Primary datum axis and secondary datum plane
Primary datum axis and secondary datum plane
(OP)
Hello,
I would be grateful if someone could help me to understand the following,
if a cylindrical axis is permitted to shift wihtin a φ0.05mm tolerance zone in relation to the primary datum (axis) and secondary datum (plane), would it really be necessary for the existence of an secondary datum plane? If i am measuring this axial positional tolerance with a CMM machine, is it really necessary to assign plane B as the secondary datum? Or i would get the same result for just assigning axis A only. To me, that secondary plane seems a bit redundant as i imagine we are simply inspecting how deviating the questioned cylindrical axis is from the primary datum axis, and if it falls outside of the φ0.05mm tolerance zone then it would not be in conformance with the requirement. Right?
Thanks in advance.
I would be grateful if someone could help me to understand the following,
if a cylindrical axis is permitted to shift wihtin a φ0.05mm tolerance zone in relation to the primary datum (axis) and secondary datum (plane), would it really be necessary for the existence of an secondary datum plane? If i am measuring this axial positional tolerance with a CMM machine, is it really necessary to assign plane B as the secondary datum? Or i would get the same result for just assigning axis A only. To me, that secondary plane seems a bit redundant as i imagine we are simply inspecting how deviating the questioned cylindrical axis is from the primary datum axis, and if it falls outside of the φ0.05mm tolerance zone then it would not be in conformance with the requirement. Right?
Thanks in advance.





RE: Primary datum axis and secondary datum plane
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Primary datum axis and secondary datum plane
If the toleranced cylinder is nominally coaxial to the primary datum axis, then a secondary datum plane would not constrain any relevant degrees of freedom.
If the toleranced cylinder is nominally parallel and offset from the primary datum axis, a secondary datum plane would constrain the last rotational degree of freedom (if it is not nominally perpendicular to the primary datum axis).
If the toleranced cylinder is not nominally parallel to the primary datum axis, then a secondary datum plane would constrain at least one rotational and one translational degree of freedom.
Evan Janeshewski
Axymetrix Quality Engineering Inc.
www.axymetrix.ca
RE: Primary datum axis and secondary datum plane
I have attached a link below to clarify my question. As shown in the picture attached, the toleranced cylinder is indeed coaxial to primary datum axis. So, from my understanding of both of your reponses, datum axis alone is enough to constrain its degree of freedom. Am i understanding correctly?
Another thing i am puzzled with is, i was told by our CMM operator that to determine if φ30 cylinder is within positional tolerance, the CMM machine will first scan the φ9.0 cylinder, obtain an axis, and project this axis onto datum plane B to acquire a point and its cordinate (X1). And scan the toleranced cylinder, obtain its axis, project it onto datum plane B to get another point and its cordinate (X2). Then CMM will automatically calculate based on how far cordinate X2 has deviated from cordinate X1 to see if its in tolerance. And he assures me that is how CMM gets its positional value. Is what he said true?
Thanks in advance.
RE: Primary datum axis and secondary datum plane
Tunalover
RE: Primary datum axis and secondary datum plane
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Primary datum axis and secondary datum plane
John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II
RE: Primary datum axis and secondary datum plane
We only see the fragment of the drawing, not the entire part.
If, for example, there is another feature related to the same datums in the same order, then pattern relationship will be implied thru sim requirement. That will make different kind of animal