![]() The gage, the pin or ring gage is the simulated datum and is the datum axis. The datum feature is the surface of the part. This simulator can be a pin gage (internal) or a ring gage (external). ![]() NOTE: Simulated datum features are used as the practical embodiment of the datums duringĪNSI Y14.5: When a diameter is designated as a datum feature, the datum axis is derived from placing the part in a datum feature simulator. A real surface of adequately precise form (such as a surface plate, a bearing, or a mandrel, etc.) contacting the datum feature(s) and used to establish the datum(s). The remainder of this topic uses the part point of view to describe how PC-DMIS solves datums. You can use either point of view to describe how geometric tolerances and datums are solved in PC-DMIS. When creating a simulated datum the specs state: of view, PC-DMIS best-fits the datum simulators to the actual part. It will be imperative that you take as many points when measuring a cylinder on a part that are equally distributed around each circle segment of the cylinder to create a more consistent best-fit cylinder.Ī datum simulator is the best method to get consistent measurement values. The orientation (See Figure 1 a) is the angular deviation of 3d line through the center of the cylinder. When measuring a part surface form, waviness, and roundness deviations will cause some variation in the calculation of the cylinder thus changing the orientation of the cylinder. The surface of the datum cylinder will never be ‘true’. ![]() When measuring a part with a datum feature requirement that requires you use a cylinder as the spatial orientation datum (base plane datum) there are several considerations you must keep in mind. Datums may by based on one or more datum features of a part. A theoretically exact geometric reference (such as axes, planes, straight lines, etc.) to which toleranced features are related. Then go back and recall or remeasure the cylinder for form or other evaluations.ĭatum Definition When the Datum Is a CylinderĪ datum is defined in ISO 5459:1981 as: This method eliminates most of the form deviation. I'd rather take multiple slices in LSQ and create a 3d Line when it comes to using the feature as a spatial orientation. Form Datum will definitely change the evaluation method when that cylinder is used as a datum. Let's say you measured in LSQ but you have Form Datum turned on. As Calypso creates a best fit cylinder, taking into account the evaluation method you have selected, things can change. Just to be clear I'm not saying that you can't or shouldn't use a cylinder but for me the form measurement deviation affects the position and orientation of the cylinder. There is an ISO spec that deals with this but I can't recall what that ISO number is right now. I would assume that form would have an effect on spatial orientation only in the case of short cylindrical primary datum features.ĭo you have a particular example that I could try to wrap my head around? I am curious about the influence of form on spatial orientation. It has been my experience that the form of the cylinder can come into play when using the cylinder as a spatial orientation. Instead I opt for a 3d Line with multiple circles making up the center points to construct the 3d Line. I always shy away from using a cylinder for spatial.
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