penjo1
Mechanical
- May 9, 2024
- 2
Hi all,
First time here and a couple months into GD&T. I have a question regarding position and it's required datums. I've attached an image that portrays the gist of what I'm working through.
TLDR: Is a datum C required to position the holes?
As you can see on the image, I've identified datum A as the top face and datum B as the midplane L to R. I initially had datum C identified as the mating face in the cutout perpendicular to A and B (C1), but it was causing some issues with locating the holes from this surface so I thought of changing it to C2. When consulting a checker, he said to remove datum C altogether and let surfaces C1, C2, and C2's opposing face be controlled purely by size tolerance, and for the holes position tolerance only to reference datum A and B, and using basic dimensions from face C2 to locate the holes. He explained it that when they go to make the holes, they will orient to A and B, touch off on that top surface using the basic dimensions, and then drill the holes. Adding datum C would introduce an extra boundary condition to C, where I thought when using position in 3 dimensions (when your datums are planes and not axes), you had to have 3 datums with basic dimensions leading back to those datums (whether its a single or chain of basic dims). It seems here that by using basic dims to C2, we're using C2 as a pseudo-datum without calling it out...
So is datum C required here or can I correctly constrain the holes without it? Do I need to make the dimensions toleranced instead of basic in that "C" plane if there is no C? Then I feel like that defeats the point of position... and if I do need it, which surface should I use?
First time here and a couple months into GD&T. I have a question regarding position and it's required datums. I've attached an image that portrays the gist of what I'm working through.
TLDR: Is a datum C required to position the holes?
As you can see on the image, I've identified datum A as the top face and datum B as the midplane L to R. I initially had datum C identified as the mating face in the cutout perpendicular to A and B (C1), but it was causing some issues with locating the holes from this surface so I thought of changing it to C2. When consulting a checker, he said to remove datum C altogether and let surfaces C1, C2, and C2's opposing face be controlled purely by size tolerance, and for the holes position tolerance only to reference datum A and B, and using basic dimensions from face C2 to locate the holes. He explained it that when they go to make the holes, they will orient to A and B, touch off on that top surface using the basic dimensions, and then drill the holes. Adding datum C would introduce an extra boundary condition to C, where I thought when using position in 3 dimensions (when your datums are planes and not axes), you had to have 3 datums with basic dimensions leading back to those datums (whether its a single or chain of basic dims). It seems here that by using basic dims to C2, we're using C2 as a pseudo-datum without calling it out...
So is datum C required here or can I correctly constrain the holes without it? Do I need to make the dimensions toleranced instead of basic in that "C" plane if there is no C? Then I feel like that defeats the point of position... and if I do need it, which surface should I use?