However a Datum Feature is a tangible surface, point or axis on a part where that theoretical datum is located. What can be a datum A Datum is a perfect point, line, plane or surface but only exists theoretically. So on that count, yes a centerline can be a datum. We control the location of the threaded holes relative to the sensor hole by specifying as our tertiary datum. A datum is a perfect plane, axis, or point. We control the hole pattern's orientation to the bottom surface by making the secondary datum. We control the perpendicularity to the surface by making our primary datum. 1 -B- Center X Feature Target Center Feature Actual Center 10. įinally we control the M3 threads with a tolerance of position. Why the Answer : The result for True Position can be thought of as the diameter of a circle, centered at the target (basic dimensions), that contains the actual center. We control its location relative to a line from to by making our tertiary datum. We control the height from the bottom surface by making our secondary datum. Now we specify our large hole for our sensor. We then label this surface datum feature. Yeah, I hate TP of datum B circle to datum A in sheet metal. Datum C position to AB - Measure datum A, Level Z+ to A, measure datum B, measure datum C, construct a line from B to C, rotate X or Y to the constructed line, set X&Y origin to B, Z origin to A. We control this surface with a profile of a surface that will orient it to and locate and orient it to and. It should be a cylinder, albeit short, called out perpendicular to A. We need to control the surface where the sensor will mount. The length of the slot has a tolerance of position that controls the location of the slot relative to datum and the orientation of the slot relative to datums and. The width of the slot is designated as datum feature. The tolerance of position on the width of the slot controls both the alignment to datum and the perpendicularity to datum. The axis of the slot must go through the center of datum feature. The slot is the same width as datum feature. We constrain the final rotation with a slot. 22.34 and no particular datum is specified, then the individual feature to which the geometrical-tolerance frame is connected is chosen as the datum. We control the perpendicularity of this net hole relative to datum. If a positional tolerance is required as shown in Fig. True Position or Position, as it is referred to in the ASME standard, is one of the most widely used symbols in GD&T. Now we create a net hole that will be barely larger than the dowel pin that goes into it. We have constrained 3 degrees of freedom (1 translation and 2 rotations) with datum. Since the block will sit on the mating part, we make the mating surface datum.
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