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What are the best techniques for ensuring accurate and repeatable GD&T measurements?

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Geometric dimensioning and tolerancing (GD&T) is a system of symbols and rules that define the shape, size, orientation, and location of features on a part. GD&T is essential for ensuring that parts fit and function as intended, and that they can be manufactured and inspected consistently. However, GD&T measurements can be challenging to perform accurately and repeatably, especially when dealing with complex geometries, tight tolerances, and multiple datums. In this article, you will learn some of the best techniques for ensuring accurate and repeatable GD&T measurements in manufacturing engineering.

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Paulo N. Dipus

BSc Mechanical Engineering & MSc Physics (Candidate)
Darrell I.

Sr Eng, Svc Engrg at Collins Aerospace
Sankar G, PMP?

Project Manager at Capgemini Engineering | Certified Project Management Professional | Certified SAFe Scrum Master

1 Choose the right instruments

The first step to ensure accurate and repeatable GD&T measurements is to choose the right instruments for the task. Depending on the type and level of GD&T control, you may need different instruments, such as calipers, micrometers, gauges, CMMs, vision systems, or laser scanners. Each instrument has its own advantages and limitations, such as resolution, accuracy, repeatability, speed, cost, and ease of use. You should select the instrument that best matches the GD&T specification, the part geometry, and the measurement environment.

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Sankar G, PMP?

Project Manager at Capgemini Engineering | Certified Project Management Professional | Certified SAFe Scrum Master
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Choosing the right instruments for GD&T involves considering factors like accuracy, resolution, speed, and cost. Calipers and micrometers offer versatility but may lack precision. Specialized gauges provide accuracy for specific features but can be costly. CMMs excel in complex 3D measurements but are expensive. Vision systems suit non-contact measurements but may be affected by lighting. Laser scanners quickly capture surface data but have limitations.

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2 Calibrate and maintain the instruments

The second step to ensure accurate and repeatable GD&T measurements is to calibrate and maintain the instruments regularly. Calibration is the process of comparing the instrument output with a known standard, and adjusting it if necessary. Calibration ensures that the instrument is measuring within the specified tolerance and error range. Maintenance is the process of cleaning, repairing, and replacing the instrument components as needed. Maintenance prevents the instrument from deteriorating or malfunctioning due to wear and tear, dust, moisture, or damage.

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Sankar G, PMP?

Project Manager at Capgemini Engineering | Certified Project Management Professional | Certified SAFe Scrum Master
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In manufacturing, calibrating and maintaining measuring instruments, coupled with Measurement System Analysis (MSA) are essential for precision. Calibrating CMM and other measuring instruments ensures accurate measurements. Also we need to do laser calibration and ball bar test in CNC machine to ensure accuracy and repeatability

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3 Follow the GD&T rules and standards

The third step to ensure accurate and repeatable GD&T measurements is to follow the GD&T rules and standards that apply to the part and the industry. GD&T rules and standards define the meaning, interpretation, and application of the GD&T symbols and modifiers. They also provide guidelines for selecting datums, measuring features, reporting results, and verifying compliance. Some of the common GD&T rules and standards are ASME Y14.5, ISO 1101, ISO 5459, and ISO 14405. You should follow the GD&T rules and standards that are specified on the part drawing or in the contract.

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Paulo N. Dipus

BSc Mechanical Engineering & MSc Physics (Candidate)
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Following GD&T rules and standard are important. However it is not enough, alignment on the interpretation of GD&T from the designer to machinist / tooling to Quality inspector to assembler should be the utmost priority for this are the stages on how parts are released. Part should be released in accordance to its design intent.

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Darrell I.

Sr Eng, Svc Engrg at Collins Aerospace
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Beyond the procedural and technical considerations, creating an environment that promotes collaboration is essential for accurate GD&T measurements. Encouraging open communication between design, manufacturing, and inspection teams helps address potential discrepancies and ensures a shared understanding of measurement goals. Cross-functional collaboration can lead to valuable insights and a more holistic approach to GD&T implementation. Moreover, promoting a culture of continuous learning and improvement empowers employees to stay abreast of evolving GD&T practices and advancements in measurement technology, contributing to sustained accuracy in measurements.

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4 Train and certify the operators

The fourth step to ensure accurate and repeatable GD&T measurements is to train and certify the operators who perform the measurements. GD&T measurements require a high level of skill and knowledge, as well as attention to detail and consistency. Operators should be trained and certified in the principles and practices of GD&T, as well as the operation and calibration of the instruments. Operators should also be familiar with the part design, the GD&T specification, and the measurement procedure. Training and certification can improve the operator's confidence, competence, and credibility.

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5 Validate and document the measurements

The fifth step to ensure accurate and repeatable GD&T measurements is to validate and document the measurements. Validation is the process of checking the measurement results against the GD&T specification, and verifying that they meet the acceptance criteria. Validation can be done manually, by comparing the results with the tolerance limits, or automatically, by using software or hardware that can analyze and display the results. Documentation is the process of recording and reporting the measurement results, as well as the instrument settings, the operator information, and the measurement date and time. Documentation can be done using paper forms, spreadsheets, databases, or software. Validation and documentation can provide evidence of compliance, traceability, and quality.

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6 Review and improve the measurements

The sixth and final step to ensure accurate and repeatable GD&T measurements is to review and improve the measurements. Review is the process of evaluating the measurement performance, and identifying any problems, errors, or variations. Review can be done using statistical methods, such as control charts, capability analysis, or measurement system analysis. Improvement is the process of implementing corrective and preventive actions, and monitoring their effectiveness. Improvement can be done using quality tools, such as root cause analysis, 5 whys, fishbone diagrams, or PDCA cycles. Review and improvement can help to reduce measurement uncertainty, variability, and waste.

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Sankar G, PMP?

Project Manager at Capgemini Engineering | Certified Project Management Professional | Certified SAFe Scrum Master
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To refine CNC measurements, integrate statistical methods like control charts, capability analysis, and measurement system analysis (MSA). Following initial inspections and advanced calibration, leverage statistical tools for ongoing monitoring. Improvement involves corrective and preventive actions, facilitated by quality tools such as root cause analysis, 5 whys, fishbone diagrams, and PDCA cycles. Rigorous documentation aids in tracking the effectiveness of interventions. This approach reduces measurement uncertainty, variability, and waste, enhancing overall CNC precision and quality.

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7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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What are the best techniques for ensuring accurate and repeatable GD&T measurements?

1

2

3

4

5

6

7

1 Choose the right instruments

2 Calibrate and maintain the instruments

3 Follow the GD&T rules and standards

4 Train and certify the operators

5 Validate and document the measurements

6 Review and improve the measurements

7 Here’s what else to consider

Manufacturing Engineering

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What are the best techniques for ensuring accurate and repeatable GD&T measurements?

1

2

3

4

5

6

7

1 Choose the right instruments

2 Calibrate and maintain the instruments

3 Follow the GD&T rules and standards

4 Train and certify the operators

5 Validate and document the measurements

6 Review and improve the measurements

7 Here’s what else to consider

Manufacturing Engineering

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