CNC cylindrical grinder grinding is a systematic project. If it is just a grinding wheel and a grinding machine, the workpiece can be grinded with electricity, it is really not difficult. If you want to make the grinding problem clear, it is necessary to participate in the entire grinding system and must be decomposed to understand.
1. Grinding force.
①. Source and decomposition of grinding force.
During grinding, forces of equal size and opposite directions act on the grinding wheel and the workpiece. This force generated during the grinding process is called grinding force (cutting force).
The grinding force is mainly composed of two parts: when the abrasive grains are cut off, the metal to be cut will have a large plastic deformation to form the cutting force; the grinding force between the abrasive grains and the surface of the workpiece during cutting.
②. Influence of grinding force on processing.
When grinding, the abrasive particles are cut with a negative rake angle, and the radius R of the edge is often larger than the amount of back-cutting, so the radial squeezing force of the abrasive particles on the workpiece is large, generally Fp = (2 ～ 3) FC . Due to the large radial force, the process system composed of machine tool ~ workpiece ~ grinding wheel has a large elastic deformation, which affects the grinding accuracy.
If the workpiece is deformed due to the radial force and tangential force, the relative movement of its axis is e, which will cause the diameter error of the workpiece.
The deformation of the process system caused by the radial force often makes the actual back eaten amount different from the tree value shown on the grinding wheel feed dial. Therefore, a reasonable grinding cycle is to stop after the feed to eliminate the deformation caused by the radial force. This kind of non-feed grinding is called smooth grinding or non-spark grinding. When grinding an elongated shaft, the workpiece is ground into a drum shape by the action of radial forces.
The characteristics of the grinding wheel, the grinding width of the grinding wheel, the workpiece material, and the amount of grinding (ap, f) have a great influence on the radial force.
2. Grinding heat.
1. Grinding heat and its conduction.
During grinding, the sharp friction of the grinding wheel against the surface of the workpiece makes the instantaneous temperature of the local grinding area reach above 1000 ℃.
Grinding sparks are the phenomenon that abrasive debris oxidizes and burns in the air. The friction between the abrasive particles and the workpiece and the shaping deformation energy of the metal layer are all converted into heat energy, so the grinding heat and its conduction can be expressed by the following formula:
Q = Q friction + Q deformation = Q workpiece + Q abrasive chip + Q grinding wheel + Q medium
2. The effect of grinding heat on processing.
①. Burn the surface of the workpiece: the surface of the workpiece may be burned under the action of instantaneous high temperature.
The so-called burn is generally the uneven annealing of the metal material on the surface of the hardened workpiece under the action of grinding heat. As a result, the surface hardness of the workpiece is reduced, which affects the performance and life of the parts.
The surface of the severely burned workpiece can be seen by the naked eye, and the surface shows a layer of burnt yellow or burnt black oxide film.
The surface of the workpiece with slight burn was light yellow. Grinding materials with poor thermal conductivity, such as bearing steel and heat-resistant alloy steel, are most likely to cause burns.
②. Residual stress and cracks on the surface of the workpiece: When the temperature of the grinding zone reaches the change of the metallographic structure of the metal material (referred to as the phase change), the metallographic structure of the metal surface layer changes and generates stress. When the local stress exceeds the strength limit of the workpiece material, a crack will occur on the surface of the workpiece.
③. Affect the machining accuracy of the workpiece: grinding heat will cause the workpiece to thermally expand and deform, affecting the shape accuracy and dimensional accuracy of the workpiece.
④. Measures to reduce grinding heat.
A. According to the material of the workpiece, select the grinding wheel elements reasonably to optimize the grinding performance.
The hardness of the grinding wheel has a greater influence on the grinding heat. When dry grinding or the arc length between the workpiece and the grinding wheel is long, in order to avoid burns, a softer grinding wheel can be selected to give play to the self-sharpening of the grinding wheel.
In the case of a particularly large amount of grinding, the large-diameter grinding wheel can be used, which is not easy to block, strong cutting force and fast heat dissipation.
B. Take good cooling measures, such as choosing the right cutting fluid or high-pressure cooling can improve the cooling conditions. At present, for mass-produced grinding machines, a cutting fluid temperature adjustment device is used to cool the workpiece at room temperature.
C. Reasonably choose the amount of grinding. When the circumferential speed of the grinding wheel increases, the thickness of the abrasive particles of a single abrasive grain decreases, but the number of friction between the grinding wheel and the surface of the workpiece increases, the grinding heat also increases accordingly, and the surface of the workpiece is easily burned.
Although the increase in the speed of the workpiece increases the grinding heat, but because it reduces the contact time of the workpiece in the grinding area, it is beneficial to heat dissipation. Therefore, the workpiece speed should be appropriately higher during rough grinding.
The amount of back-eating knives has the greatest impact on grinding heat. If a finely-trimmed grinding wheel is used to grind with a large amount of knife back, it is easy to cause burns.
Can understand the grinding force and grinding heat, then it is really one step closer to the "grinding master"! The technical support provided by Magnesian Cylindrical Grinder.
