Troubleshooting Poor GRR Results

Common causes of high %GRR and systematic corrective actions

Quick Diagnostic Guide

High Repeatability (%EV)
Equipment Variation Dominant

Symptoms:

  • %EV > %AV
  • Same operator gets different results
  • Large ranges within operator/part combinations

Primary Causes:

  • Gage calibration issues
  • Poor gage resolution
  • Environmental variation
  • Part fixturing problems
  • Measurement procedure issues
View Solutions
High Reproducibility (%AV)
Appraiser Variation Dominant

Symptoms:

  • %AV > %EV
  • Different operators get consistently different results
  • Operator-part interaction present

Primary Causes:

  • Inadequate operator training
  • Inconsistent measurement technique
  • Subjective measurement elements
  • Poor work instructions
  • Setup differences between operators
View Solutions

Solving High Repeatability (%EV) Issues

Equipment-Related Solutions

Actions to take:

  • Verify calibration status and due dates
  • Check calibration against multiple standards
  • Recalibrate if out of tolerance
  • Verify calibration procedure adequacy
  • Consider more frequent calibration intervals
Quick Check: Measure a known reference standard multiple times. Variation should be minimal.

Resolution Rule: Gage resolution should be ≤ 1/10 of tolerance or process variation

Solutions:

  • Upgrade to higher resolution gage
  • Use digital instead of analog instruments
  • Consider alternative measurement methods
  • Improve reading technique (eliminate parallax)
Example: For ±0.005" tolerance, gage should read to 0.0005" or better.
Environmental Solutions
Temperature Control
  • Maintain stable temperature (±2°F typical)
  • Allow thermal equilibration time
  • Use temperature compensation if available
  • Avoid direct sunlight and heat sources
Humidity Control
  • Control humidity (45-75% RH typical)
  • Prevent condensation on parts/gages
  • Use desiccants if necessary
Vibration Control
  • Isolate measurement area from vibration
  • Use vibration-dampening tables
  • Avoid measurements during heavy machinery operation
Fixturing Solutions
  • Improve part clamping and positioning
  • Use consistent reference surfaces
  • Eliminate part movement during measurement
  • Design dedicated measurement fixtures
  • Ensure repeatable part orientation

Solving High Reproducibility (%AV) Issues

Training Solutions
Comprehensive Training Program:
  1. Theory: Measurement principles and importance
  2. Hands-on: Proper gage operation
  3. Practice: Supervised measurement sessions
  4. Assessment: Competency verification
  5. Refresher: Periodic retraining
Key Training Elements:
  • Proper gage handling and care
  • Correct measurement technique
  • Part positioning and fixturing
  • Reading and recording procedures
  • Error recognition and correction
Procedure Standardization
Improve Work Instructions:
  • Add detailed step-by-step procedures
  • Include visual aids and photos
  • Specify measurement sequence
  • Define acceptable techniques
  • Clarify decision criteria
Standardize Setup:
  • Define standard measurement positions
  • Specify gage orientation
  • Establish consistent reference points
  • Use measurement aids/templates
  • Create setup verification checklists
Goal: Any trained operator should get the same result using the same procedure.
Automation Considerations
Manual Element Automation Option Benefits Considerations
Part positioning Automated fixtures Consistent positioning Initial cost, complexity
Gage operation Automated measurement Eliminates operator variation Equipment cost, maintenance
Data recording Digital data capture Eliminates transcription errors System integration
Decision making Automated pass/fail Consistent criteria application Programming complexity

Systematic Troubleshooting Process

Step-by-Step Investigation
  1. Analyze the Data:
    • Identify which component (%EV or %AV) is dominant
    • Look for patterns in the data
    • Check for operator-part interactions
    • Review control charts for trends
  2. Investigate Root Causes:
    • Interview operators about difficulties
    • Observe measurement process in action
    • Check equipment condition and calibration
    • Review environmental conditions
  3. Implement Targeted Solutions:
    • Address highest impact issues first
    • Make one change at a time
    • Document all changes made
    • Verify effectiveness with data
  4. Verify Improvement:
    • Re-run abbreviated GRR study
    • Compare before/after results
    • Ensure improvement is sustained
    • Update procedures if necessary
Investigation Tools
Data Analysis:
  • Control charts
  • Scatter plots
  • Interaction plots
  • Range charts
Observation:
  • Process observation sheets
  • Video recording
  • Time studies
  • Operator interviews
Testing:
  • Reference standard checks
  • Calibration verification
  • Environmental monitoring
  • Fixture repeatability tests

Common Pitfalls to Avoid

Study Design Errors
  • Insufficient parts: Use minimum 10 parts for reliable results
  • Poor part selection: Parts must span the measurement range
  • Inadequate randomization: Randomize measurement order
  • Operator bias: Blind operators to previous measurements
  • Environmental changes: Control conditions throughout study
Analysis Mistakes
  • Wrong acceptance criteria: Consider application requirements
  • Ignoring interactions: Check for operator-part interactions
  • Statistical vs practical: Consider practical significance
  • Single metric focus: Consider %GRR, NDC, and P/T together
  • Premature conclusions: Investigate root causes thoroughly

Action Plan Template

Issue Identified Root Cause Corrective Action Responsible Person Target Date Verification Method
High %EV (25%) Gage calibration drift Recalibrate gage, reduce cal interval Metrology Tech MM/DD/YYYY Reference standard check
High %AV (20%) Inconsistent setup procedure Revise work instruction, retrain operators Quality Engineer MM/DD/YYYY Abbreviated GRR study
Add your issues... Identify causes... Define actions... Assign responsibility... Set deadlines... Plan verification...

Next Steps

Re-run Study

After implementing improvements

Study Guide
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