METROLOGY

The two pillars of the six sigma breakthrough methodology are measurement and project selection. In this section we are going to discuss the issue of measurement from a metrology perspective, and in Chapter 13 we will address project selection under the umbrella of project management.

We have known for a long time that we are only as good as our measurement will allow us to be. Therefore, improving measurement systems and understanding the protocol of measurement will benefit the DFSS team immensely. It is very frustrating when you know that what you measure with is not sensitive enough, not accurate enough, or not precise enough. As though that is not enough, we compound the problem with variability ( incompatibility ) between metrology systems.

When a project has been selected for a DFSS investigation/improvement, it is imperative to understand the metrology system in place ” the advantages as well as the disadvantages/limitations of that system ” and plan accordingly .

UNDERSTANDING THE PROBLEM

Incompatibility issues are more than just a nuisance. When system A doesn't work with system B, users have to stock more than one set of spare parts , train operators on multiple software packages, and produce different types and formats of inspection reports . While these problems are significant, the real difficulty that system incompatibility creates is the inability to exchange a common inspection model between measuring devices. The time it takes to reprogram each device to inspect the same part, refixturing time and cost, and the resulting loss of accuracy in the measurement and inspection process create serious inefficiencies in a system that is, theoretically, designed to provide a high level of process control and improved inspection throughput. (In a design format, think for a moment the ramifications of a design control/test if you are not sure of its capability, accuracy, precision and so on.)

These data exchange roadblocks also create time-to-market problems. During the product development and introduction cycle, unnecessary time spent reprogramming control/inspection routines and reevaluating data contributes to lengthy product development cycles with a resulting loss of competitiveness .

The effects of system incompatibility are growing rapidly . Genest (2001) has estimated that the problem has cost the automotive industry alone about $1 billion. Cost penalties also exist for metrology system suppliers and individual users who absorb the costs of repair and training necessary when incompatible systems must work together.

It is imperative that the leadership (Champion, Master Black Belt, and Black Belt) of a designated project under DFSS understands metrology and uses it effectively. Otherwise , the results do not mean very much. Let us then examine in a cursory fashion metrology.

Metrology is a Hellenic word made up of metron = measure of length and logos = reason; logic. In its combined form it means the science of or systems of weights and measures. (It was Eli Whitney who between 1800 and 1811 proved interchangeability would be a vital part of manufacturing, thus developing the first use of the metrology system in the United States.)

METROLOGY'S ROLE IN INDUSTRY AND QUALITY

In order for metrology to exist we must recognize that we need measurements to

Make things:

Length, width and height

Eliminate one of a kind
Control the way others make things:

Designing

Building
Describe scientific ...:

Worldwide exchange of ideas

Dollar

Furthermore, we must understand that metrology is based on standards. Without standards and the ability to trace the standards, measurement would not be possible. Therefore, metrology is based on a hierarchical system of standards with a level of accuracy reflecting the level of the standard under question. The relationship of the hierarchy and accuracy is shown as follows :

In any metrology system there is a control. The control system is one or more of the following items.

Calendar elapsed time: Fixed time ” e.g., every 3 months.
Amount of actual usage: Number of products checked by equipment.
Actual operating hours: Meter actual time equipment draws power.

Depending on the control used, there is also a calibration requirement that will ensure that the measurement is "what is supposed to be." Considerations for any calibration system include the following:

Establishment and maintenance of a system:

Written description of system
Flow of system ” calibration, repair, and maintenance
Intervals identified

Established standards and correlation:

Traceable (National Institute of Standards and Technology, NIST)
Levels

Identification:

Equipment identified

Part, tool or equipment number
Calibration label

Calibration date

Next due date

Responsible party

Recall system:

Scheduled or unscheduled

Data recording and analysis:

Computerized
Manual
Characteristics

Environmental controls:

Temperature
Humidity

Just like any other system, a measuring system may develop inaccurate measurements. Some of the sources of inaccuracy are:

Poor contact ” Gages with wide areas of contact should not be used on parts with irregular or curved surfaces.
Distortion ” Gages that are spring-loaded could cause distortion of thin-walled, elastic parts.
Impression ” Gages with heavy stylus could indent the surface of contact.
Expansion ” Gages and parts should obtain thermal equilibrium before measuring.
Geometry ” Measurements are sometimes made under false assumptions. For example, part being flat when not flat, or concentric when not concentric, or round when lobed.

These inaccuracies may be the result of either accuracy and or precision problems due to:

Operator error ” The same operator using the measuring instrument on the same product will come up with a dispersion of readings .
Operator to operator error ” Two operators using the same measuring instrument on the same product will exhibit differences traceable to operator technique.
Equipment error ” Each piece of equipment has built within it its own sources of error.
Material error ” In many cases material cannot be retested, such as in cases of destruct test.
Test procedure error ” In cases where two procedures may exist which expect the same outcome.
Laboratory error ” In cases of two laboratories performing same test.

(Here the reader may want to review Gage R&R and the applicability of accuracy, repeatability , reproducibility , stability, and linearity in Volume V of this series ” especially Chapters 15 and 16.)

MEASUREMENT TECHNIQUES AND EQUIPMENT

There are many measurement techniques. However, the most typical ” at least for the DFSS ” are:

Linear
Angular
Force and torque
Surface and volume
Mass and weight
Temperature
Pressure and vacuum
Mechanical
Electrical
Optical
Chemical

To make these measurements, many types of equipment may be used, some traditional and common and some very specific and unique for individual situations. Typical types of equipment include:

Scale
Radius gages
Plug gage
Thread gage
Spline gage
Parallels
Sine plate
Surface plate
Caliper
Hardness tester
Indicator
Micrometer
Comparator
Profilometer
Coordinate measuring machine
Pneumatic gaging
Optical

PURPOSE OF INSPECTION

As surprising as it may sound, inspection sometimes is used as part of the metrology scheme. It is used with the intention of:

Distinguishing good product from bad
Distinguishing good lots from bad
Checking for process change
Comparing process to specification
Measuring process capability
Ensuring product design intent
Determining the accuracy of the inspector
Determining the precision of measuring equipment

Inspection is used primarily in three areas, called inspection points. They are:

Incoming material
- Verification of purchase order
- Checking for conformance to specification
- Verification of quantity received
- Acceptance of certification
- Identification
In-process:
- First piece setup
- Verification of process change
- Monitoring of process capability
- Verification of process conformance
Finished product
- Last piece release
- Verification of product process
- Preparation of certification of process

It is also important to know that there are three kinds of inspection. They are:

100% inspection
- Safety product
- Lot size too small for sampling
- Seventy-nine percent effective manually
Sampling
- Large lot size
- Decision making
Visual ” mechanical, sensory
- Fit and function
- Lack of standard
- Senses ” feel, smell, taste, touch.

HOW DO WE USE INSPECTION AND WHY?

Even though we know human inspection is very ineffective , sometimes it is the only option we may have. In any case, we use inspection to classify characteristics of importance and to study and evaluate testing. As for the characteristics we are interested in, they can be summarized as follows:

Critical	Will affect safety
	Personal injury
	Product not usable
	Affect consumer confidence
	Loss to company
Major A	Possible injury
	Possible product not usable
	Affect consumer
	Loss to company
Major B	Inconvenient to consumer
	Visual
	Possible loss to company
Minor	Possible visual
	Inconvenient to company

As for the testing, we are interested in

Acceptance
Reliability
Qualification
Verification

of the item tested . Perhaps the most important issue in testing for DFSS is verification. We must be sure that the test is reflective of the "real world usage" and that it addresses "customer functionality."

METHODS OF TESTING

Destructive testing
- Can only be conducted once
- Detects flaws in materials and components
- Measures physical properties
- Is not cost effective
Nondestructive testing (N.D.T.)
- Is a repeatable test
- Detects flaws in material and components
- Measures physical properties
- Is cost effective

Obviously, whenever possible methods of N.D.T. should be used. Typical methods are:

Eddy current
X-ray
Gamma ray
Magnetic particle
Penetrant dye
Ultrasonic
Pulse echo
Capacitive
Fiber optical

INTERPRETING RESULTS OF INSPECTION AND TESTING

All inspections and all tests generate reports by their nature. That means someone must evaluate them and take the appropriate action. Typical issues that the DFSS team should be looking at are:

Accuracy and precision
Sampling errors
Relation to standards
Recording documents
Tabulation and calculation
Reporting of results
Analysis and interpretation
Corrective action

Another issue in reporting is the level of reporting as well as the level of responsibility. As a team working on a DFSS project, you should be aware of that relationship. That relationship is shown as

TECHNIQUE FOR WRINGING GAGE BLOCKS

One of the most common ways to calibrate certain machinery is through gage blocks. It is very important for the DFSS team to be familiar not only with the blocks themselves but how to use them as well. In this section we are going to address both of these issues.

The following points are of special importance when working with gage blocks:

Be sure gaging surfaces are clean.
Overlap gaging surfaces about 1/8 inch.
While pressing blocks lightly together, slip one over the other.
Blocks will now adhere .
Slip blocks smoothly until gaging surfaces are fully mated.

By wringing gage blocks together, you can obtain accuracy within millionths of an inch. Caution is usually given not to use a circular action because this might cause serious wear or even damage from abrasive dust trapped between surfaces.

Gage blocks are calibrated at the international standard measuring temperature of 68 °F (20 ° C). (This is very important to keep in mind, otherwise see below.) When measurements are conducted at this temperature between blocks and parts of dissimilar materials, no correction for different coefficients of expansion is necessary providing the components have had sufficient time to adjust to the environment. If blocks and parts are made of the same material and are at the same temperature, accurate results are possible regardless of whether the temperature is high or low.

To determine the correction requirement when blocks and parts are dissimilar and at temperatures other than 68 °F, use the following formula:

E = L ( ˆ K)( ˆ T)

where E = the measurement error in microinches; L = nominal dimension in inches; ˆ K = difference of coefficients in microinches; and ˆ T = deviation of temperature from 68 °F.

Typical coefficients of expansion in microinches per inch of length per degree F are:

Hardened tool steel	6.4
Stainless steel (410)	5.5
Chrome carbide	4.5
Tungsten carbide	3.0
Aluminum	12.8
Copper	9.4

The gage blocks are typically in a set of 81 pieces and they are arranged in the following order:

9 Blocks ” 0001" Series:
.1001	.1002	.1003	.1004	.1005	.1006	.1007	.1008	.1009

49 Blocks ” .001" Series:
.101	.102	.103	.104	.105	.106	.107	.108	.109	.110
.111	.112	.113	.114	.115	.116	.117	.118	.119	.120
.121	.122	.123	.124	.125	.126	.127	.128	.129	.130
.131	.132	.133	.134	.135	.136	.137	.138	.139	.140
.141	.142	.143	.144	.145	.146	.147	.148	.149

19 Blocks ” 1.000" Series:
.50	.100	.150	.200	.250	.300	.350	.400	.450	.500
.550	.600	.650	.700	.750	.800	.850	.900	.950

4 Blocks ” 1.000" Series:
1.000	2.000	3.000	4.000

LENGTH COMBINATIONS

Do not trust trial and error methods when assembling gage blocks into a gaging dimension. The basic rule is to select the fewest blocks that will suit the requirement. To construct a length of 1.3275" using a typical 81-piece set, the following procedure may be used:

Write the desired dimension on a piece of paper:	1.3275
Begin the selection at the top of the gage block set.
Reduce the last digit of the dimension to zero by selecting a block with a 5 in the fourth decimal place. In this case, the .1005 block is selected and its length is subtracted from 1.3275 to determine a remainder still to be selected. The value .1005 may be written again in an adjacent column for subsequent proof of the selection.	-.1005
	1.2270	.1005
Select a block to reduce the third decimal place to zero.	-.107	.107
Select a block to reduce the second and first decimals to zero. Wherever possible, such double reductions are desirable to cut down the total number of blocks selected.	1.1200
	-.120	.120
	1.0000
Complete the selection with the 1,000 block.	-1.000	+1.000
Complete the selection with the 1,000 block.	0.0000	1.3275

There are times when the same gaging dimension must be assembled more than once from single set of blocks. This may unavoidably increase the number of blocks required for the specific length. Assume that a second length of 1.3275" is required from the 81-piece set:

Write the requirement.	1.3275
Select two blocks to reduce the fourth decimal to zero. In this case, .1002 and .1003.	-.2005	.1002
		.1003
	1.1270
One block may now be selected to reduce three digits.	.127	.127
Select two blocks to form the remaining 1.000. In this case, .400 and .600.	1.0000
	-1.000	.400
	+.600

	0.0000	1.3275

For some general rules and guidelines on shapes and basic calculations the reader is referred to Volume II, Part II. Also, for an explanation and examples of the SI system see Volume II, Part II.

Volume V of this series covers the issue of GR&R and its terminology and therefore here we give only brief definitions of the key terms:

Gage accuracy ” Difference between the observed average of measurements and the master value. The master value can be determined by averaging several measurements with the most accurate measuring equipment available.
Gage repeatability ” Variation in measurements obtained with one gage when used several times by one operator while measuring the identical characteristics on the same parts.
Gage reproducibility ” Variation in the average of the measurements made by different operators using the same gage when measuring identical characteristics on the same parts.
Gage stability ” Total variation in the measurements obtained with a gage on the same master or master parts when measuring a single characteristic over an extended time period.
Gage linearity ” Difference in the accuracy values through the expected operating range of the gage.