Hamar Laser created the A-511 single-axis target for use in large bore and turbine measurement and alignment. One of the critical tests of a fixture, especially one that must span large spaces, is how well the results obtained with that fixture can be repeated. This technical note describes the test to determine the repeatability of the A-511 target, and its results.

The A-511 is a single-axis, micrometer-adjustable target designed for measuring the center of a large bore or turbine. It has two legs: a measuring leg projecting down from the target's vertical axis, ending in a small pointed tip, and the brace leg, perpendicular to the measuring leg and ending in a small T-shaped tip (Figure 1). Both legs are made from standard steel electrical conduit cut to a length so that the distance from the tip of the measuring leg to the target center is equal to the radius of the bore. Both legs are the same length, determined by the formula:

Length = Radius (inches) - 2.75 inches.

The laser beam is "bucked in" to the bores to be measured, and the A-511 target set in the first bore. Measurements are taken in two positions 180 degrees apart. This is done by rotating the fixture after the first reading, pivoting it on the brace leg. As the fixture is turned, the target head is also rotated to remain facing the laser beam for the second reading. This determines the centering error for one axis. The fixture may then be rotated 90 degrees to measure the second axis; no second round of error correction is necessary (Figure 2).

The Test Set-up

The A-511 target was set up using a leg length for a 6-foot radius (a 12-foot diameter bore). The measuring leg was horizontal, the tip set against a metal plate. The brace leg rested on a flat surface so that the entire fixture was standing on the two leg tips (Figure 3, top). The test consisted of ten readings, with the fixture moved after each. This was done by pivoting it on the measuring leg in an arc along the laser beam's path and returning it to the highest point of the arc for readings. The readings were considered vertical despite the horizontal position of the target, because the movements caused the laser beam's position to change along the target cell's vertical axis.

When all ten of the readings had been taken, the results were averaged, and the standard deviation was calculated and plotted.

Results

The results are displayed in Figure 4. With leg length for a six-foot radius, the A-511 target had a 3 sigma standard deviation of less than.0002". In other words, not one of the ten vertical readings was more than .0001" from the average, excellent repeatability for a fixture. The A-511 has proven itself an exceptionally stable target for large bore and turbine work.

Both A-511 and T-218T measuring targets are equally repeatable. However, in our experience, the A-511 is much faster at taking the measurements. To get the best repeatability, some mechanism should be employed to ensure that each point on the diaphragm or other component is marked and the measuring tip is place exactly on the point. Given that the surfaces inside a turbine are usually pitted and rough, a radius tip should be used. For new turbine installations, repeatability of .001" or better is easily achievable. However, for older turbines, it becomes increasing more difficult to hold .001" repeatability because of the high level of pitting and corrosion.