Measurement Synopsis of the LBT Secondary Reference Body #2
To measure the reference body one first has to define a reference system for the part. According to the print there are two datum's: datum A is the concave surface of the part, datum B is the inside diameter of the center hole. Refer to refsetup.mac for the macro that defines the reference system.
The positive z-axis is defined by a line drawn through the center of curvature of the spherical surface and the center of the center hole.
A line going through the center of a ball bearing that has been epoxied to the surface at a distance of about 75mm from the center of the hole, and the center hole defines the positive y-axis.
The zero point is defined by x, y, z position of a circle scanned at the top of the center hole.
After that has been established, the probe reference systems must be defined. This is so that when the CMM is driven about the actuator hole, the probe is easier to control. It will drive around the axis of the holes. This is accomplished by running the holetilt.mac macro. The macro takes the tilt of the hole and rotates the part reference system to that hole, and saves if for recall later.
Once the reference systems have been defined and saved we move on to the actual data gathering. With 672 actuator holes and 9 mounting holes, doing the measurements by hand was out of the question. To overcome this problem the reference body is place on a computer controlled rotary table, the same table that was used to put the holes into it. The table was programmed to clock the part at the azimuthal angle set by the print. The table divides the rotations evenly with no error. Each ring has its own program.
This is how the measurements were made:
The data was saved in four different formats: raw data from the CMM, standard x, y, z, i, j, k; simple x, y, z; and standard transposed to part reference system.
The transposed data was imported in to MathCAD for analysis.
