QMT Features: December 2014
Keep it stable
Eric Becker of MetrologyWorks explains that setup stability is key to measurement success

In the early days of precision measurement users of optical instruments like transits, levels, and theodolites had the idea of instrument stability ingrained into them because these instruments required levelling and indexing (referred to as bucking in) prior to any measurements being taken.

An instrument that was not stable would be extremely difficult to level or to buck in therefore the operators became acutely aware of the stability of the instrument and its location in the work area. These required processes also made it extremely easy to see and understand if the instrument had moved or if the object had moved, therefore giving the operators constant feedback on the stability of their setup.
In today’s measurement world of Computer Aided Measurement Systems (CAMS) setting up to begin a measurement task has been greatly simplified, while this provides the users the ability to work 70-80% faster than the old methods it also provides the possibility for unknown measurement errors due to lack of Setup Stability.

Common sources of setup errors
When setting up any measurement system to perform precision measurements the operator must evaluate several factors; tolerances required for the measurement task, accuracy of the measurement device, location of the measurement device to the object, temperature of the object and time to complete the measurement task. However, other environmental factors that in today’s high speed manufacturing environment are frequently not accounted for during the setup of the measurement equipment greatly influence the accuracy and precision of the measurement task.
Figure 1 shows the common environmental problems for many of todays CAMS systems. Air movement from open doors and heating/cooling systems can be problematic for laser trackers and scanners. Vibration from equipment and vehicle traffic can cause movement of measurement devices and the objects. Sunlight can warm the object and their stands causing movement that goes unnoticed to the untrained eye. Cracks in the floor can cause an unstable platform for the measurement device and may cause movement when walking from one side of a cracked floor to the other.

Isolating these potential sources of measurement errors and working around them is a key part of achieving accurate and precise measurements regardless of the measurement device.

Optimizing your setup
This can be the biggest challenge of the measurement task. Depending on the environment, that you are doing your measurement work this could require little to no effort to extreme effort. In the 1980s during the building of the B-2 Stealth Bomber the Contractor in charge of building the Wing Jigs had to go so far as to dig up the entire floor of a building, drive pylons deep into the ground and create an isolation pad due to the tidal changes right outside their door. While this is an extreme case, it shows that to achieve accurate and precise products no cost is too small. Working outdoors using a Laser Tracker in the summer may require an umbrella placed strategically to shade the device from the sun to minimize the impact of heat on the stand. Sometimes it may just require the closing of outside doors, turning off heating/cooling systems or blocking off roadways from vehicle traffic, whatever the case you can be sure that minimizing these affects will result in more accurate and precise measurements.

When setting up your measurement equipment be sure that the placement of the device and stand (if required) allows you to work comfortably with the equipment. Be sure the stand is sitting on one surface (avoid cracks in the floor), the wheels (if equipped) have been raised and the device is sitting securely on the floor with all adjustment knobs properly tightened.

Note: Heavy duty rolling metrology stands and tripods like those shown in the lower left can easily slide on smooth surfaces, the use of additional weight attached to the centre or legs, double sided tape or even hot glue around the outer edges of the feet can help stabilize these stands.

Don’t forget the part – the setup and stability of the part you are measuring is equally important, it should be just as stable as the device you are measuring with!

Monitor your setup
Keeping a close eye on your setup will minimize the amount of time wasted taking ‘bad measurements’, the sooner you determine your setup has gone ‘bad’ the easier it is to recover and continue measuring. The use of ‘Drift’ points (points to determine instrument or part movement) is common in precision measurement and allows the operator to determine if the part or the instrument has moved. Measuring a ‘Drift’ point at the start of a measurement task provides you a ‘starting’ point, when you measure that point later in the job you can quickly compare the two points using a distance calculation to determine if your setup has moved. Ideally, checking your ‘Drift’ points a minimum of every couple of hours will give you confidence in the measurements you have collected.

Another common practice is to measure a series of ‘Move Device Points’ at the start of their measurement task that you can use to ‘re-bundle’ the instrument back to the original working coordinate system should the device get bumped or moved during the measurement process.

Good setup = good results
By paying close attention to all facets of your environment, equipment setup, and part setup, your measurement results will be the best that your measurement device can provide. Taking shortcuts while being faster will yield results that lack accuracy and precision.
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