An integrated process planning and robust fixture design system 4

Chapter A Hybrid Fixturing-feature-based Approach 4.1 Introduction A hybrid fixturing-feature-based approach is addressed in this chapter. It is achieved by providing an environment for the automatic fixturing feature extraction, in combination with design-by-fixturing-feature and interactive feature definition. Existing systems commonly consider process planning and fixture planning as separate activities and fixture planning is usually considered after process planning. This procedure separates fixturing features from machining features. Process planning which considers machining features should take into account fixturing features as well. A machining feature has tolerances and dimensions. Dimensions, especially position dimensions, provide precise information of datum relationships with other features in a part, which can be used to reason the fixturing features. In this chapter, the fixturing feature extraction approach is used to extract fixturing features automatically for parts of complex shapes such as castings and forgings that require multiple machining operations on 3- to 5-axis machining centers. It is realized by analyzing all the available information regarding the material and 56 Chapter A Hybrid Fixturing-feature-based Approach geometry of the workpiece, the spatial and tolerance relations between part features, the machining operations intended for the part, and the processing equipments for the operations. In particular, it takes into account the dimensional requirements and accuracy of the workpiece. The extracted fixturing features not include specifications of the actual fixturing points or the fixture components, but only location datum faces and support surfaces, if any. In case there are insufficient fixturing features for the manufacturing purposes through fixturing feature extraction, additional and necessary fixturing features can either be selected from the existing part features or designed as a new feature. The design-by-fixturing-feature approach and the interactive feature definition approach work as supplements to the feature extraction approach to obtain fixturing features for the integration of design and manufacturing. The three approaches are described in detail in the following section. 4.2 Methodology 4.2.1  Locating Principles Dimensional Requisites A machining feature on a workpiece has position relationships with other features which are referenced as position dimensions (ASME Y14.41). A location has to meet these dimensional requirements of the workpiece which are usually stated on a part drawing (Joshi, 2003). 57 Chapter A Hybrid Fixturing-feature-based Approach For example, in the workpiece illustrated in Figure 4.1, the position dimensions clearly state that the centre of the hole (a machining feature) should be at distances X from face A and Y from face C. Consequently, it must use face A and face C as datum for locating the workpiece while drilling this hole. This would ensure that the hole is at the specified distances from face A and face C. If face B is used as a stopper, the deviation in length X1 between faces A and B would cause inaccuracies in the position of the hole. If the length X1 is oversized by 1mm, the centre of the hole will be at (X+1) mm away from face A. If the length X1 is undersized, the hole would shift towards face A and would be nearer than distance X from face A. If one locates on face A, the hole would always be at the same distance from face A irrespective of the variation in length X1. Similarly, the same situation will occur when locating with face D instead of face C for dimension Y. Figure 4.1 Dimensional specifications of a workpiece Location to satisfy dimensional requirements coincides with the principle that the manufacturing datum (here referring to locating datum) should be consistent with the design datum. It is necessary to mention that, in practice, they may be different due to 58 Chapter A Hybrid Fixturing-feature-based Approach machining difficulties such as tool access, clamping position, etc, which may lead to additional manufacturing errors. This could occur and should be duly considered in set-up planning. For set-up planning, the machining environment, i.e., either 3-, 4- or 5-axis machining center, would also affect the number of set-ups and the way the part is to be fixtured.  Accuracy Location should be based on the most accurate surface of the workpiece. A machined surface is always preferred to an un-machined one. When several machined surfaces are available, always locate from the most accurate surface if feasible. For example, as shown in Figure 4.2, the small right hole has a position dimension X with respect to the center of the other three concentric circular features (A, B and C). In order to guarantee the required accuracy, it will be preferable to locate from Hole A since it is more accurate than B when drilling the small hole on the right. The accuracy of a surface is determined not only by the dimensional tolerance, but also form tolerances such as flatness and straightness (ASME Y14.41). For example, in the case shown in Figure 4.2, if the straightness of Hole B is more stringent, one should choose Hole B for locating.  Constraints A workpiece, just like any free solid body, has six degrees of freedom (some refer to this as twelve degrees of freedom by considering the +/- movements in each direction) (Nee et al, 2004): three linear and three rotary motions of the workpiece along and around the three major axes X, Y and Z as shown in Figure 4.3. Restricting all the degrees involves two primary elements: locators and clamps. Location should restrict 59 Chapter A Hybrid Fixturing-feature-based Approach as many degrees of freedom as possible, and establish a desired relationship between the workpiece and the fixture, which in turn establishes the relationship between the workpiece and the cutting tool. Z X Y Figure 4.2 Tolerance requirements of a workpiece Figure 4.3 Six degrees of freedom 4.2.2 Location Plan A location plan should consider fixturing features required for machining purposes. It is governed by the type of the machining feature being considered (Chang et al, 2005). As mentioned above, the position dimensional space of a machining feature indicates the possible location surfaces which in turn determine the location plan of the machining feature. Based on the dimensional space of a machining feature, three types of location plans, i.e., 1-face, 2-face and 3-face location, for different production requirements, are defined and shown in Figure 4.4. For prismatic parts, the popular 32-1 principle is a way to guarantee that a workpiece is correctly located in a threedimensional space with the 3-planar-face location plan in Figure 4.4c. 60 Chapter A Hybrid Fixturing-feature-based Approach a) 1-face location plan b) 2-face location plan c) 3-face location plan a, b, c: dimensions Figure 4.4 Location plans for different machining requirements 4.2.3 Locating Feature Extraction The sketch-based modeling approach is a facility provided by most 3D design software such as Inventor®, to create features. Building a model usually starts with either a 2D or 3D sketch. A sketch consists of geometries such as lines, arcs, circles, 61 Chapter A Hybrid Fixturing-feature-based Approach etc. Dimensional constraints can be added to a sketch to define the size of a feature and the positional relations with other geometries. Tolerances can be added to these dimensions if necessary, which are called dimensional tolerances. Followed by the sketch design, part features used to construct the model can be generated based on the designed sketches. When the model is designed, the dimensions and dimensional tolerances are stored with the model together with the geometrical information. They can be accessed by part features using the API provided by Inventor®. Figure 4.5 shows some position dimensional constraints on the sketches of machining features. The sketch must be located on a certain plane which is called a sketch plane. The sketch plane can be either a datum plane or a face plane on the model. a) Case b) Case c) Case d) Case Figure 4.5 Examples of position dimensional constraints 62 Chapter A Hybrid Fixturing-feature-based Approach Traditionally, manufacturing information, such as a datum which may serve as a fixturing feature, can only be obtained from the part drawing. With sketch-based modeling, datum information can now be captured. Design intents can be embodied with sketches incorporating the geometries and dimensional constraints mentioned above, and it is possible to use the dimensional constraints on the sketch to analyze the reference location features of a machining feature. The feasibility of using sketchbased datum analysis is presented in this paper. In this research, rules described in Sections 4.2.1 and 4.2.2 are applied to reason the locating features of the features to be machined. Tolerances, which include dimensional and form tolerances, are used to select the most suitable locating features from a list of candidate features. Flatness and straightness (ASME Y14.41) are the two types of form tolerances considered in this system. They are described as strings in the form of “FLATNESS: (VALUE)” and “STRAIGHTNESS: (VALUE)” respectively, and stored as attributes either with the part or the specific machined surfaces. For a casting, the flatness and straightness of the cast surfaces are values specific to a casting process and hence they are uniform throughout, so the system prompts the designer to input the two values in the part level and they are applied to all cast surfaces. For machined surfaces, the designer needs to select the specific surface to input the corresponding form tolerance type and value. The location plans described in Section 4.2.2 explains in detail the relationship between the dimensional spaces and locating features needed for a machining feature. However, the dimensional constraints on a sketch usually provide only 2D 63 Chapter A Hybrid Fixturing-feature-based Approach dimensional spaces, and hence no more than two locating features can be obtained. Having the machining features in Figure 4.5 as examples, by considering the dimensional constraints, Face and Face can be considered as locating faces in Figure 4.5a, side face and cylinder face in Figure 4.5b, and side face, cylinder face and cylinder face in Figure 4.5d. If a blind hole is drilled, as mentioned previously, three locating features are needed. In this case, another locating face is required. This dimensional space is usually embodied by the feature making process, such as extrude, punch, and cut. In some cases, a sketch itself provides a locating face by its sketch plane. For example, to plan the surface shown in Figure 4.5c, the sketch plane is the only locating face, but this rule is not always practical. For example, to make a blind hole in Figure 4.5b, the sketch plane of the sketch is where the drill starts from. It therefore cannot be considered as a locating face, while the bottom face can be considered as one. Therefore, to determine this locating face, the tool approach direction (TAD) needs to be considered together with the sketch plane direction. A surface with the normal direction parallel with the normal direction of the sketch plane but the same with the TAD can be considered as a locating face. There are cases that two TADs may exist. For example, both top and bottom faces are all accessible for drilling the hole in Figure 4.5a. In this case, face tolerances are applied and the most accurate face, i.e. the bottom face with flatness 0.01, will be selected. This rule is also used to select suitable faces shown in Figure 4.5d. There, the small machining hole has a position relation with the center of the two concentric holes. Therefore, the cylindrical faces and can be selected as the locating faces. By comparing the dimensional tolerance of these two faces, cylindrical face is abandoned. Consequently, the top face is kept and the bottom face is abandoned. The selection as a candidate face is achieved by applying both tolerance and TAD rules. 64 Chapter A Hybrid Fixturing-feature-based Approach These rules described above using the machining features in Figure 4.5 as examples are summarized in Table 4.1. Table 4.1 Candidate locating features for machining the part shown in Figure 4.5 Rules Priority Dimensional Space Fig. 4.5a face 1, face TAD Dimensional Tolerance Form Candidate Locating Features Fig. 4.5b Fig. 4.5c Fig.4.5d cylindrical side face, face, side cylindrical face 1, face cylindrical face bottom bottom top face, face face bottom face cylindrical face 2, top face bottom face Figure 4.6 shows the entire feature extraction algorithm. The machining features are extracted first by comparing the features on the raw part model and the final part model, and fixturing features are then extracted automatically for each machining feature by analyzing the dimensional constraints on the sketches of the machining feature and by considering the tolerances. Besides the geometric information, each extracted machining feature contains the information of feature type (plane or hole), dimensions with tolerances and fixturing features. Such information can be used by the downstream processes such as set-up planning and machine resource planning. Thus, machining features have sufficient information for the integration of design and manufacturing. 65 Chapter A Hybrid Fixturing-feature-based Approach Raw CAD Part Model Machining feature extraction by comparing part features on the raw part and final part. Machining Features Final CAD Part Model Find the corresponding part feature for each machining feature on the final part. Input the form tolerance for faces if necessary Fixturing Feature Extraction Rules Analyze the dimensional information of each machining feature Obtain the location fixturing features and link them to the machining feature Fixturing Feature Extraction Machining Features with Fixturing Features information Figure 4.6 Feature extraction process 4.2.4 Design-by-fixturing-feature The design of a new fixturing feature is convenient with the predefined fixturing features described in Section 3.1.1 in Chapter 3. A fixturing feature can be instanced from the predefined fixturing features. The three types of the fixturing features in Table 3.1 are parametrically defined, which make them blend to the existing model easily. Their design interfaces which contain the design parameters are shown in Figure 4.7. For plane features, a base facet needs to be defined either by points or by sketch. The designer needs to assign the required machining features or set-ups for the designed fixturing features as well. 66 Chapter A Hybrid Fixturing-feature-based Approach Figure 4.7 Design interface for fixturing features 4.2.5 Interactive Fixturing Feature Definition In this method, the designer needs to select facets as fixturing features in the existing model, and assign them to the required machining features or set-ups. Other information defined in Table 3.1 can be obtained by reasoning the geometric information of the selected facets. 4.3 System Implementation This system is implemented on the AutoDesk Inventor platform and is developed using Inventor API. Inventor is a 3D feature-based design software and uses sketches to define features. The raw and final parts in this research are modeled in Inventor, i.e., 67 Chapter A Hybrid Fixturing-feature-based Approach they are designed with feature functions such as extrude, punch, and thicken. These features are defined using sketches of the part and are accessible using Inventor API. 4.3.1 Information Representation The part, machining features and fixturing features are represented as objects in this system. Each model input into this system is described as an object “Part”. It has properties of material, tolerances and machining features. Other properties for full integration of design and manufacturing, such as information of the machining environment (machine tools used), production due date, set-ups, etc., are included in the optimal set-up planning methodology which will be described in Chapter 5. The properties and relationships of machining feature and fixturing feature have been defined in Table 3.1. Methods are implemented to store and retrieve these properties. 4.3.2 Hybrid Fixturing-feature-based Approach Six steps are involved in the extraction of the machining and fixturing features. An example shown in Figure 4.8 is used to explain the procedure step by step. The part shown in Figure 4.8 is a simplified front knuckle for an automotive chassis system. It is the joint between the brake system (caliper mounting pads), suspension system (strut) and steering system (features are not shown in the drawing for simplified representation). It is required to extract the fixturing features automatically for machining the bearing mounting hole C, bearing hole B and the base plane A. The raw part model and the final part model are modeled on the Inventor platform. The features on the part models are defined with 2D sketches. The information of these 68 Chapter A Hybrid Fixturing-feature-based Approach sketches can be obtained from the part. The flatness and straightness of the cast part are assumed to have a default value of 0.2mm. Figure 4.8 Example part Step 1: Input the form tolerances if necessary For this example, it is necessary to input the flatness and straightness values of 0.2 mm for the part and the flatness of 0.1mm for Face A. The input interface is shown in Figure 4.9. The input tolerances are stored as attributes either with the part or the specific machined surfaces. Step 2: Obtaining the machining features When clicking the button of “Extract Mac/Fix Features” in the interface shown in Figure 4.10, Boolean operation will be performed on the two CAD models of the cast 69 Chapter A Hybrid Fixturing-feature-based Approach and finished part, and the excess material of the cast part which represents the volumes of material to be removed can be obtained (Figure 4.11). By reasoning these volumes using the rules described in Table 3.2 in Chapter 3, the three machining features, Plane A, Hole B and Hole C can be obtained and are listed in Figure 4.10. Each machining feature is referenced by a part feature, and is related to a sketch which defines this part feature. Using the API provided by Inventor®, the dimensional tolerances can be obtained from the dimensions defined in the sketch, and the form tolerances can be obtained from the attributes stored with the part or the specific machined surfaces. The properties and values of the machining features are assigned at this stage except fixturing features. The following steps are used to obtain the fixturing features. Figure 4.9 Form tolerance input interface Figure 4.10 Extracted features of the example part 70 Chapter A Hybrid Fixturing-feature-based Approach Figure 4.11 Obtained machining volumes Step 3: Obtaining the sketches for the machining features The related sketches of the machining features shown in Figure 4.12 are obtained through a search algorithm among the sketches held by the part based on the machining feature types. After obtaining the sketches, the dimensional constraints on the sketches can be accessed. It is noted that there is no position dimension for Plane A. Figure 4.12 Sketches for the machining features Step 4: Reasoning the possible locating faces through the dimensional constraints of the sketches 71 Chapter A Hybrid Fixturing-feature-based Approach This procedure is to analyze the dimensional constraints to obtain the candidate locating surfaces. A dimensional constraint contains a constrained entity which is referenced by a machining feature and a constraining entity which is referenced by other features. For example, the D4 dimensional constraint shown in Figure 4.12 is for machining feature B. The center point of B is the constrained entity and the line on the face Y is the constraining entity. These entities are lines or points. Table 4.2 displays the constrained and constraining entities for the dimensional constraints shown in Figure 4.12. The analysis process starts from the constraining entities which are obtained using the method provided by the dimensional constraint objects of the Inventor API. To identify the locating surfaces, the identified constraining entities, 2D points/lines, are transformed into 3D points/lines. Next the faces containing the points/lines are analyzed and the possible locating faces are determined. After this step, the possible locating features face Z and face Y can be obtained for Hole B, and face B and face Z for Hole C. In the reasoning process, if more than one face is obtained for a dimensional constraint, the tolerances of these faces need to be taken into account. For holes B and C, this rule is not applied since there is only one identified face for their dimensional constraints. Table 4.2 Constrained and constraining entities of dimensional constraints of holes B&C Dimensional Constraints Hole B D3 D4 Hole C D1 D2 Constrained Entity Center point of B Center point of B Center point of C Center point of C Constraining Entity Line on face Z Line on face Y Center point of B Line on face Z 72 Chapter A Hybrid Fixturing-feature-based Approach Step 5: Reasoning for another locating face using TAD This step is usually necessary since a sketch only provides 2D dimensional spaces, and a third dimensional space is usually embodied by the feature making process. The locating face in this dimensional space can be obtained by reasoning the machining feature’s tool access directions. The hole or plane machining feature is made either by extrusion, punching or thickening operation based on the sketch. As the sketch lies on the sketch plane, therefore, the TAD of the machining feature is parallel to the sketch plane’s normal direction. For the through holes B and C, there are two TADs which are either from Face A or Face X. By taking into account their flatness, the system selects Face A because its flatness is 0.1mm, while Face X is 0.2mm. For Plane A, the TAD is from Face A, Face X is therefore taken as the locating face. At this stage, the system completes the feature extraction process for the knuckle part and obtains the locating features with respect to the machining features. The results are shown in Table 4.3. Table 4.3 Identified locating features for extracted machining features Machining Features Hole B Hole C Plane A Reasoning Rules D3 D4 TAD, Tolerance D1 D2 TAD, Tolerance TAD Locating faces Face Z Face Y Face A Face B Face Z Face A Face X 73 Chapter A Hybrid Fixturing-feature-based Approach Step 6: Define or design fixturing features if necessary This step is necessary when there is a lack of fixturing features. The designer can either select existing features as fixturing features or design new fixturing features. When designing a new fixturing feature, the fixturing design interface will appear as Figure 4.7. Once a fixturing feature has been defined or designed, the designer needs to assign the required machining features. 4.4 Case Study The developed system has been tested with several examples. Apart from the example shown in Figure 4.8, two more case studies are selected to demonstrate the implemented approach. It is not necessary to add new fixturing features in the first case study since the extracted fixturing features are sufficient. The second case study needs the design of new fixturing features for the extracted machining features. 4.4.1. Case Study Figure 4.13 shows a machine casing tested using this system. It is required to extract the fixturing features automatically to machine the three blind screw holes (H5, H6 and H7), four through holes (H1, H2, H3 and H4) and the base plane (P1). The raw part model and the final part model input into the system are modeled on the Inventor platform. The models are designed using features that are defined using 2D sketches. The dimensional constraints on the sketches comply with the tolerances specified in 74 Chapter A Hybrid Fixturing-feature-based Approach Figure 4.8. It is again assumed that the flatness and straightness of the part have default values of 0.2mm. The interface shown in Figure 4.9 is used to input the flatness and straightness of the part and the straightness of Face G. By performing Boolean operation on the raw part model and the final part model as described in Step in Section 4.3.2, one can obtain these eight machining features which are listed in the dialog box shown in Figure 4.14. After the machining features have been obtained, the sketches and dimensional constraints for these machining features can be accessed by the Inventor API, which are shown in Figure 4.15, and there is no position dimension for Plane A. Figure 4.13 Part drawing of the machine casing 75 Chapter A Hybrid Fixturing-feature-based Approach Figure 4.14 Extracted features of the machine casing By analyzing the position dimensional constraints, the constrained and constraining entities can be obtained. This is shown in Table 4.4. (a) Sketches and dimensional constraints constraints for Plane1and Holes 1-4 (b) Sketches and dimensional constraints for Holes 5-7 Figure 4.15 Sketches and dimensional constraints of the machining features 76 Chapter A Hybrid Fixturing-feature-based Approach Table 4.4 Constrained and constraining entities of the dimensional constraints Dimensional Constraints Hole1 d1 d2 Hole2 d3 d4 Hole3 d5 d6 Hole4 d7 d8 Hole5 d9 d10 Hole6 d11 d10 Hole7 d12 d10 Constrained Entity Center point of H1 Center point of H1 Center point of H2 Center point of H2 Center point of H3 Center point of H3 Center point of H4 Center point of H4 Center point of H5 Center point of H5 Center point of H6 Center point of H6 Center point of H7 Center point of H7 Constraining Entity Line on face C Line on face B Center point of H1 Line on face C Center point of H2 Line on face E Center point of H3 Line on face D Line on face P1 Center point of F/G Line on face P1 Center point of F/G Line on face P1 Center point of F/G By analyzing the constraining entities, the candidate locating faces can be obtained for the machining features except Plane1 which has no dimensional constraints. It needs to be mentioned that for Holes 5~7, Face G and Face F can be obtained as the locating faces based on the dimensional constraint d10. Face G is chosen instead of Face F because the straightness of Face G is 0.1mm and is smaller than the straightness 0.2mm of Face F which follows the straightness of the part. By reasoning another locating face using TAD, Face A can be obtained for Holes 1~4 and Plane and Face H can be obtained for Holes 5~7. The finally identified locating faces for each machining feature are shown in Table 4.5. 77 Chapter A Hybrid Fixturing-feature-based Approach Table 4.5 Extracted machining features and their respective locating faces Machining Features Reasoning Rules Locating faces d1 Face C d2 Face B Hole1 TAD Face A d3 Face H1 d4 Face C Hole2 TAD Face A d5 Face H2 d6 Face E Hole3 TAD Face A d7 Face H3 d8 Face D Hole4 TAD Face A Plane1 TAD Face A d9 Face P1 d10, Tolerance Face G Hole5 TAD Face H d11 Face P1 d10, Tolerance Face G Hole6 TAD Face H d12 Face P1 d10, Tolerance Face G Hole7 TAD Face H The fixturing features for each machining feature have been extracted. The designer can define any other fixturing features to replace the extracted one if necessary. He can also design new fixturing features. 4.4.2. Case Study The part used for the second case study is a simplified pump housing. Figure 4.16 (a) shows its raw cast model and Figure 4.16 (b) shows its finished model. The two end plane facets and the small holes on them are the machining features. 78 Chapter A Hybrid Fixturing-feature-based Approach Hole1 x Hole2 x Plane1 (a) Raw cast Model Plane2 (b) Finished model Figure 4.16 Models of pump housing After feature extraction, the twelve machining features can be obtained. It can be observed that there are insufficient fixturing features to machine them. Therefore it is desirable for the designer to add some fixturing features. Figure 4.17 shows a possible solution by designing two tabs on the thickest part of the left end. Facet of tab and facet of tab are the base facets of the two tabs which can work as fixturing features stated here. In this way, the fixturability can be ensured. Facet Facet Tab Tab Figure 4.17 Modified model with two designed tabs 79 Chapter A Hybrid Fixturing-feature-based Approach 4.5 Summary This chapter presents a hybrid approach to obtain fixturing features for the integration of design and manufacturing. Feature extraction is to extract fixturing features automatically for the machining operations to be performed on a part. It takes into account the dimensional requirements and accuracy of the workpiece which has received relatively little attention in reported research. This is realized by capturing the designer’s intent and extracting fixturing features through reasoning the dimensional constraints on the sketches which define the features on the part. It analyzes the available information regarding the material and geometry of the part, the spatial and tolerance relations between part features and the machining operations intended for the part. In the case that there are insufficient fixturing features for the downstream machining operations, design-by-feature and interactive feature definition method can be adopted to add the necessary fixturing features. Through this hybrid approach, the fixturability of the designed part can be ensured at the early product design stage. The output of the present development consists of machining features with rich fixturing feature information, which would be the ideal input for the optimal set-up planning which is to be addressed in the next section in Chapter 5, where tolerance requirements and the machining environment, i.e., either 3-, 4- or 5-axis machining center, which would affect the number of set-ups and the way the part is to be fixtured, will be further considered. A robust fixture layout will be presented in Chapter to recommend suitable locating and supporting points. 80 [...]... constraints, the constrained and constraining entities can be obtained This is shown in Table 4. 4 (a) Sketches and dimensional constraints constraints for Plane 1and Holes 1 -4 (b) Sketches and dimensional constraints for Holes 5-7 Figure 4. 15 Sketches and dimensional constraints of the machining features 76 Chapter 4 A Hybrid Fixturing-feature-based Approach Table 4. 4 Constrained and constraining entities... H6 and H7), four through holes (H1, H2, H3 and H4) and the base plane (P1) The raw part model and the final part model input into the system are modeled on the Inventor platform The models are designed using features that are defined using 2D sketches The dimensional constraints on the sketches comply with the tolerances specified in 74 Chapter 4 A Hybrid Fixturing-feature-based Approach Figure 4. 8... locating features face Z and face Y can be obtained for Hole B, and face B and face Z for Hole C In the reasoning process, if more than one face is obtained for a dimensional constraint, the tolerances of these faces need to be taken into account For holes B and C, this rule is not applied since there is only one identified face for their dimensional constraints Table 4. 2 Constrained and constraining entities... He can also design new fixturing features 4. 4.2 Case Study 2 The part used for the second case study is a simplified pump housing Figure 4. 16 (a) shows its raw cast model and Figure 4. 16 (b) shows its finished model The two end plane facets and the small holes on them are the machining features 78 Chapter 4 A Hybrid Fixturing-feature-based Approach Hole1 x 4 Hole2 x 6 Plane1 (a) Raw cast Model Plane2... follows the straightness of the part By reasoning another locating face using TAD, Face A can be obtained for Holes 1 ~4 and Plane 1 and Face H can be obtained for Holes 5~7 The finally identified locating faces for each machining feature are shown in Table 4. 5 77 Chapter 4 A Hybrid Fixturing-feature-based Approach Table 4. 5 Extracted machining features and their respective locating faces Machining Features... the cast 69 Chapter 4 A Hybrid Fixturing-feature-based Approach and finished part, and the excess material of the cast part which represents the volumes of material to be removed can be obtained (Figure 4. 11) By reasoning these volumes using the rules described in Table 3.2 in Chapter 3, the three machining features, Plane A, Hole B and Hole C can be obtained and are listed in Figure 4. 10 Each machining... optimal set-up planning methodology which will be described in Chapter 5 The properties and relationships of machining feature and fixturing feature have been defined in Table 3.1 Methods are implemented to store and retrieve these properties 4. 3.2 Hybrid Fixturing-feature-based Approach Six steps are involved in the extraction of the machining and fixturing features An example shown in Figure 4. 8 is used... flatness and straightness of the part have default values of 0.2mm The interface shown in Figure 4. 9 is used to input the flatness and straightness of the part and the straightness of Face G By performing Boolean operation on the raw part model and the final part model as described in Step 2 in Section 4. 3.2, one can obtain these eight machining features which are listed in the dialog box shown in Figure 4. 14. .. 4. 14 After the machining features have been obtained, the sketches and dimensional constraints for these machining features can be accessed by the Inventor API, which are shown in Figure 4. 15, and there is no position dimension for Plane A Figure 4. 13 Part drawing of the machine casing 75 Chapter 4 A Hybrid Fixturing-feature-based Approach Figure 4. 14 Extracted features of the machine casing By analyzing... optimal set-up planning which is to be addressed in the next section in Chapter 5, where tolerance requirements and the machining environment, i.e., either 3-, 4- or 5-axis machining center, which would affect the number of set-ups and the way the part is to be fixtured, will be further considered A robust fixture layout will be presented in Chapter 6 to recommend suitable locating and supporting points . combination with design- by-fixturing-feature and interactive feature definition. Existing systems commonly consider process planning and fixture planning as separate activities and fixture planning. constraints, Face 1 and Face 2 can be considered as locating faces in Figure 4. 5a, side face and cylinder face in Figure 4. 5b, and side face, cylinder face 1 and cylinder face 2 in Figure 4. 5d. If a. information can be used by the downstream processes such as set-up planning and machine resource planning. Thus, machining features have sufficient information for the integration of design and manufacturing.