How to achieve mirror surface roughness?

What is mirror grinding?
The surface roughness of the workpiece after grinding is Ra<0.01μm, and the light is like a mirror, which can image clearly, so it is called mirror grinding. The flatness of the grinding plane is not more than 3μm/1000mm. Surface machining of high precision and high added value parts requires mirror grinding.

The grinding machine for mirror grinding should meet the following conditions:
* High precision, stiffness and vibration reduction measures;

* The rotation accuracy of the grinding wheel spindle is higher than 1μm.

* The amplitude of the grinding wheel holder relative to the working table is less than 1μm.

* The transverse feed mechanism can be accurately fretted.

The following matters should be noted when dressing the grinding wheel:

1) Generally, the fine grinding allowance is 0.015~0.02mm. When the workpiece is roughened to Ra0.8, the fine grinding allowance is 0.005~0.015mm (the grinding allowance must be determined according to the properties and hardness of the steel. For the steel with high hardness and easy polishing, less grinding allowance should be left), then the grinding wheel needs to be finished.
2) When finishing the grinding wheel, first knock off the thickness of about 0.1mm, then trim twice with 0.02mm cutting depth, and then trim three times with 0.01mm cutting depth, and finally reciprocate twice without cutting deep space. During finishing, the transverse feed speed was 20-30mm /min. For mild steel workpiece, it is better to slow the transverse feed when grinding the wheel, but the opposite is true when machining the hardened high speed steel.

3) When dressing the grinding wheel, sufficient grinding fluid or other coolant must be used to wash the contact point between the grinding wheel and the diamond knife, and the grinding wheel debris should be washed away in time to avoid affecting the grinding quality.

4) The diamond knife used in finishing need not be very sharp, because the grain size of the grinding wheel is relatively coarse. The diameter of diamond tool tip is within 0.8mm. When dressing with a sharp diamond knife,

the cross feed rate must be slowed down.
5) When the newly dressed grinding wheel starts to be used, the surface roughness is slightly worse, and it will be stable after a while.

Grinding dosage
1) The back cutting deepth of the grinding wheel 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, back knife amount is too large easy to cause burns, and mild steel back knife amount is too small and not easy to polish.
2) The single stroke of the transverse feed is 0.2~0.4mm. The transverse feed speed has a great influence on the surface roughness, because the grinding wheel of mirror grinding has poor cutting ability. If the transverse feed speed increases, the grinding wheel surface will be damaged, so the very smooth surface cannot be obtained.
3) Longitudinal feed is 12-15m /min. The longitudinal feed has little effect on the surface roughness, but if it is too slow, the surface of the workpiece will produce corrugations or patterns.
4) No cutting depth polishing. Due to the poor cutting ability of the grinding wheel in mirror grinding, tool marks often appear 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 the uneven surface of the workpiece, so it needs to be polished for about two minutes without feed.

Defects and measures in mirror grinding

Mirror grinding on surface grinder often produces some defects, such as surface corrugations, burns, scratches, patterns and wear marks (thread flow), etc. 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, poor dynamic balance of motor rotor, uneven oil film between spindle and bearing, etc. In addition, the grinding wheel is not well balanced will also produce 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 ripples.

2) When machining materials with high hardness, the grinding efficiency of the grinding wheel is poor, and the grinding point will generate 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 knife should also be appropriate, not too large. The best method is to reduce the linear speed of the grinding wheel, which is about 18m/s.

3) There are roughly two kinds of scratches in mirror grinding: one kind of scratches is irregular, which is caused by impurity and abrasive particles washed between the grinding wheel and the workpiece. The method to eliminate this scratch requires grinding fluid filtration, generally using magnetic, copper wire mesh three filtering. Another kind of scratch is a row like dotted line, shallow, the length of the distance is roughly the same, this is the grinding wheel grain will fall off, with the grinding wheel rotation and scratch. At this time, the elimination method is to use sufficient grinding fluid when dressing the grinding wheel, and trim the second end face of the grinding wheel into an oblique shape with width outside and width inside. In addition, the grinding wheel should be properly selected, not too soft, do not use a wheel with good self-sharpness.

4) Mirror grinding, easy to appear in a variety of patterns. There are many factors that produce the pattern, and the main reason is vibration, including periodic vibration of the grinding wheel, external influence and vibration of the adjacent machine tool, and the pattern will also appear when the grinding wheel is too blunt and the stroke is unstable. When the adjacent machine is not started to grind, then the pattern is not easy to appear.

5) When the abrasion marks (wire flow) in mirror grinding are obvious, it is necessary to use fine-grained grinding wheels to remove such wire 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. Grinding dosage should be appropriate to prevent surface burns and other ills affecting the surface smoothness. Pay attention to leave appropriate grinding allowance.

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

4. The grinding fluid should be carefully filtered to prevent scratches.

5. When the grinding wheel is refined with a grindstone, 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 will not play the role of finishing the grinding wheel.

Moresuperhard can provide mirror surface grinding services for different industries, can provide vitrified diamond double disc grinding wheel, internal diamond grinding wheel , cylindrical diamond grinding wheel, etc.

—EDITOR: Doris Hu

—POST: Doris