An integrated process planning and robust fixture design system 4

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 mult

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

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 do 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

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

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

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

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 2-1 principle is a way to guarantee that a workpiece is correctly located in a three-dimensional space with the 3-planar-face location plan in Figure 4.4c

3-Z

XY

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,

a) 1-face location plan

b) 2-face location plan

c) 3-face location plan

a, b, c: dimensions

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 1 b) Case 2

c) Case 3 d) Case 4 Figure 4.5 Examples of position dimensional constraints

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 sketch-based 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

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 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 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 1 and 2 can be selected as the locating faces By comparing the dimensional tolerance of these two faces, cylindrical face 1 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

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

Candidate Locating Features Fig 4.5a Fig 4.5b Fig 4.5c Fig.4.5d Dimensional Space

face 1, face 2

cylindrical face, side face

cylindrical face 1, cylindrical face 2

face

bottom face

top face, bottom face Tolerance

top face Form 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

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

Machining Features

Find the corresponding part feature for each machining feature on the final part

Analyze the dimensional information of

each machining feature

Obtain the location fixturing features and link them to the machining feature

Machining Features with Fixturing Features information

Machining feature extraction by comparing part features on the raw part and final part

Fixturing Feature Extraction

Final CAD Part

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

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