Despite a laser tracker’s many capabilities, its main function presents some limitations that can make it difficult to measure certain features of a part. Line-of-sight or scanning surfaces with a small radius or contour may be a challenge when using only a laser tracker.

Before other methods were used, tool kits with extension bars provided a way to pick up hidden points where the line of sight was an issue. These bars were referred to as “offset rods” that consisted of a nominal rod, approximately 0.375-in. in diameter.

Using different tips on each end allowed measurement for scribe lines, punch marks, or a pin nest for the spherically mounted retroreflectors (SMR). The tip of the rod was used to effectively collect enough points to construct a sphere. From the sphere’s geometry, the software was able to create a center point that provided a 3-D point for the measurement that was not initially visible from the line of sight.

There are different measurement systems and processes that can be done to collect 3-D measurement data. These data can be used to apply scribe lines or locate appendages on large structured units, such as those found in the aerospace, nuclear, and shipbuilding industries. I have found the best approach is to first establish a global coordinate system and provide an efficient method to correlate with the unit during the different construction stages. An effective method for the coordinate system can be as simple as hot gluing relocation nests to the manufacturing steel nests that are welded to the unit.

Some companies may choose other instruments to collect measurement points on the unit, depending on the measurement scope and climate challenges that may exist. There have been some instances where I measured units in unfavorable weather that caused vibration issues. In other situations, I have been limited in the resources available but still managed to collect the essential data points to complete the job at hand.

When using different equipment on the same job, it is important to tie all the equipment to the coordinate system. This can be done by purchasing different targets that produce the same offset during all the measurement tasks. The basic 1.5-in. relocation nest or commonly used pin nest have adaptors that can be purchased from tool manufacturers. To accommodate probe tips for other instruments, such as a portable arm, you may use 1.5-in. sphere targets for photogrammetry systems and adaptors that accommodate a multitude of different diameters.

Through the years, I’ve stored all the valuable knowledge I've gained in my mental “tool bag,” which I always bring to every service job. For instance, when measuring a unit that is too large for the Axxis Arm and Kreon Scanner (needed for capturing small contours on the part), I initially set up a coordinate system. Using the Tracker3 laser tracker and placing relocation nests close enough is imperative to accurately update the Axxis Arm to the coordinate system.

Using a laser tracker coordinate system reduces hinging error when relocating the arm around a large unit. Another great example is when a service job requires the measurement of large objects, such as a ship’s hull, aircraft fuselage, or a flight deck of an aircraft carrier. To establish the coordinate system, I use a total station that produces accurate and precise data. This allows me to focus on the exact area to be measured, with or without the use of another instrument needed to complete the job.

Ideally, it would be nice to have a specific tool for a specific application, but this can be expensive and impractical for many companies. Building versatility with the resources and equipment that you have, can be a cost-effective means to handle larger jobs and achieve the results you need for your next measurement survey.

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