and then the two end faces and outer circles are repaired on the grinding wheel shaft. Finally, the precise static balance is carried out.
2. Dress grinding wheel. The grinding wheel is dressed well or not directly affects the smoothness of the grinding surface. In order to reduce the surface roughness, dressing the grinding wheel is the key. The following matters should be noted when dressing the grinding wheel：
1）The general fine grinding allowance is 0.015~0.02mm. When the workpiece is roughed to RA0.8 and the fine grinding allowance is 0.005~0.015mm (the remaining grinding allowance must be determined according to the properties and hardness of the steel. For the steel with high hardness and easy grinding, the grinding allowance should be properly left less), the finishing wheel is required。
2）When finishing the grinding wheel, the thickness of about 0.1mm should be knocked off first, and then the cutting depth of 0.02mm should be trimmed twice, and then the cutting depth of 0.01mm should be trimmed three times, and the final stroke should be repeated twice without cutting depth. During finishing, the transverse feed speed is 20~30mm/min. For soft steel workpiece, it is better to slow transverse feed when grinding grinding wheel, but it is opposite when machining hard quenched high speed steel.
3）When dressing the grinding wheel, sufficient grinding fluid or other coolant must be used to flush the contact point between the grinding wheel and the diamond cutter, so as to wash away the grinding wheel debris in time to avoid affecting the grinding quality.
4）The diamond dresser for finishing does not need to be very sharp, because the grain-size of the grinding wheel is coarser. Diameter of diamond tool tip is within 0. 8mm. When dressing with sharp diamond dressers, the rate of cross feed must be reduced.
5）When the new dressing grinding wheel starts to use, the surface roughness is slightly worse, and it will stabilize after a while.
Grinding Parameters
1）Grinding wheel back feed is about 0.005mm. According to the nature and hardness of the workpiece steel appropriate increase or decrease, such as high hardness of quenched steel, tool back feed is too large easy to cause burns, and soft steel back tool amount is too small and not easy to polish.
2）The single stroke of transverse feed is 0.2~0.4mm. The transverse feed speed has a great influence on the surface roughness, because the cutting ability of the grinding wheel in mirror grinding is very poor. If the transverse feed speed increases, the surface of the grinding wheel will be damaged, so it cannot obtain a very smooth surface.
3）Longitudinal feed is 12~15 m/min. Longitudinal feed on the surface roughness of the impact is relatively small, but if too slow, the workpiece surface will produce ripple or pattern.
4）No cutting depth polishing. Due to the poor cutting ability of the grinding wheel in mirror grinding, there are often knife marks on the surface of the workpiece (the circular workpiece is in the shape of a moon, the rectangular workpiece is in the shape of a belt), which reflects that the surface of the workpiece is not straight, so it needs to be polished without feed for about two minutes.
4. The defects and measures easy to happen when do mirror surface grinding
Mirror grinding on a surface grinder often produces some defects, such as surface ripples, burns, scratches, patterns and wear marks (silk flow), etc. In order to solve these problems, the following measures can be taken:
1) Most surface ripples are caused by grinding wheel vibration, such as excessive bearing clearance, spindle deviation, motor rotor dynamic balance, uneven oil film between spindle and bearing, etc. In addition, if the grinding wheel is not balanced properly, it will cause ripples. If the grinding wheel is well balanced and there are still ripples, it is necessary to overhaul or adjust the grinding wheel and its matching parts to completely eliminate the ripple phenomenon.
2) When processing high hardness materials , the grinding efficiency of the grinding wheel is poor, and the grinding point will produce high temperature, which is easy to burn the workpiece (for example, when grinding W18Cr4V, it is more likely to burn). To prevent burns, there must be sufficient grinding fluid at the grinding point. In addition, the amount of knife should also be appropriate, should not be too large. The best method is to reduce the linear speed of the grinding wheel, which is about 18m/s. At low speed of 1440r/min on M7120A grinder, the burn phenomenon can be basically eliminated. Another advantage of reducing the grinding wheel linear speed is to reduce the vibration of the grinding wheel rack, which can also reduce and eliminate the ripple phenomenon.
3) There are roughly two kinds of scratches in mirror grinding: one kind of scratches is not regular, which is that the grinding fluid is not pure, and the impurities and abrasive particles are washed to the grinding wheel and the workpiece. The method to eliminate this scratch requires grinding fluid filtration, generally using magnetic, copper wire mesh three filtration; Another kind of scratches are lines like dotted lines, shallow, the length of the same distance, this is the grinding wheel grain is about to fall off, as the grinding wheel rotates and scratches. At this time, the elimination method is to use sufficient grinding fluid when dressing the grinding wheel, and trim the two end faces of the grinding wheel into an oblique shape, which is wide outside and narrow inside. In addition, the selection of grinding wheel should be appropriate, not too soft, do not use the grinding wheel with too good self-honing.
4) When do mirror surface grinding, easy to appear the pattern shape has a variety of forms. There are many factors that produce patterns. The main reason is vibration, including the periodical vibration of the grinding wheel, the external influence and the vibration of the neighboring machine tools, etc. In addition, when the grinding wheel is too blunt and the stroke is unstable, patterns will also appear. When the grinding is carried out when the adjacent machine tools are not in operation, the pattern is less likely to appear.
5) When the trace (wire flow) in mirror grinding is obvious, it is necessary to use a fine-grained grinding wheel to remove the wire flow. However, when using a fine grinding wheel, the following points must be considered:
A. When finishing the grinding wheel, the diamond dresser should be sharp, the grinding fluid should be sufficient, and the debris should be washed away in time to avoid affecting the surface of the grinding wheel.
B. Grinding amount should be appropriate to prevent surface burns and other defects affecting surface smoothness. Be careful to leave proper grinding allowance.
C. The grinding wheel linear speed must be reduced to 15 ~ 18m/s
D. The grinding fluid is carefully filtered to prevent scratches
E. When using a grindstone to finish the grinding wheel, the contact area between the grindstone and the grinding wheel should not be too small, and the grindstone used should not be too soft, otherwise it cannot play the role of the finishing grinding wheel.