Straight grinding wheels are commonly used for workpiece outer circle and surface grinding. At present, the mainstream straight grinding wheels in the market mainly include resin bond, ceramic bond, and metal bond type grinding wheels, but ceramic bond and resin bond are mainly used in ultra-precision grinding.
During precision grinding, due to the very high requirements for surface roughness, the grinding marks on the working surface can often be observed with the naked eye. Today we will summarize and share the causes of this type of abnormality and common solutions.
1. Crossed spiral traces appear on the surface
The main reason for this trace is that the straight grinding wheel has poor straightness and unevenness. During grinding, the grinding wheel and the workpiece are only partially in contact. After the workpiece or the grinding wheel moves back and forth several times, cross spirals will reappear on the workpiece and can be observed by the naked eye.
The pitch of these spirals 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.
(I) The main reasons for the formation of spiral lines are as follows:
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 worktable, causing the worktable to float;
3. The machine tool has poor precision;
4. The grinding pressure is too large, etc.;
(II) The specific reasons for the formation of spiral lines are as follows:
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 reversing speed of the worktable is unstable and the precision 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 grains on the grinding wheel and iron filings from the grinding of the workpiece are attached to the surface of the grinding wheel. For this reason, the dressed grinding wheel should be rinsed or brushed clean with cooling water;
5. The grinding wheel is not properly dressed, with local bulges, etc.
2. Fish scales appear on the surface
The main reason for the reappearance of fish scales on the surface is: because the cutting edge of the Straight grinding wheel is not sharp enough, it "bites" during grinding, and the vibration is large at this time. The specific reasons for the appearance of fish scales on the workpiece surface are:
⑴ There is garbage and oil on the surface of the grinding wheel;
⑵ The grinding wheel is not rounded;
⑶ The grinding wheel becomes blunt and the dressing is not sharp enough;
⑷ The diamond fastening frame is not firm, the diamond shakes or the diamond quality is poor and not sharp;
⑸ The hardness of the grinding wheel is uneven, etc.
3. Roughening of the working surface
The main reason for the roughening marks on the surface is that the coarse-grained abrasive particles are sandwiched between the workpiece and the Straight grinding wheel after they fall off.
The specific reasons for the roughening of the workpiece surface during grinding are:
⑴ The traces left over from the rough grinding are not removed during the fine grinding;
⑵ The coarse and fine abrasive particles in the coolant are not filtered cleanly;
⑶ The abrasive particles are easy to fall off when the coarse-grained grinding wheel is just dressed;
⑷ The material toughness expires or the grinding wheel is too soft;
⑸ The toughness of the abrasive particles and the toughness of the workpiece material are not properly matched, etc.
4. There are straight wave traces on the surface of the workpiece.
Cut a cross section of the ground workpiece perpendicular to the axis and enlarge it, and you can see that its periphery is similar to a sine wave.
Make its center rotate and translate along the axis without rotation, and the trajectory around the sine wave is a wave cylinder, also known as a polygon.
The main reason for the straight wave is that the movement of the straight grinding wheel relative to the workpiece or the periodic change of the pressure of the grinding wheel on the workpiece 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 between the grinding wheel and the 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 grinding wheel spindle bearing is worn, the matching clearance is too large, and radial runout occurs;
6. The grinding wheel clamping mechanism or the workbench "crawls", etc.
In view of the above common problems, combined with the experience of straight grinding wheel grinding research over the years, some common prevention solutions are sorted out for everyone:
1. Pay attention to keep the grinding wheel balanced;
① The new grinding wheel needs to be statically balanced twice;
② After the grinding wheel has been used for a period of time, if it is unbalanced again, it needs to be statically balanced again;
③ Before the grinding wheel stops, turn off the cutting fluid first and let the grinding wheel run idly for dehydration to prevent the coolant from accumulating in the lower part and causing imbalance;
2. According to the properties of the workpiece material, choose the appropriate grinding wheel hardness;
3. Dress the grinding wheel in time;
4. Reasonably choose the dressing amount or turn over and re-weld the diamond, or sharpen the diamond;
5. Appropriately reduce the working speed and repair the center hole;
6. Change to grinding on a larger grinder. If this cannot be done due to equipment limitations, the grinding depth and longitudinal feed rate can be reduced and the grinding wheel can be sharpened;
7. Adjust the bearing clearance according to the machine tool manual;
8. Adjust the headstock spindle bearing clearance.
