Indian Journal of Engineering & Materials Sciences Vol. 20, February 2013, pp. 21-26
A feature-based system for CAD/CAM integration through STEP file for cylindrical parts S Sivakumara* & V Dhanalakshmib a
Department of Mechanical Engineering, B S Abdur Rahman Crescent Engineering College, Chennai 600 048, India b Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai 625 015, India Received 19 September 2012; accepted 27 December 2012
Intelligent manufacturing systems of the future produce small lot size with high flexibility, which require quick and automated decisions. To suit this environment, the integration of CAD and CAM is required. The key to the system integration is the product data exchange among various systems. This paper explains the integration of CAD and CAM by feature extraction. The system developed uses a simplified and generalized methodology of extracting manufacturing feature information from STEP files for cylindrical parts. The extracted data is used to generate controller dependent NC codes. Experimentation is carried out by turning the cylindrical parts using the generated codes and validated by inspecting them in CMM. Keywords: CAD/CAM, STEP, Feature Extraction, CMM
The global market is striving to produce high quality products at a competitive price. At the same time, the continuous changes in customer needs are to be satisfied. To suit this environment, different stages of product lifecycle are to be controlled by computers to ensure easier, faster and flexible workflow1,2. Further, an integration of computer aided design and computer aided manufacturing is required to bridge the main islands of automation. Each computer aided system has its own internal structures and usually vary from one another. Hence, the product data exchange between these systems is crucial3. The CAD model contains extensive information on product description, which will be used for various downstream applications such as process planning, NC code generation, assembly planning, inspection and so on4,5. If information is to be extracted based on features, then automatic interpretation is necessary6. Feature has different meanings at different contexts and is based on the field of applications7- 9. Various approaches and algorithms have been proposed by many researchers for feature extraction10-12. However, most researchers have developed algorithms based on the concept of internal representation of features of a particular CAD system. They often are not compatible with other CAD systems13-16. To ________________ *Corresponding author (E-mail:
[email protected])
overcome this difficulty, some researchers have used neutral file formats to extract data17. Since the neutral files such as STEP and IGES contain coordinate data, it is simple to extract feature data from them. Based on the literature, it is observed that few complex systems have been developed to extract data from a CAD model for specific applications18,19. In some studies, the NC codes were generated directly from the CAD model20. This method worked well for a particular CAD system and not for all systems. Some studies reported that NC codes were generated directly from a neutral file21. However, the neutral file contains a large amount of data in the form of coordinate information of the part. Hence, it is difficult to generate NC codes by processing the enormous data. Hence, if an interface between CAD and CAM systems is desirable, automatic interpretation of CAD data is called for 22,23. Further, this method has specific advantages of generating NC codes based on the data extracted from created CAD models using any commercially available software. Some research work concentrate on STEP NC, where in STEP file is given as input. In this work, STEP NC codes are generated as the machining working-steps in a physical file format and in an XML file format. In addition, the generated STEP NC program is controller independent and not the G and
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M codes. The STEP NC format cannot be used directly in commercial CNC system. They need editing in the CNC controller24-26. This paper explains a simplified and generalized methodology of extracting manufacturing feature information from STEP file. This method uses the extracted feature information to generate NC codes neither from the CAD model nor using neutral file. The generated NC codes can be directly executed in the CNC machine. The detailed methodology used for the integrated manufacturing system is explained in subsequent sections. Integrated Manufacturing System This paper explains the integration of CAD and CAM, the major areas of product lifecycle. The proposed integrated manufacturing system is shown in Fig. 1. The system consists of four major steps namely creation of CAD model, extraction of feature data from STEP file, turning of parts using generated NC codes and inspection using CMM for validation. Creation of CAD model
In this paper, a cylindrical part is chosen as the part model. The part model is created using Unigraphics and shown in Fig. 2. This part model has six features namely straight cylinder, right hand taper, left hand taper, right hand concave cylinder, right hand convex cylinder and hemisphere. The part model is converted into STEP AP203 file.
Fig. 1 – Procedure for the integrated manufacturing system
Identification of features from STEP-file
The part model contains geometric data, which includes coordinates of the vertices and transformation data (translation and rotation), metric information, such as distance, angle, area and volume. However, CAD information is based on geometric entities such as line, arc, circle and so on. The CAM information is based on manufacturing features such as face, taper, groove, chamfer and associated attributes. Hence, one of the major challenges in the integration of CAD and CAM is to translate the part information from CAD system to CAM system. This requires a feature extraction system for identification of features. A feature is an information collection associated with product description. It contains the geometrical data and properties such as geometry and topology, configuration parameter, default values, location and orientation parameters, relations, constraints, composite features, symbolic or skeletal representation, feature validation and so on. In this paper, the various cylindrical features are considered and the detailed method of identification of the features is given in Table 1. Extraction of feature data from STEP file
From Table 1, it is clear that the data available in STEP file is in the form of vertices, coordinates, vectors and so on. The dimensional information of the part features and their positions are extracted from STEP file using feature extraction process. A generalized program in C++ language is developed to extract features from STEP file and stored in a text file. Algorithms are written for identifying various features. Figure 3 shows the flowchart for feature extraction from STEP file. The data extracted from the STEP file are stored in a text file and this data is the reference data for all the downstream activities. Table 2 shows the extracted
Fig. 2 – Part model of the cylindrical part
SIVAKUMAR & DHANALAKSHMI.: CAD/CAM INTEGRATION THROUGH STEP FILE
Table 1 – Identification of features from STEP file
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data from STEP file. In this work, the features considered are listed from the right end of the part model, as the turning operation is carried out from the right end of the part. Turning of parts using generated NC codes
To machine the parts by turning operation, the above feature data cannot be directly used. Hence, in this work, a program in C++ language is developed to generate the NC codes from the extracted data of the part model. The input to the system includes reference data text file, type of raw material, its size and cutting parameters such as speed, feed, and depth of cut. The dimensional information of features is taken from the reference data text file. The NC codes are generated to turn the individual features one by one from the right end of the cylindrical part. The flowchart for the NC codes generation is shown in Fig. 4.
Fig. 3 – Flowchart for feature extraction
Fig. 4 – Flowchart for NC codes generation
SIVAKUMAR & DHANALAKSHMI.: CAD/CAM INTEGRATION THROUGH STEP FILE
In this work, a uniform cylindrical bar of diameter 25 mm made of aluminium is taken and is machined in XLTURN CNC lathe. The XLTURN lathe has a FANUC control system with two axis movement namely X and Z. The parts are turned using carbide tipped tool. For checking the consistency of the overall system, five similar parts are produced. The turned part is shown in Fig. 5.
Feature
Radius (mm)
Hemisphere Major 7
Minor
Length (mm) Radius of curvature (mm)
7
Fig. 5 – Turned part
Table 2 – Extracted data from STEP file Reference data of features of the part model Straight Right hand Straight Left hand concave cylinder taper cylinder 1 cylinder 2 12 12 12 8 8 8 4
25
10 -
15 -
Right hand convex cylinder 11
15 -
Right hand taper 12
8
11
3
10 -
Table 3 – Comparison of measured data in CMM and reference data
S. No.
Feature
Data Measurement data using CMM (mm) extracted Parameter from CAD Part 1 Part 2 Part 3 Part 4 Part 5
1. Hemisphere Radius Major radius Minor Right hand radius 2. concave Radius cylinder of curvature curvature Radius Straight 3. cylinder 1 Length Major radius Left hand 4. Minor taper radius Length Radius Straight 5. cylinder 2 Length Major radius Right hand Minor 6. convex radius cylinder Radius of curvature Major radius Right hand 7. Minor taper radius Length
Percentage of error
Part 1
Part 2
Part 3
Part 4
Part 5
(mm) 7
7.115
7.089
7.124
7.057
7.104
1.643
1.271
1.771
0.814
1.486
12
12.094
12.113
12.104
12.119
12.086
0.783
0.942
0.867
0.992
0.717
8
7.876
8.122
8.067
8.094
8.051
1.550
1.525
0.838
1.175
0.638
4
4.118
4.165
4.126
4.112
4.124
2.950
4.125
3.150
2.800
3.100
12 10
12.085 10.058
12.132 10.089
12.124 10.076
12.089 10.082
12.106 10.064
0.708 0.580
1.100 0.890
1.033 0.760
0.742 0.820
0.883 0.640
12
12.073
12.108
12.129
12.088
12.116
0.608
0.900
1.075
0.733
0.967
8
8.105
8.068
8.142
8.050
8.079
1.313
0.850
1.775
0.625
0.988
15 8 15
14.984 8.089 15.073
14.907 8.124 15.025
14.936 8.139 15.034
14.959 8.158 15.068
14.940 8.144 15.093
0.107 1.113 0.487
0.602 1.550 0.167
0.427 1.738 0.227
0.273 1.975 0.453
0.400 1.800 0.620
11
10.984
11.152
11.146
11.075
11.083
0.145
1.381
1.327
0.682
0.755
8
8.058
8.093
8.127
8.115
8.095
0.725
1.163
1.588
1.438
1.188
3
2.879
2.898
2.958
2.936
2.907
4.033
3.400
1.400
2.133
3.100
12
12.056
12.124
12.094
12.087
12.069
0.467
1.033
0.783
0.725
0.575
11
10.991
11.122
11.089
11.076
11.084
0.082
1.109
0.809
0.764
0.763
10
10.092
10.108
10.126
10.078
10.093
0.920
1.080
1.260
0.780
0.930
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A segment of generated NC codes is shown as: G21; [BILLET X25 Z100; G40 G98; G28 U0 W0; M06 T0606; M03 S1200; G00 X35 Z5; G01 X25 Z0 F30; G71 P10 Q20 U0.5 R1 F30; N10 G01 X0; G03 X14 Z-7 R7; G01 X16 Z-7; G02 X24 Z-11 R4; G01 X24 Z-69; N20 G00 X30; G01 X24 Z-21 F30; G90 X24 Z-36 R0 F30; X23 R0.5; X22 R1; X21 R1.5; X20 R2; X19 R2.5; X18 R3; X17 R3.5;
X16 R4; G01 X24 Z-36 F30; G90 X24 Z-51 F30; X23; X22; X21; X20; X19; X18; X17; X16; G01 X24 Z-51 F30; G70 P15 Q30 U0.5 R1 F30; N15 G01 X16; G03 X22 Z-54 R3; G01 X24 Z-64; G01 X24 Z-69; N30 G00 X30; G00 X35 Z5; G28 U0 W0; M05; M02; M30;
Inspection using CMM
generated part model information is extracted from STEP AP203 file and this information is used as input for the system to generate the NC codes. Further, the result of feature extraction leads to automatic inspection, if computer aided inspection system such as machine vision and laser inspection are introduced. References 1
2 3 4 5 6
7 8 9
Though the machining of parts using CNC machine leads to higher accuracy, surface finish and close tolerances, the parts need to be inspected. In this work, the turned parts are inspected using CMM and the results are compared with the reference data extracted from STEP file for validation.
10 11
Results and Discussion The reference data extracted from STEP file and the measured data using CMM are shown in Table 3. On comparing the above data, the deviations in dimensions of the parts are calculated as the percentage error and are given in Table 3. From Table 3, it is clear that the measurement taken using CMM has a minimum error of 0.082% and maximum error of 4.125% from the reference data. This deviation may be due to machine tool accuracy. In this work, the reference data from the CAD model is used to produce the part. Thus, the present work provides the base for the integration of CAD and CAM.
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Conclusions In this study, a generalized and simplified methodology is proposed to integrate CAD and CAM based on feature extraction for cylindrical parts. The
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16 17 18 19 20 21 22 23 24 25 26
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