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1、 What is mirror grinding

 

 

 

 

 

The surface roughness of the workpiece after grinding is Ra<0.01 µ m, and the light is like a mirror, so it can be clearly imaged, so it is called mirror grinding. The flatness of the grinding surface shall not be greater than 3 µ m/1000mm. The surface processing of high precision and high added value parts requires mirror grinding.

 

 

 

 

 

 

2、 Grinders for mirror grinding shall meet the following conditions

 

 

 

 

 

It has high precision and rigidity and adopts vibration reduction measures; The rotation accuracy of the grinding wheel spindle is higher than 1 µ m; The amplitude of the grinding wheel holder relative to the workbench is less than 1 µ m; The cross feed mechanism can move slightly; There is no creeping phenomenon when the workbench moves at low speed.

 

 

 

 

 

 

 

3、 Method of mirror grinding

 

 

 

 

 

1. Balance and trim the grinding wheel

 

 

 

 

 

(1) Balance the grinding wheel. After the grinding wheel is installed with flange, rough static balance shall be carried out first, then the grinding wheel shall be installed on the grinding wheel shaft to trim the second end face and the outer circle, and finally the static balance shall be carried out.

 

 

 

 

 

(2) Trim the grinding wheel. The dressing of the grinding wheel is good or bad, which directly affects the smoothness of the grinding surface. In order to reduce the surface roughness value, dressing the grinding wheel is the key. Pay attention to the following items when dressing the grinding wheel:

 

 

 

 

 

1) Generally, the fine grinding allowance is 0.015~0.02mm. When the workpiece is roughly ground to Ra0.8, the fine grinding allowance is 0.005~0.015mm (the grinding allowance must be determined according to the steel property and hardness. For steel with high hardness and easy to polish, the grinding allowance should be appropriately reduced), 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 a cutting depth of 0.02mm, trim it three times with a cutting depth of 0.01mm, and finally reciprocate twice without cutting depth. The cross feed speed during fine trimming is 20~30mm/min. For mild steel workpieces, the cross feed is better when grinding the grinding wheel, but the opposite is true when machining hardened high speed steel.

 

 

 

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

 

 

 

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

 

 

 

5) When the new dressing wheel is used, the surface roughness is slightly worse, and it will be stabilized after a while.

 

 

 

 

 

2. Grinding amount

 

 

 

 

 

1) The cutting amount of the grinding wheel back is about 0.005 mm. Increase or decrease appropriately according to the properties and hardness of the workpiece steel. For example, for hardened steel with high hardness, excessive back cutting is easy to cause burns, while for mild steel, too small back cutting is not easy to polish.

 

 

 

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

 

 

 

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

 

 

 

4) No deep cut polishing. Due to the poor cutting ability of the grinding wheel during mirror grinding, tool marks (moon shaped circular workpiece and belt shaped rectangular workpiece) often appear on the workpiece surface, which reflects that the workpiece surface is not straight, so it is necessary to conduct non feed polishing for about two minutes.

 

 

 

 

 

 

4、 Defects and Measures in Mirror Grinding

 

 

 

 

 

Mirror grinding on a surface grinder often produces some defects, such as surface ripples, burns, scratches, patterns and wear marks (silk flow). To solve these problems, the following measures can be taken:

 

 

 

 

 

1) The surface ripple is mostly caused by the vibration of the grinding wheel, such as excessive bearing clearance, eccentric adjustment of the main shaft, poor dynamic balance of the motor rotor, and uneven oil film between the main shaft and the bearing. In addition, a poorly balanced grinding wheel 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 repaired or adjusted to completely eliminate the ripples.

 

 

 

 

 

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 point. In addition, the amount of back stabbing should be appropriate, not too large. A better way is to reduce the linear speed of the grinding wheel, which is about 18m/s. The low speed of 1440r/min is adopted on the M7120A grinder, which can basically eliminate the burn phenomenon. Another advantage of the reduced linear speed of the grinding wheel is to reduce the vibration of the grinding wheel carrier, which can also reduce and eliminate the ripple phenomenon.

 

 

 

 

 

3) There are two kinds of scratches in mirror grinding: one is irregular, because the grinding fluid is impure, and the impurities and abrasive particles brought out are washed between the grinding wheel and the workpiece. The method to eliminate such scratches requires grinding fluid filtration, which generally adopts three layers of magnetic and copper wire mesh filtration; The other kind of scratches are lines like dotted lines, which are shallow and have roughly the same long and short distance. They are scratched with the rotation of the grinding wheel when the abrasive grains 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 trim the two end faces of the grinding wheel into an oblique shape with outer width and inner narrow. In addition, the selection of grinding wheel should also be appropriate, not too soft, and do not use the grinding wheel with too good self sharpening property.

 

 

 

 

 

4) When mirror grinding, the patterns that are easy to appear have many forms. There are also many factors that cause the pattern, the main reason is vibration, including the periodic vibration of the grinding wheel, the external influence and the vibration of adjacent machine tools, and the pattern will also appear when the grinding wheel is too blunt and the stroke is unstable. When the adjacent machine tool is not started, the pattern is not easy to appear.

 

 

 

 

 

5) When the wear mark (silk flow) in mirror grinding is obvious, the fine grain grinding wheel must be used to remove the silk flow. However, when using fine grain grinding wheel, the following points must be considered:

 

 

 

 

 

① When finishing the grinding wheel, the diamond knife 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.

 

 

 

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

 

 

 

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

 

 

 

④ The grinding fluid is closely filtered to prevent scratches.

 

 

 

⑤ When the grinding stone is used for finishing the grinding wheel, 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.

 

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