The generation and prevention of spiral shape in t

2022-07-27
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Generation and prevention of spiral shape during grinding cylindrical workpieces there are two cases of common spiral defects of cylindrical workpieces (Fig. 1):

spiral shape consistent with the relationship between workpiece speed and workbench speed

spiral shape inconsistent with the relationship between workpiece speed and workbench speed

Fig. 1 the reason why the cylindrical workpiece

has spiral shape is that the busbar of the grinding wheel has poor contact with the workpiece during grinding. The cause of poor contact is: poor dressing of the grinding wheel: the workpiece or grinding wheel is offset due to the grinding force during grinding. However, it has to be analyzed according to the specific situation

the main causes of the first kind of spiral shape and its elimination measures:

1 the busbar of the grinding wheel is unevenly repaired during rough trimming, but it cannot be straightened due to less trimming during slight trimming. During grinding, the grinding wheel is in partial contact with the workpiece, and one edge of the grinding wheel often contacts the workpiece, resulting in a spiral on the surface of the workpiece, as shown in Figure 2. Therefore, the cutting depth of rough grinding wheel in precision grinding, ultra precision grinding and mirror grinding should not be too large, and it can generally be within 0 01mm/time

Fig. 2 helical shape caused by uneven dressing of grinding wheel

2 no coolant is used when dressing the grinding wheel, or the coolant is only directed at one part of the grinding wheel, and the other part is not directed. When trimming the part that is not impacted, the diamond will cause expansion and cause the busbar of the grinding wheel not to be straight. Therefore, when dressing the grinding wheel, there should be sufficient coolant, and the nozzle position and coolant should be aligned with the entire width of the grinding wheel

3 because the spindle of the grinding wheel rack is tilted or lowered, the center line of the grinding wheel is not horizontal with the guide rail of the bed in the vertical plane, so the grinding wheel busbar will be concave during trimming (see Figure 3)

Figure 3 the working surface of the grinding wheel is made into a concave shape

the concave grinding wheel surface has poor contact with the workpiece, resulting in a spiral shape. Therefore, when installing the grinding wheel rack, it is expected that the grinding wheel shaft has a bit of warping head. The measurement accuracy is about 0.005 mm every 10 mm, which can be achieved by scraping the guide rail or rotary table of the grinding wheel rack. For machine tools with short three pad bearings, the height of the bearing bush can be adjusted to achieve certain requirements. After the grinding wheel is specially installed, the heavy basin of the wheel can lower the warped head a little, which is almost just horizontal

4 the lubricating oil pressure of the worktable is too high, which makes the worktable unstable and floating, resulting in unilateral contact during grinding and spiral shape of the workpiece. Therefore, the guide rail lubricating oil pressure should be lower and the flow rate should be smaller

5 due to the inconsistent rigidity of the head and tailstock system. When the grinding force is basically unchanged, the deformation due to poor rigidity of the system is large, resulting in poor contact between the grinding wheel and the workpiece, resulting in spiral shape, as shown in Figure 4. In this case, attention should be paid to reducing the grinding amount

Fig. 4 inconsistent rigidity of head and tail frames causes poor rigidity of spiral

6 grinding wheel frame. When subjected to the grinding normal force, the horizontal deflection around the vertical axis is generated. Fig. 5 shows that the grinding wheel bears little force and basically does not deflect during dressing; FIG. 5 shows that the grinding wheel bears great force and produces certain deflection, resulting in the contact between the edge of the grinding wheel and the workpiece, resulting in spiral shape. This phenomenon is more likely to occur in China's plastic machinery market, especially when graphite sand wheel is used for mirror grinding. Because the normal impulse is large during mirror grinding (that is, the contact pressure between the grinding wheel and the workpiece)

Figure 5. The deflection of the grinding wheel causes the helical

7 guide rail of the grinding wheel rack to distort. However, there is too much correlation between the position of the diamond during dressing and the position of the workpiece during grinding, which will also cause one-sided contact between the grinding wheel and the workpiece during grinding, resulting in helical defects. Therefore, it is necessary to improve the accuracy of the transverse guide rail. The position of the grinding wheel frame when dressing the grinding wheel should be close to the grinding position

8 grinding wheel hardness is uneven in the width direction Replace the grinding wheel. The hardness on both sides of some grinding wheels is inconsistent with that in the middle. Therefore, when grinding with high finish, both sides of the grinding wheel should be trimmed lower (see Figure 6)

Fig. 6 the two sides of the grinding wheel are trimmed into shoulder shape

9 because the grinding wheel is trimmed during the thermal deformation process of the machine tool. During grinding, the thermal deformation caused by the rotation of the grinding wheel deflects the grinding wheel, resulting in poor contact between the grinding wheel bus and the workpiece, resulting in spiral shape. Therefore, it is required to trim and grind the grinding wheel when the machine tool enters the temperature rise saturation state

the main reason for the second kind of spiral is that the lubricating oil of the workbench is too bad, which makes the workbench float up and shake during operation. This kind of sloshing frequency is very low, and there is no certain product to make up for the blank law in the field of special-shaped copper strip with 50ram end distance between high clamps in China. This kind of spiral shape is generally wide, and the elimination measure is to properly lower the lubricating oil and pressure of the guide rail to reduce the flow. (end)

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