Analysis And Preventive Measures For Ring Grinding Burns (1)

Machining And Safety Of Magnesium Alloy Parts
22/04/2024
Analysis And Preventive Measures For Ring Grinding Burns (2)
24/04/2024

During the grinding process of the bearing ring, due to the cutting, scoring and frictional effects of the abrasive grains on the workpiece, the metal surface is plastically deformed, and the relative displacement between the metal molecules inside the workpiece generates internal friction and heat; when the grinding wheel is cutting, relative to the workpiece The speed is very high, causing severe external friction with the workpiece surface to generate heat. And because the cutting of each abrasive is instantaneous, the heat is generated instantaneously and cannot be dissipated in time, so the instantaneous temperature in the grinding area is relatively high. High, generally reaching 800~1500℃. 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 will occur on the surface of the workpiece (generally tens of microns to over microns), which will cause damage. The structure on the surface of the workpiece has been removed, and severe burns can be seen with the naked eye. A severe burnt yellow or black oxide film appears on the surface. Minor burns can only be observed after being etched with dilute acidic solution. The burned area will appear black. Burns can reduce the service life of the workpiece.

  1. Mechanism of burns
    During the grinding process of the bearing ring, due to the cutting, scoring and frictional effects of the abrasive grains on the workpiece, the metal surface is plastically deformed, and the relative displacement between the metal molecules inside the workpiece generates internal friction and heat; when the grinding wheel is cutting, relative to the workpiece The speed is very high, causing severe external friction with the workpiece surface to generate heat. And because the cutting of each abrasive is instantaneous, the heat is generated instantaneously and cannot be dissipated in time, so the instantaneous temperature in the grinding area is relatively high. High, generally reaching 800~1500℃. 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 will occur on the surface of the workpiece (generally tens of microns to over microns), which will cause damage. The structure on the surface of the workpiece has been removed, and severe burns can be seen with the naked eye. A severe burnt yellow or black oxide film appears on the surface. Minor burns can only be observed after being etched with dilute acidic solution. The burned area will appear black. Burns can reduce the service life of the workpiece.
  2. Methods to prevent burns
    Since burns are caused by a large amount of heat generated in the grinding area but not dissipated in time, to avoid burns it is necessary to reduce the generation of heat and accelerate the dissipation of heat, that is, to reduce the internal and external friction during grinding, and to ensure that the workpiece is Full and effective cooling. For coolant, its component concentration must be appropriate, and the flow rate and pressure must be sufficient to truly play a cleaning role (scouring the friction of the grinding wheel and workpiece, cooling and lubrication).
    To reduce the generation of frictional heat, we must ultimately reduce the internal and external friction during grinding. This requires comprehensive consideration of the following factors: reducing the grinding thickness, reducing the plastic deformation of the metal, and reducing the internal friction, thereby reducing wear and tear. Cutting heat; increasing the rotational speed of the workpiece shortens the time it takes for the grinding surface of the workpiece to pass through the grinding area, which can reduce the accumulation of grinding heat. In terms of grinding wheel characteristics, high abrasive hardness means good cutting performance and can reduce heat generation, but the abrasive hardness cannot be If it is too hard, the structure cannot be too fine, otherwise the dull abrasive will not easily fall off, and the micropores between the abrasives will be easily plugged, which will reduce the cutting performance of the grinding wheel, increase the contact area between the workpiece and the grinding wheel, and strongly squeeze the surface of the workpiece, causing friction to increase the temperature. . Therefore, in order to avoid burns and ensure abrasive efficiency and workpiece accuracy, a grinding wheel with low hardness and a large texture number is used during rough grinding, and a larger grinding thickness is selected; a harder grinding wheel with a smaller texture number is used during final grinding. Ensure effective dressing of the grinding wheel and select a smaller grinding thickness; for materials with high grinding strength, high hardness and poor thermal conductivity, it is easy to use a smaller feed amount and increase the workpiece speed, which can effectively avoid burns.
  3. Types of burns
    In actual production, there are the following specific factors that can cause burns:
    (1) Process system vibration, machine tool vibration and hydraulic system pressure instability. The amount of grinding will increase at the moment of vibration, causing burns. At this time, the burns are distributed along the surface of the workpiece in the form of vibration patterns, which is also called vibration pattern burns.
    (2) The grinding wheel is poorly dressed and the abrasive is not sharp, causing burns. At this time, the burns are distributed along the grinding marks of the grinding wheel, also called scratch burns.
    (3) If the centerless clamp has insufficient suction or loose support, the workpiece will instantly slide, causing burns.
    (4) Uneven grinding amount and inconsistent grinding thickness may cause local burns.
    (5) The grinding wheel is too hard, the feed rate is too large, and the workpiece speed is too high (too high a workpiece speed will reduce the cutting ability of the grinding wheel), which will cause uniform burns on the surface of the workpiece.