In the grinding process of bearing, its working surface is ground by high-speed rotating grinding wheel. Surface Grinding Therefore, if the equipment is not operated and adjusted according to the operation instructions, various defects will appear on the working surface of bearing,
which will affect the overall quality of bearing. During precision grinding of bearings, due to the high Surface Grinding requirements for roughness, the grinding marks on the working surface can often be observed with the naked eye. The surface grinding marks mainly include the following.
It shows signs of cross helix
The reason for this trace is mainly due to the poor flatness of the bus of the grinding wheel and the concave Surface Grinding convex phenomenon. During grinding, the grinding wheel is only in partial contact with the workpiece. When the workpiece or grinding wheel moves back and forth several times,
the cross helix will reappear in the workpiece and can be observed by the naked eye. The pitch of these spiral lines is related to the speed of the Surface Grinding worktable and the rotating speed of the worktable. At the same time, it is also related to the non parallelism of the axis line of the grinding wheel and the guide rail of the worktable.
（1） Main causes of helix formation
1. The grinding wheel is poorly trimmed, the edges and corners are not chamfered, and the coolant is not used for trimming;
2. There is too much lubricating oil in the guide rail of the workbench, causing the workbench to float;
3. Poor accuracy of machine tool;
4. Excessive grinding pressure, etc.
（2） There are reasons for the formation of helix
1. The rigidity of V-shaped guide rail is not good. When grinding, the grinding wheel is offset, but the edge of the grinding wheel is in contact with the Surface Grinding working surface;
2. When trimming noisy wheels, the reversing speed of the worktable is Surface Grinding unstable and the precision is not high, so that the trimming of one edge of the grinding wheel is slightly less;
3. Poor rigidity of the workpiece itself;
4. There are broken and peeled sand particles on the grinding wheel and iron filings under workpiece grinding on the surface of the grinding wheel. Surface Grinding Therefore, the trimmed grinding wheel shall be washed or brushed with cooling water;
5. The grinding wheel is not well trimmed, with local bulges, etc.
Fish scales appear on the surface
The main reason for the reproduction of fish scale marks on the surface is that the cutting edge of the grinding wheel is not sharp enough, and the phenomenon of “gnawing” occurs Surface Grinding during grinding, and the vibration is large at this time. The specific causes of fish scale marks on the workpiece surface are:
1. There are garbage and oil dirt on the grinding wheel surface;
2. The grinding wheel is not rounded;
3. The grinding wheel becomes blunt. Trimming is not sharp enough;
4. The diamond fastening frame is not firm, the diamond shakes or the diamond quality is not sharp;
5. Uneven hardness of grinding wheel, etc.
Working face roughening
The main reason for the reproduction of galling marks on the surface is that after the coarse-grained abrasive particles fall off, the abrasive particles are clamped between the workpiece and the grinding wheel.
The specific reasons why the workpiece surface is roughened during grinding are as follows:
1. Traces left during rough grinding are not removed during fine grinding;
2. Coarse abrasive particles and micro abrasive particles in the coolant are not filtered clean;
3. When the coarse-grained grinding wheel is just trimmed, the abrasive particles are easy to fall off;
4. Material toughness expiration date or grinding wheel is too soft;
5. Improper matching between abrasive toughness and workpiece material toughness, etc.
There are straight wave marks on the workpiece surface
When we cut a cross section of the vertical axis line of the ground Surface Grinding workpiece and enlarge it, we can see that its periphery is similar to sine wave. Make its center move along the axis without rotation and translation, and the track around the sine wave is the waveform cylinder, also known as polygon.
The straight waveform is caused by the movement of the grinding wheel relative to the workpiece or the periodic change of the grinding pressure of the grinding wheel on the workpiece. This vibration may be forced vibration or self-excited vibration, so there is often more than one direct wave frequency on the workpiece.
The specific reasons for the straight waveform trace are:
1. The clearance of grinding wheel spindle is too large;
2. The hardness of grinding wheel is too high;
3. Poor static balance of grinding wheel or dull grinding wheel;
4. The workpiece speed is too high;
5. The horizontal Qi knife is too large
6. The grinding wheel spindle bearing is worn, and the matching clearance is too large, resulting in radial runout;
7. Grinding wheel pressing mechanism or workbench “crawling”, etc.
The workpiece surface is often burned during grinding. There are several types of burns. First, the burns are dark black patches along the machining direction of the grinding wheel; Second, it is in line or intermittent line shape.
The workpiece surface is burned during grinding, which can be summarized into the following reasons:
1. The grinding wheel is too hard or the particle size is too fine and the structure is too dense;
2. Excessive feed rate, insufficient supply of cutting fluid and poor heat dissipation conditions;
3. The workpiece speed is too low and the grinding wheel speed is too fast;
4. The grinding wheel vibrates too much and burns due to the continuous change of grinding depth;
5. The grinding wheel is not trimmed in time or well;
6. The diamond is sharp and the grinding wheel is not well trimmed;
7. The workpiece is burned too deeply during rough grinding, and the fine grinding allowance is too small to be worn off;
8. The clamping force or suction of the workpiece is insufficient, and the workpiece stops rotating under the action of grinding force.
So how do you know whether the workpiece surface is burnt during grinding? This can be checked by regular pickling.
After pickling, when the surface of the workpiece is wet, it shall be visually inspected under the astigmatism lamp immediately. The normal surface is uniform dark gray. If it is a software point, it presents cloud like dark black spots,
and the perimeter is not set; If decarburized, it will show gray or dark black spots; In case of grinding crack, the crack is cracked. In case of burn, first, the surface presents dark spots along the grinding wheel processing direction, and second, it presents line or intermittent line shape.
If the above burns occur during grinding, the causes must be analyzed in time and effective measures must be taken to solve them to eliminate batch burns.
The surface roughness does not meet the requirements
The surface roughness of bearing parts has standard and process requirements, but in the process of grinding and superfinishing, due to various reasons, it often can not meet the specified requirements. The main reasons for the failure of workpiece surface roughness to meet the requirements are:
1. The grinding speed is too low, the feed speed is too fast, the feed is too large, and the grinding time without feed is too short;
2. The rotating speed of workpiece is too high or the vibration of workpiece shaft and grinding wheel shaft is too large;
3. The grain size of grinding wheel is too coarse or too soft;
4. The dressing speed of grinding wheel is too fast or the clearance of dressing mechanism is too large;
5. The diamond dressing the grinding wheel is not sharp or of poor quality;
6. The quality of ultra precision oilstone is poor and the installation position is incorrect;
7. The quality of super refined kerosene fails to meet the requirements;
8. The ultra precision time is too short;