The selection of the machine-tools and tooling within the generative CAPP systems.

Doicin, Cristian-Vasile ; Tonoiu, Sergiu ; Ionescu, Nicolae 等

1. INTRODUCTION

There are two main directions regarding the process of generate different variants of process plan in a computer aided environment: Variant Process Planning (VPP) and Generative Process Planning (GPP) (Allada & Anand, 1996). Both of them take into account specific tools and machine-tools, according with the constructive and technological parameters describing the part to be machined. First direction, the most used within the CAPP systems, is based on large databases storing the process plan variants for different groups of parts, developed in a previous stage. The second direction, more difficult to define, but much more flexible, is based on a method of defining the variants of process plan (Allada & Anand, 1996) in a generative way. Following the second direction, a method to create a generative CAPP system was developed by the authors.

The paper presents an important step of the third stage of the process of generating the variants of process plans, those of establishes the tools and machine-tools which could be used in a certain machining operation (Doicin & Gheorghe, 1999), (Doicin, 2001).

2. PROCESS STAGES WITHIN THE CAPP SYSTEM

The CAPP system needs detailed information in order to complete his work and to generate variants of process plans (Doicin & Gheorghe, 1999).

Thus, in the first stage, the 3D model of the part is defined, usually outside the TehnoCIN system, using a CAD package. Then, using specific routines specially developed, geometrical data are extracted from the CAD model and translated in a specific language, used by the system. The data are stored in a transfer file (Doicin, 2001), (Doicin & Tonoiu, 2002).

After that, in the second stage, a lot of technological data are added to the transfer file, by attaching a list--containing technological attributes--to the geometrical description of each geometric Distinctive Feature (Doicin, 2002). Thus, the constructive description (extracted from the CAD model) of the part is completed with technological data, all the information being stored in the transfer file. Based on these data, some preliminary variants of process plans are generated.

Then, during the third stage, the variants of process plans are generated. The algorithm which generates process plans uses a database containing information about part materials, machining parameters, cutting forces, tools, machine-tools etc.

The TehnoCIN system provides a minimal initial database (containing, in the machine-tools tables, data for 41 lathes and 5 milling machines), but user can add new data to the database in order to enlarge the set of parts that may be analysed.

3. ALGORITHM FOR SELECTING THE TOOLS AND THE MACHINE-TOOLS

The next stage is that of establishing the technical possible variants of process plans for the analysed part.

In order to detect the appropriate cutting tools and machine-tools for each variant of process plan generated by the system, a specific algorithm is used, composed from the following steps:

1) Based on the data extracted from the database and using the formulas presented in the literature (Vlase, 1996; Fabian et.al., 2007), for each machining phase within each machining operation, the depth of cut (t) is calculated;

2) Depending on the previously calculated value for the depth of cut, the axial feed rate values (mm/rev) are extracted from the database. There are a lot of restrictions which influences the value of the feed rate (Vlase, 1996), their influence being captured in the values contained in the database. Due to the fact that the database contains only certain discreet values--acquired from the literature--an interpolation (automatically made by the system) is usually required at this point, in order to calculate an exactly feed rate value corresponding to the cutting depth value.

According to the way the values are given in literature, an interval of feed rate values will be obtained during this activity. Let's call it recommended interval ([I.sup.r.sub.s]) of feed rate values (mm/rev),

3) The recommended interval is restrained by about 10% (value considered as safety coefficient: the experience shows that there are difficult to continue the calculus with the marginal value of the recommended feed rate values due to the lack of data required further on). The restrain is applied only to the maximum recommended values.

4) Based on the previous identified values of t and [I.sub.sr], for each machining phase, the values of cutting speed, cutting force and the power consumed by the machine-tool engine are extracted from the database. The system will use them in the next phases.

5) Taking into account the already known values for depth of cut and feed rate, two sets of values of machining parameters, an average and a maximum one, are calculated for each machining phase. These values are obtained by multiplying the recommended values of the cutting speed, cutting force and engine power with the average / maximum values of the correction coefficients described in the literature (Vlase, 1996), (Fabian et. Al, 2007). Let's name the obtained values as average / maximum machining parameters.

6) Let [L.sup.r.sub.UT] be the list of available machine-tools and Sij the set of part's surfaces machined during the Pj machining phase, component of the i operation. For each machine-tool (UT) from [L.sup.r.sub.UT] list, some checks are made in order to verify if each available machine-tool is also an acceptable one (it may be used for machining the part, respecting all the prescribed dimensional and quality characteristics of the Sij part surfaces). As a first step, this is established by searching the feed rate tables in the database for some real values in the restrained recommended interval of feed rate (identified during the step 3). The search is made for each m machine-tool in the [L.sup.r.sub.UT] list and for each Pj machining phase in each i operation. Initially, all the available machine-tools (compounding the [L.sup.r.sub.UT] list) are considered to be also accepted ones.

After the above-mentioned verification, the machine-tools which do not contain the values of the feed rate within the restrained recommended interval ([I.sub.sr]) are removed from the [L.sup.r.sub.UT] list. Let [L.sup.a.sub.UT-Pij] be the list of acceptable machine-tools assigned to the Pj machining phase within i operation. We may define this list as in the equation 1:

[L.sup.a.sub.UT-Pij] = [L.sup.r.sub.UT-Pij] - [UT.sub.e] (1)

where [L.sup.r.sub.UT-Pij] is the list of available machine-tools for Pj machining phase within operation i and [UT.sub.e] is the machine-tool current removed from the [L.sup.r.sub.UT-Pij] list);

7) For each machining phase and each acceptable machine-tool, the values of the determined feed rate are verified from the point of view of the feed mechanism resistance.

As a result, the machine-tools whose feed mechanisms don't accomplish the resistance criteria are removed from the list of acceptable machine-tools. Finally, for each machining phase within each operation a new (restrained) list of acceptable machine-tools is obtained;

8) For each machining phase and each acceptable machine-tool, the values of the engine consumed power are also verified. The machine-tools whose engine power is less than the power required for machining a certain surface is removed from the list of the acceptable machine-tools.

As a result, the final list of machine-tools, assigned to each Pj machining phase within each i operation will contain only those machine-tools capable to machine the Sij surfaces using the maximum recommended values of the machining parameters (depth of cut, feed rate, speed), by respecting the prescribed characteristics for Sij surfaces.

9) For each i operation, the [L.sup.a.sub.UT-Pij] lists assigned to each Pj machining phases are intersected. The result is a list of the acceptable machine-tools, not only for a machining phase, but for entire i operation (see equation 2).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where nu is total number of acceptable machine-tools for machining Sij surfaces in operation i;

10) Depending on the type (turning, milling, drilling etc) and the nature (roughing, finishing etc.) of each Pj machining phase, on one hand, and on the dimensional and quality characteristics of the Simple Distinctive Feature (Doicin, 2001; 2002) to whom the Pij is assigned, on the other hand, the list of acceptable tools are extracted from the database.

11) For each Pij machining phase from each i operation and each acceptable machine-tool from the [L.sup.a.sub.UT-Opi] list, each tool from the acceptable tool list is verified from the point of view of tool body resistance. The calculus is made based on the assumption of using the maximum values of the machining parameters. The tools which don't fulfill the imposed condition are removed from the list of acceptable ones.

12) Based on the average values of the machining parameters, the unit values of the setup and machining time are extracted from the database;

13) The total time required for machining each Sij set of surfaces is calculated. The machining time (which is function of feed rate and speed--with no values at this moment) is estimated based on the maximum values of the machining parameters;

14) The system displays, in a specific window of the system interface, all the acceptable variants of process plans, ordered by the values of machining time, previously establish;

15) The user selects, from the list of acceptable machine-tools--assigned to each i operation, and from the list of acceptable tools--assigned to each Pj machining phase, one machine tool for each i operation and one or more tool(s) for each Pj machining phase.

As a result, lists of acceptable machine-tools and lists of acceptable tools, proposed by the CAPP system to the user, are obtained for each variant of process plan previously generated.

4. CONCLUSION

The paper presents an algorithm for automatically establishing the cutting tools and the machine-tools for each manufacturing phases within each operation of each variant of process plan made in a generative CAPP system. The algorithm proved his functionality within the TehnoCIN package, a generative CAPP system developed by the authors.

Starting from technical principles, the algorithm provides a list of acceptable tools, respectively, machine-tools and user action is required in order to select a single tool / machine-tool for the current machining phase or operation.

Efforts are being made in order to reduce at minimum the user intervention, and the system is continuously evolving. Thus, a friendlier interface it is planned for this module in the future.

5. REFERENCES

Allada, V.; Anand, S. (1996). Machine Understanding of Manufacturing Features, International Journal of Production Research, Vol. 34, No. 7, July 1996, pp. 1791-1819, ISSN 0020-7543;

Doicin, C.V.; (2001). Contributions to the CAPP of Shafts from Machine Building Industry (in Romanian), PhD Thesis, University POLITEHNICA of Bucharest, pp. 130-145, Bucharest, 2001.

Doicin C.V. (2002). Concepts regarding the Description of the Revolution Parts through Constructive--Technological Entities, Proceedings of the International Conference on Integrated Engineering, G. Draghici & S. Tichkiewitch (Ed.), pp. 94-98, ISBN 973-8247-92-6, Timisoara, April 2002, Ed. Politehnica, Timisoara;

Doicin, C.V.; Gheorghe, M.; (1999), A Feature-Based CAPP Methodology for Rotational Parts, Proceedings of the International Regional DAAAM-CEEPUS Workshop on INTELLIGENT MACHINES and TECHNOLOGIES in the 21-st CENTURY, Kalmar L. (Ed.), pp. 97-102, ISBN 963 661 3710, Miskolc, May 1999, Miskolc.

Fabian, M; Spisak, E.; Dovica, M.; Draganovska, D.; Seminsky, J. (2007). The study of the effect of pre-set CAM parameters for cutting surface quality, Ovidius University Annual Scientific Journal: Mechanical Engineering Series, Vol. 9, No. 1, July 2007, pp. 33-38, ISSN 1224-1776.

Vlase A. (1996). Machine Building Technology (in Romanian), E.T., Bucharest, ISBN 973-648-228-7, 1996;