Introduction to Simulation for Variation Reduction · PDF fileIntroduction to Simulation for Variation Reduction 1 ... Introduction to Simulation for Variation Reduction 2 ... •Sums

Product Excellence Using Six Sigma 1/24/2009

Introduction to Simulation for Variation Reduction 1

PEUSS 2008/2009©Warwick Manufacturing Group

Introduction to Simulation for Variation Reduction

Page 1


Product Excellence Using Six Sigma Module



Page 2

Contents

Introduction

Tolerance Stacking

Tolerance Simulation Overview

Review & Close

Quality Maturation

Measurement Planning

Datums & LocatorsGeometric Dimensioning &Tolerancing

Break





Page 3

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 4

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 5

Simulation Types

• Product• Process• Effects



Page 6

Tolerance Stacking





Page 7

Tolerance Definitions

• Tolerance is the ‘limit of allowable variation’• Tolerance Zones are defined by the annotation

12.512.0

Limit Tolerance

12.25 ±0.25

Plus/MinusEqual Bilateral

12.00+ 0.5

0

Plus/MinusUnilateral

- 0.2+ 0.3

12.20

Plus/MinusUnequal Bilateral

• Tolerancing can be very confusing

• Ideally work to International Standards



Page 8

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 9

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 10

Tolerance Stacking

• Better described as Dimensional Variation Analysis (DVA)

• Prediction of potential scale of variation• Different Methods– Manual– Computer based– 1D– 3D





Page 11

Timing

Concept Design Prototype Production

ConceptApproval

ProjectApproval

Job 1

Product Requirements Defined

Design to Meet Product Requirements

Documentation

Measurement Plan

Achieve Capability

Ongoing ControlContinuous Feedback



Page 12

Variation Analysis Methods

• Straight Stack• Root Sum Square• Statistical Tolerancing• Simulation





Page 13

Straight Stack

• Worst Case

Variation = A + B + C …



Page 14

Root Sum Square

• Gives an approximation of SPC(Assumes Cp = Cpk = 1.0)

Variation = √ (A² + B² + C² …)





Page 15

Comparison of Results

Results• Straight Stack• RSS

± 1.00F

±0.75E

± 0.50D

± 1.50C

± 0.25B

±1.00A

ToleranceComponent

± 5.00± 2.26



Page 16

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 17

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 18

Variation Simulation





Page 19

• Wide variety of software tools available• Wide variety of sophistication and cost• There is no ideal simulation software• Use appropriate tool to situation

Variability Simulation Tools

Increasing Sophistication

3 D

1 DSimple

Basic

General

Specialised



Page 20

Variability Simulation Tools - Simple


Linear Tolerance Stack (Worst Case or RSS)

e.g. MS Excel, ‘fag packet’





Page 21

Variability Simulation Tools - Basic



e.g. MS Excel Linear Tolerance Stack with statistical modelling (Monte Carlo)

e.g. 1DCS, Crystal Ball



Page 22

Variability Simulation Tools – 3D




e.g. 1DCS, Crystal Ball3D simulation of assembly variation (Monte Carlo)

e.g. VisVSA, 3DCS





Page 23

Variability Simulation Tools - Aesthetic





e.g. VisVSA, 3DCSVisualise effects of assembly variation

e.g. Aesthetica



Page 24

Variability Simulation Tools – FE Based





e.g. VisVSA, 3DCSVisualise effects of assembly variation

e.g. Aesthetica

FE element modelling of component distortion during assembly

e.g. TAA





Page 25

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 26


• Randomly takes one part from each ‘bin’ and performs straight stack analysis

+ ++ +

+ =

Source E

Result

Source A

NominalLSL USL

Source B

NominalLSL USL

Source C

NominalLSL USL

Source D

NominalLSL USL

NominalLSL USL

Source F

NominalLSL USL

• Stores result for each run and repeats





Page 27


• Sums up all runs to give overall result– Mean– Range– Distribution– Cp & Cpk

– 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 28

Simulation Examples

Assembly Sequence

Define Measurements

Run Simulations

Review Results





Page 29

Assembly Sequence

VisVSA



Page 30

Define Measurements

VisVSA





Page 31

Run Simulations

VisVSA



Page 32

Review Results

VisVSA





Page 33

Tapers and Misalignments

Aesthetic Results Review

Icona aesthetica



Page 34

Aesthetic Results Review

Flushness and See-through

Icona aesthetica





Page 35

Simulation Results



Page 36

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 37

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 Cp = Cpk = 1.0– Variation is actually a combination of offset mean and

range

• Requires Skilled Analysts and TIME!



Page 38

Break





Page 39

Datums & Locators



Page 40

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 41

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 42

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 ofTx – Translation in X Plane

Lateral Translationalong X Axis

Lateral Translationalong Y AxisTy – Translation in Y Plane

Lateral Translation along Z Axis

Tz – Translation in Z Plane

Rotation aboutX Axis (Roll)

Rx – Rotation about X Axis

Rotation about Y Axis (Pitch)

Ry – Rotation about Y Axis

Rotation about Z Axis (Yaw)

Rz – Rotation about Z Axis





Page 43

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 ‘3-2-1’• Generally labelled ‘ABC…’



Page 44

3-2-1 Datum Locator Scheme

Primary DatumIdeally 3 Points of ContactStops Z Translation, Roll & Pitch Rotation

Secondary DatumIdeally 2 Points of ContactStops Y Translation &Yaw Rotation

Tertiary DatumIdeally 1 Point of ContactStops X Translation

Z

Y

X





Page 45

Hole & Slot Datums

• Alternative to surface datums• Use pins through holes to locate parts• Tertiary datum slots more effective than

diamond pins

Variation



Datum Definition

A A125 x 25

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





Page 47

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 48

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 49

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



Example of a ‘321’ Datum Scheme

Main Floor Panel





Page 51

Datum Transfer

Datums retained from PanelDatums retained from Panel Datums transferred from Datums transferred from

symmetrically opposite symmetrically opposite

PanelPanel



Page 52

Basic GD&TInterpretation & Specification





Page 53

GD&T Standards

• GD&T Standards used to ensure– Common Language– Common Interpretation– No Ambiguity

• ISO – BS8888 consolidation ‘kit’• ANSI – Y14.5M 1994



Page 54

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 55

Feature Control Frame

• Box divided into compartments used to define:– Geometric

Characteristic– Tolerance Value– Modifiers– Datum References

• Order of Datums is important

First Compartment always contains the geometric characteristic symbol

0.2

0.2

0.2 M

Second Compartment always contains the tolerance value & any modifiers

A

A B C

Third, Fourth & Fifth Compartment always contain the datum information



Page 56

Tolerance Zones

Variation lost if design cannot accept extremes of tolerance in both directions.

Additional variation allowed, without changing specification, if design allows extremes of variation in both directions. (57% Greater Tolerance Zone)

Positional Tolerance specified as ±1.5mmNominal

1.5

1.5

1.5

1.5

2.12 1.5





Page 57

GD&T Characteristic Symbols

CHARACTERISTIC

Angularity

SYMBOLStraightness

Flatness

Circularity (Roundness)

Concentricity

Profile of a Line

Profile of a surface

Perpendicularity

Parallelism

Position

Cylindricity

Circular Runout

Symmetry

DATUM REF. USECATEGORY

Form

Profile

Orientation

Total Runout

Location

Runout

Never

Sometimes

Always

Indi

vidu

al F

eatu

res

Rel

ated

Fea

ture

s



Page 58

Denotes for Information Only( )Reference

As above, with no flats or reversals in ZoneCRControlled Radius

Creates a Tolerance Zone defined by 2 arcsRRadius

Modifies shape of Tolerance ZoneDiameter

Only Tangent Plane needs to be in Tolerance Zone

Tangent Plane

Tolerance Zone Projected above part surfaceProjected Tolerance Zone

e.g. Smallest Shaft or Largest HoleLeast Material Condition (LMC)

e.g. Largest Shaft or Smallest HoleMaximum Material Condition (MMC)

UsageSymbolTerm

Modifiers

M

L

P

T





Page 59

Measurement Planning



Page 60

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 61

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 62

Measurement Direction Requirement

Part Surface

CAD Surface

Target

Point

Probe DirectionSame result on

Nominal Geometry gives different result

on physical part





Page 63

Measurement Point Structure

4 4

Level 4 Process Development Part Level

3 3

Level 3 Process Proving Sub-Assemblies

2 2

Level 2 Process Capability Major Assemblies

ProductComponent

Frequency Volume

1 1

Level 1 Process Monitoring Craftsmanship



Page 64

Measurement point / Edge point

Edge point

Hole / Slot / Nut hole

Locating pins

Locating datum

Measurement Planning Example

4 4

Process Development





Page 65

4 4

3 3

Process Proving


Edge point


Locating pins

Locating datum




Page 66


Edge point


Locating pins

Locating datum

4 4

3 3

2 2

Process Capability






Page 67


Edge point


Locating pins

Locating datum


4 4

3 3

2 2

1 1

Process Monitoring



Page 68

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 69

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 70

Quality Maturation





Page 71

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 72

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 73

Review & Close



Page 74

References

• Achieving Dimensional Quality by Applying the Technology of Dimensional Management, by Curt Larson, Right Tech Inc. 1994

• Fundamentals of Geometric Dimensioning & Tolerancing, 2nd Edition – Metric, by Alex Krulikowski, Effective Training Inc. 1997

Documents

Introduction to Simulation for Variation Reduction · PDF fileIntroduction to Simulation for Variation Reduction 1 ... Introduction to Simulation for Variation Reduction 2 ... •Sums