Feature-Based Reasoning in Fixture Design

Feature-Based Reasoning in Fixture Design X. Dong, W. R. DeVries (2). M. J. Wozny, Rensselaer Polytechnic institute, Troy, New YorkfUSA Received on January 16, 1991

Absmct. Features for mechanical design are generally defined 3s entities which descnbe both the form 3nd function of the design. This geomemc and process related design information faciliutes the design of :ooling and hxtures :'or manubctunng a pan. This paper investigates the use of features for fixture design. concenuating on the selection o i !ocatin: dements and the ;dentiAcation oi locating surfaces ior workpiece positioning Keywords: design. fixture. feature

1 Introduction

Fixture design is one of the tasks that adds to the lead time in the design to manufacture cycle. In recent years, one of the approaches that has been developed to link and reduce rhis lead time is the use of features. rather than rely solely on the CAD geometry to describe a product. Features encompass more :han geometry. They can be used to specify product design intent :hat can be used to aid in rhe manufacturing design task. usually referred to AS process planning. This paper will concenmle on applications of features for one pamcular aspect of process planning. namely fixture design. A feature 3 'any named entity with attributes of both form and func:ion" [ 101. In the context of product design, features are shape elements serving certain funcrions; for example, ribs are used to strengthen a component. pin joints provide a constrained movement between two pars. I n the conrex: oi machining. features a n surfaces that are producible by a known mochining process. Some examples of machining features ar:: holes that can be produced by drilling, boring or reaming. or slots or steps that are produced by miiling. These are "common sense'. definitions of features that have been used in design and manufacturing for many years. However. in the subsequent discussions. the term feature relates specifically to the data structures used to represent these "common sense" feotures i n a CADiCAhl environment. Fearures have h e n used in number of application domains such as the generation of group technology codes [ 151, generative process planning [Y]. mnnufactuwbility evaluarion for ciisring [XI. finite element analysis [ 16). and assembly planning Is]. Fixture design is a much older field. but traditionally was done by exprienced tool desi-ners. The impetus to look into more formal methods for fixture design are to gduce the lead time in moving from CAD product designs to C A M XC progams. Some of these effons are based on kinematically locking a workpiece, trcaung both the workpiece and fixture as rigid bodies [I]or by assunng that complex workpieces will be suble when they are loaded on a fixrure 122. 771. Considering the direction of loading during processing. in particulu machining. is the approach in [7. 221. Building on the work of Lee and Haynes [I91 which considered the deflection of the workpiece subject to different point clamping loads. techniques to select locatine surfaces and optimize support positions have been developed [23, 21,251. Wkle machining forces and workpiece deflections are important for achieving accuracy. determining locating surfaces bJsed on tolerance stackup or to avoid damage to the surface finish produced in previous operations is an imponant part of the fixture design system of Bwrma and Kals 12. 31. To implement these design criteria. some researchers have used the techniques from artificial intelligence and expert systenis [ 13, 21. 261. Atrempts 10 use features in the product design and process planning for manuhcture exist. In most cases. their use is aimed at programming CXC machine tools, for example [17. IS]. i n this work. tooling as well as fixturing is a given. so the machining center or FMS is resrricted in what it can produce. Actual practice is to design tooling for a product. and this paper will report on preliminary attempts to do this using features. The objectives of this paper are: 1. To outline some of the design criteria used for designing fixtures and study how fearures may be used to aid i n this task; 7. To develop a feature-based representation for a machined par? and intermediate workpiece for selecting locating and supponing surfaces; 3. To demonstrate the use of features for fixture design through an example. 2

Potential Application of Features in Fixture Design

Many of the criteria used by an experienced tool designer art based 011 establishtd principles of kinematics and mech:inics The basic purpose of 3 finrtire is to be able to repcatably locate a workpiece using surfaces of the workpiece as datums. One of the initial design problems is to identify surfaces that can be used for this purpose. Features come into play hcrr in several ways. The geometric atniburzs of a features c a n dzfine surfaces, their orientation. area. and position. This can be used to select a primary reference plane and positions for supporting points. e.g., as in [a. 221. When selecting surfaces of a workpiece for locating, the primary concerns are to arrest the degrees of freedom and minimize tolerance stackup. Features. which carry along more than geomemc specification. can enhance the different approaches that have already been developed for selecring locating surfaces. For example. once the cutting plan has been established based on a slot or pocket feature. much of the necessary machining technology miormation is available to estimate force direcnons and magnitudes. This in turn allows techniques like those in [:, 211 to be used to pick locating surfaces so that the degrees of freedom of the workpiece x e arrested. Since features carry along tolerance and suriice finish specification. this permits other design cnteria to be used. For example. the stackup of tolerdnces IS one the key synthesis rechniques [3]. Funhermore, Features that convey the final

Annals of the CIRP, Vol. 40/1/1991

inrent of a surface, for example. a tribological surface that has been heat treated and precision finished should nor be used for either clamping or supponing. This serves as 1 way to prune the possibilities that are inherent in the heunstic process of set-up planning and fixture layout. Other fixture design criteria. while mechanics based. 31e often tw complicated to calculate so they are handled by heuristic rules. This may include not using a finished surface for locating if the intended use is as beanng surface. This limits, at the very least, the sequence of fixturing set-ups. If a design feature is a rib *mthright tolerances on thickness. ihis dictates special fixturing provision to keep the rib from deflecting while machining. In selecting surfaces and fixtiinng elements for either suppon or locating. determination of overconsuaining due to redundant suppons and deflections caused by exctssive clamping is 3 check that can be used to assess a fixturing plan.

3 Representation of Part and Machining Process

For fisture design. in addition to the geometry. additional data that should be carried along with the part description are machining features, their associated accuracy. tolerance and surface finish. and intended uses. This data will determine the order that should be used to generate surfaces. 3s well as how to suppon. clamp or locare. There are several ways that machining features can be defined: they can be directly input by the user; they can be reiognized automatically from the solid geometric description of a machined pan by a computer program, as in [ l l ; ; or they can be convened from design features. We have taken the last approoch. In our approach. a pan is designed using design features in a teaturebased design svstem FREDS. developed at Rensselaer Design Research Center. Rensselaer' Polytechnic Institute [Brand, 19901. The current implementation of FREDS supports nine types of design features: LAND, THROUGHHOLE, BLIND-HOLE. THROUGH-SLOT, BLISD-SLOT. POCUT, 0PE.USTEP, BLIND-STEP. and BOSS. These features m depicted in F i g m I . I

~

----

i I

LAND

THROUGH-HOLE

BOSS

,

BLIND-HOLE

THROUGH.SLOT

BLWD-SLOT

i

F'OCKET

OPEN-STEP

RLW-STEP

L--

-

.

~

_

_

Figure 1 Design Features For fixture design, we have chosen nine types of machining features: THROUGH-HOLE, BLIND-HOLE. THROUGH-SLOT, BLIWD-SLOT, POCKET,,OPEN-STEP, BLIND-STEP, FACE, and 2D-CONTOUR. These features are illustrated tn Figure 2. Although most of these machining features are the same as their corresponding design features in Figure I. some machining features have to be deduced from design features, For example. machining feature 2D-CONTOUR can sometimes be deduced from d e s i p features BLOCK and BOSS. 3.1 Representation of Part, Stock and Intermediate Workpiece

The scheme for representing the workpiece and feature information IS shown 3. Atnibutes of the final part. the stock and each intermediate workpiece geometry are descnbed in Figures 4, 5 and 6 respecnvely. Among these atmbutes, some can be derived from the others. For example, the geomerry in Figure

111

3.2 Representation of Feature and Intermediate Feature State

2DCONTouR

FACE

BLIND-HOLE

The attributes of machining feature type THROUGH-HOLE is given in Figure 7. For each machining feature, its size, position and relation to other features need to be defined. Tolerance specifications on size and position are required. Furthermore, it is important to represent the functional intent of a feature. Each feature instance is associated with at least one feature state.

THROUGH-HOLE

THROUGH-SU)T

Attribute ID Intendeduse Featurestates Diameter I Deuth I Axis I FaceList I DiameterTolerance I DeuthTolerance SurfaceFinish PositionDatums

BLIND-SLOT

OPEN-STEP

BLIND-STEP

Figure 2 Machining Features

PositionDimensions mapping

i r k opuytion

I PositionTolerances

I

ConcenhicityDatum

I ConantricityTolerance

I PerpcndicularityDatum

a

Description

~

_

_

_

_

~

~

1

Diameter of the hole

I Deuth of the hole I Hole axis direction

I List of constituent faces I Tolerance on the diameter I Tolerance on the depth

I I I I I I

Surface finish specification on the hole Datum references used to specify the position of the hole Dimensions used to specify the position of the hole I Tolerances on the wsition of the hole I

I

Datum reference used to specify the concentricity tolerance

I Concentricity tolerance on the hole I Datum reference used to specify the perpendicularity tolerance

PerpendicularityTolerance

ilh lnlemdiou Workpiece

_

List of intermediate feature states

I I I

Perpendicularity tolerance specified on the axis of the hole

Infermediale Fealure Stare

EntranceFaces

Pan faces on which the hole is located

ApproachDirections

Cutting tool approach dircction(s) Other features on the part which are connected to the hole

AdjacentFeatures

Figure 7 Attributes of Machining Feature Type THROUGH-HOLE

Amibute ID MaterialPropcnyList

I FeatureList I Batchsize

Descriphn Identification of part List of final mechanical propemes of the material: t w , hardness. strength)

I List of final constituent machinine features

I

IBatch size of the uan to be machined

I

I Identification of the solid model for the part

Geometry

I Identification of the stock

Stock

Figure 4 Attributes of Type PART

I Athibute I ID

I FeatureList MaterialPropcnyList Geomeny

I Descriution I Identification of stock I List of initial features on stock

~

1 ~~~

I

Attribute ID FinalFeature Diameter SurfaceFinish

IDescriphn

IIdentification of an intermediate THROUGH-HOLE state ldenafication of final THROUGH-HOLE feature Intermediate hole diameter intermediate surface finish

I

List of mechanical properties: (type, hardness, strength) Identification of the solid model for the stock

AUribute ID FeatureList MaterialRopenyList

Description Identification of intermediate workpiece

Geometry

Identification of the solid model for intermediate workpiece

List of intermediate feature states List of mechanical propemes: (type, hardness, strength)

the workpiece and the fixturing tools as well as between the cutting tool and the fixturina - tools. The part, stock and each intermediate workpiece arc defined in terms of the list of machining features and other attributes. The initial workpiece can be a block or a piece of bar or roll stock, or it may be forging or casting with features already present Each intermediate workpiece is defined in terms of intermediate feature states. The geometry of each intermediate workpieces and feature state can be generated based on the process plan, and their associated faces serve as potential locating and supporting surfaces.

112

FEATURESTATE describes the intermediate state of a feature instance during the machining process. It contains those information that may be different from one feature-state to another. For example, the diameter of a hole is changed after a drilling operation; therefore, it must be described in the feature-state for the hole. Another example is surface finish. The axis of a hole, on the other hand, does not change and therefore is not included in the definition of the featun-state. The intermediate values of a hole feature are necessary for such tasks as the selection of the size of a locating pin. The amibutes of feature-state type THROUGH-HOLE is given in Figure 8.

MachineTool

Description identificanon of the process Number of setups in this machining task List of subprocesses Identification of the machine tool used

PartID

Identification of the machined pan

Aftribute ID NumberSetUps SubRocesses

i

The amibutes of these process types are described in Figures 9-12. Although we have attempted to develop a general shucture such that it can also be used by other process planning modules, the information included are mainly from the viewpoint of fixture design. For example, status of a feature and cutting force directions can be derived from this representation.

Attribure ID SubPrwsses Features

Description Identification of the process List of subprocesses List of features to be machined in this setup

SupportFaces

List of support faces used in this seNp

one tool point

two tool points

three tool poinu

two narrow plates

one large plate

Figure 10 Attributes of Process Type OPERATIONS-IN-ONE-SET-UP one narrow plare

Attribute ID SubProcesses MachincdFeature

ICuttingTool

Description Identification of the process List of subprocesses Identification of the machined fCaNre of the cutting tool used in this

~

I

I Descrivtion I Identification of the orocess

wor!qiece

I locating element

I

X

X

Tz. Rx, RY

Tz

one large circular ptare

I I I I I

one small circular plate

Y X

I Feed I DeothOfCut I Deoth of cut I CuttineForceDirection 1 Direction of cutting force in this oass

I

lZ

0

Figure 11 Attributes of Process Type OPERATION-ON-ONE-FEATURE

I Attribute

I

IZ

Y

short pin

Y X

X

lapcrd shon pin

long pin

Figure 15 Locating Elements Applicable on Planar Surfaces and Hole Surfaces

Figure 12 Attributes of Process T y ~ ePASS These processes are o r g a n i d into a hierarchical shucture, as illustrated in Figure 13, where circles denote machining processes, arcs connecting p r e cesses at different levels denote CONSIST-OF relationships among machining processes, and arcs connecting processes at the same level denote sequences among these processes. A similar hierarchical s h u c m was proposed by Brown and Gyorog for representing inspection processes [Brownand Gyorog, 19901. Operatwm-On-One-Machine

Operaion-On-One-Fcwc

x --*M---Y h

Pas

Figure 13 Hierarchy of Machining Processes

In addition, representations have also been developed for machine tool, cutting tool and fixture. 4

Using Features for Fixture Design: An Example

In this section, we will illustrate the use of features in two subtasks of fixture design, namely the selection of locating elements and locating surfaces. The sample part in Figure 14 is composed of five design features found in Figure 1: one LAND, one BLIND-SLOT, one OPEN-STEP, and two THROUGH-HOLES. All of these design features ma into InanufaCtunng features except for LAND which maps into eight FACE gatures. FACE-3

4.1 Selection of Locating Elements

Features can be used to select candidate locating elements. For example, FACE features which have large surface area and good surface finish suggest the use of tool points or plates as locating elements; THROUGH-HOLE features with good surface finish suggest the use of pin-type locating elements. Some common locating elements applicable on planar and hole surfaces are given in Figure 15 (adopted from [29]). Also shown in the figure are the degrees of freedom constrained by each locating element. For example, a short pin ("short" or "long" is defined relative to the depth of the hole) constrains the translational degrees of freedom in the two dixctions perpendicular to the hole axis (i.e., Tx and Ty);a long pin constrains not only the translational degrees of freedom along the same axes, but also the rotational degrees of fnedom about these axes (i.e., Rx, Ry); a shop taper pin constrains the translational degrees of M o m along all the axes (1.e.. Tx, Ty and Tz). Although Figure 15 gives the degrees of freedom constrained by each locating element when applied alone, it does not indicate the aggregate effects of a set of locating elements. For this purpose, the constraint reduction method developed by Turner and Shrikanth [28] is used. Developed for variational modcling of assembly for computer-aided tolerance analysis, the procedure is able to determine a single aggregate constraint having the same net effect when two parts or subassemblies me related by multiple constraints. Four types of translational and three types of rotational freedoms are classified and twelve simple rules are used to deduce the aggregate constraint. When applying this constraint reduction method, we treat the workpiece as one subassembly and the set of locating elements as another subassembly. Then, the aggregate effects of these locating elements can be deduced. For example, a candidate locating plan is to use a large plate, a short pin and a tool point, as shown in Figure. 16. According to Figure 15, the large plate constrains Tz, Rx, and Ry, the short pin constrains Tx and Ty, and the tool point constrains Ty. Applying the constraint reduction method. the short pin and the tool point together effectively eliminates the rotation about Z at any origin (although the short pin or the tool point alone docs not eliminate the rotation about Z). As the result, all degrees of freedom of the workpiece are eliminated. Underconsmining can bc easily detected using this method. For a given set of locating elements, the constraint reduction method enables us to determine the degrees of freedom that are still permitted by the locating elements. Overconstraining can also bc easily detected for a given set of locating elements. If we use DOF, to denote the degrees of freedom of the workpiece constrained by the ith locating element, the workpiece is overconstrained when

DOF, n DOFj

Figure 14 Sample Part

# 0, i # j

For example in Figure 16, since both the short pin and the tool point constrain the translational degree of freedom of the workpiece along Y axis, there is an overconstraint. Overconstraining the workpiece reduces locating accuracy and repeatability, causes the distortion of the workpiece when large clamping forces are applied, and often makes it difficult to load and unload the workpiece.

113

Figure 16 Selecuon of Lociting Elements 4.2 Selection of Locating Surfaces

I

Features can be used for selecting fixture locating points using the heunstic This work was resmcred to using tooi points and the 2-2-i rules in [3]. locating principle. Process planning analysis has indicated that the sample pm m Figure I4 can be machined in one set-up with three tools: 2 face mill to generate FACE-7. a twist dnil for the two THROUGH-HOLES,and an end mill for SLOT-I. The stock. identified as STOCK-1, i s 6061-T6 aluminum bar stock cut to the correct length with a hardness of H @ = l?Okg;mm'!. Based on Ihe geomemc attributes of STOCK-I, FACE-5 or FACE4 would be idenufied 2s potential suppornng faces. but F,9CE-6 is eliminated because :I design feature SLOT-I is defined From it. As a resuit. FACE-5 is the supporting face for the three tool points. Techniques in 1231 can be used to optimize final positions. with the two THROUGH-HOLES consrraining the solution. Forces used to select locating faces can be estimated from machining features. For example. the force vector for making SLOT-l can be estimated From the feeds and speeds determined in process planning and the matenal attributes of STOCK-I. The same can be done for the face milling cut. Because the force vectors for both cuts intersect FACE-4. the heunstics would pick it as a locating face. The teniary datum would use the heuristic in [24) based on the frequency of the force components intersecting a face. Figure 16 shows a top view of how the tool points would be positioned. The THROUGH-HOLES would have no direct effect on !a-anng. With these heuristic rules, machining features provide a data structure that can easily be used to estimate the forces needed to apply these rules, speeding up this part of fixture design. 5

Conclusions

In this paper. the uses of features are investigated in the domain of fixture design. A representation scheme has been developed to describe a m:ichined part.

I

4

1

vectors SLOT-1 FACE-6

I

Figure 17 Locating Points Synthesis intermediate workpiece geometry and material properties. machined features. and their intermediate states. The information represented about the intermediate workpiece and features enables a fixture design program to determine the surfaces available for locating and supponing and will facilitate the detection of interference between the workpiece, the cutting tool and the fixture. The representation of machining processes descnbes the operations between intermediare workpiece states and provides the process information that allows the generation of such information as cuttine force directions. A sample part has been used to demonstrate the uses of fe;&res in tw'o fixture design tasks. It has been shown that feature information are very useful for the selection of locating elements and surfaces. Future work for this research inChdeS the encoding of fixture design knowledge and the development of an inference system for synthesizing a tixture plan. One imponant requirement for this system is the ability to deal with conflicting factors and make design nade-offs.

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