Measurement datum and positioning datum.
(1) Assembly datum: the datum used to determine the position of parts in parts or products during assembly, which is called assembly datum.
(2) Measurement datum: the datum used to check the size and position of machined surface, which is called measuring datum.
(3) Positioning datum: the datum used for workpiece positioning during machining, which is called positioning datum. The surface (or line or point) processed by computer gong as the positioning reference can only select the rough surface in the first process. This positioning surface is called rough datum. The machined surface can be used as the positioning reference in the following processes, and the positioning surface is called the precise reference.
In the process of turning a blank into a finished product, the total thickness of the metal layer cut off on a machining surface is called the total machining allowance of the surface. The thickness of the metal layer cut by each process is called the machining allowance between processes. For the rotating surface such as outer circle and hole, the machining allowance is considered from the diameter, so it is called symmetrical allowance (i.e. bilateral allowance), that is, the actual thickness of the metal layer cut off is half of the machining allowance on the diameter. The machining allowance of the plane is the unilateral allowance, which is equal to the actual thickness of the metal layer removed. The purpose of leaving machining allowance on the workpiece is to remove the machining errors and surface defects left by the previous process, such as the surface cold and hard layer, air hole and sand inclusion layer on the casting surface, oxide scale, decarburization layer and surface crack on the forging surface, internal stress layer and surface roughness after cutting. The precision and surface roughness of the workpiece are improved. The size of machining allowance has great influence on machining quality and production efficiency. If the machining allowance is too large, it not only increases the labor of machining, reduces the productivity, but also increases the consumption of materials, tools and power, and increases the processing cost. If the machining allowance is too small, it can not eliminate all kinds of defects and errors in the previous process, and can not compensate for the clamping error in this process, resulting in waste products. The selection principle is to make the allowance as small as possible on the premise of ensuring the quality. Generally speaking, the more finish machining, the smaller the process allowance.
Applicability:
In the process of turning a blank into a finished product, the total thickness of the metal layer cut off by the computer gongs on a machining surface is called the total machining allowance of the surface. The thickness of the metal layer cut by each process is called the machining allowance between processes. For the rotating surface such as outer circle and hole, the machining allowance is considered from the diameter, so it is called symmetrical allowance (i.e. bilateral allowance), that is, the actual thickness of the metal layer cut off is half of the machining allowance on the diameter. The machining allowance of the plane is the unilateral allowance, which is equal to the actual thickness of the metal layer removed. The purpose of leaving machining allowance on the workpiece is to remove the machining errors and surface defects left by the previous process, such as the surface cold and hard layer, air hole and sand inclusion layer on the casting surface, oxide scale, decarburization layer and surface crack on the forging surface, internal stress layer and surface roughness after cutting. The precision and surface roughness of the workpiece can be improved by computer gonging. The size of machining allowance has great influence on machining quality and production efficiency
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