德语
1. What is mirror grinding
After grinding, the surface roughness of the workpiece is Ra<0.01µm. 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 3µm/1000mm. The surface processing of high-precision and high-value-added parts requires mirror grinding.
2. Grinding machines for mirror grinding must meet the following conditions
It has high accuracy, stiffness and vibration-absorbing measures; the rotation accuracy of the grinding wheel spindle is higher than 1µm; the amplitude of the grinding wheel frame relative to the workbench is less than 1µm; the transverse feed mechanism can move slightly; the workbench does not crawl when moving at low speed.
3. Methods for mirror grinding
1. Balance and trim grinding wheels
(1) Balanced grinding wheel. After the grinding wheel is equipped with flange, a thick static balance is first performed, and then the two end faces and outer circles are installed on the grinding wheel shaft, and finally a static balance is performed.
(2) Trim the grinding wheel. The good or bad grinding of the grinding wheel directly affects the smoothness of the grinding surface. In order to reduce the surface roughness value, trimming the grinding wheel is the key. The following things should be paid attention to when trimming the grinding wheel:
1) The general fine grinding margin is 0.015~0.02mm. When the workpiece is roughly ground to Ra0.8 and the fine grinding margin is left 0.005~0.015mm (the grinding margin must be determined according to the properties and hardness of the steel. For steels with high hardness and easy-to-grind polish, the grinding wheel needs to be repaired.
2) When refining the grinding wheel, first remove the thickness of about 0.1mm, then use 0.02mm deep cut twice, then use 0.01mm deep cut three times, and finally repeat twice without cutting deep space stroke. During refining, the lateral feed speeds are all 20~30mm/min. For mild steel workpieces, the lateral feeding is better when repairing the grinding wheel, but the opposite is true when processing very hard high-speed steel.
3) While trimming the grinding wheel, you must use sufficient grinding fluid or other coolant to wash the contact point between the grinding wheel and the diamond knife, and wash away the grinding wheel debris in time to avoid affecting the grinding quality.
4) The diamond knife used during refining does not need to be very sharp, because the grinding wheel has a relatively thick particle size. The diameter of the diamond knife tip is within 0.8mm. When trimming with a sharp diamond knife, the cross-feeding speed must be slowed down.
5) When the newly trimmed grinding wheel starts to be used, the surface roughness is slightly worse and it will stabilize after use for a while.
2. Grinding quantity
1) The grinding wheel back is about 0.005 mm. Appropriately increase and decrease according to the properties and hardness of the workpiece steel. For example, quenched steel with high hardness, excessive amount of the back-loading knife can easily cause burns, while soft steel is too small and not easy to polish.
2) The single stroke of the transverse feed volume is 0.2~0.4mm. The lateral feeding speed has a greater impact on the surface roughness, because the grinding wheel cutting ability of mirror grinding is very poor. If the lateral feeding speed increases, the surface of the grinding wheel will be damaged, so a very smooth surface cannot be obtained.
3) The longitudinal feed is 12~15 m/min. The longitudinal feed has a relatively small impact on surface roughness, but if it is too slow, corrugation or pattern will occur on the surface of the workpiece.
4) No-cut deep light grinding. Due to the poor cutting ability of the grinding wheel during mirror grinding, cuts often appear on the surface of the workpiece (the round workpiece is moon-shaped and the rectangular workpiece is belt-shaped), which reflects that the surface of the workpiece is not straight, so it is necessary to perform feed-free light grinding for about two minutes.
4. Defects and measures that are prone to occur during mirror grinding
Mirror grinding on a flat grinder often produces some defects, such as surface corrugation, burns, scratches, patterns and obvious wear marks (wire flow). In response to these problems, the following measures can be taken:
1) Most surface corrugations are caused by vibration of the grinding wheel, such as excessive bearing clearance, spindle deflection, poor dynamic balance of the motor rotor, uneven oil film between the spindle and bearing, etc. In addition, poor balance of the grinding wheel will also cause ripples. If the grinding wheel is well balanced and there are still corrugations, the grinding wheel and its matching parts must be inspected or adjusted to completely eliminate the corrugation phenomenon.
2) When working with materials with high hardness, the grinding wheel has poor grinding efficiency, and high temperatures will occur at the grinding point, which can easily burn the workpiece (for example, when grinding W18Cr4V, it is more likely to cause burns). To prevent burns, sufficient grinding fluid must be present at the grinding point. In addition, the knife should be measured appropriately and should not be too large. A better way is to reduce the linear speed of the grinding wheel, which is suitable for about 18m/s. The M7120A grinder uses a low speed of 1440r/min, which can basically eliminate burns. Another advantage of reducing the linear speed of the grinding wheel is to reduce the vibration of the grinding wheel frame, which can also reduce and eliminate corrugation.
3) There are roughly two types of scratches in mirror grinding: one is that the scratches are irregular, which is impure in the grinding fluid, and the impurities and abrasive particles brought out are scratched between the grinding wheel and the workpiece. The method to eliminate this kind of scratch requires grinding fluid to filter, which generally uses three lines of filtration through magnetic and copper wire mesh; the other type of scratch is rows of lines that look like dotted lines, are shallow, with roughly the same length and distance. This is scratched as the grinding wheel abrasive particles are about to fall off. The method of elimination at this time is to use sufficient grinding fluid when trimming the grinding wheel, and to make the two end faces of the grinding wheel into an inclined shape with wide outside and narrow inside. In addition, the grinding wheel should be selected appropriately, not too soft, and do not use grinding wheels with too good self-hardening properties.
4) When mirror grinding, there are many types of pattern shapes that are likely to appear. There are many factors that produce patterns. The main reason is vibration. There are periodic vibrations of the grinding wheel, external influences and vibrations from adjacent machine tools. Patterns will also appear when the grinding wheel is too dull or the stroke is unstable. When grinding is performed when the adjacent machine tool is not running, the pattern is less likely to appear.
5) When the grinding marks (filaments) in mirror grinding are obvious, to remove such filaments, a fine-grained grinding wheel must be used. However, when using fine-grained grinding wheels, 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 amount of grinding should be appropriate to prevent surface burns and other defects from affecting the surface finish. Pay attention to leaving appropriate grinding allowance.
③The linear speed of the grinding wheel must be reduced to 15~18m/s.
④The grinding fluid is strictly filtered to prevent scratches.
⑤ When using a grinding stone to fine-dress the grinding wheel, the contact area between the grinding stone and the grinding wheel cannot be too small, and the grinding stone used cannot be too soft, otherwise it will not function as a fine grinding wheel.
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