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WELCOME TO TRAINING PROGRAM ONMEASUREMENT SYSTEM

ANALYSIS (MSA)

BY CHINTAMANI KULKARNI

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What is Measurement system A measurement system is a process by which

we assign a number to a characteristic of a product / service.

A measurement system includes Standard Item of interest (Work piece) Measurement equipment (Instrument) Personnel / Procedure to use the equipment Environment under which to conduct the

measurement i.e. SWIPE

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Effect of Measurement system error on Product Decisions A wrong decision may

be taken whenever any part of the measurement distribution overlaps a specification limit.

A “Good” part will sometimes be called as “Bad” (type I error, producer’s risk or false alarm)

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Effect of Measurement system error on Product Decisions A “Bad” part will

sometimes be called as “Good” (type II error, consumer’s risk or miss rate)

This is most risky as Consumer will be suffering as “Bad” part will be delivered to him as “Good” part.

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What is MSA?

Measurement System Analysis (MSA) Deals with analyzing the effect of the

Measurement system on the measured value Emphasis is on the effect due to equipment &

personnel We test the system to determine numerical

values of its statistical properties and compare them to Accepted Standards

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Measurement is not ALWAYS ExactMeasurement system variation affects Individual measurements Decisions based on data

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Attribute measurement system example

Could this ball hit the stumps? (Decision for LBW)

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You require measurement system which will give enlarged

view

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Ball just misses the stump. But will player will be given out or not? (OK or NOT OK) Can umpire give repeatedly CORRECT decisions?

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Repeatability

Variation in measurements obtained with one measurement instrument when used several times by one assessor while measuring identical characteristic on same part.

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Reproducibility

Reproducibility is variation in average of measurements made by different assessors using same measuring instrument when measuring identical characteristic on same part.

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Stability

Stability (or drift) is total variation in measurements obtained with a measurement system on same master or parts when measuring a single characteristic over an extended time period.

(a time period is days, not hours)

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Linearity

Difference in bias values through expected operating range of gage.

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Bias

Difference between observed average of measurements and reference value. Reference value, also known as accepted reference value or master value, is a value that serves as an agreed‑upon reference for measured values. A reference value can be determined by averaging several measurements with a higher level of measuring equipment.

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MSA Plan

Sr. No.

Measurement System

MSA Type

Elements of system Planned in

1 Thickness measurement

R & R Equipment – Micrometer 0.01 L.C.

Component – Dial 2 +/- 0.08

Environmental conditions – Ambient

Appraiser / Trial – 3 / 2

April 2006

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Gage R&R

Gage R&R is conducted for Variable measurements Attribute measurements

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Conducting Gage R&R study (variable) Sample of Ten units are selected from

manufacturing process. Three assessors who usually do

measurements are selected to conduct study. Each part is measured Two / Three times at

random by three assessors and results are collected & tabulated.

Equipment variation & Appraiser variation is calculated

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Conducting Gage R&R study (variable) Calculations R = 0.3417, n = 10, r = 3, Xdiff = 0.4446, Rp = 3.511 Repeatability = Equipment variation (EV) = R * K1 = 0.3417*0.5908 = 0.20188 Reproducibility = Appraiser variation (AV) = Sq. root {(Xdiff * K2) –

(EV*EV / nr)} = 0.22963 n = parts, r = trials

Trials 2 3

K1 0.8862 0.5908

Appraisers 2 3

K2 0.7071 0.5231

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Conducting Gage R&R study (variable) Repeatability & Reproducibility (GRR) = Sq. root (EV*EV + AV*AV) GRR = 0.30375 Part variation (PV) = Rp*K3 = 1.10456 Total variation (TV) = Sq. root (GRR*GRR + PV*PV) = 1.14610 % EV = 100 [EV / TV] = 17.62 % % AV = 100 [AV / TV] = 20.04 % % GRR = 100 [GRR / TV] = 26.68 % % PV = 100 [PV / TV] = 96.38 %

Parts 2 3 4 5 6 7 8 9 10

K3 0.7071 0.5231 0.4467 0.4030 0.3742 0.3534 0.3375 0.3249 0.3146

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Attribute Gage Study

Select 20 parts Include some parts that are marginal on

upper & lower specification limits Select two / three assessors who regularly

use the gage as part of their daily work Take two / three measurements by each

assessor on each part at random

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Parts in Gray areas

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Analysis of GRR studies

If Repeatability is large compared to Reproducibility

The instrument needs maintenance The gage may need to be redesigned to be

more rigid The clamping or location for gaging needs to

be improved There is excess within part variation

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Analysis of GRR studies

If Reproducibility is large compared to Repeatability

The Appraiser needs to be better trained in how to use & read gage instrument

Calibrations on the gage dial are not clear A fixture of some sort may be needed to help

the Appraiser use the gage more consistently

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Gage stability study

Obtain sample & establish its reference value relative to traceable standard, identify as master sample for stability analysis

If not possible select production part in mid range of process / tolerance

Measure master(s) three to five times (based on knowledge of measurement system) at different times of the day for a week

Plot data on X bar & R chart Compare standard deviation for measurements

with process to determine suitability for the application

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Bias study

Obtain accepted reference value for part Use tool room or layout inspection equipment Measure same part minimum 10 times using

gage under evaluation Calculate average of readings Bias = observed average – reference value % Bias = 100 [Bias / Process variation (or

tolerance)]

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Gage Linearity Study

Select 5-8 parts that can be measured at different operating ranges of measurement system

Determine reference value for each part using layout inspection

Use one appraiser to measure parts Take 10-12 repeated measurements on each part Calculate part’s bias Bias = observed average – reference value

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Gage Linearity Study

Plot bias average against reference values Linearity represented by slope of best fit line

of these points Gage linearity index = Slope X Process

variation or Tolerance % Linearity = 100 [Linearity / Process

variation or Tolerance]

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Charting Linearity

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