Hey everyone, first time poster here. I've got a bit of a GD&T quandary and I'm hoping someone can offer some suggestions. I've designed a fixture for a customer and I'm trying to figure out how we're going to qualify it.
I've attached a print screen of the fixture. Basically, it's a plug gauge for to check hole positions on a formed part with pins to make sure it'll assemble. I've got a flat block with two locators in it that are "square to the world" and will locate the part, so those features are what I'll used to establish the coordinate system for the gauge assembly. So the top of the green block would be Datum A, and the locator holes will be Datum B and C to constrain the coordinate system.
To qualify the gauge, I need to know that two drill jig bushings in the angled block are in the right location and orientation. So my understanding of GD&T says that there exists a perfectly straight line somewhere in space that reflects where the "basic" location is for a perfectly located bushing and that the value in the control frame box represents the diameter of a cylindrical tolerance zone that's required to encompass the axis of the measured feature. I get the theory, or at least I think I do. ;P
So theoretically speaking, I could "legally" just put up a feature control frame tying the tolerance zone back to A/B/C and be OK, in theory. However, my CMM software apparently isn't capable of directly showing me the diameter of the required tolerance zone cylinder needed to fully encompass the axis of the measured part as discussed above. So I'm not sure how I'm going to evaluate the accuracy of this thing once it's made and assembled.
Some noteworthy points:
- The top plane of the blue "door stop" block doesn't touch the part by design. So I can't really just bump that plane and level a new coordinate system on it because we really don't care a lot about where that plane is.
- The print for the piece part that this fixture checks is no help. It has no datums, only rectangular dimensions and it's laid out with respect to the outer profile, which we won't be checking, so it's kinda no good to me since we're purely checking hole to hole relationships.
- A method I'm considering: I could take the customer part model and make a print using my A/B/C coordinate system to locate the hole entry and exit points in 3D space. Then in my CMM program, I could mathematically define a work plane that represents the "basic" location of the part's planar surface. I could then check the two holes as cylinders and project their axes onto my work plane to create a point so that I could compare X/Y/Z coordinates for the two points to my created print. But that's pretty involved and it'd probably be tough to sell it to the customer as a means of qualification. But I don't really have a better idea.
Does anyone have any suggestions? To be honest, when I've looked through Y14.5, it doesn't really seem to dedicate much time to a situation such as this. Most positional tolerance examples are pretty straight forward black and white parts, like a plate with three holes in it.
I'd like to hear any recommendations, war stories, or whatever you may have to share that's relevant. Thanks!
You may find your answers in ASME Y14.43, Dimensioningg and Tolerancing Principles for Gages and Fixtures and / or ANSI B4.4M Inspection of Workpieces
Posting Permissions
Reply With Quote