GRR Studies on CMM Accuracy Hex
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Transcript of 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
*Defini tion from the ASQC Automotive Divis ion SPC Manual, 1986
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.
*Defini tion from the ASQC Automotive Divis ion SPC Manual, 1986
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