A System for Automated Fixture Planning with Modular Fixtures

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6 New and existiog analytical mls are developed and used to genenue multiple candidare hture plans and check their validity against various funrre requkements. For faster interference checldng, Simple polyhedra with a small number of faces have been USXI to model fixture components and swept volumes of cutter paths, and fast interference checking methods have been employed m avoid a computahodly intensive 3D algorithm. Screw theory has been applied to check part rotation due m clamping during the assembly. To annpute the leasr clamping force to maintain a part’s fodmoolent equilibrium, problems are ireated as a problem of minimizing clamping force. and a hear programming method is used as a mathematical tool to solve this problem. An algodrhm for fixme planning with.owerhead.clamp is devehpd to generate fume plans with a minimal number of clamps to reduce d l y time and !he b clamping force in order to minimize the problem of pan deflection. For a selected clamp type, the algorithm identifies clampable locations by discretizing the edges of the chpble part into points and then checks each discretized point’s feasibility as a candidate clamp location against hxturing consmints Using thme candidate clamp locations, the algorithm exhaustively generam all valid damping plans with the minimum number of clamps. For each clamp plan. the types and poSirions of locators to amnately locate the part rn derermined according to the 3-2-1 lacating rule, and suppolts are placed under damps when the plnt needs to be lifted from the fixture base. For fume planning with a vise, an algorithm is develaped to identify every psible area for jaw contact onthem Thea,thealgorithmgeoeratesafuaaeplanbyde~gthelocationofbothjawsoneach xcesible area. A 2D geometric reasoning scheme has been developed to identify the area fa the jaw CUII~~CL In each plan, the minimum damping force to hold the pan is eshated to reduce the chance of part dektion due to clamping force. When there are multiple futare plans wih the same number of clamps 01 !he same type of vise, a fixture plan with a smaller clamping force is typically more f a d hn one with a greats clamping force because it has less chaocc of part deflection. In each algorithm fixm plans are rad and ordered according to their magnitude of clamping force. Thus a human exp~ can select the most fawrable fvcture plan wih either clamps and/or a vise while considering the problem of part defkction and other corsrratnts. Due to the capability of geometric reasoning in tbe. algorithms, the system is applicable to any generic part shape. Also, the system is designed to accommodate various modular hxutring systems. A future data base for a modular fixming system can be quickly established and easily modified. The system requires the assistance of human expert to evaluate a futlare plan far the possibility of deformation of part due to damping force when clamping or cutting force is applied to ill supported areas of a thin part. A human expert may visually identify any unacceptable pan deflection due to clamping or cutting force by inspecting a simulated prt/fumre assembly displayed on a graphics screen. When there is pan deflection in a fixture plan with clamps, the human expen may add exW suppnns or use a subplate m prevent unacceptable part deflection. In order to automatically detect part deflection, an accumte force model is required and an expert system using a mathemarid analysis such as a finite element method (FEM) needs to be developed in the future.
1.7. Contributions Majmmnmbutions of this thesis can be summarized as follows: 1. Development of an algorithm for automated fixture planning with overhead Clamps 2. Development of an algorithm for aummated fixrure planning with a vise 7 3. Development of 2D force analysis to estimate clamping force when damp locations and the direction and magnitude of the cutting forces are. known. 4. Generation of practical fixme plans 5.Comple.te interference checking between fixture components and parr/fkture componen Wcutter paths 6. Application of screw theory m check part rotation due to the cuing location 7. Geoeration of fixture plans with a minimum number of clamps and minimum clamping force 8. Extendibility of algorithms to various modular fixtun systems 1.8. Organization of Thesis This thesis is mganized as follows. In Chaptcr 2, an ovaview of previous work on automated Fvrnne planning and related research has been givm In chapter 3, variws analytical tools used to autMnate the fixturing process are inaoduced. Mathematical tods used to check interfaem between polyhedra, m cbeck part rorarion due to clamping during assembly, and to compute. the least clamping force with the specified clamp lacatim are. described InChapter4,asystMlma~ti~yg~efticientfixrureplanswirhe~aclamp4oraviseina modular furtwing system is presented. Assumptions and inputs required for the system are a h described. In chapter 5, an algorithm to generate fixture. plans with clamp is pnsented and the reasoning and analytical tools used in the various steps are described in detail. Io Chapter 6, an algorithm to generate fixture. plans with a vise is presenred and the reasoning and procedures usedin rhevarious stepsare described in detail. In Chapter 7. the implemeotation of the system is deserikd and several examples of tixuue plans with Mmnt part geomemes and cuning operations are used to demonstrate the system’s capability. Also, complete sehlp sequences generated with the system for two sample. parts are demonstrated. A setup sequence fora test part similar m one of the sample. parts is genmtedand machined. In Chapter 8, we conclude this thesis and describe the future research that needs to be pursued to furrher automate the fixture p hnq process.
Page 1: A System for Automated Fixture Plan
Page 5: Abstract This thesir desribes a sys
Page 9: Acknowledgements First of all, I wo
Page 12 and 13: 4.7.1. Position and Orientation of
Page 15 and 16: List of Figures Figure 1-1: A modul
Page 17: Figure 6-26: A force model to estim
Page 20 and 21: describes how the pan is held on .t
Page 22 and 23: 1.3. Effect of Fixture Plans on Cos
Page 27 and 28: Chapter 2 Literature review Two maj
Page 29 and 30: 11 Inim 2-2: A fixnae plan by C~OU
Page 31 and 32: 13 Thaoreridy, grasp planning and f
Page 33 and 34: 3.1. Introduction 15 Chapter 3 Nece
Page 35 and 36: Y A A x . X mul max 17 - x P i e 3-
Page 37 and 38: 19 are projected onto the same plan
Page 39 and 40: Z X 21 Wrenches from bodom referenc
Page 41 and 42: For toque: For thrust M Kp]8d1.8A T
Page 43 and 44: 2s The equation (3.5) conservativel
Page 45 and 46: n @ friction force _.. .- normal I
Page 47 and 48: Minimize N1 -p - N
Page 49 and 50: 31 area is 1 square inch. the stres
Page 51 and 52: 4.1. Introduction 33 Chapter 4 A Sy
Page 53 and 54: 35 checking part deflection and sel
Page 55 and 56: 4.4. Assumptions 37 The future plan
Page 57 and 58: 39 A touch probe should be used to
Page 59 and 60: 4.6.2. Cutting plan 41 1 major refe
Page 61 and 62: Cutting force and direction 43 Chap
Page 63 and 64: Clamp bar 45 , .....-, . strap clam
Page 65 and 66: aata base for supports> Type No. R
Page 67 and 68: 49 thickness. If the part is suppor
Page 69 and 70: 51 While the z coordinate of the pa
Page 71 and 72: 5.1. Introduction 53 Chapter 5 Fixt
Page 73 and 74: 55 Because the goal is to generate
Page 75 and 76:
Accessible edge 57 Inaccessible edg
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clamps are oriented so that they am
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61 unacceptable part deflection due
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Enlarged f I ! 63 Y 4 Figure 5-9: E
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clamp location I 65 Pipre 5-11: par
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67 desirable contact area Y Zt X
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69 When every candidate plan genera
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71 5.9.1. Generation of Candidate L
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73 Egure 5-20 shows the actual loca
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5.10. Rating Fixture Plans 75 Typic
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n not be possible to install a clam
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L jaw geometry in the local c odite
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m = no. of contact planes for a mov
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84 The following rearming is used m
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86 F i €4 shows obstacles mapped
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88 In our reasoning. each pan face
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i obstacles Figure 6-11: Globally f
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7 obstacles I case 3: If xiax < 92
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6.7. Determining the loeation of vi
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xo Case 1 96 X common contact area
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98 Obr Option 1: adjust the jaw loc
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100 Parallel bars should be placed
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When LCfc Lbor+ 2 d Case 1: When %=
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movable jaw 1W . . . . . . There is
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X m Pigore 6-28 Invalid fume plans
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planner getg cutting plans for a se
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0.150 110 ; IMW Teat Part 2 -- boon
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112 For a clamp libmy, we have cons
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114 *** Plan with clamp type with 5
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116 II II Fipre 7-6: F m plans with
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118 77 1 Y II’ .- Figure 7-7: Fut
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120 I i Figum 7-9 Fuhue plans with
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The system has generated multiple f
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,170 Chamfered hole 124 Chamfer 1 C
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I1 i Figure 7-14: Fixture plans for
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I 0.150 - 2582 - 7.7.1. Speed of Fi
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130 Figure 7-18: F’art geometry o
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134 8.1.1. Fixfure Planning with Ov
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for holding rectangular parts due t
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138 series of fixture plans fm each
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the fixture plan viokea any fixture
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142
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144 Ohwovoride [Ohwovoriole 801 app
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[Ahmad 871 t m 851 [Goldman 561 Ref
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Menassa, R. J. and Devries, W. R. O
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A System for Automated Fixture Planning with Modular Fixtures

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