I. Machining precision concept
Machining accuracy is the degree to which the three geometric parameters of the actual size, shape, and position of the surface of the processed part conform to the ideal geometric parameters required by the drawing.
II. What are the factors that affect the accuracy of machining?
1. The original error of the process system: the geometric error of the process system, such as the machining principal error, the geometric accuracy of the machine tool, the manufacturing error of the tool and the fixture, the installation error of the workpiece, etc. Errors caused by force deformation, execution deformation, and internal stress of the process system will affect the accuracy of parts processing. Machining principal errors such as production approximate processing, forming processing, etc. As long as the processing principal errors are limited to allowable tolerances, productivity can often be greatly improved.
2. Therefore, many precision machine tools are placed in the temperature workshop, (slow wire machine CNC machine tools, etc.) the deformation caused by the internal stress of the workpiece. The residual stress in the cutting process causes the workpiece to deform. Therefore, to reduce the machining accuracy caused by the original error, we usually think of the machine tool for self-turning, self-milling, self-grinding, and self-cutting to try to recover the machining error caused by the original error.
III. What are the methods to ensure the accuracy of the mutual position of the surfaces?
For example: the coaxiality of the inner and outer surfaces of the sleeve parts and the perpendicularity of the end face and the axis of the hole are generally high, and the following methods can generally be used to meet:
1. Complete all the processing of the inner and outer surfaces and end faces in one installation, which can eliminate the installation error of the workpiece and obtain a high mutual position accuracy. However, due to the concentrated process, it is inconvenient to install the larger sleeve, so it is mostly used for the turning of the smaller sleeve.
2. The main surface is processed in several installations (holes are processed first), the holes are processed to the size of the part drawing first, and then the outer circle is processed based on the hole as a fine reference. Because the used fixture (usually a mandrel) has a simple structure and small manufacturing and installation errors, it can ensure a high mutual position accuracy, and it is widely used in the processing of sleeve parts.
IV. Questions about material selection for machining
The determination of the blank not only affects the economics of blank manufacturing, but also affects the economics of machining. Therefore, when determining the blank, it is necessary to consider not only the thermal processing factors, but also the economic factors, but also the cold processing requirements, so as to reduce the manufacturing cost of the parts from the determination of the fault.
The rough parts with complex shapes should be manufactured by casting methods. At present, most castings are made of sand casting, which is divided into wooden mold manual molding and metal mold machine molding. Wooden mold manual molding castings have low precision, large machining surface allowance, and low productivity. They are suitable for single-piece small batch production or large-scale parts casting. The metal mold machine has high molding productivity and high casting precision, but the equipment cost is high, and the weight of the casting is also limited. It is suitable for mass production of small and medium castings. Secondly, a small number of small castings with higher quality requirements can be used in special casting (such as pressure casting, centrifugal manufacturing, investment casting, etc.).
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