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Ken Vakil

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Integrated Composite Parts Thickness Measurements in Aerospace Manufacturing and Assembly

Published: Thursday, November 12, 2015 - 12:17

In aerospace vehicle manufacturing and assembly, including that at Northrop Grumman Aerospace Systems, measurements of composite part thickness are required for ensuring that underlying processes are within tolerance, and for achieving outer-mold line-control surface requirements.

This article describes an integrated process involving a robust and reliable method of thickness measurement followed by wirelessly transferring the data into a quality database, and directly populating measured thickness points into an electronic planning document. Without such a system, the process would consist of three steps: 1) manually taking measurements, 2) hand-documenting the measured data, and 3) manually entering the measurement data into a planning document within the manufacturing execution system. Obviously, the manual approach involving three steps is time consuming and prone to data entry errors.

This article will discuss the details of the mechanical gauge with ZigBee data transfer capabilities, composite part thickness-measurement system architecture, gauge reproducibility and repeatability (gauge R&R) study, software features, how the new thickness measurement process works, and the benefits of the system.


Composite parts play an important role in aircraft manufacturing and assembly operations, and can include side panels, doors, and aircraft skins. Control of part feature thickness is critical to achieving outer-mold line requirements and ensuring proper assembly. In industry, feature thicknesses are measured by depth gauges, Vernier calipers, micrometers, and other specially designed gauges—none of which provide an integrated solution that take the measured data from the point of measurement all the way into a quality database or manufacturing planning system. Measured thickness points are also typically entered into statistical process control (SPC) software for analysis and trending.


The need for an effective thickness measurement system arose as Northrop Grumman Aerospace Systems (located in El Segundo, California) wanted to use thickness measurements, not only for composite part fabrication control, but also to ensure outer-mold line control at the skin/structure assembly level. The thickness measurement devices available in the industry were accurate by themselves, but when it came to repeatability, they exhibited significant variation from operator to operator. The newly designed gauge discussed here reduces operator variability significantly by recording when the measurement is taken. In addition, the data taken by the gauge are wirelessly recorded into the system laptop, and periodically uploaded to a quality engineering server.

System components

The system consists of the IntelliMic gauge developed by Physical Optics Corp. (located in Torrance, California) with the systems components mounted on a mobile cabinet. The mobile cart also houses calibrated thickness gauges to verify system accuracy at the beginning of the each shift. See figure 1 for a schematic of the composite thickness measurement system architecture, and figure 2 for a picture of the mobile system cabinet.

Figure 1: Composite thickness measurement system architecture

Figure 2: System cabinet

IntelliMic thickness gauge

The gauge, seen in figure 3, was specially designed and developed by Physical Optics Corp. under a Small Business Innovative Research project grant. It is fundamentally an electro-mechanical gauge in which the measurement jaws open upon applying pressure to (i.e., hand-squeezing) its operating handle. When the handle motion concludes and the handle is released, the jaws come down on the part to take the thickness measurement. This method of taking measurements when the handle is released contributes to significant reduction of operator effects during the measurement process.

Figure 3: IntelliMic thickness gauge

The measurement values are displayed on the display screen seen in figure 4. The gauge also has ZigBee wireless transmission capability. Once the data are taken and accepted, they are transmitted to the appropriate file in a laptop computer, and then to the manufacturing execution system. Ultimately, the data are then uploaded into the electronic planning document via a quality engineering server.

The gauge comes in two different throat configurations: a 1-in. throat and a 3-in. throat. The use of either version depends on how far back off the edge the thickness measurements are required to be taken. Due to an increased lever effect, the 3-in. throat gauge is slightly less accurate than a 1-in. throat gauge.

Gauge R&R

A gauge R&R study was conducted using a 0.2500-in. gauge block as well as an actual production part. The accuracy on the gauge block was 4.5 percent against the thickness tolerance requirement of ± 0.002 in., as seen in figure 4. The gauge R&R on the part itself was less than 12.5 percent.

Figure 4: Gauge R&R results on gauge block

Composite part thickness measurement system (CPTMS) software

The system laptop contains proprietary software that allows the measurement plan of the part to be displayed, guiding the mechanic to the measurement point position on the part, and takes the measurement when the gauge jaws are released on the part. The software provides the flexibility to correct measurement errors before measurements are accepted and loaded into the quality engineering server.

The main features of this software include:
• Communication with the composite part measurement device via the 802.4.15 (ZigBee) wireless interface
• Display of image-based measurement plans with visual guidance to the operator and automatic checking of out-of-tolerance conditions for the purposes of quality control
• Display of CATIA-based measurement plans with visual guidance to the operator
• Automatic export of data to file for import into the manufacturing execution and InfinityQS SPC system
• Automatic tracking of gauge calibration, and visual indication of expired calibration and soon-to-expire calibration status
• Tracking of job statistics, which includes the amount of time taken for each set of measurements and number of deletions
• Automatic creation of HTML reports for email and online viewing of all out-of-tolerance conditions
• Auditory alerts for in-tolerance and out-of-tolerances readings
• Support of bar code scanners

Thickness measurement graphical user interface

When the CPTMS software is run, the opening screen contains the following key elements (as seen in figures 5 through 7):
• Clear button: Clears all data, including the current measurement plan and all unsaved thickness readings
• Save button: Saves all the data and clears the interface; this function will only proceed if all measurements have been taken and all fields filled
• Delete button: Deletes the last measurement
• Operator: Field for identifying the operator taking the measurements
• Sequence number: Field for identifying the sequence number for the measurements taken
• Part number: Part number of the composite part being measured
• Y order number: Y order number of the composite part being measured
• Gauge serial number: Serial number of the gauge being used for measurement
• Calibration display: Shows the calibration status of the selected gauge
• Active reading display: Shows the next point to measure and the current reading
• Measurement results: Shows a list of measurements
• Measurement plan display: Shows a graphical representation of the measurement plan

Figure 5: Main menu, CPTMS system

Image-based measurement plans for quality control

The software is directed to a measurement plan by entry of a part number into the part number field. This tells the software to look for a measurement plan (a *.dat file) for that part number, which the user interface displays. One type of measurement plan supported by the software is image-based, in which a *.jpg file (as seen in figures 6 and 7) is displayed on the screen for guidance to the operator, and a list of numerical measurement points with tolerance values and screen locations specified.

Figure 6: Image-based measurement process

When the measurement plan has been loaded, the user interface indicates the next point to measure both by graphically highlighting the point with a yellow circle and by displaying the number of the measurement point above the active thickness gauge reading. As readings are taken, they are added to the thickness gauge reading list, with out-of-tolerance readings shown in red (as seen in figure 7). Out-of-tolerance readings are also highlighted by use of a different auditory beep than what is used for in-tolerance readings. If a reading was taken in error, the delete button can be pressed to remove the last reading.

Figure 7: Out-of-tolerance reading

The data taken by the gauge with part number, serial number, and planning document number attributes are transmitted to the system’s laptop first. There is an uploading program created that periodically uploads the data into a quality engineering server. When the operator performs a particular thickness measurement sequence identified in a planning document, it is automatically loaded into the planning document, eliminating the need to hand-write the data. (See figure 8.)

Figure 8: Data transfer in planning document


The system provides several benefits:
1. Time required to take measurement is reduced because the image-based plans guide the mechanics in the measurement process (as seen in figure 9)
2. More reliable and accurate measurements because the measurement process is not operator-dependent
3. Data are uploaded into the quality database for SPC analysis
4. Measurement data are automatically loaded into an electronic planning document, eliminating errors caused by manual entry
5. Measurement results are instantly available for corrective action

Figure 9: Comparison, manual vs. integrated process

Lessons learned

An integrated system such as the one described here can be a useful and efficient tool in gathering thickness feature data. However, there are a few things to remember for its effective application on the production floor, including:
1. Monitor and control measurement plans for each part
2. Provide adequate IT support due to software changes and network issues
3. Dedicated Ethernet drops can limit flexibility in terms of where the system can be placed; a wireless approach is desirable
4. Annual maintenance contracts with the supplier
5. Consider both the 1-in. and 3-in. throat versions of the gauge


The author would like to thank Chris Ulmer, director of engineering, and David Miller, vice president and general manager, both of photonics systems at Physical Optics Corp., for developing the CPTMS based on the requirements and Manufacturing Readiness Level testing defined by Northrop Grumman Aerospace Systems.


About The Author

Ken Vakil’s default image

Ken Vakil

Ken Vakil, manufacturing technology engineer at Northrop Grumman Aerospace Systems, has more than 45 years experience in manufacturing engineering, industrial engineering, and financial systems.