Improvements in the design of the machines-tools and the impact on the manufacturing systems.

Ivan, Ioana Carmen ; Ispas, Constantin ; Paraschiv, Marius Daniel 等

1. INTRODUCTION

New research directions in the mechanical field are the study of the parallel kinematic structures and the implementation study of the high speed machining in the industrial applications. This fact is due to the fact that there are necessary to be done some improvements in the technological systems.

As industrial companies are more and more interested to offer high quality products, competitive on the market, the research regarding design and technology focuses on the finished product and on its utility resented by the customers; the utility is measured by the accomplishment of their clients' needs and demands. Never the less, companies are also interested in the new manufacturing methods in order to lower the production costs and in the same time to increase their profit.

Classical machine-tools are often behind the times in the industrial competition for offering the best product on the market with the lowest production costs and with a high profitability rate. As this classical structure of machine-tools overcome sometime with difficulty the industrial market demands, improvements need to be brought in the machine-tools domain (Neugebauer et al. 2000).

2. IMPROVEMENTS IN THE DESIGN AND TECHNOLOGY

Improvements need to be achieved in every field of the machine-tools, like accuracy, precision, speed, flexibility in operations.

A competitive industrial product means a product with a shorter time for the development phase, a high quality product which fulfils the client's demands with success and last but not at all least, especially for the manufacturing companies, a product with low production costs, as low as possible in a strong correlation with the two points mentioned above.

Trying to achieve some of the objectives mentioned previously, the researchers have obtained important results like the appearance of new structures in the conception of machine-tools and new machining methods.

The parallel kinematic machine-tools structure is one of the new structures used in the recent design of the machine-tools.

[FIGURE 1 OMITTED]

This structure, best known as the Stewart platform, has six degrees of freedom. It consists of a rigid base (A1, ... A6) connected through six identically, extensible jointed arms to a mobile spindle platform, where it is placed the end-effector (B1, ... B6); the structure is pointed in Figure 1.

Several prototypes of machine-tools with the structure based on a Stewart platform, commonly called hexapods, have been produced and tested by machine-tools manufactures from different state, like Germany, US, France, Sweden.

The parallel structures are naturally more resistant then the serial ones, as the load is distributed on all arms and because for certain architectures the arms are loaded only with axial loads (Merlet, 2006).

The high speed machining is a method of a high productivity for the rough machining and the final machining of the small dimension pieces and also for the final machining and super final machining of all type dimensions pieces (e.g. obtaining cavities in aluminium structures for aero-spatial industry) (Ispas, 2006).

High speed machining does not represent only a spindle rotation, but an assembly that involves every component of the machine-tool. The structure of the machine-tool must be as rigid as possible to be able to support, in certain cases, the machining efforts, and especially the strong accelerations. The mobile parts should be as light as possible and in the same time they have to be strongly ribbed to obtain a good balance between rigidity and inertia.

3. STRONG AND WEAK POINTS OF EACH TYPE OF THE IMPROVEMENTS

The newly structures with parallel kinematics have significant advantages relative to conventional structures based on serial kinematics, advantages like:

* large field of activity domains; * higher speed; * higher accuracy, because they are more rigid and because the arm errors are calculated as an average, in comparison with the ones from the classical machine-tools, errors which cumulate; * high structural rigidity; * low friction in joints, offering a longer life time (Wavering, 1998).

The parallel structures, in spite of their advantages regarding the accuracy, the rigidity and the large area of applications, they have also disadvantages that need to be considered, some weak points like:

* singularities and low stiffness outside the working volume,

* large machine size for a given working volume,

* programming and calibration difficulties of the kinematics linkages for achieving the desired tool path accuracy.

The implementation of high speed machining in the production process brings along some important modifications regarding different components of the manufacturing system:

* tools: are the first step that allowed the progress toward the high speed machining; the tool has to prove a very high chemical stability for increasing the number of the machined pieces but it also increases the prices of the tool,

* spindles: are essentials for the high speed machining and they have to function for a long period of time with very high speed and to provide a high power not only for high speed, but also for a lower speed,

* machining centres: the machine-tool has to take and to absorb important accelerations and decelerations compared to the classical machining, without endangering its base structure and preserving a long life time function and a high precision also,

* numerical control: its quickness is imperative. If the machine stops for waiting dates from the computer, unacceptable vibrations appear for the quality of the surface,

* lubrication: can be realised through the centre of the tool at a high pressure, as the classical system is inefficient because of the air ring which is created around the tool, caused by the high speed,

* human training: appears the necessity for training the human operators and readapting their working habits for a high speed machining centre.

* noise and price: noises are higher then the ones in the classical machining and the price of a high speed machining centre is higher, more then twice, but the price for the production time is at least twice less.

There are two important characteristics of the high speed machining which influence the manufacturing process: the first characteristic is the increase of dynamic efforts; the second characteristic is the quality of the machined pieces.

Regarding the application domains, the high speed machining has revolutionised the production in some industrial fields like the automobiles production, the aero spatial field (light alloy, composite materials).

4. IMPACT ON THE MANUFACTURING SYSTEMS

The improvements brought to the manufacturing systems change the approach of the companies towards the market and the approach to the engineers training.

Companies are interested into producing competitive products, which is translated by the diminution of the production development cycle, by a higher product quality and last but not least by a lowering of the manufacturing costs.

Those three important objectives make important changes in the design and manufacturing methodologies and work tools of products and systems.

These three important parameters, time quality and costs, that need to be taken into consideration for a successful company, bring along important modifications and improvements to the manufacturing methodologies and the fabrication systems. Each one of these parameters can and will be translated into technical terms for a clear comprehension, as it follows.

A shorter time for obtaining a product is gained by improving and increasing the speed of the machining, of the production system (Chanal et al. 2006).

A higher quality of a future product, compared to the ones which can be produced with existing equipments of a manufacturer, is provided by improving and increasing the accuracy of the operations from the technological chains.

Lower costs for production, in the same time not diminishing the quality, are obtained by improving and increasing the life cycle of the machining tools.

5. CONCLUSIONS

Following the actual design methodology and the level of research and development in engineering on high speed, high quality and low costs, training of future engineers has to speed up in quality, reactivity, team work and flexibility, in order to satisfy the new needs that appear and to find new better solutions for old and present problems. The new achievements and demands in the technical and technological area bring changes and improvements on the engineers training, which is an important part of the manufacturing systems.

It is needed that human operators are properly trained to use, to maintain the actual technical and technological level and to bring new ideas, to create and to innovate.

The advantages of the parallel kinematic structures must allow in the future the achievement of the objectives established for the high speed machining, including high speeds and accelerations.

Although it can be noticed a fast increase of the parallel structures number, only a very small number of them are functioning into the production systems from the industry, while the majority of these structures is remaning in the university or researching centres laboratories which have realised the study and researches. This discreapancy has not yet been explaind so far and a serie of questions appears, like: which are the reasons of the industrial manufacturers' actions or better inactions? or, what is holding the industrials to use parallel structures more often in the production systems.

This study will be the subject and the study for a future paper in which will be closely studied the connexion, if it exists one between the industrial applications and the universitary researches.

6. REFERENCES

Chanal, H. E., Duc, E,. & Ray, P. (2006). A study of the impact of machine tool structure on machining processes, International Journal of Machine Tools and Manufacture, Vol.46, Issue 2, (Febr. 2006), Pp 98-106, ISSN 0890-6955

Ispas, C. (2006). Machines outils pour l'usinage a grande vitesse (Machine-tools for high speed machining), 4eme Assies Machines et Usinage a Grande Vitesse, 2006, ENSAM, Aix-en-Provence, France.

Merlet, J.P., (2006). Parallel Robots, ISBN-10 1-4020-4132-2, Springer Publisher.

Neugebauer, R., Wieland, F. & Ihlenfeldt, S., (2000). Comparison of parallel structure concepts for five-axis machining, Journal of Manufacturing Prrocesses, Society of Manufacturing Engineers 2000, ProQuest Information and Learning Company.

Wavering, A. J.(1998). Parallel Kinematic Machine Research at NIST: Past, Present, and Future, First European-American Forum on Parallel Kinematic Machines Theoretical Aspects and Industrial Requirements, August 1998, Milan