Section 6: Reference Frames Establishing a reference frame and datuming is accomplished using any of a variety of alignment operations. These operations may be used individually or in combination, and will be based upon the capability of the measuring machine as well as the particular alignment requirements of the application. Datuming operations can be performed in the form of explicit datum feature measurements, by modifying the individual coefficient values of previously measured features or by constructing datums from existing features. The M2 reference frame is established by performing, a part skew alignment and then a datum point which is the origin of the XY coordinate system. When measuring a datum-dependent part, establish a datum or skew alignment first, and then measure the features associated with that datum. The skew can be offset by the user when the skew line position is located where it cannot be measured directly by the system. This offset skew position is identified by values from other measured or constructed features. The skew alignment can also be rotated to specified angular values by the user to facilitate measurements based on angular secondary alignments. Establishing a Reference Frame The M2 measurement reference frame consists of: • Skew alignment • Part Datuming (measurement coordinate system origin) Skew alignment When a part is misaligned (twisted) in the XY plane, small cosine errors can be generated during the measurement process. These errors can be eliminated by creating a skew alignment for the part. The skew alignment performs a precise measurement of the part of misalignment in the XY plane. Once the misalignment is known to the system, subsequent feature measurements are automatically compensated to eliminate cosine errors. Measurement data in the Feature Detail screen and DRO viewports will reflect measurements of a perfectly aligned part.
To create a skew alignment: 1 Select the desired probe from the probe menu.
Measure toolbar > Probe menu > Desired probe
2 Press the Skew button of the Datum menu.
Measure toolbar > Datum menu > Skew button
3 Probe a minimum of two points well distributed along the entire length of the desired part reference edge. 4 Press the Done button to complete the measurement and enter the skew line into the Feature List. Measure toolbar > Done button NOTE Previously measured or constructed features can also be used to perform a skew alignment by manually zeroing the theta angle coefficient and the X or Y axis coefficient. Pressing the label buttons to the left of these coefficients in either the Feature Detail screen or the DRO viewport will set the corresponding values to zero.
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