When machining parts on a CNC machine tool, in order to ensure the machining accuracy and processing quality of the workpiece, the workpiece must be located in the correct position on the machine, which is commonly known as “positioningâ€, and then it is fixed, which is commonly referred to as “clipsâ€. tight". The process of positioning and clamping the workpiece on the machine is called the clamping process of the workpiece.
1. Workpiece positioning principle
(1) Six-point positioning principle
The workpiece has six degrees of freedom in space, that is, the freedom of movement in the three coordinate directions of X, Y, and Z, and the rotational degrees of freedom A, B, and C around the three axes of X, Y, and Z, as shown in the following figure.
To determine the position of the workpiece in space, it is necessary to arrange six supporting points according to certain requirements, which are commonly referred to as positioning elements, in order to limit the degree of freedom in machining the workpiece. This is the “six-point positioning principle†of workpiece positioning. It should be pointed out that the shape of the workpiece is different, the positioning surface is different, and the arrangement of positioning points is also different.
Figure axis of CNC machine tool
(2) Limiting the relationship between degrees of freedom and workpiece processing requirements
According to the different processing requirements of the workpiece surface, some degrees of freedom affect the processing requirements. Some degrees of freedom have no influence on the processing requirements. The degrees of freedom that affect the processing requirements must be limited, and the degrees of freedom that do not affect the processing requirements need not be limited.
(3) Fully positioned and incompletely positioned
The six degrees of freedom of the workpiece are all limited to the positioning of the complete positioning, the workpiece is limited to less than six degrees of freedom, but does not affect the processing requirements of the positioning, incomplete positioning, full positioning and incomplete positioning is the actual processing of the workpiece The most commonly used positioning method.
(4) Basic principles of workpiece installation
The principle of workpiece mounting on CNC machine tools is the same as that of ordinary machine tools. It is also necessary to reasonably select positioning reference and clamping solutions. In order to improve the efficiency of CNC machine tools, the following points should be noted when determining the positioning reference and clamping scheme:
1 Strive to unify the design basis, process baseline, and programming calculation benchmark.
2 Minimize the number of chucking operations. After all positioning and mounting, the entire surface to be machined can be machined.
3 Avoid the use of machine adjustment programs to give full play to the effectiveness of CNC machine tools.
2. Clamping of the workpiece
In the metal cutting process, the workpiece must be clamped in order to ensure the correct position of the workpiece during positioning and prevent the workpiece from being displaced and vibrated under cutting force, centrifugal force, inertia force, or gravity. This kind of device that guarantees processing accuracy and safe production is called a clamping device.
(1) Basic requirements for clamping
1 When the workpiece is clamped, the correct position occupied by the workpiece cannot be changed.
2 The size of the clamping force is appropriate. It is necessary to ensure that the position of the workpiece cannot be changed during the machining process, and that the workpiece does not undergo large clamping deformation; at the same time, the processing vibration phenomenon should be as small as possible.
3 Easy to operate, labor-saving and safe.
4 The degree of automation and complexity of the clamping device should be compatible with the batch size of the workpiece.
(2) Determination of clamping force direction and clamping point
1 The clamping force should be as far as possible towards the main positioning reference. This ensures that the positioning of the workpiece will not be damaged when the workpiece is clamped, and the machining accuracy of the workpiece will be affected.
2 The direction of the clamping force should be conducive to reducing the clamping force. It is required to be able to complete the processing of the part under the minimum clamping force.
3 The point of action of the clamping force should be selected in the direction and orientation of the workpiece with good rigidity. This principle is particularly important for parts with poor rigidity, which can ensure the minimum amount of clamping deformation of the part.
4 The clamping force application point should be as close as possible to the machining surface of the part to ensure that the distance between the action point of the main clamping force and the machining surface is the shortest, which can effectively increase the rigidity of the part clamping and reduce the vibration during the processing.
5 The direction of action of the clamping force shall be within the effective range of the positioning support, without destroying the positioning requirements of the part.
3. Fixture selection
The characteristics of NC machining put forward two basic requirements for fixtures. One is to ensure that the coordinate direction of fixtures is relatively fixed with the coordinate direction of the machine tool; the other is to ensure the accurate dimensional relationship between the parts and the machine coordinate system. According to the status of parts wool and the installation requirements of CNC machine tools, fixtures that can ensure the processing quality and meet the processing needs should be selected. In addition, consider the following points:
(1) When the parts processing volume is not large, it should try to use combination fixtures, adjustable fixtures and other universal fixtures to shorten the production preparation time and save production costs. Special fixtures can be considered when producing in batches, and the structure of the fixture is also required to be simple.
(2) The installation of parts must be convenient and reliable, and avoid the use of manual adjustment of the clamping method to shorten the auxiliary time, try to use hydraulic, pneumatic or multi-position fixtures to increase production efficiency.
(3) The fixtures used on CNC machine tools must be able to be installed accurately and ensure the relative position and size of the workpiece and the machine coordinate system, and strive to unify the design datum, process datum and programming origin in order to reduce the error of non-coincidence of reference and NC programming. The calculation workload.
(4) Minimize the number of fixtures, complete the surface machining of all parts or the processing of most surfaces after a fixture, to reduce the clamping error, and improve the mutual position accuracy between the processing surfaces to fully improve the efficiency of CNC machine tools. the goal of.
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