Knowledge representation database for the development of tolerances and fits in design.

Romanescu, Maria Camelia ; Pruna, Liviu ; Antonescu, Ion 等

1. INTRODUCTION

The research presented in this paper allows the authors to bring theoretical contributions on the rules specific for structuring data bases characteristic to fits, tolerances fields and ISO limit deviances values from the perspective of using them, to create software programs able to run under CAD systems.

From a practical perspective, we have created a program able to run under CAD systems, that allow the user to choose and automatically mark on technical drawings the data related to the tolerances fields and the limit deviances values.

2. RELATED WORK

Unfortunately, design references do not give clear information of principles involved in fits and tolerance field establishing. Instead, papers present general design criteria and rules of thumb that should be used in design process. Parkinson (Parkinson, 1982) views tolerances and basic dimensions as parameters that affect the overall cost of machining and assembly. He does not address the parts dimensions and tolerances required for its function, but only those needed for assembly. M. Anjanappa (Anjanappa et al., 1991) discusses the identification, development, and operation of an approach that uses a global data base of flexible manufacturing.

A. Desrochers (Desrochers & Riviere, 1997) presents a matrix-based method for defining the constraints for tolerance representation function of the imposed rules for computer aided design and computer assisted manufacturing. Tarek M. Sobh (Tarek et al., 1999) addresses the problems related to the representation and analysis of tolerances in the fields of quality control, computer aided design and production.

M. Kalta (Kalta & Davies, 2001) describes the EXCAP program for cast parts that interacts with the CAD system. The program extracts the geometrical data and the dimension devices, necessary for the parts manufacturing.

In the field of mechanical design, a module developed in the Czech Republic is available, for the automatic computation of tolerances and fits incorporated in the MITCalc program.

The research status reveals that currently in the field of workpiece dimensional precision there is no program for establishing the fits type, the tolerance fields or the values of limit deviances that can run within a CAD software.

3. OVERVIEW OF CURRENT WORK

The expert program TFDES aids the designer by automating a specific design task while having also the capability to store new knowledge, as a result of experience that can be used latter (Romanescu et al., 2007). The authors' experience in both production and CAD domain will lead to the development of new solutions for structuring the data base such that it can be used by a program able to run on 2D CAD systems.

The successful planning requires the following activities:

1) Creating a data base incorporating the fits, tolerance fields and limit deviances values and structuring it in a quantifiable format that works in 2D CAD system;

2) Creating a base of knowledge in a quantifiable format;

3) Developing of expert system computer code;

4) Testing the computer program.

The authors describe developments in the first three activities, which have been made possible by their experience.

4. SYSTEM ARCHITECTURE

Block diagrams for incorporating the fits, tolerance fields and limit deviances values strategy can be divided into three sections: the input stage, the knowledge base, and the output stage. An algorithm developed is depicted in a flow chart, as shown in figure 1.

Input stage

Several important factors that may affect a blank layout in choosing the tolerances field and fits are first discussed:

* Establishing a nominal dimension;

* Choosing the fitting system;

* Choosing the precision class;

* Choosing the type and group of the fitting;

* Marking on the drawing tolerance fields and values of the limit devices.

Knowledge representation and decomposition

The base structure of these systems includes the following:

* a component that contributes to the development in the computer science field;

* a component constituting the knowledge base.

The requirements imposed on the system are the following: the utilization of the real knowledge about the tolerances and fits system, the extension ability, the transparent characteristic of the system, the user-friendly representation of the knowledge, the capacity for user-computer integration.

As an user-aiding mechanism, the system simplifies the designer reasoning--the design project is reduced to selecting automatically without operator intervention and placing the tolerances, the times for detailing are reduced, the tolerances can be efficiently reused and the constructions can be CAD systems.

[FIGURE 1 OMITTED]

System formulation

The user calls the tolerance input program, which assign default tolerances. Each module interacts with another to arrive at the final design. Tools to assist this process were written in the AutoLISP programming language. Figure 2 presents a fragment of the index_filter function that establishes the index that must be saved after the user selection:

(defun index_filter(/ parameter searchList
indexes positions)
[...]
(if (= step 2) ; after we know the Domain, prepare to
save the corresponding indexes
 (progn ; for the item selected by the user
 (setq searchList domList) ; determine in what list to
search for the parameter
 (setq parameter (cadr (assoc 'Dom state))) ;
save the selected parameter
 (setq positions (cdr (assoc 'Diameter
indexes_list))) ; save in <<positions>> the indexes that
we are searching for in <<searchList>>, to check if they match
the ones selected by the user
 (filter parameter)
 (if (/= indexes nil)
 (progn
 (if (> (nth 0 indexes) (nth 1 indexes)) ;
order ascending the indexes that match the user selection
[...]
 (if (not (assoc 'Dom indexes_list))
 (progn ; if the article is not <<indexe s_list>>
 (setq indexes_list (cons (cons 'Dom indexes)
indexes_list)) ; save it in "indexes_list"
 ) ; end progn
 (progn
 (save_indexes 'Dom)
 ) ; end progn
 ) ; end if
 ) ; end progn
) ; end if step 2
[...]

Figure 2. The index_filter function

The output stage

The selection of the fits function of the conditions set (economic, manufacturing and functioning) by TFDES for the assemblies made as CAD applications will determine the choosing and subsequent positioning of the tolerances and limit devices corresponding to the ISO standards values for shop drawings components. Determining the algorithms needed, the structure of the program and developing an expert type program at code level led to finalizing the representation knowledge in an expert system building process in CAD system.

5. CONCLUSION AND FUTURE WORK

The work described in this paper has been also implemented in educational process of our students. The major conclusions are:

a) The program TFDES developed considers most of the requirements needed in a selection of tolerances and fits in design.

b) Solid groups and subgroups information with open access to its data structure and functionality is necessary for an expert system used for ISO tolerances field and jigs.

The following future work is recommended:

a) Developing a user manual for the software.

b) Turning the expert program commercial into a commercial product that can be subsequently integrated into the complex CAD systems.

c) Developing a manual containing the expert system for dimensional tolerances in AutoCAD.

6. REFERENCES

Anjanappa, M.; Courtright, M. J. & Anand, D. K. (1991). Manufacturability Analysis for a Flexible Manufacturing Cell, Journal of Mechanical Design, Vol. 113, December 1991, pp. 372-376

Desrochers, A. & Riviere, A. (1997). A Matrix Approach to the Representation of Tolerance Zones and Clearances, The International Journal of Advanced Manufacturing Technology, Springer-Verlag London Ltd, Volume 13, Number 9, September 1997, pp. 630, ISSN: 0268-3768

Kalta, M. and Davies, B. J. (2001). Integration of 2D CAD Models of Turned Components with CAPP, Engineering with Computers, Springer-Verlag London Ltd, Volume 17, Number 3, October 2001, pp. 315-325, ISSN: 0177-0667

Parkinson, D. B. (1982). Assessment and Optimization of Dimensional Tolerances, Computer Aided Design, Vol.17, No.4, May 1982, pp. 191-199

Romanescu, C. & Pruna, L. (2007). Expert Systems Methodology, The 2nd International Conference on Engineering Graphics and Design 2007, June 2007, Galati

Sobh, T. M. & Henderson, T. C. (1999). Tolerance Representation and Analysis in Industrial Inspection, Journal of Intelligent and Robotic Systems, Springer Science + Business Media B.V., Formerly Kluwer Academic Publishers B.V., Volume 24, Number 4, 1999, pp. 387-401, ISSN: 0921-0296