What is a fixture? How is the technology developing?

Why do we need fixtures?

Whether in traditional manufacturing or modern flexible manufacturing systems, fixtures play a vital role in the manufacturing and processing process. Its main function is to fix and position the workpiece so that it remains stable and precise during processing. Specifically, there are several important reasons for using fixtures:

1. Ensure processing accuracy and quality

Precise positioning: The fixture can accurately position the workpiece to ensure the accuracy of each processing position during processing. This helps to keep the size and shape of the workpiece consistent and ensure the quality of the final product.

Stable clamping prevents workpiece movement and vibration, reducing errors and surface defects.

2. Improve production efficiency

Using fixtures reduces clamping and adjustment time, improving production efficiency. For example, you can quickly adjust a combination fixture for different workpieces.

Increase speed: A stable fixture allows higher processing speeds, boosting productivity.

3. Improve worker safety

Avoid holding the workpiece: Holding the workpiece is unsafe during high-speed cutting or heavy processing. The fixture can securely fix the workpiece, reducing the risk of workers directly contacting tools and workpieces.

Using fixtures reduces manual adjustments and clamping, lowering worker fatigue.

4. Improve workpiece consistency

Repeat accuracy: The fixture makes the processing position and method of each workpiece consistent, thereby ensuring the consistency of processing. This is particularly important for mass production, which can ensure that the specifications and performance of each batch of products are the same.

Standardized production: The fixture ensures standardized clamping and processing, simplifying production and reducing errors and defects.

5. Save manufacturing costs

Reduce scrap: Fixtures improve accuracy and consistency, minimizing scrap from inaccurate positioning or unstable clamping, and cutting production costs.

Reduce processing time: Fixtures reduce clamping and adjustment time, boosting speed and shortening the production cycle, saving costs.

6. Adapt to multi-variety and small-batch production

We can quickly adjust flexible fixtures to meet the processing needs of different workpieces, ideal for multi-variety, small-batch production.

We can quickly replace and adjust fixtures, reducing changeover time and improving production flexibility.

7. Enhance the controllability of the production process

Process control: Fixtures can ensure consistency and repeatability during the processing, making the process more controllable, and easy to monitor and adjust.

Using fixtures to secure the workpiece improves quality traceability at each production stage, aiding in quality control.

Fixture design is crucial in manufacturing, as it directly impacts processing quality, efficiency, and cost.

What is a flexible fixture?

A flexible fixture is a workpiece fixing device designed to adapt to multi-variety and small batch production. You can flexibly adjust and reconfigure it to adapt to the shapes and sizes of different workpieces. The design concept of flexible fixtures is to improve production efficiency, reduce cost, and reduce equipment adjustment time to adapt to the needs of modern manufacturing for flexibility and rapid response.

Flexible fixtures came into being with the development of flexible manufacturing systems and computer-integrated manufacturing systems (CIMS). With the introduction of CIMS, multi-variety small batch production has received increasing attention, and flexible fixtures that adapt to the needs of such products and production changes are even more indispensable.

Basic steps of fixture design

Fixture design generally includes the following steps:

Installation planning: confirm the positioning, number of clamping times, positioning method of the workpiece in each clamping,g, and processing surface during processing.

Planning: confirm the positioning surface, positioning poi, not, and clamping surface and clamping point on the workpiece during processing.

Configuration design: select fixture components and assemble them into a fixture that can clamp the workpiece.

Fixture configuration design has become an important issue in the field of CAFD. With the application of more and more CNC machine tools and machining centers, many processes can be completed in one clamping, which requires es reliable fixture configuration design.

Research and development of flexible fixtures

1. Overview of the development of flexible fixtures

The process equipment of the manufacturing process includes the selection and design of tools, fixtures, testing equipment, and molds. To reduce the cost of clamping and production preparation time, flexible clamping is essential.

Generally speaking, a flexible fixture adapts to changes and remains usable even when the shape and size of the workpiece change to a certain extent.

The workpiece change can be either small, with similar shapes and sizes, or large, with significant shape and size differences.

The definition of flexible fixtures remains vague, with no clear boundaries or definitions. Generally speaking, they are fixtures that machine tools and machining centers use to clamp a variety of different workpieces.

Flexible fixtures include combined fixtures, programmable fixtures, general flexible fixtures, phase change material flexible fixtures,s and other fixtures.

Using different phase change materials allows the creation of various types of fixtures.

Mechanical fixtures clamp only small areas or single points, while phase change material fixtures effectively clamp complex, deformable workpieces.

1.1 Innovative flexible fixtures

Including three types

1) Authentic phase-change fixtures and pseudo-phase-change flexible fixtures.

This type of fixture leverages the material’s physical properties to transition between liquid and solid phases. When the material is in its liquid phase, the user places the workpiece in a container filled with the material. External stimulation then triggers the material to quickly solidify, clamping the workpiece in place. After processing, the user reverts the material from its solid phase back to its liquid phase to release and retrieve the workpiece.

Pseudo-phase change material simulates the bidirectional properties of true phase change material using a particle fluidized bed. This approach employs a physical method to force the material to transition between a “solid state” and a “flow state.” This process, called pseudo-phase change, mimics the appearance of a phase change without an actual phase change occurring.

The basic principle involves filling the fluidized bed with fine metal particles and introducing air through inlets at the bottom of the bed. Fine metal particles bury the standard components placed inside. The process begins with clamping the workpiece and then introducing gas from the air inlets to loosen the particles. Finally, workers remove the workpiece to complete the processing.

2) Adaptable fixtures:

This refers to fixtures whose clamping elements can automatically adapt to the shape of the workpiece, that is, they can change their shape when clamping to adapt to the changes of the workpiece.

3) Modular program-controlled fixtures:

A servo system drives the positioning, clamping, and supporting elements. CNC programs install these elements on a bidirectional movable guide rail and design different fixture configurations.

1.2 Innovation of traditional fixtures

Adjustable fixtures and combined fixtures divide traditional fixtures into two categories.

Adjustable fixtures include general adjustable fixtures and special adjustable fixtures. The latter mainly uses grouping technology as a technical tool.

Component standardization has led to the development of combined fixtures as a new type of process equipment.

2. Combination fixture

This fixture is a new type of process equipment developed based on parts standardization. The combination fixture system includes a variety of components and can be used for various purposes, such as positioning and clamping vertically or horizontally.

You can divide combination fixtures into different types based on the occasions they are used：

Lathe fixtures, drilling machine fixtures, boring machine fixtures, and bed fixtures.

It is divided into eight categories: basic parts, support parts, positioning parts, guide parts, clamping parts, fasteners, auxiliary parts, and assembly parts.

Combination fixtures shorten production preparation time, ensure quality, and improve economic benefits, making them ideal for multi-variety, small-batch production.

With the rapid development of flexible equipment and processing systems, highly flexible combination fixtures with standardized components have become essential for manufacturing systems.

There are generally two types of combination fixture systems:

T-slot-based and pin-based (also called hole-based) systems.

The manufacturing industry has developed and used the combination fixture based on T-slot for more than 50 years.

It realizes the precise positioning of the workpiece through the mutually perpendicular and parallel T-slots on the base.

In this system, when fastening the fixture components on the same row of T-slots, we should arrange the assembly order of the fixture components reasonably.

There are many combination fixture systems based on T-slots in the world, and the more famous ones are: Wharton Unitool (England), YCT (Russia), CATIC (China), and Halder (Germany) systems.

The combination fixture based on pins mainly realizes the precise positioning and fastening of the fixture components through the positioning holes on the base.

The main systems are:

BlucoTechnik (Germany, the United States), Kipp (Germany), Qu-Co (the United States), Stevens (the United States), CarLane (the United States), SAFE (the United States), TJMGS (China), CPI (Russia).

Modular fixtures are flexible clamping methods with great potential in FMS and CIMS. A key technology for their application is computer-aided fixture technology, which includes CAFD, verifies fixture designs, and manages fixtures.

To adapt to agile manufacturing, a new fixture concept and clamping system has emerged, namely agile fixtures, which are the extension and development of traditional modular fixtures and flexible fixtures.

The fixture industry increasingly values it due to its good reconfigurability, scalability, openness, strong adaptability, fast response to new products, and rapid reconstruction speed.

Improving manufacturing systems’ ability to quickly respond to product changes, shorten design and manufacturing cycles, increase flexibility, reduce costs, and enhance product quality is crucial.

Computer-aided fixture design (CAFD)

1. Development of CAFD

In the past decade, manufacturing research has focused on improving CAD/CAM and CAPP.

Only in the past 20 years has CAFD developed into an important component of CAD/CAM integrated technology and an important aspect of CAPP. It is the link between design and manufacturing in the CIMS environment.

With the establishment of CAD/CAM systems, designers apply CAFD to fixture design, aiming to reduce manufacturing costs and cycles, with shortening the preparation cycle as a key goal.

Fuh et al. introduced the first generation of an interactive CAFD (I-CAFD) fixture design system, which integrates with CAD software, utilizes CAD’s graphic processing capabilities, and includes a fixture component library and assembly sequence menu.

The purpose of developing I-CAFD is:

1) More designers can interactively design fixtures to complete more complex fixture design tasks;

2) Shorten the development cycle by providing geometric operation tools;

The second generation of CAFD developed in the mid-1980s produced two types of CAFD systems based on variant and generative methods:

Group technology (GT) and knowledge (KB).

In the variant fixture design method, designers adapt existing designs for similar workpieces within a component family by modifying processing features, operations, and installation sequences.

Through the coding system of group technology, we adjust the original similar fixture elements in the fixture library to adapt to the processing requirements of the new workpiece.

When it is impossible to restore a similar fixture, you can use the generative fixture design method to create a new fixture design scheme, and then add the newly generated fixture code to the fixture library for reference in the next design.

The CAFD developed after the 1990s is the third generation. The characteristic of CAFD in this era is that it has turned to the software design of fixtures with the purpose of product fixture structure and actual production application as the orientation.

At present, the development of CAFD has greatly reduced the manufacturing development cycle, optimized the manufacturing process, verified the manufacturing process flow, and is playing an increasingly important role in FMS and CIMS.

Computer-aided fixture design is mostly focused on fixture planning research, including the algorithm proposed by Demeter for selecting positioning and clamping positions:

Menasa and DeVries studied fixture planning based on kinematic analysis;

The rule-based system developed by European scholars Pham, de Sam Lazaro, etc., is used to design combined fixtures for prismatic workpieces:

Hargrove SK and Kusiak A made a comprehensive review of the development of contemporary CAFD in the literature and proposed future development trends and needs.

Knowledge-based CAFD organizes the knowledge of experts into rules and obtains various decisions based on the reasoning mechanism of the expert system.

Since rule application is not very realistic, ShuHuang Sun and JaHau Lewis Chen proposed an intelligent fixture design system based on case-based reasoning (CBR) to address the problems that rule-based expert systems cannot solve:

1) The knowledge in the design is too complex to be simplified into rules.

Rules are accumulated from similar situations in the past, while design is carried out for specific cases. It is difficult to complete the design of specific cases through rules;

2) Experts cannot express their knowledge clearly;

3) For knowledge-based expert systems, knowledge acquisition is still an obstacle.

The proposed system stores experience as cases. When it faces new problems, it finds the most similar case from previous cases and modifies the case to meet the new situation.

After the problem is solved, this new solution is stored as a case in the case library and can be applied again when similar situations arise.

2. Main research areas in the field of CAFD

1) Classification methods based on group technology and case-based reasoning in fixture design;

2) Determination of positioning points and clamping points through kinematic analysis;

3) Selection of positioning surfaces and clamping surfaces using knowledge-based expert systems;

4) Fixture planning based on geometric analysis;

5) Precision relationship analysis for positioning reference selection;

6) Configuration design of combined fixtures.

Development of Automated Fixture Design (AFD)

In recent years, the fixture industry has widely focused on the design of modular fixture systems, and some literature has reviewed the latest developments in this field.

Tao, Kumar, and Nee et al. verified the problem of force locking in fixture configuration by geometric calculation method and proposed a geometric reasoning method for determining the optimal clamping point and clamping sequence.

This method is very simple and effective in determining the optimal clamping point from the candidate clamping point layout after considering the force locking.

Utpal Roy and Jianmin Liao introduced a geometric reasoning mechanism for automatic fixture design, combining qualitative and quantitative reasoning. They analyzed two types of deformation: contact deformation from clamping and bending from cutting forces. Based on this, they optimized the support and clamping positions, ensuring firm and accurate workpiece fixation during processing, and implemented the system in a prototype.

The AFD system provides an intelligent automatic fixture design environment, which consists of 4 main modules:

1) fully informationized product model;

2) knowledge base;

3) reasoning mechanism;

4) final fixture configuration.

According to the degree of automation, we divide fixture design systems into interactive, semi-automatic, and automated types.

An interactive fixture design system provides a user interface to help users select suitable components based on the designer’s knowledge. However, it is time-consuming as users must choose clamping surfaces, points, and components based on the workpiece’s geometry and processing requirements, and it has not fully developed its computer functions.

The improvement of the semi-automatic fixture design system is based on the interactive system, which reduces the designer’s requirement for professional knowledge. The automated fixture design system further enhances the efficiency and quality of the fixture design process by automatically determining the clamping points and selecting accurate clamping points from a list of candidate points.

Kumar, Fuh, Kow, etc. developed an automatic modular fixture design system based on the application of advanced CAD and AI technology in modular fixtures, using CAD methods and 3-DCADICAM software technology.

The system is integrated with an interactive and semi-automatic fixture design system. In addition to automatically determining the clamping point, it can also use a machining interference detection submodule to prevent interference between the cutting tool and the fixture during machining.

Hu and Rong proposed a fast interference detection algorithm, which simplifies the fixture component model into a two-dimensional wheel model with high information. It is different from the detection process using filled entities and is more effective in fixture verification.

Conclusion

Looking back at the development of the fixture industry and the latest research results, despite more than a decade of research, there is still no mature commercial fixture software in the world.

Therefore, the development of practical fixture CAD software should be the top priority in the current research of manufacturing software. A comprehensive and comprehensive automatic fixture design system that integrates CAD/CAMI/CAE technology with CNC programs and can detect interference between cutting tools and combined fixtures is the future development trend.

Due to the complexity of fixture design and the ever-changing actual situation, it is difficult for AFD research to make a breakthrough in a short period. There is still a lot of work to be done to achieve application in actual production.