The workpiece is positioned on the machine tool or in the fixture. If several positioning support points repeatedly limit the same or several degrees of freedom, it is called repeated positioning. Whether the positioning of CNC machine tool workpieces allows repeated positioning should be analyzed according to the different conditions of the workpiece. Generally speaking, for CNC machine tools, when the blank surface with low shape accuracy and positional accuracy is used as the positioning base, repeated positioning is not allowed; In order to improve the stability and rigidity of workpiece positioning, repeated positioning is allowed under certain conditions.
Use one clamp and one top to clamp the workpiece. When the clamping part of the chuck is long, it is equivalent to four supporting points, which limits the four degrees of freedom of 7, z, ‘}, and the rear top is equivalent to two supporting points , the two degrees of freedom of Xi and ‘≯ are restricted, and ‘≯ and Jin are repeatedly restricted. Therefore, when the chuck is clamped, the rear center is often not supported at the center. If it is forced, the workpiece will be easily deformed. Therefore, when using one clamp and one top to clamp the workpiece, the method to prevent repeated positioning is to shorten the clamping part of the chuck, and only limit the two degrees of freedom of y and .
The reason for the repeated positioning is that the positioning elements on the central tool simultaneously and repeatedly limit one or several degrees of freedom of the workpiece. Repeated positioning of CNC machine tools can make the positioning of workpieces unstable, destroy the consistency of the position of a batch of workpieces, cause workpieces or positioning components to deform under the action of clamping force, and even make some workpieces unable to be clamped. In order to reduce or eliminate the adverse consequences caused by repeated positioning, CNC machine tools can take the following measures:
- Change the structure of the positioning element;
- Cancel the positioning element that repeatedly restricts the degree of freedom;
- Improve the positional accuracy between the workpiece positioning datum surfaces and between the working surfaces of the positioning element.
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