¡¾×ªÌû¡¿question about datums

yang686526

question about datums
hello,
if i have a geometric control box(perpendicularity) located to "a", "b", & "c" where "a" is a surface(three point contact) and "b"(two point contact)is a diameter, and "c"(last datum 1 point contact) is a diameter.
how is the datum established if diameter "b" is not 90 deg to surface "a". it is impossible to establish 2 point contact.
robert
  perpendicularity doesn't work like this. is the feature that you are trying to control a feature of size or a surface feature? you can control a surface feature by calling out it out wrt one or two datums, but not three.
  to answer your question; you cannot control a surface or feature that is not perpendicular to the desired datums using a perpendicularity control. you can use angularity or even profile depending on what the feature is. can you post a drawing of what you are looking at?
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
perpendicularity of a surface or feature of size (hole or pin) will reference a primary datum (datum a in this case) and sometimes a secondary datum but never a tertiary datum.
get clarification from the designer in your situation. i wonder if they meant positional rather than perpendicularity??  only the designer knows.
by the way, if you have a hole as a secondary and another hole as a tertiary, it is not a 2 point set up on the secondary and a 1 point on the tertiary. this is only true on surfaces that become secondary and tertiary datums.
dave d.
if the secondary datum feature is a cylinder and defined at an angle other than 90 degrees from the primary plane then it is capable theoretically of restraining (@mmc) or constraining (@rfs) the remaining three degrees of freedom (two translation and one rotation). as the angle approaches 0 degrees the defined tertiary becomes (more irrelevant if the secondary is @mmc) and (erroneous if the secondary is @rfs). as the angle approaches 90 degrees the defined tertiary becomes (more significant if the secondary is @mmc) and (remains erroneous or immeseaureable...requiring the use of the defined tertiary... if the secondary is @rfs).
paul   
thank you for your posts.
i believe i have sorted out my error. i was thinking in terms of surfaces.
i have received clarification and understand that because the secondary and tertiary datums are parallel the perpendicularity can have three datums.
thanks pauljackson. my constraint had mmc for the secondary and tertiary.
robert
what necessitates the use of three datums in your case to establish  perpendicularity.  seems peculiar.  can you provide a sketch perhaps?
ringman,
i wish i could provide a drawing. it would clear up a lot. unfortunately, proprietary issues arise.
for the third datum it was required to relate the clocking position to another diameter feature.
robert
my recollection is 'thru a given point on a line there is only one plane that is perpendicular to the line, and that thru a given point on a plane there is only one line that is perpendicular to that plane.'
if that is still the case, there should be no need for secondary and tertiary tatums for the perpendicularity requirements.
geometry in 1953.
imnotfrommars,
you can provide an example of what you are talking about without providing any actual insistance.  
the soon-to-be-replaced asme y14.5m-1994 shows that when more than one datum is applied to a perp tol, then both datums are primary, that is to say, the surface must be perpendicular to both datums to the same tolerance (fig 6-35).  though the standard allows some creativity with perp tol, it is clear that any datum referenced is solely to establish primary perpendicularity and provides no secondary or tertiary references.   (these are my words as interpretation of the standard, not the actual words in the standard; since the standard does not normally discuss what something is not, only what it is.)
given that, it is impossible to have a single geo tol for perpendicularity to three different datums, as conflicts in tolerancing would arise between competing surfaces.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
btw, as a point of clarification, fig 6-35 does label one datum as primary and one as secondary, but the instructions are "the surface must lie between two parallel lines 0.12 aprt which are perpendicular to the datum planes a and b.  the surface must be within the specified limits of size."  so, the primary and secondary plans are treated the same.
matt lorono
cad engineer/ecn analyst
silicon valley, ca
datum features are like bill tandler says�and i am not necessarily quoting his words exactly but “whatever degrees-of-freedom a datum feature can and is capable of constraining in its order …it must!
as primary…Whatever the degrees-of-freedom a feature is capable of constraining:
a point (sphere)…three degrees �all translation, no rotation
a surface (plane)…three degrees �one translation, two rotation
a cylinder (surface of revolution) four degrees- two translation and two rotation
a cone (surface of revolution with an apex) five degrees -three translation and two rotation.
a torus (doughnut) five degrees-three translation and two rotation.
a hole pattern- five degrees three translation and two rotation.
etc.
note: surfaces should have significant breadth as compared to other features to serve as primaries for measurement stability. cylinders, cones, and hole patterns should have significant depth as compared to breadth to serve as primaries.
   
as secondary: whatever the feature is capable of constraining that is not already constrained by the primary!!!
as tertiary: what ever is left to constrain (if anything).
your comments started out explaining (if i understand them correctly) that the secondary datum feature was a cylinder…and was not defined perpendicular to the primary datum feature. if so “it can�and must�constrain the as many of the remaining three degrees of freedom that it is capable of�because it is capable of constraining the remaining three degrees of freedom …it must �according to geometry)!!!
just wait until the new standard comes out (if it is as i suspect) where a cylinder declared as a secondary is at an angle to the primary surface. it is capable of constraining the remaining three degrees-of-freedom but may be limited to constraining only translation (by the rules). therefore it must have a position callout to the primary, secondary and tertiary rather than just an orientation to the primary.   
paul