Pattern as a Datum; dimension origin
Pattern as a Datum; dimension origin
(OP)
(Pls see attached drawing)
Balloon 1:
Datum B is the pattern of 4 bosses
(delineated in this way for illustration only)
Balloon 2:
Dimensions originate from Datum pattern axis
Balloon 3:
Dimensions originate from "features" of pattern Datum B
Balloon 4:
Dimension is interpreted as equal distant from the
axis of Datum B; or is "half dimension" required
as shown 29.00 BSC in Balloon 2 area?
Question 1:
In our company there are some who say dimensions relating to a Datum pattern
originate at the axis of the pattern. Others say dimensions originate from the features
of the pattern. I don't see any example figures in the standard (1994) that make this clear.
Where should basic dimensions originate from in relationship to the referenced pattern as a Datum;
Pattern axis, or feature ?
Question 2:
Are dimensions that are related to an axis of pattern Datum assumed to be equidistant?
Or are half dimensions required?
"4.2.2.1 Mutually Perpendicular Planes. The
planes of the datum reference frame are simulated
in a mutually perpendicular relationship to provide
direction as well as the origin for related dimensions
and measurement."
I believe the dimensions should originate from (or "trace back to") the axis of Datum B in this case, because it is part of the DRF.
I also believe that dimensions should not be "assumed" to be equidistant and should have
"half" dimensions that relate back to the DRF.
Comments?
Balloon 1:
Datum B is the pattern of 4 bosses
(delineated in this way for illustration only)
Balloon 2:
Dimensions originate from Datum pattern axis
Balloon 3:
Dimensions originate from "features" of pattern Datum B
Balloon 4:
Dimension is interpreted as equal distant from the
axis of Datum B; or is "half dimension" required
as shown 29.00 BSC in Balloon 2 area?
Question 1:
In our company there are some who say dimensions relating to a Datum pattern
originate at the axis of the pattern. Others say dimensions originate from the features
of the pattern. I don't see any example figures in the standard (1994) that make this clear.
Where should basic dimensions originate from in relationship to the referenced pattern as a Datum;
Pattern axis, or feature ?
Question 2:
Are dimensions that are related to an axis of pattern Datum assumed to be equidistant?
Or are half dimensions required?
"4.2.2.1 Mutually Perpendicular Planes. The
planes of the datum reference frame are simulated
in a mutually perpendicular relationship to provide
direction as well as the origin for related dimensions
and measurement."
I believe the dimensions should originate from (or "trace back to") the axis of Datum B in this case, because it is part of the DRF.
I also believe that dimensions should not be "assumed" to be equidistant and should have
"half" dimensions that relate back to the DRF.
Comments?





RE: Pattern as a Datum; dimension origin
RE: Pattern as a Datum; dimension origin
The reason I say not to dim to the axis: think of another four-boss pattern that is not a nice rectangular pattern; maybe they are scattered at different locations. We can still call them datum B, but it would be nearly impossible to dimension to the "centroid" of the pattern.
I don't have the standard handy today, so I'll have to look later for an exact statement to back me up...
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Pattern as a Datum; dimension origin
I would also try to use pattern center as an origin whenever possible (just for clarity), but as J-P said if irregular patterns are considered it could be really hard to find centroids of those.
Unfortunately, as you noticed, Y14.5M-1994 gives very few examples of defining pattern of features as a datum feature so it would be quite hard to find proper example.
RE: Pattern as a Datum; dimension origin
John-Paul,
What about the DRF origin. The center of the pattern is the axis with 2 planes intersecting at 90 deg.,
right? Wouldn't the center planes be relatively easy to find by touching of the virtual pin locations?
Are you saying that you would measure from "a" boss and then add a "half the distance between bosses to get back to the DRF origin?
Pmarc,
I do agree that this has been my way of thinking also.
Since a DRF should be the result of datums selected because of part function and interface; my view point is that if the dimensions come from the pattern axis, then it would be apparent that the part features are being centered in this case. Vs. selecting edges for example that serve little function relative to the mating parts.
Your comments are appreciated
RE: Pattern as a Datum; dimension origin
The same thing goes for the DRF origin. It doesn't really matter where it goes, to me it's purely arbitrary. I realize that this doesn't completely agree with what Y14.5 says, particularly in '94. Some of my biggest pet peeves with Y14.5 are in this section, particularly the concept of the "axis of a pattern". This is a flawed concept that doesn't work (IMHO) and has led to a lot of confusion.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Pattern as a Datum; dimension origin
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems