That’s it for now - feel free to send comments/suggestions about these or other common mistakes that you encounter. The full explanation of this was covered in an earlier blog entry, found here. In other words, the only thing being controlled is perpendicularity. Finally, sometimes you may see the position symbol used on a single feature (a hole, for instance) and then the only datum referenced is a single planar datum that happens to be perpendicular to that hole.That’s where the critical flag should be noted. This shouldn’t be done, because basic dimensions themselves have no tolerance - there is nothing to track! Instead, the real variation to be measured is shown in a feature control frame. With F free to translate, hole E will take out the translations in the X and Y directions and hole F will take out only the rotation about the Z axis. When imposing these ideas onto GD&T, however, a common mistake is to flag a basic dimension as a critical characteristic. The datum translation symbol allows a gage to be used that has pins for both holes E and F with the pin for hole F being free to translate. This gets into things like standard deviation, six-sigma, Cpk values, and other lovely terms from statistics. This allows the long-term statistical trends for that dimension/tolerance to be tracked. Many of you may be familiar with the practice of identifying “critical characteristics” which require the inspector to measure something and keep a log or spreadsheet for these measurements. The next error is related to quality control and statistics.While that is sometimes easily forgiven, the bigger problem is when this misuse of the datum symbol creates ambiguity, such as this one on the left, and the corrected version on the right: However, the triangle symbol doesn’t identify the true, theoretical datum it is supposed to identify the physical feature from which the theoretical datum will be derived. And that is TRUE! The theoretical datum is very often a center line or axis. Too many times I’ll see this triangle symbol tagged to a center line, because the designer/engineer thinks that the datum will be the center line. Another error - probably the most common one I’ll be listing here - is the improper location of the datum feature symbol.This one isn’t as egregious, because there is only one way to interpret these symbols, and thus the meaning isn’t different. It might be tempting, because each of these is applied to a round feature, but the tolerance number given is NOT a distance across a circle (which is the definition of diameter), but a radial bandwidth. The next common error I’d like to review is similar to the first - using a diameter symbol when it shouldn’t be there! I see this in feature control frames for circularity, cylindricity, circular runout, and total runout.ASME Y14.5-2009 The axis of a datum feature simulator established from the datum feature.This is probably what the original intent was: A cylindrical zone indicated by the diameter symbol “Datum axis” is defined as follows in ISO and ASME standards: ISO 5459 : 2011 A datum that is a theoretically exact straight line. This section explains cases where a datum axis is established on a cylinder bore and on a cylinder axis. Learning GD&T From Scratch, provided by KEYENCE, walks you through the basics of geometric dimensioning and tolerancing, datums, and measurements by coordinate measuring machines. A datum line is generally established on the center of a target’s cylinder bore or cylinder shaft, and is rarely established on a ridge. This page explains the indication symbols and notation used for common datums and datum systems as well as how to set datums. A datum line can be a datum axis or datum axis line. Datum LineĪ datum line is the theoretical straight line established on the center of a cylinder bore or cylinder shaft, or a ridge such as the target’s edge. The theoretical plane generated from these features is the datum center plane. The two supporting planes are the simulated datum features. The datum center plane can be established by supporting the surface set up as a datum from both sides using precision planes.
In the same way as a datum plane, the surface of the target (datum feature) specified as a datum may be uneven or warped. B Datum = Plane set up by two flat contact surfaces
0 kommentar(er)
