【转帖】gdt datum selection

yang686526

gd&t datum selection
i am having difficulty setting up the three datum planes for gd&t. i am trying to dimension this wrt to the way the part will be held down during insepection.
it is my understanding that the three planes must be perpendicular to each other. my primary and secondary planes (a & b) are perpendicular. the part is such that there is no plane perpendicular to both a & b. in real life, there will be a jig pin constraining the last dof for the part.
how do i achieve this? can i create datum c normal to a & b and through the point where the pin will be even though it won't lie ony any real surface? do i use datum targets for this?
any help is greatly appreciated.
if you can show a picture, it would be helpful.
chris
solidworks/pdmworks 08 3.1
autocad 06/08
i created a quick sketch. i don't have the actual print w/ me right now. i hope this helps. if not, i can provide a better post tomorrow.
i've never posted pics before. i hope it works.
thanks.
from your sketch, it would appear that a target point for atum feature c would serve the purpose.
ringman,
is it allowable to represent datum c with only a datum target and not show it as a datum plane?
thanks!
yes
ringman is correct.  since the first two planes are defined, only one point is needed to define the third.  imagine a line the shortest distance from the point to datum a and another to datum b.  those lines will be perpendicular and will lie on the plane of datum c.
clear as mud?
believe it if you need it or leave it if you dare.fff"> - robert hunter

i got it. thanks for the help.