【转帖】primary secondary and tertiary datum planes

yang686526

primary secondary and tertiary datum planes
what are the rules to locate which features as the primary, secondary, and tertiary datum features?  
weisusu, take a look at asme y14.5m-1994 section 4 assuming that's what you work to.
perhaps the most important one that sprins to mind is that they should be selected based on function..
again it seems you need to do some reading and hopefully get some training.
kenat,
one of the things that i was taught in gd&t training was to think of the planes in the order you would mount the part in a fixture for inspection.
  
"wildfires are dangerous, hard to control, and economically catastrophic."
ben loosli
related to the original question, do the primary, secondary and tertiary datums have to follow a-b-c order?
no
you can place the part on c (primary) locate it to a (secondary) and position it to b (tertiary).
  
"wildfires are dangerous, hard to control, and economically catastrophic."
ben loosli
can someone cite a paragraph reference, i get beat-up about this all the time by our wonderful qa folks.
a quick google search:
mad,
see para. 4.3, at least that's where it was in the '82 std.  from that para., "the desired order of precedence is indicated by entering the appropriate datum reference letters, from left to right, in the feature control frame".
i.e. the oop can vary from feature to feature.   
pretty close trueblood. in y14.5-1994, paragraph 4.4 "specifying datum features in an order of precedence" covers it.
thanks folks, my argument is won for the day.