During precision grinding of imported bearings, due to the extremely high surface finish requirements, grinding marks on the working surface are often visible to the naked eye. The main types of surface grinding marks are as follows. The primary cause of intersecting spiral marks on the surface is the poor straightness of the grinding wheel’s generatrix, resulting in unevenness. During grinding, the grinding wheel makes only partial contact with the workpiece. After several back-and-forth movements of the workpiece or grinding wheel, intersecting spiral marks reappear on the workpiece surface and are visible to the naked eye. The pitch of these spiral lines is related to the feed rate and rotational speed of the workpiece, as well as to the lack of parallelism between the grinding wheel’s axis and the worktable guideways.

(1) Causes of spiral line formation

1. Poor rigidity of V-shaped guideways; during grinding, the grinding wheel shifts, resulting in contact only at the wheel’s edge with the workpiece surface. 2. Unstable table reversal speed and low precision during wheel dressing result in one edge of the grinding wheel being dressed slightly less;

3. Poor rigidity of the workpiece itself;

4. Broken or flaking abrasive grains on the grinding wheel, as well as metal swarf from the workpiece, accumulate on the wheel’s surface; therefore, the dressed grinding wheel should be rinsed with cooling water or brushed clean;

5. The grinding wheel has not been dressed properly, resulting in localised protrusions, etc.

(II) Main causes of helical marks

1. Poor dressing of the grinding wheel, with edges not chamfered and no coolant used during dressing;

2. Excessive lubricant on the worktable guideways, causing the worktable to float;

3. Poor machine tool accuracy;

4. Excessive grinding pressure, etc.

1. Seized bearings:

The ferrograph shows a significant number of large-sized alloy abrasive particles and black metallic oxides.

2. Fretting wear on the bearing raceway:

Spectral analysis reveals abnormal iron concentrations, with numerous sub-micron wear particles of iron components in the ferrograph, as well as abnormal moisture content and acid value in the lubricating oil.