A knowledge based system for improving design and manufacturing process for Ultrasonic machining in CE environment

Morteza Sadegh Amalnik,

Computer and Automation R&D Center of ACECR-Sharif Branch

Department of Mechanical and Industrial Engineering, University of Qum and  Tabriz, Iran

                  E-mail: sadeghamalnik@yahoo.com

Abstract

Concurrent engineering is a systematic approach to the integrated concurrent design of product and their related processes. This approach considers all elements of the product life cycle from conceptual stages to the final stages of product development including quality, cost, manufacturability, schedule and user requirements. It is an organizational strategy which creates an environment that people who design or manufacture products work to gather under the same goals and the same sense of values to tackle the same problems from the early stages of product development to the final stage of product development. The goals are reduction of product development time, cost and shorter time to market. This paper addresses the concept and development of a knowledge-based-system (KBS) in computer based concurrent engineering environment for hard and brittle material, such as glass, quartz, diamond, carbides, semi conducting materials, ceramic and graphite etc. which can be machined with ultrasonic. An advisory system in computer integrated manufacturing and concurrent engineering environment for Ultrasonic machining (USM) based on object oriented technique is developed.  The system links with A feature based CAD system in order to extract design. The knowledge base is linked with databases. The machining cycle time, cost, penetration rate, efficiency and effectiveness, of each selected design feature are estimated. The system provides useful information such as machining cycle time and cost, penetration rate, efficiency and effectiveness of machining of the selected design feature for product designers at the conceptual stages of design process and also advises manufacturing engineers to select optimum machining parameters.

1- Introduction

The limitation of conventional and some of the unconventional machining have led to the development of ultrasonic machining for hard and brittle materials [1]. The history of USM traced back to Lewis Balamuth, who invented the process about forty three years ago [2].  The benefits of discovery of USM to industry were quickly realized, and in 1950 the production of USM-tools began [3]. A wide range of material specially hard materials (e.g. tungsten and titanium carbides, die and tool steels etc.) and brittle materials (e.g. germanium, silicon, ferrites, ceramics, glass, quartz etc.) could be effectively machined by this method [4, 5]. The attraction of USM is

unlike ECM and EDM the material removal rate is affected by brittleness and hardness of materials. USM is used in wide range of industry including aerospace, electronic, optics, and automobile industries [6].  The rapid progress in this field can be seen  from the number of published papers.  It is reported that up to 1960s about

 

350 papers had been published.  Ultrasonic machining (USM) is a

mechanical unconventional machining process by which material is removed through direct hammering of the abrasive particles on the work piece by the vibration of tool and flow of the abrasive particle. The mechanisms involved in material removing by USM have been described in previous studies [3, 7, 8].  USM includes the flowing three activities, (i) direct hammering of the abrasive grains between tool and work piece, (ii) micro chipping by impact of the flow of the abrasive particles, (iii) Erosion of work piece for some metals such as graphite by cavitations in the slurry stream.  It has been reported  that the fist two are the most essential factors for material removal,

While the third process is applied only for some of the material such as graphite [5, 9, 10].  Influence of different parameters on material removal rate is reported by [10-14]. These parameters can be classified into the following categories: (i) frequency and amplitude of vibration and tool pressure, are the major importance in USM, (ii) type and grit size and slurry concentration and volume of the abrasive slurry, (iii) material type (iv) geometric shape and description of feature  for tool and work piece, (v) machining time, productivity and penetration rate.

  The resonance transducer or vibrator converting the electrical power received from the oscillator into mechanical vibration. this is the main source of mechanical oscillation. The output of transducer is inadequate for USM operation.  It should be amplified so that the output  and the amplitude is sufficiently large for USM operation. To overcome this problem a device so called horn is used. The horn or tool holder is a waveguide or concentrators which is fitted from one side to the end of transducer and from the other side  is stick to the tool head. Its cross sectional area is decreases from the transducer to the out put tool. The tool shape is complementary to the design feature and attached to the end of the horn. The abrasive slurry is fed between tool and work piece.  A special mechanism is used to maintains the static pressure between tool and work piece. For effective USM operation, the machine tool must provide vibration of the tool at maximum amplitude at a given frequency [2].  The effects of process parameters such as static pressure, ultrasonic vibration amplitude and frequency, abrasive size etc. on the process performances such as material removal rate, tool wear, etc. have been also investigated experimentally by varies researchers, the conclusions are summarized in [15]. Static pressure has a great effect on tool wear and material removal rate (MRR).  As static pressure increases tool wear increase and MRR increase to the maximum level and then decreases. Vibration amplitude has also effect on MRR.  As the vibration amplitude increases up to some point , MRR increase, but further increasing of the vibration amplitude above this point results in a reduction in MRR. Other process parameters including parameters such as amplitude and frequency of tool vibration, abrasive slurry characteristics (type, size, concentration), magnitude of the  applied force and material specification of the work piece.  Holes as small as 0.076 mm up to 89 mm diameter with  maximum 64 mm depth can be produce with USM [10].

  Surface finishes for the USM range from 0.2 to 1.5 μm Ra with no chemical, thermal or electrical alterations of the surface.  Accuracies of 0.013 mm are typical, tolerances of 0.005mm can be achieved for specialized applications, with good machining condition, machining depth of 64 mm can be obtained [16] the sonotrode tool is made by using electro-discharge machining (EDM). Grin size yields essential effect on surface roughness, where changing from 600 to 280 mesh increases the surface roughness more than twice (Ra from 1 to 2.5 μm) [14].  In case of ceramic, it has been reported that any increase in the amount of work/energy imparted onto the ceramics in terms of the amplitude of the conic wave, the static weight applied and the size of the abrasive, will result in (i) increase in material removal rate, (ii) a roughening of machined surface [17].   The process can be controlled by varying the gap between tool and work piece. During the machining the actual distance between tool and work piece should be kept constant [18].    USM has many benefits as discussed before, but it has some drawbacks such as material removal rate reduce as the penetration depth increase, the slurry may wear the wall of the machined hole as it passes back toward the surface which limits the accuracy specially for small hole, the action of slurry causing considerable tool wear which in turn makes it very difficult to hold close tolerances.  In order to overcome the USM problems, rotary ultrasonic machining (RUM)  is introduced in 1964 by Percy Legge, a technical officer at United Kingdom Atomic Energy Authority (UKAEA) [15].   Ultrasonic polishing has been developed by the GrafEx Division of Extrude Hone Corporation for variety of polishing applications.  In this type of process, t he extend of polishing determined by the initial surface roughness of the work piece and the finishing requirement after  polishing. Typical surface improvements range  from 5:1 to 10:1; finishes as low as 4 μ inch (0.1μm) Ra can be obtained [19]. Advanced ceramics are increasingly used in industry for their superior properties such as high strength, resistance to chemical degradation, wear resistance and low density, but the only problem is high cost of machining with current technology. for instance the cost of machining of a part with a high  accuracy can be as high as 90 % of the total cost.  Among the various processes, rotary ultrasonic  machining has the potential for high material removal rate  with low machining pressure resulting in less surface damage [20]. Ceramic materials have a wide range of application  from insulations to very complex applications such as artificial teeth, bones, joints, internal combustion engines, thermal barrier coatings, tougher metal cutting tools etc. [21], USM techniques can be used to machine a wide variety of ceramic components, some of the USM applications are shown in [22].

  RUM is a hybrid machining process that combines the material removal mechanisms of diamond grinding and USM.  Experimental results have shown that the machining rate of RUM is nearly 6-10 times higher than from a conventional grinding process under similar conditions, and 10 times faster than USM [15]. Other worker reported the machining time of RUM is half to one third of USM [23]. In RUM slurry is replaced with abrasives bounded to the tool.  The efficiency of RUS (ultrasonic diamond) milling and drilling  of deep holes depends on the mechanical properties of the work piece material, the design of the diamond tool and the machining conditions.  With increasing specific load, removal rate  increases significantly [24].  RUM including a rotary tool metal bonded diamond abrasives vibrates while the work piece is fed towards the tool at a constant pressure.  The coolant pumped through the internal hole of the drilling tool, washes away the swarf,  reduce the tool temperature and prevents jamming [20], [25]. Experimental investigations have been conducted on the productivity and surface quality and tolerances in ultrasonic machining of ceramics [26].

 The tool is shaped based on design feature specification. A constant flow of slurry which is automatically  cooled and recirculated between tool and work piece to carry away the chips from the work piece. As a result of that material is  removed and work piece crushing. This is shown in Figure1. The components of USM is also shown which including  elements such as Oscillator; the resonance transducer; the horn, tool holder; tool; clamping system; abrasive slurry; and static pressure system.

Figure.1. An ultrasonic machine apparatus

 

  At present, most procedures are based on personal knowledge and judgment. The complexity of the process, and the interrelationship between its process variables mean that designer or even general process planners have limited knowledge of USM. In planning they have to turn to the literature or experts.  The information required by the former is often difficult to obtain. moreover, the training of both process planners and manufacturing operators in USM technology is time-consuming and expensive. Consequently if the knowledge is not available from a reliable source, the USM product development cycle time and cost increases, and both quality and productivity is likely to decrease. Expert system and intelligent knowledge-based system (IKBS) provide a route to overcoming these hurdles to the further advancement of USM, although little has been achieved in these approaches.

Knowledge-based system becomes attractive for USM because it provides information that can be used to minimize machining time, cost and increase productivity and benefits. it could provide a ready, on-line knowledge consultancy system guiding product designers and manufacturing engineers to select appropriate product parameters and process conditions. The more effective design and manufacture of an USM product by IKBS also needs evaluation of manufacturability.  At present however, no computer-based systems have yet been reported that advice designers and manufacturing engineers and apply manufacturability evaluation concepts to USM. In the present paper, the implementation of IKBS for manufacturability evaluation in concurrent engineering environment for USM is investigated. The need for manufacturability evaluation is based on the following concept that the most significant manufacturing time and cost are those that result from poor design.  Design account for only small fraction of product development cost, but the vast majority of total product life cycle cost is influenced by the design decisions.  Therefore designers need feedback from manufacturing engineers and other functional area at the various stages of design processes. On the other hand, all functions including manufacturing engineers should be able to contribute to the product development  from the early stages of design process . This is based on the philosophy of concurrent engineering (CE) environment.  In computer based CE environment, various intelligent knowledge based system can be used instate of  team of experts to communicate with each other through blackboard architecture system. This is based on computer based concurrent engineering environment. A computer-based CE approach to USM is the subject of this paper.

2- Concurrent engineering

Manufacturing systems comprise of a large number of different stages that affect product cost, product quality and the productivity of the overall system. The interactions between these various processes of a manufacturing system are complex. In conventional manufacturing systems, concept decisions, product design, and prototyping are performed before manufacturing system design, process planning, and production. Sequential engineering is the name given to the traditional engineering design, when each stage is taken in turn and the next step in sequence does not start until the previous step has been completed. If an error is detected during this process then the design goes back one or more steps to correct it. Although the tracking of design is easy, the process is slow and expensive in terms of time to develop products. This is also called the serial engineering or over the wall approach in which inter-departmental cooperation is minimal.  Disadvantages of conventional systems including longer development time, High level of development time and costs,  and low quality. According to the U.S. Institute of Defence Analysis ” Concurrent Engineering is a systematic approach to the integrated, concurrent design of products and their related processes, including manufacture and support. This approach is intended to cause the developers, from the outset, to consider all elements of the product life cycle from conception through disposal, including quality, cost, schedule, and user requirements." As in many other quality approaches, the implementation mechanism of CE is a team work. Members from different departments including marketing, design, engineering, and manufacturing,  share information and insights about the product to adjust the design and eliminate the problems in early stages of product development. The use of multifunctional teams is critical to concurrent engineering success. Japanese companies are pioneers of using CE philosophy in developing the products with  higher quality and much more quickly than their western competitors. They incorporate different stages much more concurrently. Since CE uses multi-disciplinary team and considers the product and processes at the same time, it supports right-first-time designs which address all the product requirements such as customer attributes, functionality, reducibility, assimilability, maintainability, and recyclables. Using the CE approach for product development, not only the number of redesigns is reduced, but also changes are made at early stages of product development.

Integration of KBS system for USM  machining in computer based CE environment is illustrated in Figure 1.

 

Figure1. Integration of all KBS, and d machines in a computer based concurrent engineering  environment

3. KBS  in computer based CE environment

A knowledge based system (KBS)  for ultrasonic machining (USM) has been developed in a computer based CE environment, the latest version (3) of an expert system shell (NEXPERT), based on object oriented techniques (OOT) is used to develop the knowledge-base.  A Hewlett Packard (HP) model 715/80 workstation was used as the hardware for development of the expert systems.  A geometric specification of the features of the component sent for manufacturability evaluation for the various stages of its design.  Within the manufacturability procedure, the cost and cycle time and penetration rate of USM is estimated.  In the design of a part, its features can be described in terms of its geometry, its particular its volume and the amount of material has to be subsequently removed. The attributes of six different classes of work piece materials, three type of abrasive and two type of tool material are stored in database. The IKBS can retrieve information from databases and advise the designer on the appropriate choice of material, design feature description and machine type for his decision. The IKBS also contains information for manufacturability evaluation, Knowledge of design representation in three dimensions in terms of features, rules for good practice, machine and process capabilities and constraints of features that can be manufactured by a particular process. For the present KBS knowledge has been gathered from experiments on USM at Edinbrough Universities and also from technical journals and handbooks. For each design feature undergoing evaluation for manufacturability by USM, the cost and time  of the machine cycle, and penetration rate and productivity is a major consideration. 

4. Architecture of KBS

The KBS system contains USM expertise gathered from experiment and from general knowledge about the process that can be provided to designers and manufacturing engineers. The  KBS including:

3.1 Feature library:  Feature library, containing different classes of design features such as holes, slots and pocket, each of which can be produced by USM.

3.2 Work piece material: Material library contains seven different classes of material for work piece including glass, ceramics, hard metals with hardness of (40 to 60 R), composites (e.g. glass epoxy), tungsten carbide, graphite and stone that can be accepted by the system are stored in the system.

3.3 Abrasive solution: Properties of three main USM abrasive including boron carbide (B4C), silicon carbide (SiC) and aluminium oxide (Al2O3) are stored, so that the expert system can deliver information on process conditions such as abrasive type, size, concentration and carrier fluid.

3.4 Tool material: Tool material library contains two different classes of material  including stainless steel and mild steel.

3.5 Ultrasonic machines parameters: Information related to the other machining parameters including wear ratio, mrr, frequency, amplitude vibration, power range, and so on for each type of material for work piece are stored in process data base.

3.6 Machining cycle time module: The knowledge base provides estimates of cycle time and costs for each selected design feature, based on the selected work piece and tool materials, abrasive and process conditions.

3.7 Manufacturability: The three elementary quantities associated with a design feature is its size, machining time and cost are used to obtain the penetration rate and productivity of each design feature or machining operation.  The created feature size is depend on tool cross sectional area and  path needed to produce the design feature.  The size of these features is specific in terms of their volume which is equal to the amount of material removed from work piece.  The penetration rate shows how fast a feature can be machined and expressed in unit of depth of operation per unit time. Productivity expresses the volume of material removal per unit of time. In this system, manufacturability is assessed by estimates of the design features, machining time and cost, penetration rate, and productivity.

5.     Experimental Verification

  Results are presented in Table 1.  The results of intelligent system described above  was compared with the experimental result of ultrasonic hole drilling.  The tool diameter is 15 mm and the depth of holes are 2.6, 7.5, 10.0 and 12.0 mm.  In practical USM, estimates of machining time and cost, penetration rate and productivity is time-demanding on experienced personnel.  In contrast the knowledge-based system can provide these estimates usually in less than one minute.  For example, the intelligent result of a circular hole making with different material type for work piece, abrasive and tool for the same design feature specification is presented in Table 2.

  Designers of manufacturing engineers select work piece material and design feature from the work piece and feature library. Then work piece specification and design description for each selected design feature are obtained interactively by the IKBS.   The system estimates all necessary parameters such as spindle force, abrasive size, concentration, carrier fluid, frequency, power, machining time and cost, penetration rate and efficiency.   Intelligent results of different design features with different material type for work piece and different tool and abrasive are presented in Table 3. All necessary parameters including machining time and cost, penetration rate and efficiency is estimated by the KBS. Other parameters such as wear ratio, spindle force, abrasive size, concentration, carrier fluid, frequency, power are also recommended by the KBS.

Data for experimental: Frequency 20 kHz, Amplitude 40 μm, Static force 3, Abrasive BC, Tool steel.  Data for intelligent system: Frequency 20 kHz, Amplitude 38 μm, tool mild steel.

Conclusion

An Intelligent advisory and manufacturability evaluation for Ultrasonic machining in concurrent engineering environment based on object oriented technique has been developed. A feature based approach is used to obtain design feature description.  A feature library is designed which contains different classes of design features.   Attributes of six different type of material for work piece, three most important type of abrasive, two different type of material for tool, and description of different process parameters for each type of material for work piece are stored in process data base. The intelligent system is linked with these databases and able to advice designers and manufacturing engineers. Design specification is interactively acquired by the system. Then it gives some advice for optimal selection of process parameters for each  design feature. Then the system automatically estimates machining (cutting) time, and cost, penetration rate, and productivity for each individual feature and operation. The current  intelligent system can be used to assist product designers to estimate machining cycle time and cost and all other machining parameters mentioned above at the early stages of design process and give some advices for improvement of design specification.  It also assist manufacturing engineers to select the optimal process parameters. In the developed system, both heuristic and algorithmic procedures have been implemented. An experimental verification has been conducted on USM. The developed system allows for additional more detailed function modules or databases without altering the rest of the knowledge base.  The system is user-friendly and can be used either by designer or USM experienced or  those who need many guidance.


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