symmetry and position together?
symmetry and position together?
(OP)
I have a fairly unusual situation that I hope to find a resolution for. We have a project going on right now, with some fairly large parts as well as purchased parts of unknown size (until we get them, anyway). The issue is this: in both cases, the engineer simply wants a feature (say, a hole pattern) to be on center. Dimension from one side isn't important, especially since the overall is either very loose or unknown. In the attachment, let's say we want the hole pattern to be on center horizontally, but tolerance is not too close. This pattern has to be a little closer than that from the bottom (datum C), but hole to hole has to be closer than that. Is the feature control frame I constructed legal, or is it totally inappropriate? What I'm trying to say is "symmetric to A within .1, position of hole pattern to C within .030, and hole to hole within .010". If it's not the best way of doing this, what is?





RE: symmetry and position together?
The other two callouts (position) are also fraught with issues, mostly regarding the datum references and the diameter symbols.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: symmetry and position together?
RE: symmetry and position together?
RE: symmetry and position together?
RE: symmetry and position together?
A small note... your original post said that you wanted the distance to datum C to be a little tighter than the horizontal position.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: symmetry and position together?
The tough part here is selling the idea. It's been the practice of a number of folks here to simply throw a center line up on a view with no datums or feature control frames with the implication that something is on center. I'm trying to put forth a better way, but I'm sure I'm going to meet resistance. Not only that, but some of these parts in question are large weldments, and I'll bet anything the average weld shop isn't going to understand it... sigh...
RE: symmetry and position together?
RE: symmetry and position together?
2JL
RE: symmetry and position together?
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: symmetry and position together?
Exactly my point. That's what I've been trying to explain to the engineers who do this. Not only that, but depending upon the shape of the part, it may also be ambiguous as to what a centerline is the center of. I'm fighting a difficult battle, though. Battle #2: I'm also confronted with this statement, "well, if we send a drawing with all this GD&T to a weld shop, they're not going to understand it" or, "that will automatically make them quote it higher". Sigh...
RE: symmetry and position together?
Best part - no pesky symbols, so no excuse for not understanding or automatically quoting higher.
It still doesn't dodge what is often the real problem - a supplier that doesn't want to be responsible for meeting drawing requirements.
RE: symmetry and position together?
That's not a bad idea, although it's not like our vendors are horrible, to me it's a matter of making sure the drawing is clear and can only be interpreted one way. Don't want to get into one of those situations where the drawing could be interpreted more than one way, and then having an issue with a vendor that we can't defend (due to incomplete drawing). Give plenty of tolerance where you can, and hold things down when you need to.