Mirror grinding is a special machining process suitable for high-precision grinding machines. Through this grinding technique, you can achieve a surface roughness of Ra ≤ 0.1 μm, resulting in a mirror-like reflective surface.
It is widely used in mold manufacturing, aerospace, and various high-end precision parts processing fields.
This article will introduce the concept of mirror grinding, the corresponding grinding machine requirements, the grinding method process, and solutions to common problems.
After grinding, the surface roughness of the workpiece Ra < 0.1µm, resulting in a mirror-like surface that allows for clear imaging; hence the name mirror grinding. The flatness of the ground surface is no greater than 3µm/1000mm.
Mirror grinding is commonly required for the surface processing of high-precision, high-value-added parts.
1. The machine tool for mirror grinding requires high precision. Spindle runout must be ≤ 0.001–0.002 mm, and table flatness ≤ 0.002 mm, with vibration reduction measures implemented.
2. The rotational accuracy of the grinding wheel spindle must be higher than 1µm.
3. The amplitude of the grinding wheel head relative to the table must be less than 1µm.
4. The transverse feed mechanism must be able to make precise micro-movements.
5. The table must not crawl during low-speed movement.
6. Fine-grit grinding wheels must be used to achieve a mirror finish.
(1) Balancing the Grinding Wheel. After mounting the grinding wheel on the flange, perform a rough static balance first. Then, dress the two end faces and the outer diameter on the grinding wheel shaft, and finally perform a precise static balance.
(2) Dressing the Grinding Wheel. The quality of grinding wheel dressing directly affects the smoothness of the ground surface. Dressing the grinding wheel is crucial to reducing surface roughness.
1) The general finishing allowance is 0.015~0.02mm. When the workpiece is rough ground to Ra0.8, leaving a finishing allowance of 0.005~0.015mm (the grinding allowance must be determined according to the steel properties and hardness;
for high-hardness, easily polished steels, the grinding allowance should be appropriately reduced), the grinding wheel needs to be dressed.
2) When dressing the grinding wheel, first remove about 0.1mm of thickness, then dress twice with a depth of cut of 0.02mm, then three times with a depth of cut of 0.01mm, and finally perform two reciprocating strokes with no depth of cut.
The transverse feed rate during finishing is 20~30mm/min. For soft steel workpieces, a slower transverse feed is better when dressing the grinding wheel; the opposite is true when machining very hardened high-speed steel.
3) While dressing the grinding wheel, sufficient grinding fluid or other coolant must be applied to the contact point between the grinding wheel and the diamond cutter to remove grinding debris promptly and prevent affecting grinding quality.
4) The diamond cutter used for finish dressing does not need to be very sharp, as the grinding wheel grit is relatively coarse. A diamond cutter tip diameter of 0.8mm or less is acceptable. When dressing with a sharp diamond cutter, the transverse feed rate must be slowed down.
5) When a newly dressed grinding wheel is first used, the surface roughness will be slightly poor, but it will stabilize after a period of use.
1) The depth of cut should be approximately 0.005 mm. Adjust the depth of cut appropriately according to the properties and hardness of the workpiece steel.
For example, for high-hardness quenched steel (HRC50–65), an excessive depth of cut can easily cause burning, while for soft steel, a too-small depth of cut makes it difficult to grind smoothly.
2) The single-stroke transverse feed rate should be 0.2~0.4 mm. The transverse feed rate has a significant impact on surface roughness because the grinding wheel in mirror grinding has poor cutting ability.
If the transverse feed rate increases, the grinding wheel surface will be damaged, thus preventing the achievement of a perfectly smooth surface.
3) The longitudinal feed rate is 12~15 m/min. The longitudinal feed has a relatively small impact on surface roughness, but if it is too slow, ripples or patterns will appear on the workpiece surface.
4) No-depth-of-cut finishing. Due to the poor cutting ability of the grinding wheel during mirror grinding, tool marks often appear on the workpiece surface (crescent-shaped on round workpieces, band-shaped on rectangular workpieces), indicating an uneven surface.
Therefore, no-feed finishing for about two minutes is necessary.
Mirror grinding on a surface grinder often produces defects such as surface ripples, burns, scratches, patterns, and obvious grinding marks (thread flow). The following measures can be taken to address these problems:
(1) Most surface ripples are caused by grinding wheel vibration, such as excessive bearing clearance, spindle misalignment, poor dynamic balance of the motor rotor, and uneven oil film between the spindle and bearings.
Poor wheel balancing can also cause ripples. If the grinding wheel is well-balanced but ripples still appear, the grinding wheel and its mating parts must be inspected or adjusted to completely eliminate the ripple phenomenon.
(2) When machining high-hardness materials, the grinding wheel has poor grinding efficiency, and the grinding point generates high temperatures, easily burning the workpiece (for example, burning is more likely to occur when grinding W18Cr4V).
To prevent burning, sufficient grinding fluid must be present at the grinding point. Additionally, the depth of cut should be appropriate and not too large.
The best method is to reduce the linear speed of the grinding wheel to approximately 18 m/s. Using a low speed of 1440 r/min on the M7120A grinder can basically eliminate the burning phenomenon.
Another advantage of reducing the linear speed of the grinding wheel is that it reduces the vibration of the grinding wheel head, which in turn reduces and eliminates the ripple phenomenon.
(3) There are generally two types of scratches in mirror grinding: one type is irregular scratches, which is caused by impurities and abrasive grains carried out by the impurities in the grinding fluid and washed between the grinding wheel and the workpiece.
To eliminate this type of scratch, the grinding fluid needs to be filtered, generally using a three-stage filtration system of magnetic filtration and copper wire mesh filtration; the other type of scratch is like rows of dotted lines, which are shallow and roughly the same length.
This is caused by the abrasive grains on the grinding wheel about to fall off, scratching as the grinding wheel rotates.
The elimination method in this case is to use sufficient grinding fluid when dressing the grinding wheel and to dress the two ends of the grinding wheel into a bevel shape that is wider on the outside and narrower on the inside.
In addition, the grinding wheel should be selected appropriately, not too soft, and not a grinding wheel with too good self-sharpening properties.
(4) There are various forms of patterns that are likely to appear during mirror grinding. Many factors contribute to the formation of grinding patterns, primarily caused by vibration.
These include periodic vibration of the grinding wheel, external influences, vibrations from nearby machine tools, and dull grinding wheels with unstable strokes. Grinding is less likely to produce patterns when nearby machine tools are not in operation.
(5) When grinding marks (thread flow) are significant during mirror grinding, a finer-grit grinding wheel must be used to remove them.
1) When finishing the grinding wheel, the diamond cutter must be sharp, and sufficient grinding fluid must be used to promptly flush away debris to avoid affecting the grinding wheel surface.
2) The grinding amount must be appropriate to prevent surface burns and other defects that affect surface finish. Leave an appropriate grinding allowance.
3) The grinding wheel linear speed must be reduced to 15–18 m/s.
4) The grinding fluid must be thoroughly filtered to prevent scratches.
5) When finishing a grinding wheel with a whetstone, the contact area between the whetstone and the grinding wheel should not be too small, and the whetstone should not be too soft; otherwise, it will not achieve the desired finishing effect.
Mirror grinding typically involves four steps. The first step is rough grinding, using a #60–#120 grinding wheel with a depth of cut of 0.01–0.02 mm to quickly remove excess material.
The second step is semi-finish grinding. In this stage, a finer grinding wheel (#120–#400) is selected with a feed rate of approximately 0.01 mm per stroke. This step improves the flatness of the workpiece.
The third step is finish grinding, using a #600–#1500 grinding wheel with a depth of cut of approximately 0.002–0.005 mm. This step reduces the surface roughness of the workpiece.
The fourth process is mirror grinding, using a #2000–#3000 grinding wheel with a feed rate of approximately 0.001–0.003 mm/stroke, ultimately achieving a high surface finish of Ra 0.05–0.1 μm.
Problem 1: Unable to achieve a mirror finish
Cause: Grinding wheel is too coarse or insufficiently fine-ground
Solution: Dress the grinding wheel every 15–20 minutes for fine grinding.
Problem 2: Surface burns
Cause: Improper feed or insufficient cooling
Solution: Keep the coolant clean and stable and use micro-feed.
Problem 3: Vibration marks appear
Cause: Machine tool vibration or unstable clamping
Solution: Ensure machine tool rigidity, reduce vibration, and improve stability.
Achieving mirror grinding in the field of high-precision grinding is not easy. It requires a high-precision surface grinding machine tool of excellent quality and a suitable machining process and solution.
YASHIDA 3060AHD and 4080APS grinding machines are high-quality grinding machines suitable for mirror finishes.
By optimizing process parameters, high-quality mirror finishes can be consistently achieved.
Learn more:
How to Control Surface Roughness of High-Precision CNC surface grinding machines
Common Faults and Quick Repair Guide for CNC surface grinding machines
The Influence of Temperature on Machining Accuracy of Surface Grinding Machines
Basic Mechanical Knowledge – Understanding Flatness in Surface Grinding
Main Safety Risks and Preventive Measures for Surface Grinders
This website uses cookies to ensure you get the best experience on our website.
Español
Portugues
Pусский
العربية
Phone
Comment
（0）