【转帖】asme y14.5 draft 2 4.12.7 translation modifier

yang686526

asme y14.5 draft @ 4.12.7 translation modifier
in the new asme y14.5 draft @ 4.12.7 translation modifier. "when it is necessary to indicate that the basic location of the tgc is unlocked and the tgc is able to translate within specified geometric control to fully engage the feature, the translation modifier is added to the feature control frame following the datum feature referenced and any other applicable modifiers. when the translation modifer is applicable, and direction of movement is not clear, movement requirements shall be specifed."
what is that all about? can anybody give me a clear explanation of what they are realy after? is that similar to datum shift? and when is it not clear? or when is it clear? any help?  
eng-tips forums is member supported.
what is tgc?
matt lorono
cad engineer/ecn analyst
silicon valley, ca
?    maximum material boundary (mmb)
?    least material boundary (lmb)
?    true geometric counterpart (tgc)
?    regardless of feature boundary (rmb)
?    degrees of freedom (dof)

sorry, you did say that before.
next, what is meant by "unlocked" and "fully engage the feature"?  
sorry for being stupid here.  i just haven't seen this type of terminology in my daily life.   
matt lorono
cad engineer/ecn analyst
silicon valley, ca
now you get it-that is part of the problem.  what the heck are they trying to say? it is not clear to me either.
you should not be expected to understand the new terminology till you have have had a course and got the textbook that accompanies it.
ringman,
i'm not sure if you are being sarcastic.   anyway, my take on this is that standard should fully explain itself without additional materials unless it refers to materials that are themselves fully explained.  that is why there is a lengthy definitions section within each of the standards.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
i agree that a standard should fully explain itself without additional materials unless it refers to materials that are themselves fully explained. i am simply asking anybody that has this standard, if they understand it to please, if they could maybee explain it in a clearer manner, to give me and others a better understanding of what they are saying. the lengthy definitions i understand.
here is what james meadows had to say: "this is the result of a long drawn out fight between individuals and committees.  it's known as the "tertiary datum problem".  some of us said (as it currently says in the y14.43 standard) that since the tertiary datum feature of size is only to give angular orientation to measurements taken from the secondary location datum axis (of a hole or shaft) that the datum feature simulator (real gage or fixture element) and the tgc (imaginary datum feature simulator) should translate (move) toward or away from the secondary simulator.  others said that both simulators should be represented as stationary, separated by their basic dimension, as is currently done for pattern datums.  the stationary folks won the vote.

then some of us pointed out that the result of this vote is that we aren't really measuring distance from the secondary hole or shaft, but are really measuring distance from both the secondary and tertiary datum features of size.  what are we supposed to do when we want to actually have the secondary locate and the tertiary give us an angle of measurement?  they came up with the translation modifier.  it says that the tertiary (or sometimes secondary) datum feature it is attached to can be represented by a simulator that moves toward or away from the secondary (or sometimes primary) datum feature that precedes it in the feature control frame.  that way the tertiary datum feature of size only gives orientation to the measurements.

it's a mess created to solve a problem that rarely existed.  but the faction got what it wanted and that was a default rule that all datum feature simulators (real) or true geometric counterparts (imaginary) are stationary and separated by their basic dimensions (unless the translation modifier is used).

since you didn't mention the planar datum features shown with a profile tolerance that can be referenced at mmc and represented at their virtual condition distances/sizes or the rfs ones that can grow from their basic dimensions until they contact the part, i won't either.  
things have gotten out of control, but there is no stopping this runaway train."  

thanks jim
brandy,
that does explain it.  now they just need to add that text (without the history behind it) to the standard. lol
  
matt lorono
cad engineer/ecn analyst
silicon valley, ca
in defense of the standard, there is a "means this" diagram (fig 4-37) that illustrates what happens when the translation modifier is applied.  it's difficult to picture what's going on from the text alone.
evan janeshewski
axymetrix quality engineering inc.