Some help on GD&T and Datums on drawings?
I think I understand the basics of datums and the difference between it and datum features: datums are a perfectly flat plane derived from the real part (assuming CAD of this part is perfectly flat), while a datum feature is the actual surface is self- which won't be perfectly flat with the microscopic imperfections.
What I don't understand is
- The difference between placing the "datum marker" directly on the surface or on a FCF. Take this image:
I think I know that the dashed vs solid lines denote which "side" the datum is on and that #5 is creating a theoretical center plane as the datum, but I'm not sure on the differences between the rest- if there is one.
Or this image as well:
Are #3 and #4 the same thing? (Again, just looking at the symbol)
- What is the actual use of a datum feature?
Even in the 2nd image, the surface of C becomes the datum feature and the theoretical centerline/axis becomes the perfect datum? How is the datum feature any different than a normal surface here? A different book I own says that "Geometric tolerances" are always related to the perfect datum- which I assume to be talking about the theoretical datum and not the datum feature/real surface. This makes sense to me, because how would you dimension and measure a couple million spots with slightly different dimensions, right? Are there "tolerances" that are not considered a "geometric tolerance"? The book didn't do a good job at defining that.
I feel like I understood datum markers placed on a FCF to be denoting datum features, but then how are we referencing the datum feature on dimensions? Strange open loop that I can't wrap my head around.