Introduction to Simulation for Variation Reduction. Product Excellence Using Six Sigma Module. Contents

Transcription

1 Variation Reduction Product Excellence Using Six Sigma Module Page 1 Contents Introduction Tolerance Stacking Tolerance Simulation Overview Break Datums & Locators Geometric Dimensioning &Tolerancing Measurement Planning Quality Maturation Review & Close Page 2 1

2 What is Simulation? Simulation is using mathematical tools to imitate real-life conditions and predict the likely results Why Simulate? To confirm theories To test assumptions To reduce number of physical tests required To reduce number of changes to production tools To reduce development time Page 3 Why Simulate Variation? Variation is a normal part of any manufacturing process Need to quantify if quality targets can be met Need to design manufacturing processes as well as nominal geometry Need to quantify likely product conditions to ensure that performance is maintained Page 4 2

3 Simulation Types Product Process Effects Page 5 Tolerance Stacking Page 6 3

4 Tolerance Definitions Tolerance is the limit of allowable variation Tolerance Zones are defined by the annotation ± Limit Tolerance Plus/Minus Equal Bilateral Plus/Minus Unilateral Plus/Minus Unequal Bilateral Tolerancing can be very confusing Ideally work to International Standards Page 7 Tolerance Zones All tolerancing is based upon theoretical zones of acceptability Within the zone any component form is acceptable, unless a modifier has been specified Tolerance Zones describe a theoretical feature parallel to the toleranced feature which defines the limit of the tolerance Tolerance Zones can be lines, planes, cylinders etc. Virtual Boundary Page 8 4

5 Influence of Tolerance Tolerance defines the limits of allowable variation Design must be capable of accepting the variation Product must function at limits of expected variation Test to limits, not Nominal design condition Page 9 Tolerance Stacking Better described as Dimensional Variation Analysis (DVA) Prediction of potential scale of variation Different Methods Manual Computer based 1D 3D Page 10 5

6 Timing Concept Approval Project Approval Job 1 Concept Design Prototype Production Product Requirements Defined Design to Meet Product Requirements Documentation Measurement Plan Achieve Capability Continuous Feedback Ongoing Control Page 11 Variation Analysis Methods Straight Stack Root Sum Square Statistical Tolerancing Simulation Page 12 6

7 Straight Stack Worst Case Variation = A + B + C Page 13 Root Sum Square Gives an approximation of SPC (Assumes C p = C pk = 1.0) Variation = (A² + B² + C² ) Page 14 7

8 Comparison of Results Component A B C D E F Tolerance ±1.00 ± 0.25 ± 1.50 ± 0.50 ±0.75 ± 1.00 Results Straight Stack RSS ± 5.00 ± 2.26 Page 15 Statistical Tolerancing Specifies Control Limits for the process Assumes a normal distribution, centred on nominal Still analyse by stacking tolerance ranges Not widely used or understood Page 16 8

9 Limitations Manual tolerancing works best in 1D Usually over simplifies analysis Variation only considered in perpendicular planes Difficult to calculate 2D & 3D variation Misses sources of variation Ignores effect of assembly sequence Page 17 Variation Simulation Page 18 9

10 Variability Simulation Tools Wide variety of software tools available Wide variety of sophistication and cost There is no ideal simulation software Use appropriate tool to situation Simple 1 D Increasing Sophistication Basic General Specialised 3 D Page 19 Variability Simulation Tools - Simple Linear Tolerance Stack (Worst Case or RSS) e.g. MS Excel, fag packet Increasing Sophistication Page 20 10

11 Variability Simulation Tools - Basic Linear Tolerance Stack (Worst Case or RSS) e.g. MS Excel Linear Tolerance Stack with statistical modelling (Monte Carlo) e.g. 1DCS, Crystal Ball Increasing Sophistication Page 21 Variability Simulation Tools 3D Linear Tolerance Stack (Worst Case or RSS) e.g. MS Excel Linear Tolerance Stack with statistical modelling (Monte Carlo) e.g. 1DCS, Crystal Ball 3D simulation of assembly variation (Monte Carlo) Increasing Sophistication e.g. VisVSA, 3DCS Page 22 11

12 Variability Simulation Tools - Aesthetic Linear Tolerance Stack (Worst Case or RSS) e.g. MS Excel Linear Tolerance Stack with statistical modelling (Monte Carlo) e.g. 1DCS, Crystal Ball 3D simulation of assembly variation (Monte Carlo) Increasing Sophistication e.g. VisVSA, 3DCS Visualise effects of assembly variation e.g. Aesthetica Page 23 Variability Simulation Tools FE Based Linear Tolerance Stack (Worst Case or RSS) e.g. MS Excel Linear Tolerance Stack with statistical modelling (Monte Carlo) e.g. 1DCS, Crystal Ball 3D simulation of assembly variation (Monte Carlo) Increasing Sophistication e.g. VisVSA, 3DCS Visualise effects of assembly variation e.g. Aesthetica FE element modelling of component distortion during assembly e.g. TAA Page 24 12

13 How Simulation Software Works Creates bin of parts for each tolerance specified Uses Monte Carlo Random Number generation Creates a normal distribution for parts Mean centred upon nominal and ±3ó coincident with specification limits Page 25 How Simulation Software Works Randomly takes one part from each bin and performs straight stack analysis LSL Nominal USL LSL Nominal USL LSL Nominal USL LSL Nominal USL Source A Source B Source C Source D LSL Nominal Nominal USL LSL USL + = Source E Source F Stores result for each run and repeats Result Page 26 13

14 How Simulation Software Works Sums up all runs to give overall result Mean Range Distribution C p & C pk Numbers and percentage out of tolerance Can also evaluate influence of features Re-runs analysis with only one variable Compares predicted variability with global prediction Page 27 Simulation Examples Assembly Sequence Define Measurements Run Simulations Review Results Page 28 14

15 Assembly Sequence VisVSA Page 29 Define Measurements VisVSA Page 30 15

16 Run Simulations VisVSA Page 31 Review Results VisVSA Page 32 16

17 Aesthetic Results Review Tapers and Misalignments Icona aesthetica Page 33 Aesthetic Results Review Flushness and See-through Icona aesthetica Page 34 17

18 Simulation Results Page 35 Simulation Requirements Quality Targets CAD Data Datums & Locators Part Tolerances Material Specification Assembly Sequence Joining Positions and Sequence Fixture Designs and Tolerances Measurement Points Page 36 18

19 Simulation Limitations Garbage in, Garbage Out! Usually ignore flexibility, stresses, clamping forces, thermal effects etc. Firstly assume all parts are rigid Point representations of surfaces Simulations assume part variation C p = C pk = 1.0 Variation is actually a combination of offset mean and range Requires Skilled Analysts and TIME! Page 37 Break Page 38 19

20 Datums & Locators Page 39 Datums All measurements must have a start point Choice of datums can be hugely influential Physical datums are also how parts are held during assembly Datums need to be clearly identified Datums can be global or local Page 40 20

21 Global and Local Datums Global Datums Refer all features to a Master Set of References Determine absolute position or variation Features considered in isolation Local Datums Refer limited number of features to a singe reference Determine relative position or variation Used to control a group of features Page 41 Six Degrees of Freedom A Datum scheme must constrain all 6 Degrees of Freedom unless the part is required to move Movement is made up of Tx Translation in X Plane Ty Translation in Y Plane Tz Translation in Z Plane Rx Rotation about X Axis Ry Rotation about Y Axis Rz Rotation about Z Axis Lateral Translation along Y Axis Rotation about X Axis (Roll) Rotation about Z Axis (Yaw) Lateral Translation along Z Axis Lateral Translation along X Axis Rotation about Y Axis (Pitch) Page 42 21

22 Datum Definition Datums are theoretically exact features or surfaces Create framework of three mutually perpendicular planes Primary Datum constrains three degrees of freedom Secondary Datum constrains two degrees of freedom Tertiary Datum constrains the final degree of freedom Known as Generally labelled ABC Page Datum Locator Scheme Secondary Datum Ideally 2 Points of Contact Stops Y Translation & Yaw Rotation Tertiary Datum Ideally 1 Point of Contact Stops X Translation Z X Y Primary Datum Ideally 3 Points of Contact Stops Z Translation, Roll & Pitch Rotation Page 44 22

23 Hole & Slot Datums Alternative to surface datums Use pins through holes to locate parts Tertiary datum slots more effective than diamond pins Variation Page 45 Datum Definition A 25 x 25 A1 This Symbol is used to identify the continuous Datum Plane This Symbol is used to identify the size and Position of the Datum Targets that make up the Datum Plane 23

24 Datum Documentation Datums should ideally be identified on the CAD Model Datums should be used irrespective of process undertaken Datum definition is an instruction to toolmakers, fixture designers, etc. Care should be taken not to over-constrain parts Page 47 Datum Selection Datums need to be logical Datums should apply to multiple processes Component manufacturing Inspection Assembly Need to be on most stable part of product Page 48 24

25 Coincident & Transferred Datums Datums should be reused as much as possible between processes Not reusing datums introduces another source of variation Pick Master Datums for an assembly and carry through Page 49 Example of a 321 Datum Scheme Main Floor Panel 25

26 Datum Transfer Datums retained from Panel Datums transferred from symmetrically opposite Panel Page 51 Basic GD&T Interpretation & Specification Page 52 26

27 GD&T Standards GD&T Standards used to ensure Common Language Common Interpretation No Ambiguity ISO BS8888 consolidation kit ANSI Y14.5M 1994 Page 53 Features of Size Full extent of element identified Tolerance applies to whole of feature Tolerance Zones projected from feature Default action unless a modifier is specified Page 54 27

28 Feature Control Frame Box divided into compartments used to define: Geometric Characteristic Tolerance Value Modifiers Datum References Order of Datums is important A 0.2 M First Compartment always contains the geometric characteristic symbol Second Compartment always contains the tolerance value & any modifiers Third, Fourth & Fifth Compartment always contain the datum information A B C Page 55 Tolerance Zones Positional Tolerance specified as ±1.5mm Nominal Additional variation allowed, without changing specification, if design allows extremes of variation in both directions. (57% Greater Tolerance Zone) Variation lost if design cannot accept extremes of tolerance in both directions. Page 56 28

29 GD&T Characteristic Symbols CATEGORY CHARACTERISTIC SYMBOL DATUM REF. USE Straightness Form Profile Flatness Circularity (Roundness) Cylindricity Profile of a Line Profile of a surface Never Sometimes Individual Features Angularity Orientation Location Perpendicularity Parallelism Position Concentricity Always Related Features Symmetry Runout Circular Runout Total Runout Page 57 Modifiers Term Symbol Usage Maximum Material Condition (MMC) M e.g. Largest Shaft or Smallest Hole Least Material Condition (LMC) Projected Tolerance Zone Tangent Plane L P T e.g. Smallest Shaft or Largest Hole Tolerance Zone Projected above part surface Only Tangent Plane needs to be in Tolerance Zone Diameter Modifies shape of Tolerance Zone Radius R Creates a Tolerance Zone defined by 2 arcs Controlled Radius CR As above, with no flats or reversals in Zone Reference ( ) Denotes for Information Only Page 58 29

30 Measurement Planning Page 59 Measurement of Variation Need to quantify variation Not practical to measure entire feature Assume measured variation is consistent across whole feature Need to specify how variation is to be quantified Page 60 30

31 Measurement Points Measure Feature at Point of Control X,Y,Z of point specified I,J,K of measurement direction specified Structure of points gives opportunity to reduce measurements as maturation progresses Correlation across product Points defined on CAD Model Part & Process Capability maintained Page 61 Measurement Direction Requirement Part Surface Probe Direction Same result on Nominal Geometry gives different result on physical part CAD Surface Target Point Page 62 31

32 Measurement Point Structure Component Product Level 1 Process Monitoring Craftsmanship Level 2 Process Capability Major Assemblies Level 3 Process Proving Sub-Assemblies Level 4 Process Development Part Level Frequency Volume Page 63 Measurement Planning Example Process Development 4 4 Measurement point / Edge point Edge point Hole / Slot / Nut hole Locating pins Locating datum Page 64 32

33 Measurement Planning Example Process Proving Measurement point / Edge point Edge point Hole / Slot / Nut hole Locating pins Locating datum Page 65 Measurement Planning Example Process Capability Measurement point / Edge point Edge point Hole / Slot / Nut hole Locating pins Locating datum Page 66 33

34 Measurement Planning Example Process Monitoring Measurement point / Edge point Edge point Hole / Slot / Nut hole Locating pins Locating datum Page 67 Link to Simulation Target Points in Simulation Become Measurement Points Utilise Level 1 and Level 2 Points in Simulation Must use common labelling system Corporate system to give each point an unique ID Page 68 34

35 Documentation & Reporting Must use measurement results to drive; Product Quality improvement Simulation Improvements Management must understand SPC Use IT networks to eliminate decoration of CMM rooms Page 69 Quality Maturation Page 70 35

36 DVA Calibration DVA studies are not 100% accurate Combine Quality Maturation activities with DVA Study Calibration Look at General Results and focus on Areas of excessive variation Areas of significant discrepancies between simulation and measurement results Use Slow Build technique for next build phase Page 71 Slow Build Know what goes into the process Understand what happens within the process Know condition of tooling Take intermediate stage measurements Re-run DVA simulation with part offset means and variation range Useful problem investigation technique Time consuming & takes preparation Page 72 36

37 Review & Close Page 73 References Achieving Dimensional Quality by Applying the Technology of Dimensional Management, by Curt Larson, Right Tech Inc Fundamentals of Geometric Dimensioning & Tolerancing, 2 nd Edition Metric, by Alex Krulikowski, Effective Training Inc Page 74 37