Types and causes of bearing surface roughness defects

During the grinding process, the working surface of the bearing is ground by a high-speed rotating grinding wheel. Therefore, if the equipment is not operated and adjusted according to the operating instructions during grinding, various roughness defects will appear on the working surface of the bearing, which will affect the overall quality of the bearing.

During precision grinding of bearings, due to the high surface roughness requirements, the grinding marks on the working surface can often be observed with the naked eye. There are mainly the following types of surface grinding marks. (It is recommended to use: roughness meter for roughness detection)

Cross-helical traces on the surface

The main reason for this mark is that the generatrix of the grinding wheel is poorly straight and has concave and convex phenomena. During grinding, the grinding wheel and the workpiece are only partially in contact. After the workpiece or grinding wheel moves back and forth several times, cross-helical lines will reappear on the workpiece surface and can be observed by the naked eye. The pitch of these spiral lines is related to the workpiece table speed and workpiece rotation speed, and is also related to the non-parallelism of the grinding wheel axis and the worktable guide rail.

The main reasons for the formation of spiral lines:

1. The grinding wheel is poorly dressed, the edges and corners are not chamfered, and coolant is not used for dressing;

2. There is too much lubricating oil in the guide rail of the workbench, causing the workbench to float;

3. The machine tool accuracy is poor;

4. The grinding pressure is too high, etc.

Specific reasons for the formation of spiral lines:

1. The rigidity of the V-shaped guide rail is not good. When grinding, the grinding wheel is offset, and only the edge of the grinding wheel contacts the working surface;

2. When dressing the grinding wheel, the workbench reversing speed is unstable and the accuracy is not high, so that one edge of the grinding wheel is slightly less dressed;

3. The rigidity of the workpiece itself is poor;

4. There are broken and peeled sand particles and iron filings from the grinding of the workpiece on the grinding wheel. For this reason, the dressed grinding wheel should be rinsed or brushed with cooling water;

5. The grinding wheel is not well dressed, and there are local bulges, etc.

Fish scales appear on the surface

The main reason for the reappearance of fish scales on the surface is that the cutting edge of the grinding wheel is not sharp enough, and the "biting" phenomenon occurs during grinding, and the vibration is large at this time, causing the workpiece.

The specific reasons for the appearance of fish scales on the surface are:

1. There is garbage and oil on the surface of the grinding wheel;

2. The grinding wheel is not rounded;

3. The grinding wheel becomes blunt and the dressing is not sharp enough;

4. The diamond fastening frame is not firm, the diamond shakes or the diamond quality is poor and not sharp;

5. The hardness of the grinding wheel is uneven, and the working surface is roughened.

The main reason for the reappearance of roughening marks on the surface is that after the coarse-grained abrasive particles fall off, the abrasive particles are sandwiched between the workpiece and the grinding wheel.

The specific reasons for the roughening of the workpiece surface during grinding are:

1. The traces left over from the rough grinding are not removed during the fine grinding;

2. The coarse and fine abrasive particles in the coolant are not filtered cleanly;

3. The abrasive particles are easy to fall off when the coarse-grained grinding wheel is just trimmed;

4. The material toughness expires or the grinding wheel is too soft;

5. The abrasive toughness and the workpiece material toughness are not properly matched, etc.

There are straight wave traces on the workpiece surface

We cut a cross section of the vertical axis of the ground workpiece and enlarge it, and we can see that its periphery is similar to a sine wave. The center of the workpiece is not rotated and translated along the axis, and the trajectory around the sine wave is a wave cylinder, also known as a polygon.

The reason for the straight wave is the movement of the grinding wheel relative to the workpiece or the periodic change of the pressure of the grinding wheel on the workpiece, which causes vibration. This vibration may be forced vibration or self-excited vibration, so there is often more than one straight wave frequency on the workpiece.

The specific reasons for the straight wave marks are:

1. The clearance of the grinding wheel spindle is too large;

2. The hardness of the grinding wheel is too high;

3. The static balance of the grinding wheel is not good or the grinding wheel becomes blunt;

4. The workpiece speed is too high;

5. The transverse cutter is too large;

6. The grinding wheel spindle bearing is worn, the matching clearance is too large, and radial runout occurs;

7. The grinding wheel clamping mechanism or the workbench "crawls", etc.

Burn marks reappear on the workpiece surface

The workpiece surface is often burned during the grinding process. There are several types of burns. One is that the burn is along the grinding wheel processing direction, showing dark patches; the other is in the form of lines or intermittent lines.

There are several reasons why the workpiece surface is burned during grinding:

1. The grinding wheel is too hard or the grain size is too fine and the structure is too dense;

2. The feed rate is too large, the cutting fluid supply is insufficient, and the heat dissipation conditions are poor;

3. The workpiece speed is too low and the grinding wheel speed is too fast;

4. The grinding wheel oscillates too much, and the grinding depth changes continuously and burns;

5. The grinding wheel is not dressed in time or the dressing is not good;

6. The diamond is sharp and the grinding wheel is not dressed well;

7. The workpiece is burned too deeply during rough grinding, and the fine grinding reserve is too small, so it is not ground off;

8. The clamping force or suction force of the workpiece is insufficient, and the workpiece stops under the action of the grinding force.

So how do you know whether the workpiece surface is burned during the grinding process? This can be checked by regular pickling.

After the workpiece is pickled, when the surface is wet, it should be visually inspected under a diffuse light immediately. The normal surface is uniform dark gray. If it is a soft spot, it will appear as a dark cloud-like spot with irregular circumference; if it is decarburized, it will appear gray or dark mottled; if it is a grinding crack, the crack will be cracked, and if it is a burn, one is that the surface will appear dark patches along the grinding wheel processing direction, and the other is that it will appear as lines or intermittent lines.

If the above-mentioned burn phenomenon occurs during the grinding process, it is necessary to analyze the cause in time and take effective measures to solve it to prevent batch burns.

Surface roughness does not meet the requirements

The surface roughness of bearing parts has standards and process requirements, but in the grinding and superfinishing process, due to various reasons, it often does not meet the specified requirements.

The main reasons for the workpiece surface roughness not meeting the requirements are:

1. The grinding speed is too low, the feed speed is too fast, the feed amount is too large, and the no-feed grinding time is too short;

2. The workpiece speed is too high or the workpiece axis and the grinding wheel axis vibrate too much;

3. The grinding wheel particle size is too coarse or too soft;

4. The grinding wheel dressing speed is too fast or the dressing mechanism gap is too large;

5. The diamond for dressing the grinding wheel is not sharp or the quality is poor;

6. The oilstone for superfinishing is of poor quality and the installation position is incorrect;

7. The quality of kerosene for superfinishing does not meet the requirements;

8. The superfinishing time is too short, etc.