Cylinder can be evaluated either in 2D as circle center point locations or in 3D as an axis comparison along a length. Settings in this section control the way that the specific feature is evaluated.
The position of measured cylindrical holes or pins can be established in either of 4 ways depending on the users needs. These methods are as follows:
Trans-Axis Center Position Measurement. (Single Point Method) This method takes either a single point or averages a set of points and compares the resulting point to the nominal cylinder axis while using the nominal direction and radius for the evaluation. An example of this type of check is resting a probe tip of a larger diameter on a hole.
Center & Diameter at Nominal Direction. (Circle Center Method) This method projects the associated measurements to the end cap of the nominal cylinder. The results of the projection are then used to fit a circle. The circle then determines the center point and the diameter while the nominal cylinder is used for the direction. This is an excellent choice in situations where you have very little coverage of the cylinder on either side of a cross section plane to define the cylinder axis.
Center and Surface Axis Measurement. (Axis Measurement Method) This method assumes that multiple measurements of the cylinder axis were taken at varying depths. A line is fit to the measurements and that line determines the center and direction of the cylinder while the nominal cylinder is used for the radius. For example, using a probe resting on a hole with and without a spacer could be used to define a cylinder center axis.
Full Cylinder Evaluation. (Full Cylinder Fit Method) This method does a cylinder fit from the provided measurements. The points are assumed to be measurements of the cylinder walls (pin or hole), and the length of the cylinder is determined by the extent of the measurements along the cylinder axis. In this case the measurements are used to determine the central axis and radius/diameter of the cylindrical feature.
Extrapolate axial extents. This option provides the ability to extrapolate the GD&T evaluation beyond the extents of the measured cylinder to the full extent of the nominal cylinder. This option is more true to the letter of the standard but the extrapolation can lead to false part rejection, particularly if there is sparse data coverage along the length of the pin or hole.
Exclude from auto eval. This option allows prevents the Full Cylinder Evaluate mode from being used. This option is helpful when measuring with a limited number points and expecting a 2D result. With this option off and you measure 10 points for example we will do a full cylinder fit which can produced an unexpected result.
Choose for me (Automatic). This is the default used when creating datums and checks with cylindrical features. In this mode, the evaluation method is determined automatically based on the number of measurements. When set to this option the following designations will be made: if one point is associated, trans-axis Center pos. Meas. will be chosen; if two points are associated, then Center and Surface axis Meas. will be selected; if three to nine points are associated, then Center & diam at nom. direction will be selected; and if ten or more points are associated then Full Cylinder eval will be chosen.
Enable Pin Nest Detect & Comp. With greater than 3 measurements (use to define a planar direction) pin nest compensation can be used. The planar offset is applied in the direction of the plane defined by these measurements and then the radial off- set is applied laterally based upon the cylinder type designation, either pin or hole.
Enable Actual Diameter Override. This option allows the user to manually stipulate a diameter for a check. It was added to allow quick evaluation of other checks such as True Position checks that have a material modifier where only a single point was taken (and therefore the actual diameter is unknown).