In the bottom half of the datum target frame, you write the letter/symbol indicating the same datum as that of the entire feature, along with the number of the datum target. Specifying a Datum TargetĪ datum target is described using a circular frame with a horizontal line drawn through the middle (datum target frame). A datum simulator is what we call the theoretical surface that we use to approximate the datum feature for measurement purposes. On the drawing shown below, datum feature B, in the position callout of the four hole pattern, is modified at MMC. In such cases, you can indicate a datum target to define the minimum required part as a datum. When datum features are modified at MMC or LMC, the datum feature and the features being controlled may be able to shift relative to the datum axis or centerplane. Using a concaved bottom as a datum plane may also cause problems in placing it directly on the surface plate. Describe two ways datum targets are symbolized. Datum plane A can be established by supporting the surface set up as a datum using a surface plate or other higher precision plane. The designer suspects the part may rock or wobble when the datum feature contacts the datum feature simulator. Note: The dimension controls the size of the physical surface it does not. Only a portion of the feature is used in the function of the part. The datum dialog opens and you can change the label. Figure 1: Typical section from a drawing showing casting datums. They are typically designated as X-Y-Z or Z-Y-X. It would be my suggestion to use datum simulators for the rough edges such as you stated, parallels, 1-2-3 blocks, squares, anything that would allow you to use the best geometric mating fit of the part and take the measurement lines/points on these artificacts instead of the actual rough surfaces. Measuring under such conditions can result in deteriorated repeatability and reproducibility. It is not practical to use the entire surface as a datum feature. This where casting datums come into play. If a plane defined as a datum is warped or otherwise shaped inappropriately, using that entire surface as a datum feature for indication can cause a significant error during processing or inspection. A datum target is a point, line, or limited zone on a target with which machines and instruments for processing, measuring, and inspection come into contact in order to establish a datum.
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