Detailed explanation of the technical scheme for grinding force and grinding heat by the Engineering Technology Center of the External Grinding Machine
Time：2019-10-12 Visits：1274
CNC cylindrical grinding is a systematic engineering process. If it's just a grinding wheel and a grinding machine, and you can grind the workpiece with electricity, it's really not difficult. If you want to solve the grinding problem thoroughly, you need to participate in the entire grinding system and decompose it in order to solve it.
1、 Grinding force.
① The source and decomposition of grinding force.
During grinding, there are forces of equal magnitude and opposite directions acting on the grinding wheel and workpiece, and this force generated during the grinding process is called grinding force (cutting force).
The grinding force mainly consists of two parts: the abrasive particles need to cause significant plastic deformation of the metal being cut during cutting, resulting in cutting force; The grinding force generated between the abrasive particles and the surface of the workpiece during cutting.
② The influence of grinding force on machining.
When grinding, the abrasive particles cut at a negative rake angle, and the radius R of the edge corner is often larger than the back feed, so the radial extrusion force of the abrasive particles on the workpiece is very large, usually Fp=(2-3) FC. Due to the large radial force, the process system composed of the machine tool, workpiece, and grinding wheel undergoes significant elastic deformation, thereby affecting the grinding accuracy.
If the workpiece undergoes deformation due to radial and tangential forces, and its axis moves relative to e, it will cause a diameter error in the workpiece.
The deformation of the process system caused by radial force often results in a difference between the actual back cutting amount and the tree value shown on the feed scale of the grinding wheel. Therefore, a reasonable grinding cycle is to stop for a while after the feed to eliminate deformation caused by radial forces. This type of non feed grinding is called smooth grinding or spark free grinding. When grinding a slender shaft, the workpiece is ground into a drum shape by radial force.
The characteristics of the grinding wheel, the grinding width of the grinding wheel, the material of the workpiece, and the grinding amount (ap, f) have a significant impact on the radial force.
2、 Grinding heat.
1. Grinding heat and its conduction.
During grinding, the intense friction between the grinding wheel and the surface of the workpiece causes the instantaneous temperature in the local area of grinding to reach over 1000 ℃.
Grinding sparks refer to the phenomenon of grinding debris oxidizing and burning in the air. The friction between the abrasive particles and the workpiece, as well as the plastic deformation energy of the metal layer, are all converted into thermal energy. Therefore, the grinding heat and its conduction can be expressed as follows:
Q=Q Friction+Q Deformation=Q workpiece+Q debris+Q grinding wheel+Q medium
2. The impact of grinding heat on machining.
① Causing surface burns on the workpiece: The surface of the workpiece may be burned under instantaneous high temperature.
The so-called burn generally refers to the uneven annealing of the metal material on the surface of the quenched workpiece under the action of grinding heat. As a result, the surface hardness of the workpiece decreases, which affects the performance and lifespan of the parts.
The surface of severely burned workpieces can be seen with the naked eye, presenting a layer of burnt yellow or burnt black oxide film.
The surface of the workpiece with minor burns appears light yellow. Grinding materials with poor thermal conductivity, such as bearing steel, heat-resistant alloy steel, etc., is most likely to cause burns.
② Residual stress and cracks on the surface of the workpiece: When the temperature in the grinding zone reaches a level that causes a change in the metallographic structure of the metal material (referred to as phase transformation), it causes a change in the metallographic structure of the metal surface and generates stress. When the local stress exceeds the strength limit of the workpiece material, cracks appear on the surface of the workpiece.
③ Affects the workpiece
1、 Grinding force.
① The source and decomposition of grinding force.
During grinding, there are forces of equal magnitude and opposite directions acting on the grinding wheel and workpiece, and this force generated during the grinding process is called grinding force (cutting force).
The grinding force mainly consists of two parts: the abrasive particles need to cause significant plastic deformation of the metal being cut during cutting, resulting in cutting force; The grinding force generated between the abrasive particles and the surface of the workpiece during cutting.
② The influence of grinding force on machining.
When grinding, the abrasive particles cut at a negative rake angle, and the radius R of the edge corner is often larger than the back feed, so the radial extrusion force of the abrasive particles on the workpiece is very large, usually Fp=(2-3) FC. Due to the large radial force, the process system composed of the machine tool, workpiece, and grinding wheel undergoes significant elastic deformation, thereby affecting the grinding accuracy.
If the workpiece undergoes deformation due to radial and tangential forces, and its axis moves relative to e, it will cause a diameter error in the workpiece.
The deformation of the process system caused by radial force often results in a difference between the actual back cutting amount and the tree value shown on the feed scale of the grinding wheel. Therefore, a reasonable grinding cycle is to stop for a while after the feed to eliminate deformation caused by radial forces. This type of non feed grinding is called smooth grinding or spark free grinding. When grinding a slender shaft, the workpiece is ground into a drum shape by radial force.
The characteristics of the grinding wheel, the grinding width of the grinding wheel, the material of the workpiece, and the grinding amount (ap, f) have a significant impact on the radial force.
2、 Grinding heat.
1. Grinding heat and its conduction.
During grinding, the intense friction between the grinding wheel and the surface of the workpiece causes the instantaneous temperature in the local area of grinding to reach over 1000 ℃.
Grinding sparks refer to the phenomenon of grinding debris oxidizing and burning in the air. The friction between the abrasive particles and the workpiece, as well as the plastic deformation energy of the metal layer, are all converted into thermal energy. Therefore, the grinding heat and its conduction can be expressed as follows:
Q=Q Friction+Q Deformation=Q workpiece+Q debris+Q grinding wheel+Q medium
2. The impact of grinding heat on machining.
① Causing surface burns on the workpiece: The surface of the workpiece may be burned under instantaneous high temperature.
The so-called burn generally refers to the uneven annealing of the metal material on the surface of the quenched workpiece under the action of grinding heat. As a result, the surface hardness of the workpiece decreases, which affects the performance and lifespan of the parts.
The surface of severely burned workpieces can be seen with the naked eye, presenting a layer of burnt yellow or burnt black oxide film.
The surface of the workpiece with minor burns appears light yellow. Grinding materials with poor thermal conductivity, such as bearing steel, heat-resistant alloy steel, etc., is most likely to cause burns.
② Residual stress and cracks on the surface of the workpiece: When the temperature in the grinding zone reaches a level that causes a change in the metallographic structure of the metal material (referred to as phase transformation), it causes a change in the metallographic structure of the metal surface and generates stress. When the local stress exceeds the strength limit of the workpiece material, cracks appear on the surface of the workpiece.
③ Affects the workpiece