Research on the Composition and Automatic Configuration Design Method of Agile Fixture CAD System
Abstract: This paper introduces the composition of agile fixture system, expounds the hierarchical representation structure of agile fixtures, and proposes a feasible research method for the automatic configuration of computer-aided agile fixtures.
Key words: agile fixture; computer aided fixture design feature modeling; automatic configuration design
Fixtures are an important basic component in manufacturing systems and equipment. With the development of CAD/CAM technology, traditional fixture design methods cannot meet the flexible and fast requirements of modern manufacturing systems. Agile fixture is a new fixture concept and clamping system to meet this requirement. It is the extension and development of traditional combination fixtures and flexible fixtures. Agile fixtures are of great importance in improving the ability of manufacturing systems to respond quickly to product changes, shortening product design and manufacturing cycles, increasing the flexibility of manufacturing systems, reducing costs, and improving product quality.
1 Hierarchical representation of agile fixtures
There are two standard clamping systems that are commonly used today: T-slot-based clamping systems and pin-based clamping systems. Since the pin system is easier to realize the automatic assembly of the fixture, the pin clamping system is selected as the research object in the current agile fixture research. In the pin clamping system, all of the clamping elements are positioned and attached to the substrate in a certain manner, and the substrate is uniformly provided with a standard connection hole. In the pin system, when the workpiece and the component are fixed on the substrate, the entire clip is obtained.
The specific structure of the entire clip can be expressed in three levels: the clamping unit layer, the component layer, and the functional surface layer.
1.1 clamping unit layer
The clamping unit is a collection of several interconnected clamp elements, at least one (and usually only one) of the clamp elements being in direct contact with the workpiece through a functional surface for positioning, support or clamping. We define this functional surface as the active surface of this clamping unit. The clamping unit has two planes and a cylinder. The plane is uniquely identified by the plane midpoint Pc and the plane normal vector Vc; the cylinder surface can be uniquely identified by a point Pa on the central axis of the cylinder and a central axis vector Va. The midpoint of the action surface of the clamping unit is called the action point of the clamping unit, and the distance from the working point to the substrate is defined as the working height H. The height of action is an important parameter in the design of the fixture, which will be discussed later.
1.2 fixture component layer
The configuration process of an agile fixture can be seen as a process of selecting components and assembling components. There are four types of fixture components: substrate, positioning component, clamping component, and support component. The latter three types of components are assembled to form a clamping unit, and the clamping unit is mounted on a substrate to constitute a clip specific to meet design requirements. A library of components containing a large number of standard fixture components and a library of component assembly relationships are the premise and basis for the configuration design. The fixture component library and the component assembly relationship library record three aspects of information: component geometry information, component assembly feature information, and assembly relationship between components. The component geometry information is obtained by directly reading the component graphic by the CAD platform; the component assembly information is obtained by defining and reading the component functional surface parameters of the component; the assembly relationship between the two components (including the assembly relationship between two identical components) Description, first of all, according to the assembly characteristics of the two components can judge whether the two can be assembled, the standard fixture component assembly relationship model is shown in Figure 1 (V1, V2, ... V8 in the figure represent the clamping components in the agile fixture component library). If the assembly requirements are met, the assembly features of the components and the assembly between the two components are recorded in a library of assembly components and stored. When the assembly relationship between all the components in all the agile fixture component libraries is traversed, the system's agile fixture component assembly relationship library is obtained.
Figure 1 Standard fixture component assembly relationship model diagram
The functional surface is the surface on the fixture element that is in contact with the workpiece and other components for positioning, clamping or assembly.
2 System Introduction
We have developed a computer-aided agile fixture design system. The system module is shown in Figure 2.
Fig. 2 Module structure diagram of computer aided automatic configuration design system for agile fixture
(1) The clamping planning module provides a user-friendly interface for the user to input a clamping plan for determining the positioning clamping surfaces of the workpiece to be machined and the positioning clamping points on the relevant surface.
(2) The fixture component library is a database based on feature modeling. The feature modeling technology is used to design the parameters, and the physical features and assembly features of the fixture components are recorded.
(3) Fixture component assembly relationship library, recording component assembly relationship.
(4) The automatic configuration module automatically generates an agile fixture assembly drawing according to the clamping plan, the workpiece information, and the fixture component library and the component assembly relationship library. The module includes three sub-modules: component selection, clamping unit generation and clamping unit installation.
3 Agile fixture automatic configuration
Based on the workpiece information, the agile fixture automatically generates an agile fixture assembly drawing based on the workpiece information. The configuration process can be divided into the following two steps.
3.1 Select the fixture component to generate the clamping unit
Firstly, the working height H of each clamping unit is calculated from the coordinates of the clamping point and the positioning point in the clamping plan, and the positioning member or clamping member that directly contacts the workpiece is selected according to the positioning or clamping requirements, and then according to the component assembly relationship. The constraint relationship in the library sequentially selects the positioning member (or clamping member) to the clamp member between the substrates. The component V1 → V2 → ... → Vn constitutes a candidate component group and satisfies the formula (1). (h(Vi)i=1, 2,...n is the i-th component height of the clamping unit).
H=h(V1)+h(V2)+...+h(Vn)(1)
Since each clamp element may have a plurality of components assembled therewith, a clamping unit has a plurality of candidate component groups, and the component group having a smaller number of components and a lighter total component weight constitutes a clamping unit.
3.2 Installing the clamping unit to generate an agile fixture assembly drawing
The mounting of the clamping unit starts from the substrate. Before installation, a suitable substrate is selected according to the workpiece size, and the substrate coordinates are taken as global coordinates, and the substrate center is taken as the coordinate origin. The substrate is provided with a threaded hole and a light hole. Taking a square substrate as an example (as shown in FIG. 3), the hole coordinate (x, y) on the substrate can be expressed by the following formula:
x=T×u(2)
y=T×v(3)
T is the spacing between two adjacent holes.
u=-N,...,-1,0,1,...N;
v=-M,...,-1,0,1,...M;
When u, v are both even or identical, the hole is a threaded hole.
When u, v parity is different, the hole is a light hole.
Figure 3 Schematic diagram of the substrate
After the bottom components of the clamping unit are mounted on the substrate, other components are sequentially installed. Since the two jig components may be assembled in various forms, the position of the jig components is not fixed, and thus various forms of the clamping unit are obtained. Although there may be multiple assembly forms of the two clamp elements, the transformation matrix Ai of the coordinate system of the two clamp elements is fixed for each assembly form. Assuming that the coordinates of the action point of the clamping unit are (x, y, z), the local coordinates (x1, y1, z1) of the top clamping member (or positioning member) of the clamping unit are known to have the following formula. :
(x, y, z, 1) = (x1, y1, z1, 1) A1A2...AN(4)
A1 is a coordinate transformation matrix between the bottom element and the substrate; Ai is a coordinate transformation matrix between the i-th element and the i-1th element;
Equation 4 can calculate the coordinate of the action point (x, y, z) in different forms of the clamping unit, and calculate the distance d between the point and the coordinate of the clamping point (x*, y*, z*):
D2=(x*-x)2+(y*-y)2+(z*-z)2
When d2→min, the assembly scheme is the solution.
The clamping unit is mounted to the substrate as described above, and an agile fixture assembly drawing is obtained.