The fixturing pyramid
Keywords: Fixtures, Fixture design, Automated fixturing
Fixture design is a practical problem and is crucial to product manufacturing. During the course of manufacturing processes, such as machining,assembly, or inspection, it is necessary to immobilize, support, and locate the workpiece. This is also referred to as workholding. Fixturing technology, as its stands today, has many limitations and remains a big stumbling block in flexible manufacturing.
A fixture is comprised of a number of basic workholding elements such as locators, supports, and clamps. A primary task of fixture design is the fixture layout design to determine the number, type, and location of the workholding elements. An appropriate fixture assembly is then designed and constructed from the layout. This second task is referred to as fixture setup, and its main concern is to avoid collision and interference between the machine tool and the fixture.
The problem of fixture design is characterized by a blend of technical and practical issues that must be considered together. A design must satisfy a set of fundamental requirements of kinematic localization and total constraint of the workpiece. In addition, it should yield desirable characteristics in the processes of fixture operation such as fixture setup and part loading and unloading. Therefore, a system for automated fixture design must be capable of incorporating the multi-functional requirements and constraints in finding an overall optimal fixturing solution.
In the standard terminology basic fixture components are classified as locators, supports, and clamps. Locator pins or buttons are essential for providing unique and accurate location (both position and rotation) of the workpiece with respect to a fixture reference frame. They also provide support for stable resting when the workpiece is initially loaded in the fixture and for force- closure after clamping. This part-locating function is known as part localization. A support refers to a movable anvil that is used for providing additional rigidity to the workpiece. It is usually actuated by spring force(pop-up support), screw thread (jack support), or by hydraulics. In all cases,it is engaged only after localization and is locked into place once it makes contact with the workpiece, transforming it into a passive element. In this way,supports will not create a redundancy in part localization. A clamp is represented as a force applied on the workpiece to provide a complete restraint of the workpiece against any external forces on the workpiece during its manufacturing process. Clamps are typically engaged manually or pneumatically.
From a layout point of view, fixtures have five basic functional requirements: (1) stable part resting, (2) accurate localization, (3) support reinforcement, (4) stable clamping, and (5) force-closure (or total restraint). It is important to recognize that these functions have a strong condition of sequential realization. When a workpiece is placed into a fixture, it must first assume a stable rest against gravity. Then, the locators should provide accurate localization. Next, support anvils (if any) are moved in place, and finally clamps are activated for the part immobilization (force-closure). The part location must be maintained in the process of the instantiating clamp without workpiece lift- off. Furthermore, in the presence of friction, the contact forces at the locators and supports will depend on the sequence of application of the clamping forces and applied loads on the workpiece. Even when simultaneous clamping is possible, it is not desirable.
The ultimate measure of a good fixture design is the fixture performance defined as workpiece geometric error during the manufacturing stage. The geometric error is primarily determined by the fixture localization accuracy in terms of the spatial relationship between the workpiece datum reference frame with respect to the axial reference frame of the machine tool. The workpiece static and dynamic deformation during manufacturing may also have a significant contribution to the error. In addition to the workpiece and fixture elements,the fixture assembly will have a major influence on static and dynamic deflections. Likewise, the fixture assembly will ultimately determine the set up time, fixture cost, easiness of loading and unloading, and the probability of tool-path interference. These are primary constraints to be satisfied during the phase of fixture set up design.
Thus, the entire set of fixturing requirements can be illustrated as a fixturing pyramid shown in the figure below. The problem of fixture design is complex with all these multiple functional requirements, performance objectives,precedence conditions and practical constraints. In a typical production plant today, fixturing is a black art and is usually carried out by relying on design experience or by using trial-and- error methods. The lag in fixture design automation can be attributed largely to the complexity involved. In order to bring fixture design into the arena of flexible manufacturing, an automated and more systematic approach is required with the capability of providing the fixture designer with analysis and design tools based on sound engineering principles and computational techniques.
Figure 1 The fixturing pyramid
Michael Yu Wangis based at the Department of Automation and Computer-Aided Engineering, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong. Tel: 852-2609-8487; Fax: 852-2603-6002; E-mail: yuwang@acae.cuhk.edu.hk
