Datums BC Sometimes A
Datums BC Sometimes A
(OP)
On a simple bock with a hole or boss, I’ve seen three datum references ABC and a single positional tolerance. I understand the need for the xy placement tolerances of B and C relative to the two side planes, but not the top surface A.
Obviously a feature has to be on the surface referenced as A, but the positional tolerance relative to datum A does not make sense. it’s not like it is saying it can be some amount from the surface.
Separate parallel and perpendicular control frames make sense. I’ve read where perpendicularity is assumed from the positional, but in that case the actually tolerance value does not seem to unless the value is simply a catch-all tolerance for features.
I’ve also seen just B and C, with A treated separately
Obviously a feature has to be on the surface referenced as A, but the positional tolerance relative to datum A does not make sense. it’s not like it is saying it can be some amount from the surface.
Separate parallel and perpendicular control frames make sense. I’ve read where perpendicularity is assumed from the positional, but in that case the actually tolerance value does not seem to unless the value is simply a catch-all tolerance for features.
I’ve also seen just B and C, with A treated separately
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RE: Datums BC Sometimes A
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Datums BC Sometimes A
Fox Manufacturing Group http://foxmfggroup.com/Home.html
RE: Datums BC Sometimes A
If you lop off the reference to datum A, then you would be saying that datum B is primary, and that would not accurately simulate that functional contact situation in 3-D space.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Datums BC Sometimes A
Fox Manufacturing Group http://foxmfggroup.com/Home.html
RE: Datums BC Sometimes A
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Datums BC Sometimes A
Perhaps attached pdf will be useful.
http://files.engineering.com/getfile.aspx?folder=5...
Of course this does not mean that there are no real life situations where |B|C| couldn't be used - I just wanted to show a difference between these two tolerancing schemes.
RE: Datums BC Sometimes A
Fox Manufacturing Group http://foxmfggroup.com/Home.html