The premise of reasonable design: understand the characteristics of the material and select the appropriate fixture material.
Positioning method one
A positioning method based on a plane. Positioning components include support plates, adjustable supports and floating self-positioning supports, etc. The methods for positioning on the product surface are as follows.
Positioning method two
A positioning method and positioning components based on the outer cylindrical surface of the workpiece. When an outer cylindrical surface of the workpiece is used as a positioning reference, the commonly used positioning method is to install the outer cylinder in a round hole, a semi-circular hole, a V-shaped block or a centering clamping mechanism.
2.1 Install the outer cylinder in the round hole
2.2 Install the outer cylinder into a V-shaped block
2.3 Install the outer cylinder in the centering clamping mechanism
Positioning method three
The positioning method and positioning components of the workpiece take the circular hole as the positioning reference. When the positioning reference is a circular hole, the positioning pin and the positioning mandrel are commonly used for positioning. In addition, it can be positioned by means of a centering clamping mechanism.
3.1 Locating pin positioning
3.2 Positioning mandrel positioning
3.3 Positioning of the centering clamping mechanism
The main reasons for the error of the machining size in CNC machining are: due to the size and geometric shape error of the positioning datum itself and the gap between the positioning datum and the positioning element, the above two causes of the positioning datum of the same batch of workpieces along the direction of the processing size The maximum displacement is called the positioning reference displacement error. Therefore, the following positioning elements should be paid attention to when designing fixtures for CNC machining:
1) The six degrees of freedom of the workpiece are all restricted by the positioning elements in the fixture, and occupy a completely determined unique position in the fixture, which is called complete positioning.
2) According to the different processing requirements of the workpiece processing surface, the number of positioning support points can be less than six. Some degrees of freedom have an impact on processing requirements, and some degrees of freedom have no effect on processing requirements. This positioning situation is called incomplete positioning. Incomplete positioning is allowed.
3) According to the processing requirements, the degree of freedom that should be restricted is not restricted, which is called under-positioning. Undertargeting is not allowed. Because under-positioning cannot guarantee processing requirements.
4) The positioning where one or several degrees of freedom of the workpiece are repeatedly restricted by different positioning elements is called over-positioning. When over-positioning causes deformation of the workpiece or positioning elements and affects machining accuracy, it should be strictly prohibited. However, over-positioning does not affect the processing accuracy, but improves the processing stability.
Positioning Design Example Analysis
The lathe tee clamp in the above picture is composed of chassis, spring, moving block, jaw, positioning pin and so on. From the safety point of view, it is not difficult to see that the fixture has defects: when the fixture is installed on the lathe to start processing, the centerline of the spindle rotation is perpendicular to the positioning pin installed on the fixture. If there is no fool-proof, the spindle rotating positioning pin will be thrown out to cause danger. Therefore, in addition to focusing on the design elements, the positioning of the workpiece also requires an understanding of the process.
