1、 What is mirror grinding

After grinding, the surface roughness Ra of the workpiece is less than 0.01 µ M. the light is like a mirror and can be clearly imaged, so it is called mirror grinding. The flatness of the grinding plane shall not be greater than 3 µ M / 1000mm. Mirror grinding is needed for the surface machining of high-precision and high value-added parts.

2、 The grinding machine for mirror grinding shall meet the following conditions:

Have high precision and stiffness, and take vibration reduction measures; The rotation accuracy of grinding wheel spindle is higher than 1 µ m; The amplitude of the grinding wheel frame relative to the workbench is less than 1 µ m; The transverse feed mechanism can move accurately; There is no crawling phenomenon when the workbench moves at low speed.

3、 Method of mirror grinding

1. Balance and trim grinding wheel

(1) Balance grinding wheel. After installing the flange on the grinding wheel, conduct rough static balance first, then install it on the grinding wheel shaft, trim the two end faces and the outer circle, and finally conduct accurate static balance.

(2) Dressing the grinding wheel. Whether the grinding wheel is trimmed 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. Pay attention to the following items when dressing the grinding wheel:

1) The general fine grinding allowance is 0.015 ~ 0.02mm. When the workpiece is roughly ground to Ra0.8 and a fine grinding allowance of 0.005 ~ 0.015mm is reserved (the grinding allowance must be determined according to the steel properties and hardness, and the grinding allowance shall be appropriately reduced for the steel with high hardness and easy polishing), the fine grinding wheel is required.

2) When finishing the grinding wheel, first knock off the thickness of about 0.1mm, then trim it twice with the cutting depth of 0.02mm, and then trim it three times with the cutting depth of 0.01mm. Finally, the empty stroke without cutting depth reciprocates twice. During finishing, the transverse feed speed is 20 ~ 30mm / min. For mild steel workpieces, it is better to slow down the transverse feed when repairing the grinding wheel, but it is the opposite when machining quenched high-speed steel.

3) While dressing the grinding wheel, enough grinding fluid or other coolant must be used to wash the contact point between the grinding wheel and the diamond knife, so as to wash away the grinding wheel debris in time, so as not to affect the grinding quality.

4) The diamond knife used in finishing does not need to be very sharp because the grain size of the grinding wheel is relatively coarse. The tip diameter of diamond cutter can be within 0.8mm. When dressing with a sharp diamond knife, the cross feed speed must be reduced.

5) When the newly trimmed grinding wheel is used, the surface roughness is slightly worse and will stabilize after a while.

2. Grinding amount

1) The back draft of grinding wheel of grinder is about 0.005 mm. According to the nature and hardness of the workpiece steel, it is appropriate to increase or decrease. For example, for the quenched steel with high hardness, the back knife is too large, which is easy to cause burns, while the back knife of mild steel is too small and difficult to polish.

2) The single stroke of transverse feed is 0.2 ~ 0.4mm. The cross feed speed has a great impact on the surface roughness, because the cutting ability of the grinding wheel in mirror grinding is very poor. If the cross feed speed increases, the grinding wheel surface will be damaged, so a very smooth surface cannot be obtained.

3) The longitudinal feed rate is 12 ~ 15 m / min. Longitudinal feed has little effect on surface roughness, but if it is too slow, ripple or pattern will appear on the workpiece surface.

4) No cutting depth polishing. Due to the poor cutting ability of the grinding wheel during mirror grinding, tool marks often appear on the workpiece surface (the circular workpiece is in the shape of moon and the rectangular workpiece is in the shape of belt), which reflects that the workpiece surface is not straight, so it needs to be polished without feed for about two minutes.

4、 Defects and measures in mirror grinding

Mirror grinding on surface grinder often produces some defects, such as surface ripple, burn, scratch, obvious pattern and wear mark (wire flow), etc. to solve these problems, the following measures can be taken:

1) Most of the surface ripples are caused by grinding wheel vibration, such as excessive bearing clearance, main shaft misalignment, poor dynamic balance of motor rotor, uneven oil film between main shaft and bearing, etc. In addition, if the grinding wheel is not well balanced, it will also produce ripples. If the grinding wheel is well balanced and there are still ripples, the grinding wheel and its mating parts must be overhauled or adjusted in order to completely eliminate the ripple phenomenon.

2) When machining materials with high hardness, 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 easier to burn). In order to prevent burns, there must be sufficient grinding fluid at the grinding points. In addition, the amount of back knife should also be appropriate, not too large. The best way is to reduce the linear speed of the grinding wheel, which is about 18m / s. The low speed of 1440r / min on M7120A grinder can basically eliminate the burn phenomenon. Another advantage of reducing the linear speed of grinding wheel is to reduce the vibration of grinding wheel frame, and then the ripple phenomenon can be reduced and eliminated.

3) There are roughly two kinds of scratches in mirror grinding: one is that the scratches are irregular, which is caused by the impurities and abrasive particles brought out by the impure grinding fluid being washed between the grinding wheel and the workpiece. The method of eliminating this scratch requires grinding fluid filtration, which generally adopts three-way filtration of magnetic and copper wire mesh; The other kind of scratch is like a dotted line, which is shallow, and the long and short distance is roughly the same. This is the scratch caused by the rotation of the grinding wheel when the grinding particles of the grinding wheel are about to fall off. At this time, the elimination method is to use sufficient grinding fluid when dressing the grinding wheel, and repair the two end faces of the grinding wheel into an oblique shape with wide outside and narrow inside. In addition, the selection of grinding wheel should also be appropriate, not too soft, and do not use grinding wheel with good self sharpening.

4) During mirror grinding, there are many patterns and shapes that are easy to appear. There are also many factors that produce patterns, which are mainly caused by vibration, including periodic vibration of grinding wheel, external influence and vibration of adjacent machine tools, as well as patterns when the grinding wheel is too blunt and the stroke is unstable. When grinding is carried out when the adjacent machine tool is not started, the pattern is not easy to appear.

5) When the grinding mark (wire flow) in the mirror grinding of the grinder is obvious, the fine-grained grinding wheel must be used to remove this wire flow. However, when using fine grain grinding wheel, the following points must be considered:

① When finishing the grinding wheel, the diamond knife shall be sharp, the grinding fluid shall be sufficient, and the debris shall be washed away in time to avoid affecting the surface of the grinding wheel.

② The grinding amount shall be appropriate to prevent surface burns and other defects from affecting the surface smoothness. Pay attention to leave appropriate grinding allowance.

③ The linear speed of grinding wheel must be reduced to 15 ~ 18m / s.

④ The grinding fluid is tightly filtered to prevent scratches.

⑤ When finishing the grinding wheel with a grinding stone, the contact area between the grinding stone and the grinding wheel shall not be too small, and the grinding stone used shall not be too soft, otherwise it will not play the role of finishing the grinding wheel