04-19-2012, 08:41 PM
(This post was last modified: 04-19-2012, 08:57 PM by Tony Wells.)
The latest standard I am aware of for us is (ASME) Y14.5-2009, but I believe the Symmetry callout was there in the previous version. It is still in use.
Ken is correct about the -C- being incorrectly drawn. If it is specifying Datum C it should simply be a capital C in a rectangular box.
Basic dimensions do have a tolerance, but not the same tolerance as would be given in the title block of the drawing, such as +/- 0.005 for 3 place decimals. The tolerance is according to the feature definition block referenced back to the datum controlling the basic dimension. In this case, the basic dimensions are given with modifiers on actual size of the feature and Datum C as manufactured. This time we see a positional tolerance zone of 0.006 on the outside edges (the 0.835 -0.005 dimension), modified to enlarge the tolerance zone up to 0.015, by allowing the feature to shrink within its limit. That comes from the -0.005, (the tolerance of the feature), plus 0.006 (True Position tolerance), and 0.004 (datum C @ it's LMC). The way it works is basically that the smaller each control feature is, the more error is allowed in location of the basic dimensioned feature. The intent is to actually give more overall tolerance than could be given with hard dimensions and the standard tolerance conventions. The smaller the features get, the sloppier you can work, in other words.
Ken is correct about the -C- being incorrectly drawn. If it is specifying Datum C it should simply be a capital C in a rectangular box.
Basic dimensions do have a tolerance, but not the same tolerance as would be given in the title block of the drawing, such as +/- 0.005 for 3 place decimals. The tolerance is according to the feature definition block referenced back to the datum controlling the basic dimension. In this case, the basic dimensions are given with modifiers on actual size of the feature and Datum C as manufactured. This time we see a positional tolerance zone of 0.006 on the outside edges (the 0.835 -0.005 dimension), modified to enlarge the tolerance zone up to 0.015, by allowing the feature to shrink within its limit. That comes from the -0.005, (the tolerance of the feature), plus 0.006 (True Position tolerance), and 0.004 (datum C @ it's LMC). The way it works is basically that the smaller each control feature is, the more error is allowed in location of the basic dimensioned feature. The intent is to actually give more overall tolerance than could be given with hard dimensions and the standard tolerance conventions. The smaller the features get, the sloppier you can work, in other words.