An integrated process planning and robust fixture design system 3

In this research, integrative features, which include machining features, fixturing features, set-up features and machine resource features, are defined to integrate the process planning

System Structure

3.1 Integrative Feature

Feature-based technology has been found to be feasible for the integration of CAD/CAM/CAPP segments due to its ability to capture the designer’s intent from one stage to the other of the product development cycle (Shah, 1990)

One approach is design-by-feature, which is to design a part using manufacturing features directly This approach has some drawbacks because: 1) designers need to work in terms of functional features, which are linked to product performance requirements, 2) manufacturing features are closely tied to specific processes and therefore restrict one’s ability to select the best manufacturing methods Another approach is feature recognition and extraction Feature extraction is considered as a key element in linking design and process planning It is used to recognize feature information (shape/form) and retrieve technological information (dimensions, tolerances, etc.) from a product’s solid model

The design-by-feature approach is used in most of the current commercial software tools in product development to create component models Examples are Inventor®,

UG®, Pro/ENGINEER®, SolidWorks®, SolidEdge®, etc Their features are usually at a

high-level of abstraction and the commands may be very generic such as punch, extrude, cut, thicken, etc., but they are not suitable for downstream applications, and feature extraction is yet to be established in commercially available CAD tools Researchers mostly agree that an ideal feature-based system should provide an environment for the design-by-feature approach, in combination with feature extraction and interactive feature definition

In this research, integrative features, which include machining features, fixturing features, set-up features and machine resource features, are defined to integrate the process planning and robust fixturing design processes Integrative features, in the manufacturing view, represent shapes and technological properties associated with manufacturing (here refer to machining) operations and tools The integrative features are obtained through a hybrid approach: feature extraction is used to extract machining/fixturing features directly from the 3D product which is modelled using a commercial CAD software, and the design-by-fixturing-feature approach is used to design fixturing features on the product model if necessary Machine resource features represent the machining environment, and the data can be obtained from libraries

Set-up features represent the possible orientations of a workpiece during the machining processes The detailed definitions of each type of integrative feature are presented in the following sections

3.1.1 Fixturing Feature

Proper fixturing is based on features which can be considered for location, clamping, and supporting These features, defined as fixturing features, must be present in a product If not, then the product cannot be fixtured and hence cannot be machined

successfully It would mean that the product will need to be redesigned with certain fixturing features to be included, which may or may not be part of the original shape of the product

In simple cases, the fixture layout will use datum which is a theoretical feature that is assumed to be exact (Robert 2003) A datum is an idealized plane, point, or a set of points, lines, axes, or other sources of information on a part They are used for geometric dimensioning and frequently imply information on the locations of fixturing surfaces However, in more complex cases, functional features, such as tooling holes, bosses, or tabs, may need to be added to the component design, providing features of sufficient size and geometric form to allow proper part fixturing The fixture layout then takes on additional geometric characteristics in order to provide the locating and clamping surfaces necessary for workholding It is therefore necessary either to enhance the existing features in a product to make them suitable for fixturing, or to design special fixturing features which can be removed later on A typical example for the latter is the die casting method called FIXTUREBLOCKTM (Street, 1986), which is used to ease the machining of hard-to-clamp parts such as turbine blades This practice

is more costly and is usually avoided

To address these issues which are more likely to happen to the cast and forged parts due to their irregular shapes, fixturing feature extraction, interactive fixturing feature definition and design-by-fixturing-feature approaches are proposed to obtain fixturing features The former is to extract fixturing features, especially for locating fixturing features, from the 3D model of a concept product design directly The latter two let the

users define or design fixturing features in case the extracted fixturing features are not sufficient

It can be seen from Figure 1.1 in Chapter 1 that the fixturing features on a cast or forged part are usually holes and tabs Therefore in this system, these two categories of fixturing features are considered Some possible types of holes and tabs for location and clamping on a casting are shown in Figure 3.1 For castings, there are many categories: (1) Sand casting; (2) Investment casting and (3) Die casting Each will have slightly different characteristics Die castings and investment castings are usually smaller with thin sections, while sand castings are usually large with thick sections To ensure the fixturing features can be blended to the existing dimensions of these castings, they are to be parametrically defined, and their sizes can be varied according

to the different castings Their properties are listed in Table 3.1

Figure 3.1 Examples of fixturing features for location and clamping

Fixturing Feature

Fixturing Feature

Fixturing Feature

Fixturing Feature

Fixturing Feature

The fixturing method may vary based on the distribution and types of fixturing features For instance, if there are small hole fixturing features on a precision die casting, hole-locating method may be considered If the fixturing features are plane surfaces, 3-2-1-locating method is selected If the base surface is large, 4-2-1-3-2-1-locating type can be selected If a cylinder fixturing feature is obtained, v-block locating is usually used

Table 3.1 Properties of integrative features

Machining

Feature

Small Hole

Id, Diameter, Position, TAD, Diameter_tol, Depth_tol, Fixturing Features, Operation, Set-up, Machine Resource Large

Hole

Id, Diameter, Position, TAD, Diameter_tol, Depth_tol, Fixturing Features, Operation, Set-up, Machine Resource Plane Id, Length, Width, Depth, Position, TAD, Length_tol,

Width_tol, Depth_tol, Fixturing Features, Operation,

Set-up, Machine Resource

Fixturing

Feature

Hole Id, Diameter, Position, Dir, Diameter_tol, Depth_tol,

Set-up, Machining Feature, Fixturing Method Cylinder Id, Diameter, Position, Dir, Diameter_tol, Depth_tol,

Set-up, Machining Feature, Fixturing Method Plane Id, Area, Position, Dir, Length_tol, Width_tol, Depth_tol,

Machining Feature, Fixturing Method Set-up Feature Id, Dir, Machining Features, Fixturing Features

Machine Resource

Feature

Id, Axis (3-/4-/5-axis), orientation(horizontal or vertical), set-up, schedule, tooling, operation cost

Each fixturing feature will have a link to the machining features and set-up features,

and have a unique fixturing function, i.e., location, clamping or supporting An

example to show the links among these integrative features is illustrated in Section 3.1.5

The application of fixturing features may vary according to different machining environment, e.g 3-, 4- or 5-axes machining centre, vertical or horizontal, etc Some optimizing strategies will be required in applying these fixturing features, e.g whether

it is meant for minimum number of set-ups and minimum tool changes, which are realized using a cost model, while satisfying tolerance requirements with respect to some datum definition during machining, etc

The extracted/designed fixturing features will be stored in the information structure which can be accessed by each segment of the product development cycle

3.1.2 Machining Feature

A machining feature can be considered as the portion of a part having some functional significance and can be created by common machining operations, such as drilling, boring, reaming, milling, shaping, planning, broaching, etc In this research, castings are considered as raw workpieces, and machining features usually represent holes or planes in a casting Therefore, hole and plane features, which commonly exist on a cast part, are considered, and they are shown in Figure 3.2 Their properties are listed

in Table 3.1 There are two types of hole features One is a smaller hole feature which

is generated purely by machining, and it is the “Small Hole” shown in Table 3.2 The other is the larger hole generated by casting and requires finish machining This type of hole is usually quite large, and it is the “Large Hole” shown in Table 3.2 The TAD of

a feature is determined by searching whether there are any intersection entities in the candidate direction with a ray which has a radius similar to the cutter If the result is negative, the candidate direction can be considered as a TAD Otherwise, this candidate direction should be discarded For a hole feature, the candidate directions are

the two directions of the hole axis For a plane feature, the candidate direction is the direction of the face normal

An extracted machining feature will be assigned a machining operation automatically based on its type and tolerances For example, rough or finish machining is assigned to

a plane feature, drilling assigned to a small home, and reaming assigned to a larger hole Each machining feature will finally be produced using a feasible machining operation within a set-up with an appropriate fixturing method and on a suitable machine Except for the properties which describe its shape, tolerance and material, it must have the link to the fixturing feature, set-up feature, operation and machine resource feature

The machining features are obtained using feature extraction The heuristics used for

reasoning the hole and plane features are shown in Table 3.2 The inputs to the feature

Figure 3.2 Hole and plane machining features

extractor are the 3D cast part model and the 3D machined part model Both of them are designed using part features available in Inventor® Part feature in Inventor® has the geometrical and dimensional information Boolean operation will be performed on the two models, and the excess material of the cast part which represents the volumes of material to be removed can be obtained The excess material has only geometrical information By reasoning the geometrical information on the excess material and the part features in the machined part model, the excess material can be linked to a part feature in the machined part model In this way, the machining feature can be obtained

by analyzing the linked part feature The extracted machining features will be stored

in the information structure which can be accessed by each segment of the product development cycle

Table 3.2 Reasoning heuristics for machining features

3.1.3 Set-up Feature

The features of a workpiece are machined in a certain sequence, as given in its process plan These features (planes, slots, holes, etc) are usually located on the different sides

Small Hole Large Hole Plane

Reasoning Heuristics

1 It has three faces: one cylinder face and two plane faces;

2 The cylinder face is machined

1 It has four faces, two cylinder faces and two plane faces;

2 The outer cylinder face is machined

1 It has one machined plane face

(faces) of the workpiece and not all the features can be accessed (machined) while the workpiece is in a given orientation An orientation, or a set-up, refers to a unique location, clamping and supporting configuration Every machining operation has a fixturing configuration that is best for the operation, but may not be a practical one One would like to find subsets of such feasible fixturing configurations whereby all the operations can be performed within those configurations Therefore, it is necessary that the machining operations can be grouped into set-ups Each set-up will have a unique location/clamping/supporting scheme, and requires a separate fixture in most cases

The set-up feature is a segment of the integrative features It represents a particular orientation of a workpiece during a machining process Its properties are shown in Table 3.1 Each set-up has a link to the machining features which will be machined in

this set-up, and also has a unique fixturing configuration, i.e., basing on particular

fixturing features

Set-up planning relies on the geometry, dimension and tolerance of the individual machining features and the fixturability of fixturing features (Ong and Nee, 1994, 1996,

1997, 1998) It must also be based on the availability and the specification of the machine tools A different machine tool would have a different effect on set-up planning, and therefore the fixture plan The operations that can be performed depend

on the number of axes of a machine Many CNC machines, especially machining centres, can perform a variety of operations in one set-up position A machining centre with five axes would be able to perform more operations in one set up The fixture must then be designed to the requirements of all the specified operations

3.1.4 Machine Resource Feature

Common job shops are assumed to have 3- to 5-axis vertical and horizontal machining centres These machining centres can be distributed and located in different places Each machining centre has its own tools with their corresponding capabilities Each machine has its ID, axis (3-, 4- or 5-axis), location, orientation (vertical or horizontal), power, set-up, schedule, tooling, operation cost, etc., which are listed in Table 3.1 Each tool has its ID, geometry, material, hardness, potential operations, tolerance specification, etc

It is also assumed that there are always available tools to machine the part, i.e., each

machining operation can be processed on a certain machine As stated before, machining features considered for cast parts in this system are mainly hole and plane features, correspondingly, several possible operations to machine them are considered They are rough and finish milling of faces, drilling, boring and reaming of holes

Five axis machining is most commonly used to make highly contoured part, such as aircraft components, dies/molds, and engine components in the automotive industry Machining with 5-axis machines could help manufacturers enhance productivity with far fewer set-ups on a smaller number of machines Doug Gale, vice president and general manager, Handtmann CNC Technologies (Buffalo Grove, IL) noted "The true beauty of the five-axis machine is the quick set-ups, for low-volume work." (Patrick, 2004) As castings have irregular shapes and low production volumes, they are more likely to be machined using a 5-axis machining centre, so as to reduce the set-ups and thus improve the machining quality due to the ability to control a tighter tolerance arising from fewer set-ups