Question about Datums
Question about Datums
(OP)
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
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





RE: Question about Datums
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
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RE: Question about Datums
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.
www.qmsi.ca
RE: Question about Datums
Paul
RE: Question about Datums
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
RE: Question about Datums
RE: Question about Datums
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
RE: Question about Datums
If that is STILL the case, there should be no need for secondary and tertiary tatums for the perpendicularity requirements.
GEOMETRY IN 1953.
RE: Question about Datums
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
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
RE: Question about Datums
Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
RE: Question about Datums
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
RE: Question about Datums
Paul
RE: Question about Datums
How many degrees of freedom can a "deep" parallel hole pattern constrain when used as a primary?
Five, all but one translation perpendicular to the axes of the holes.
Paul