FISHER CONTROLS
PRAGMATIC APPLICATION OF
GAUGE-MANPOWER CORRELATION
Alexandria Stewart, Michael Stinn, Justin Thede, Alicia Wieland
FISHER CONTROLS
• Fisher Controls is a manufacturer of
control valves
• Headquartered in Marshalltown, IA
• These valves control elements like
pressure, flow, and liquid level
• They are used in many industries,
including nuclear power plants, oil and
gas refineries, pulp and paper
factories, etc.
• Due to these applications, having valves
that are within engineering specification is
important
PROCESS
• Fisher’s operators machine parts to specifications
set by their engineers
• After each machining operation, the dimension
machined is measured by the operator and verified
as acceptable by an inspector
• When machining is complete, each part undergoes
a final inspection process
• The inspector measures the part, either with a micrometer or
a caliper
PROBLEM
• Parts are being rejected at ±0.001” out of tolerance
• Inspectors use their own discretion as to which
measurement device is appropriate in a given situation
• It is suspected that the current procedures are not
producing this kind of accuracy
• The objective of this study is to analyze measurement
capability and suggest appropriate changes
PROCEDURES
• Gauge R&R Study
• Three operators measured 10 small parts three times each
with a caliper
• This was repeated using a set of 10 larger parts
• Three measurements per part were taken using a micrometer
on the 10 small parts by the same three operators
Figure 2: 8” Ring Measurement Process Chart
Figure 1: 1” Stem Measurement Process Chart
DATA
ANALYSIS
• The measurements were analyzed using JMP
statistical software
• The resulting output was used to find
σ𝑅𝑒𝑝𝑒𝑎𝑡𝑎𝑏𝑖𝑙𝑖𝑡𝑦, σ𝑅𝑒𝑝𝑟𝑜𝑑𝑢𝑐𝑖𝑏𝑖𝑙𝑖𝑡𝑦, σ𝑅&𝑅 , distributions, and
Gauge Capability Ratios for each set of data
ANALYSIS
• Gage Capability Ratio (GCR)
Item – Device
Gauge Capability Ratio
1” Stem – Caliper
0.266
1” Stem – Micrometer
0.152
8” Ring – Caliper
0.209
• GCR < 0.1 wanted for a gage to be considered
appropriate
• GCR < 0.01 is preferred for most applications
• Micrometer had smallest GCR of either device
ANALYSIS
• The micrometer variation is consistently smaller than
those of the caliper
• Caliper variation was consistent
Figure 3: Repeatability, Reproducibility, and R&R
ANALYSIS
• Micrometer measurements are more precise than
the caliper
• Calibration for accuracy is possible, but imprecision
cannot be fixed
Figure 4: Range and Distribution of Stem Measurements
ANALYSIS
• Comparison of operator consistency between 1”
stems and 8” rings
• Operator 3 consistently lower than 1 and 2
• Variation was independent of part size
Figure 5: Average measurements of 1” stems with caliper
Figure 6: Average measurements of 8” rings with caliper
ANALYSIS
• Comparison of measurement devices
• Variation between operators was larger using the caliper
• Measurements of each part were consistent regardless of
the device
Figure 5: Average measurements of 1” stems with caliper
Figure 7: Average measurements of 1” stems with micrometer
ANALYSIS
• We can say with 95% confidence, for any single measurement
the σ𝑅&𝑅 error will be within 0.00068 and 0.00096 inches
• For the calipers we are 95% confident that the σ𝑅&𝑅 error will
be within 0.00083 and 0.00398 inches
Figure 8: σ𝑅&𝑅 Estimates and 95% Confidence Intervals
RECOMMENDATIONS
• Create and implement standard protocol for measurement
tools
• For part features with tolerances less than ±0.005”, always use a
micrometer
• With calipers, we are 95% sure that a measurement could be skewed nearly as much as
0.004 inches
• By using a micrometer, our 95% confident maximum error is less than 0.001”, allowing
for a much more accurate measurement
• Use this data as an awareness tool to further establish the
importance of using the appropriate tool for specific situations
• Conduct a training event for the inspectors and establish
standard measurement procedures
• Standardization across all inspectors on how to hold the device
• Digital caliper shows value during measurement – can bias recorded
measurement if inspector views screen during process