Primary datum axis and secondary datum plane

[softserve1020]

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.

 

[Belanger]

A sketch might help clarify your question, but here's a general answer... The primary axis alone would not stop all necessary "degrees of freedom." I can establish a datum axis, but the rest of the universe can still rotate around that axis, and also translate longitudinally along that axis. If your toleranced axis only needs to be held a a particular radial distance from the datum axis, then the secondary plane would not be needed. But to fully control its location, the primary axis alone isn't enough.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[axym]

The answer depends on the basic relationship between the toleranced feature and the datum features.

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.

http://www.axymetrix.ca

 

[softserve1020]

Thank you both for your answers!

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.

 

[Tunalover]

If Datum B is a surface the you've got it presented wrong but your hole seems to be sufficiently toleranced since Datum A creates two datum planes that are free to rotate. But in your application it doesn't matter that they do (e.g. no clocking datum required). As a secondary datum, Datum B is sufficient. There is nothing in the rule book that says that all degrees of freedom have to be constrained for a tolerance to be sufficient, that is, a clocking datum is not always (and I dare to say rarely) required.


Tunalover

 

[Belanger]

The difficulty might stem from the common idea that a CMM needs to stop the part fully in all directions before it can measure a part. So the CMM person might want to see the part's rotation around those axes stopped -- thus the addition of the datum B that we're all saying is meaningless. For this particular GD&T callout that is would be overconstraining the part.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[powerhound]

So I think the direct answer to your question is that, in this case, reference to datum B has no effect.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[CheckerHater]

Safe way to say will be that it has no impact on this particular position relationship.

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