Note: Please refer to the attached information at the end of the article.
First, the basic principle and composition of the fixture in February** dynamic incentive plan The fixture is an additional device for the machine tool and a process equipment for installing the workpiece. In order for the workpiece to be formed, there should be relative motion between the workpiece and the tool. Therefore, the position of the workpiece relative to the tool is determined, and the relative position of the workpiece needs to remain constant during the cutting process.
Second, the principle of fixture positioning
1. Six-point positioning principle
There are six degrees of freedom in the space Cartesian coordinate system The fixture uses six supporting points that are reasonably distributed to limit the six degrees of freedom of the workpiece respectively, so that the position of the workpiece in the fixture is completely determined, which is called the "six-point positioning principle".
2. Application of the six-point positioning principle - complete positioning of the workpiece.
The six degrees of freedom are all limited by the positioning component of the fixture, so that the workpiece is in a completely determined position in the fixture, which is called complete positioning.
3. Application of the principle of six-point positioning - incomplete positioning of the workpiece.
If some degrees of freedom are allowed not to be limited by the positioning component of the fixture and do not affect the machining requirements, this positioning is called incomplete positioning.
3. Positioning structure
1. Locate according to the round hole.
2. Position according to the outer cylinder.
One more thing to say here, we will also use the commonly used fixture knowledge and pin positioning when designing the structure.
The diameter of the working part of the cylindrical positioning pin specified in the national standard D is usually manufactured according to the processing requirements and considerations for easy clamping, and is manufactured according to G5, G6, F6 or F7. The connection between the dowel pin and the clamp body can adopt interference fit (as shown in Fig. a, b, c), and can also adopt gap fit (as shown in Fig. d); Cylindrical dowels usually limit the workpiece to 2 degrees of freedom of movement.
1) Diamond pin and tapered pin.
2) Anterior and posterior apex.
3) One pin positioning on one side.
4) Positioning of two sides and one pin.
5) One side and two pins positioning. When one side is positioned with two pins, if it is two cylindrical pins, it will produce an overpositioning phenomenon. Solution: One pin to reduce the diameter; Diamond pins.
Fourth, the classification of fixtures
1) Universal fixture.
For example, the three-jaw self-centering chuck and four-jaw single-action chuck commonly used on lathes, and the flat-mouth vise, indexing head, and rotary workbench commonly used on milling machines all belong to this category. This kind of fixture has a certain universality, so it gets its name. General fixtures are generally standardized, and produced by specialized professional factories, often provided to users as standard accessories of machine tools.
2) Special fixtures.
This kind of fixture is specially designed for a certain process of a certain workpiece, and is named because of its specific purpose. The drilling fixture in the example above is a special fixture.
3) Universal adjustable fixtures and group fixtures.
This type of fixture is characterized by the fact that some elements of the fixture can be replaced, and part of the device can be adjusted to adapt to the processing of different parts. The jigs used for group machining of similar parts are called group jigs. Compared with group fixtures, the general adjustable fixture is less clear about the processing object and has a wider range of applications.
4) Accompanying fixtures.
This is a type of fixture that is used in automated lines or flexible manufacturing systems. The workpiece is installed on the accompanying fixture, and in addition to completing the positioning and clamping of the workpiece, the workpiece is also sent to each machine tool by the transport device, and is positioned and clamped on each machine tool.
5) Combination fixture.
This type of fixture is a set of completely standardized components, assembled according to the processing requirements of the part. Just like building blocks, different combinations and connections of different components can form fixtures with different structures and uses. This type of fixture is characterized by flexibility, versatility, short cycle times and reusable components.
Summary of references:
It mainly refers to a courseware of fixture design, but most of the courseware talks about theoretical knowledge.
There is also an abb's "Fixture Design Principles and Methods", which strongly recommends this, which combines engineering reality and tolerance analysis, which is very valuable!