#### Rotary Table Calibration

Rotary table compensation is disabled until all table parameters have been entered in the Configuration Tool and Kinematics Setup screens.

Rotary Table Calibration is performed using machine coordinates, consequently all location parameters are defined using machine coordinates.

Prior to calibrating a rotary table, the user must know where the machine coordinate centers of rotation are for the axes and enter them in the kinematics setup screen. These numbers are used to transform the points so that NC Gage knows approximately where the sphere is at different table positions. Once these values have been defined, the user can proceed with calibrating the rotary table.

1. Press the Probe soft key from the Main screen.

2. Press the Rotary Table Calibration soft key to display the Rotary Table Calibration screen.

3. Follow the on-screen instructions then click the Confirm soft key to complete the procedure or press the Return soft key to return to the previous screen without saving.

Parameters Definition:

- Feed Rate - Sets the speed of positional moves in millimeters per minute (or inches per minute if in inch units) that the probe will travel during calibration.

- Prehit Distance - Specifies the distance from the theoretical hit location to which the probe will move before taking a hit. This is used during probe calibration, defined in millimeters or inches.

- Safe Rotation Height - The Z axis height to which the probe will be moved between table positioning moves. This value is the tip center height in machine coordinates on the safe plane. It is the Z value on machine coordinate minus the probe Z offset while the tip center is on the safe plane.

WARNING - It is critical that the Safe Rotation Height value is appropriately set in order to prevent tip crashing into rotary table or part. Unless previously set, the default value for this parameter is 0 (zero).

Properties Definition:

- Number of Positions - The number of places where the sphere will be measured. The locations will be evenly distributed between the start angle and the end angle.

- Start Angle - The first table angle used in the calibration.

- End Angle - The last table angle used in the calibration.

Note: If the start angle and end angle are both the same or differ by 360 degrees, then the positions will be evenly spaced, starting at the start angle and ending not at the end angle, but one increment short of it. In the screen shown above, the end angle will actually be 270.

- Points Per Sphere - The number of points used to measure each sphere.

- Upper Angle - Normally this is 90 which means the north pole hit. It’s possible this could be a smaller angle if there is an obstruction on the top of the sphere.

- Lower Angle - Normally this is 0 which means the equator. This may be a larger angle if equator hits are not possible.

Note: NCGage will avoid the stem which is defined by the calibration sphere tool properties.

Rotary Table Definition:

The Nominal Column defines the center location and are the values which exist in the NCGage Kinematics setup screen prior to entering the rotary table calibration screen. The Measured Column defines the center location and are determined by the calibration process. When the user accepts a calibration, the measured values are written to the NCGage setup and will become the new Nominal Column values the next time a calibration is performed and will appear in the Kinematics setup screen.

- Axis ID - The letter that identifies the axis - A, B or C depending on the number of rotary tables defined in the system.

- Center X, Y, Z - The X, Y and Z machine coordinate values that define the center of the rotary table.

- Nominal Column - Displays the theoretical location when the table is initially calibrated. Each time the rotary table is to be calibrated, the previous values will be displayed.

- Measured Column - Displays the location of the current calibration values.

- Rotary Axis I, J, K - The vector I, J and K values that define the axis in which the table rotation is to be defined. Available options depend on what table axes exist on the machine and is selected from the drop-down list. Default values will populate the fields in the Nominal column which can be manually set if necessary.

- Nominal Column - Displays the theoretical direction when the table is initially calibrated. Each time the rotary table is to be calibrated, the previous values will be displayed.

- Measured Column - Displays the direction of the current calibration values.

Calibration Sphere Tool Definition:

These values define the location, size and orientation of the calibration sphere artifact. The user can type these values in directly, or learn them by pressing the calibration sphere definition button. A description of the calibration sphere definition process is given later in this document. If the sphere definition button is pressed, the user will be directed to define the sphere and upon exiting the sphere definition screen the learned values will populate the calibration sphere screen.

- Center X, Y and Z - The X, Y and Z machine coordinate values that define the center of the Sphere that is measured. This is the position when the table is at zero.

- Diameter - The diameter of the Sphere measured and used to calibrate the table.

- I, J and K - The vector axis values the rotation of the table will defined for the Sphere that is measured. This vector defines the stem of the Calibration Sphere. The vector points from the stem to the center of the sphere.

Probe(n) Definition:

- Pocket - The tool pocket index of the probe tool.

- Diameter - The Nominal and Calibrated diameter of the probe tip.

- Length - Specifies the length of the probe.

Tip Offset Definition:

- X, Y, Z - Specifies the Calibrated X, Y and Z tip offset coordinates of the probe. These values represent the direction from the gage point to the center of the probe tip.

When all fields have valid values, the user presses the execute button and NCGage performs the calibration sequence. If no errors are encountered, each sphere location is measured and the set of sphere centers is used to compute a plane. A circle is computed with the sphere centers in the computed plane. The center of this circle is the new measured center of the rotary table axis. The normal vector of the plane is the new IJK vector of the rotary table axis.

Navigation Bar Soft Keys for this Screen:

|  |  | View Calibration Report Select this soft key after the calibration has successfully completed to view the associated report.
| Define Sphere Tool Information Displays a screen that the user can use to manually enter the values that define the location, size and orientation of the calibration sphere artifact. The user can also learn these values using the Read Probe Position button. |  |
|  |  | Print This soft key prints the active calibration report displayed on the screen.

Calibration Process:

Prior to entering this screen, the user must know where the centers of rotation are for the axes and enter them in the kinematics setup screen. These numbers are used to transform the points so that NC Gage knows approximately where the sphere is at different table positions.

The nominal X, Y, Z, I, J and K values are initialized to the Setup screen values. The measured X, Y, Z, I, J and K values will be replaced by the existing values from the Kinematics Setup screen after the table is calibrated and the user accepts the calibration.

A previously calibrated probe must be selected prior to performing rotary table calibration.

Note: The quality of the probe calibration is very important. Any errors in the probe calibration may be transferred to the table calibration.

After the Kinematics Setup screen values have been entered, the user needs to do the following to calibrate a table:

1. Enter the Feed Rate, Prehit and Safe Plane Tip Height values. The Feed Rate is for non-measurement moves. The Safe Plane Tip Height value is very important. NC Gage moves to this height before the each table positioning move and sphere measurement. If this value isn't set correctly, you may find that NC Gage drives the machine way up or down into the table. This field should define a safe Z value to move the tip of the probe to before the table is rotated. All locations in this process are in machine coordinates.

2. Set the sphere measurement values. Enter the Number of Positions, Start Angle, End Angle, Points Per Sphere, Lower Level Angle and Upper Level Angle values.

- The Number of Positions defines the total number of measurements that will be taken on the sphere.

- The Start Angle is the first angle for the table being calibrated and the End Angle is the last angle. NC Gage will divide this range evenly. For example, if the Start Angle is set to 30 and the End Angle is set to 210 and you have defined 5 positions, NC Gage will measure the sphere at 30, 75, 120, 165 and 210 degrees. In the special case where the Start Angle is 0 and the End Angle is 360, the End Angle is adjusted based on the number of spheres. For example if you enter 0-360 and 4 measurements, then NC Gage will measure at 0, 90, 180 and 270.

- Points Per Sphere is the number of points to measure on each sphere.

- The Upper Angle and Lower Angle defines how much of the sphere will be measured. 90 means the North Pole while 0 means the Equator. 90 is the most often used value for the Upper Angle and may never need to be changed, but it can be changed if necessary. The Lower Angle value may need to be changed to help avoid the stem of the calibration sphere. NC Gage does provide some stem avoidance so adjusting this value may not be necessary either.

3. Ensure the correct probe being used to calibrate the table is correct. The Pocket, Diameter, Length and Tip Offset values of the currently active probe can be found in the lower-right corner of this screen. If needed, the probe can be changed by pressing the Select Probe soft key . Note that it is a requirement that the probe being used for table calibration was calibrated on a sphere. Tips calibrated on a ring cannot be used.

4. Select the appropriate Axis ID from the drop-down list. The Nominal column will automatically fill in.

5. The user must define the sphere that is being used. NC Gage needs to know the center location, diameter, stem vector and rotary table positions of the sphere. If these values are not known, press the Define Sphere Tool Information soft key to access the Find Sphere Tool screen. The Read Probe soft key can be used to learn the current probe position. Some notes on these values:

- Center X, Center Y, Center Z are the coordinates of the center of the sphere when the tables are positioned at values in this form (A and C in this example). NC Gage actually needs to know where X, Y and Z are when the tables are all at zero and will use the information defined in the Kinematics Setup screen to compute these values. Unless there is a reason that it can’t be measured at that position, it is recommended that the sphere is defined in coordinates where the rotary tables are at zero.

- I, J, K define the direction from the center of the stem through the center of the sphere. Normally this vector will be (0, 0, 1), which means the sphere is pointing “straight up”. NC Gage will use this vector to try to avoid the stem when the sphere is measured.

- When the upper table (C in the image below) is being measured, the lower table (A in the image below) must be at zero.

- When the lower table (A in the image below) is being calibrated, the upper table (C in the image below) can be at any position.

6. When all of this information is entered, press the Execute soft key to calibrate the table.

If no errors are encountered, each sphere location is measured and the set of sphere centers is used to compute a plane. A circle is computed with the sphere centers in the computed plane. The center of this circle is the new measured center of the rotary table axis. The normal vector of the plane is the new IJK vector of the rotary table axis.

Notes:

The original rotary table axis point will be projected onto the axis determined by the calibration process. Thus the new axis point should not differ from the old one by much and not be confusing to the user.

All of the information that is entered in the form is saved to a file. Each rotary axis will have its information stored separately in the file. After a calibration is executed the file will be rewritten. When a new calibration is performed, the form will be populated with values saved from the last calibration of that axis.

Calibration is done separately for each axis. Initially it is required that the rotary axis that is not being calibrated is forced to zero. This restriction may be removed at a later time.

Rotary Table Calibration Inspection Report

After the table is calibrated, the user can create a calibration report by pressing the View Calibration Report soft key .

The report will contain the following information:

- The report shows the time of measurement, axis ID and the computed center of the table.

- The report will display the shift values which show the difference between the current measurement and the values defined in the Kinematics Setup page. If this is the first calibration, the shift values show the deviations from the position defined by the user. For subsequent calibrations, this shows the change from the previous calibration. If the user accepts the calibration, then the rotation center will be written to the Kinematics Setup page.

- The Rotation Axis shows the “Tilt of the Table”. It is expressed as a vector and as a pair of angles. The distance between the rotary axes displays the distance between the axes of the two tables at their closest.

- The positioning error of each sphere measurement is displayed in both table and graphical formats. The components of the positioning errors are:

- Radial Error - This is the amount that the sphere location deviates from the computed circle in the radial direction (from center of circle outward)

- Tangent Error - This is the error of the sphere location based on its angle within the circle and the rotary axis position when the sphere was measured. These deviations are “clocked” to the sphere location. So the first sphere will always have zero for tangent error, all other sphere tangent errors are relative to the first measurement. This error essentially measures the accuracy of the rotary table positioning.

Note: Radial and Tangent errors are depicted graphically by the mis-location (which is exaggerated) of the sphere relative to its nominal position.

- Axial Error - This error measures the deviation of the sphere location from the best fit plane that was computed using all of the sphere centers. Plus values mean the error is “up”, minus values down. This is depicted graphically by scaling the errors so that you can see the differences relative to each other. Positive deviations are solid blue disks, negative deviations are red rings. Solid vs. rings was done in case the report is printed out to a monochrome printer.

- The user can press the Print soft key to get a hard copy printed to the default printer.