1 Mechanism of grinding burns
In the grinding process, due to the grinding, scoring, and friction of the abrasive grains on the workpiece, the metal surface is plastically deformed, there is heat caused by internal friction which is generated by the relative displacement between the metal molecules inside the workpiece. When grinding by wheels, the speed relative to the workpiece is very high. And there is heat caused by intense external friction with the surface of the workpiece.
Because every abrasive grain grinding is instantaneous, the heat generation is instantaneous and cannot be transmitted in time. Therefore, the instantaneous temperature in the grinding area is high and it can reach 800 ~ 1500 ° C. If the heat dissipation measures are not good, it is easy to cause burns on the surface of the workpiece, that is, secondary quenching and high temperature tempering on the surface of the workpiece (generally tens of microns to hundreds of microns), destroying the tissue of the surface of the workpiece.
It can be seen as severe burns by the naked eye. There is severe burnt yellow or black oxide film on the surface, and the slight burn is observed with the dilute acidic solution. The burned area is black, and burns can reduce the life of the workpiece.
2 Prevention of grinding burns
Since the burn is caused by a large amount of heat generated in the grinding area and is not released in time, reduce the heat to avoid the burn, accelerate the heat dissipation, that is, reduce the internal and external friction during grinding, making the workpiece fully effective cooling. For the coolant, the composition concentration should be appropriate, the flow rate and pressure should be sufficient, and the cleaning effect( washing the grinding wheel and workpiece friction, cooling, and lubrication) can be achieved.
Reducing the generation of frictional heat can reduce internal and external friction during grinding. This should take into account the following factors: reduce the grinding thickness, making the reduction of plastic deformation of the metal, reducing the internal friction and grinding heat.
Increase the workpiece rotation speed, shorten the time that the workpiece grinding surface passing through the grinding area, and reduce the accumulation of grinding heat. In terms of the features of the grinding wheel, the hardness of abrasive is high, the cutting performance is good, the heat can be reduced, but the hardness of the abrasive is not too hard, the tissue should not be too thin, otherwise the blunt abrasive is not easy to fall off, the micropores between the abrasives are easy to block, reducing the cutting performance of grinding wheel.
And increase the contact area between the workpiece and the grinding wheel. The surface of the workpiece is strongly pressed, and the friction cause s the temperature to increase.
Therefore, in order to avoid burns and ensure the abrasive efficiency and the accuracy of the workpiece, the grinding wheel with low hardness and large tissue number is used for rough grinding, and choose larger grinding thickness. In the final grinding, the harder grinding wheel with smaller organization number is used to ensure effectively dress the grinding wheel and choose smaller grinding thickness. For materials with high grinding strength, high hardness, and poor thermal conductivity, small feed rate is applied to increase the workpiece rotation speed and avoid burns.
3 Specific factors that cause grinding burns
In actual production, the following specific factors can cause burns:
Process system vibration, machine vibration and hydraulic system pressure unstable. In the moment of vibration, the amount of grinding will increase, which causes burns. At this time, the burn is distributed along the surface of the workpiece, which is also called vibrating burn.
The grinding wheel is poorly dressed, so that the abrasive is not sharp, which cause burns. At this time, the burn is distributed along with the grinding scratching of wheels, also called scratch burn.
The centerless clamp has insufficient suction or loose support, and the workpiece will slip instantaneously and cause burns.
The grinding amount is not uniform, and the grinding thickness is inconsistent, and local burns may occur.
The grinding wheel is too hard, and the feed rate is too large, and the workpiece rotation speed is too high(Too high workpiece speed will reduce the cutting ability of the grinding wheel), which will cause uniform burn on the surface of the workpiece.
3.1 Surface grinding burns
In the case of the surface process such as grinding flat surface on the vertical single-head grinder, there are several factors that can cause burns under the condition of the right operating specifications.
The grinding wheel shaft is swayed to increase the instantaneous grinding amount to cause burns, at this time, tighten the bearing of the grinding wheel and replace the bearing.
The table disc jumps or the rotation speed is uneven, which increases the instantaneous grinding amount and causes burns. Repair the various components of the disk reducer such as the worm gear, worm and etc., replace the worn parts.
The magnetic force of the disc is small, which will cause the workpiece to slip on the disc and cause burns. For maintenance, adjust the charging and demagnetizing lines.
The reciprocating stroke is not well-positioned. If the contact switch is not in good contact or the reciprocating reducer assembly is worn, the worm gear and the worm mechanism cannot be self-locking, which may cause burns by punching, impacting and jumping in the carriage.
Burns can be caused by double-face surface grinder of M7675 series:
The feeding guide is worn, not straight, and the feeding is uneven.
The guide plate is not parallel to the working surface of the grinding wheel.
The inlet of the grinding wheel end face is too small so that the outer edge of the grinding wheel bears most of the cutting amount, and the grinding wheel becomes dull.
3.2 Cylindrical grinding Burns
Centerless cylindrical grinding machines such as M1083 generally have the following special cases: The feeding plate is crawled and punched, and the feeding amount is suddenly increased to cause burns. Adjust the wedge iron, clean the lubrication plate or scrape the guide plate, repair the screws and the nut, replace the worn parts.
The spark is not adjusted properly. There is too much ground at the entrance, the front end burns easily, and the workpiece is spirally traced when the grinding is too much at the outlet. If the feed amount is too large or the electrical components are in poor contact, the thermal replay will trip in the workpiece and make the sensitive pause. At this time, the tool should be quickly retracted.
3.3 Internal grinding burns
When internal grinding, the contact arc between the grinding wheel and the workpiece is longer, and there are more heat, and the coolant is less likely to hit the grinding area, and the heat dissipation condition is poor, and it is difficult to remove the chips and the grinding wheel blocks easily, which causes the grinding high temperature.
Therefore, the coolant should be fully effective, and effectively dress the grinding wheel in time. For example, the MZ208 series internal grinding machine, the compensation mechanism of the grinding wheel should be reliable, otherwise, it will cause burns due to not dressing the grinding wheel surface.
Because the compensation mechanism of this type of machine is realized by the roller friction transmission, it cannot be ensured that the same compensation amount is obtained every time. At present, the compensation mechanism is changed from the roller friction mechanism to the stepping motor mechanism.
3.4. Raceway grinding Burns
For the swinging head grinding, the self-sharpness of the grinding wheel is strong, and the rubber bond grinding wheel is usually used. Because the rubber bonding agent is easy to age and become brittle in the high temperature of the grinding region, the blunt abrasive grains are easy to fall off under the swing, so the grinding wheel should always be sharp.
For the in-feed grinding, the performance of the grinding wheel to maintain the shape is strong. Usually, the hard grinding wheel is used, and the self-sharpness is poor, so usually, effective dress the grinding wheel, and the cooling liquid is sufficient. In terms of the external raceway process, it is the most likely process to cause burns. Cooling is particularly important.
In addition, due to the different surface speed of the grinding wheel, the wear of the grinding wheel is not uniform, and the dressing effect is required to be good. The rotation speed of the dresser is uniform and stable, and the compensation mechanism should be reliable, at the same time, diamond should be sharp, the position of the tip of single point diamond dresser should be lower than the center of the grinding wheel by 1-2mm, otherwise it will cause vibration when dressing and affect the dressing effect.
Conclusion of grinding burns
In the actual grinding process, we should be familiar with the causes of grinding burns and avoid the occurrence of grinding burns to the most extent.