Some applications contain measurements of locations that are difficult to place the Spherically Mounted Retroreflector (SMR) on directly requiring use of the Bar Extension commonly called the Hidden Point Bar.
When a Bar Extension is combined with the 1.5” Shankless Nest and Point Tip or Sphere Adaptor, a sphere can be measured, and the desired point is the center point of the sphere. The measurements should be taken very carefully to avoid any movement or slippage of the bar’s tip while ensuring adequate coverage of the sphere. The diameter of the measured sphere can be determined by combining the lengths of the bar, tip, nest and SMR and this information can be used to evaluate the measurement to validate it.
Some common scenarios for an inaccurate measurement are listed below. Five or more readings on the sphere measurement are required to obtain this information.
Inaccurate Sphere Diameter, High Standard Deviation of Sphere Fit, and/or Poor Form of the Sphere
This indicates that the measurement of the sphere is not accurate or repeatable. T
he point bar tip may have moved during the measurement. If the sphere fit adaptor is being used, the Tooling Ball being measured may be moving in hole it is inserted. Another possibility is the 1.5” Shankless Nest is not securely attached to the top of the bar and is moving during the measurements.
Inaccurate / Inconsistent Sphere Diameter with a low Standard Deviation of Sphere Fit and good Form of Sphere
This indicates that not enough the sphere is being measured.
Sphere fit algorithms require a certain amount of a sphere's surface be measured to calculate its diameter and center point accurately. If not enough of this surface is measured, then the sphere fit can return a low standard deviation and form but with an incorrect diameter.
For example, NIST testing showed Measure Q can produce accurate results with partial spheres that contained 30 degree bands of data. When using the 10” Bar with the Point Tip, moving the SMR only around 1 to 2 inches produces only 5 to 10 degree band and can produce this effect. Moving the SMR around 4 to 5 inches would be required to obtain the 30 degree band and would produce the correct sphere diameter and center point.
Hidden Point Bar Problems, Sphere Fit Problems, Bar Extension Problems, Hidden Point Bar Accuracy, Bar Extension Accuracy