grr studies on cmm accuracy hex

8/12/2019 GRR Studies on CMM Accuracy Hex

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Gage R&R Studies onCMM Accuracy

6/14/2005


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What is a GR&R ? GR&R is an abbreviation for Gage

Repeatability and Reproducibility

A GR&R is one component of a

Measurement System Evaluation

A Measurement System Evaluation isa component of a Statistical Process

Control system


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Gage Accuracy Gage Accuracy* is the difference between

the observedaverage of measurements and

the trueaverage.

*Defini tion from the ASQC Automotive Divis ion SPC Manual, 1986

ACCURACY

True

Average


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Gage Repeatabili ty Gage Repeatability* is the variation in

measurements obtained when one operator

uses the samegage for measuring the

identicalcharacteristics of the sameparts.

True

Average


REPEATABILITY


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Gage Reproducibility Gage Reproducibility* is the variation in the

average of measurements made by different

operators using the samegage when

measuring identical characteristics of the same

parts.

True

Average

*Defin ition from the ASQC Automoti ve Divis ion SPC Manual, 1986

REPRODUCIBILITY

A

C

B


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Gage Stability

Gage Stability* is the difference in the averageof at least two sets of measurements obtained

using the samegage on the same parts taken

at different times.

*Defin ition from the ASQC Automotive Divis ion SPC Manual, 1986

True

AverageSTABILITY

Time 1

Time 2


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Gage Linearity Gage Linearity* is the difference in the

accuracy values through the expected

operating range.


True

Average

10mm

Accuracy over

short distance

Accuracy over

long distance

True

Average

1000mm


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The Purpose of a GR&R Designed to provide a user with an

assurance that the measurement

system will repeat and reproduce toacceptable levels


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Why is it neededEARLY AND CORRECT RECOGNITION OF PROCESS TRENDS

TOLERANCE

UCL

LCL

FREQUENT, UNNECESSARY

PROCESS INTERVENTION

POOR REPEATABILITY GOOD REPEATABILITY

LESS FREQUENT, NECESSARY

PROCESS INTERVENTION

- PROCESS CHANGE INDICATED


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Long Method Parameters

NUMBER OF PARTS:5 10 NUMBER OF TRIALS:2 3 NUMBER OF OPERATORS:2 - 3

TYPICAL


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Typical GR&R Procedure DECIDE ON NUMBER OF PARTS, TRIALS

AND OPERATORS

OPERATOR A MEASURES ALL PARTS ASTRIAL 1

REPEAT THE ABOVE STEP FOR THETOTAL NUMBER OF OPERATORS

REPEAT THE ABOVE TWO STEPS FORTHE TOTAL NUMBER OF TRIALSSELECTED


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Mathematically Reduce Results to: REPEATABILITY - EQUIPMENT VARIATION

(EV) THE AVERAGE AVERAGE RANGE - R-double bar

REPRODUCIBILITY - APPRAISERVARIATION (AV) DIFFERENCE BETWEEN AVERAGE VALUES FROM

EACH OPERATOR - X bar difference

USE THE ABOVE VALUES IN SOMENASTY LOOKING FORMULAS WITH

STATISTICAL CONSTANTS BASED ON

THE NUMBER OF OPERATORS AND

TRIALS


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SIMPLY TAKE BETWEEN 20 AND 90 OBSERVATIONS

ON MULTIPLE PARTS

WITH MORE THAN ONE OPERATOR

AND PERFORM THE FOLLOWING

CALCULATION:

GR&R= 100 X(R x K1)= 2

+ [Xdiff x K2] - (R x K1) / (n x r)=2

( )2

OF A FEATURE ,

TOLERANCE

Gage R&R Made Simple


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Acceptable Limits LESS THAN 10% - Very Good

Measurement System

10 to 30% - May be acceptable based

upon the importance of the application,

cost of gage, cost of repair, etc..


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The Other Meaning of 10 ALTHOUGH IT IS BASED ON 10% OF

TOLERANCE, IT IS A STATISTICAL 10%.

FOR THE DATA SAMPLE RECORDEDDURING THE STUDY, THE GAGE MUST

REPEAT ON AVERAGE TO ONLY 2.2 to 3.3 %

OF TOLERANCE !

STATISTICAL MULTIPLIERS ARE USED TO

PREDICT LONG TERM VARIATION TO A 5.15

SIGMA LEVEL


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Did He Just Say Approximately 2-3 ?YOU HEARD IT CORRECTLY

JUST 2-3% OF TOLERANCE


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10 GR&R - Dont Confuse It DO NOT CONFUSE 10% GR&R WITH THE

OLD 10:1 METROLOGY RULE.

IN CASE YOU FORGOT, THATS THE RULE

WHICH STATES A GAGE SHOULD BE 10TIMES MOREACCURATETHAN THE

TOLERANCE IT WILL BE USED TO CHECK.

REMEMBER - 10% GR&R MEANS THATTHE GAGING SYSTEM NEEDS TO

REPEAT AND REPRODUCE TO

APPROXIMATELY 3% OF THE

TOLERANCE DURING THE STUDY


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So Whats the Big Deal ? ALL CMMs THESE DAYS REPEAT TO 80

MILLIONTHS OR BETTER, RIGHT ?

THE BIG DEAL IS THAT THIS IS NOT SIMPLE

B89 REPEATABILITY.

YOU WILL SEE THAT

MANY SOURCES OF ERROR COME INTOPLAY.


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Influences on Results CMM MECHANICS (Stability)

PROBE SYSTEM - Type of Probe Head

CONTROLLER ACCURACY ANDREPEATABILITY

PROBE STYLUS CONFIGURATION

FIXTURE - Design and Stability

PART PROGRAM

WHAT CAN INFLUENCE THE RESULTS ?


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Influences on Results CLEANLINESS OF PARTS

SURFACE FORM AND FINISH OF PARTS

PROJECTED TOLERANCES

OPERATOR

TEMPERATURE VARIATION

WHAT CAN INFLUENCE THE RESULTS ?


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Top Four Influences FIXTURES R US

Cleanliness

Temperature

Form and Finish of the

Workpiece


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Survival Tools - CMM SelectionBest to use the ISO 10360 E

1st term as a starting point for

predicting GR&R

performance

Compare the E value to thetarget R-double bar required

to meet the 10% GR&R

Predicting GR&R is not an

exact science Remember the 3%

rule


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Survival Tools - Fixturing Use a 3 - 2 - 1 Nest concept

Use clamps in sequence to nest the

part

Assure stability of fixture

Avoid locating on cast surfaces where

possible


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Survival Tools - Form Understand the allowable level of Form

error to be expected in the part

Consider most advantageous probingstrategy and data density

The higher the Form error, the more

critical the fixture

Use Scanning if possible


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Survival Tools - Cleanliness Provide Clean and Dry parts for the

study


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Survival Tools - Feature Projection Be aware of feature projections

Eliminate them all, except those cases

where the projection is per the designintent


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Feature Projection - not possible

-A-


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Feature Projection - possible

-A-


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Run Preliminary Tests A GR&R can be time consuming

10x3x3 on a 10 minute inspection cycle equates

to 15 hours of run time

Run 1 part 10 times leaving it in the fixture

Compare range to the target R-double bar

Run 1 part 10 times removing it from the

fixture Compare range to target R-double bar


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What About Accuracy?

Leave the absolute accuracy certification to

known traceable standards such as Gage

Blocks and Ring Gages

XXX

Ring GagesGage Blocks

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