Conical datum features/ Linear Extruded Shape/ Complex Shape 2

[greenimi]

Can a primary datum feature such as Conical datum features/ Linear Extruded Shape/ Complex Shape be specified at MMB?

In other words, can a cone be specified primary datum feature with an MMB modifier?
4.17-2009 states that the “irregular features of size” can be specified at MMB, LMB and RMB, but does not specify anything about any conditions (such as being secondary or tertiary ---not primary---) in order to be called at MMB and all the examples shown (unless I am missing something, which is entirely possible) that have the irregular features of size at MMB/ LMB are depicted with this datum feature being secondary and tertiary (not primary at MMB)

Any condition, that you are aware of, is applicable here?

I know the standard cannot cover every possible scenario, but I am wondering, if something is “forbidden” by other rules (and regulations) in the standard that I am not aware of.

 

[Nescius]

drawoh,

Yes, good catch, it is a sphere. Solid piece.

Let's say that the partial sphere fits into a matching pocket and that the cross hole is clearance for a pin. A loose fit is OK; we just want to guarantee assembly.

The flat face is not controlled in my drawing, so it seems like the hole could orbit right off the part, but that's really an issue of the flat face chopping off the portion of the partial sphere where the hole was/is. So, the hole could intersect the part at an apparently remarkable angle and still satisfy the position FCF. A neat interaction there. Although, if the flat face is uncontrolled, I suppose you could be clever and deliver a full sphere!

 

[drawoh]

Nescius,

The next thing I see is that datum[ ]A is not defined. I assume it is the spherical feature. If this were my drawing, I would call up datum targets, or call up a ring at some arbitrary diameter. You have to control the flat face somehow. Either it, or your through hole must be a second datum.

How difficult this all is depends on how accurate your tolerances are.

--
JHG

 

[Nescius]

I don't know what happened to my datum leader, but sure enough, it's not there.

Regarding the flat face, yes of course it needs to be controlled. I was merely commenting on that interesting interaction

The part is purely hypothetical, only meant to explore the notion of "size".

 

[axym]

Nescius,

I think that your sphere example makes sense, and shows some interesting things.

If the function of the part was as you described, with a clearance-fit pin holding the part to a mating part with a spherical cavity and corresponding clearance holes, then the position tolerance works. (other than the datum feature label being missing as drawoh mentioned).

I also like your comments on the "handles" that we use to describe certain shapes such as spheres and cones, and that these sometimes conflict with the way GD&T really works. These handles relate to the datum points, axes, and planes shown in Fig. 4-3 of Y14.5-2009. In the case of your partial sphere example, the spherical surface has a nominal center point and a basic radius (spherical radius? But that center point and radius really have no relevance to the part and its function - it's all about the surface, and its relationship to the hole. The feature functions like a surface, that happens to be nominally spherical. Just because the surface s nominally part of a sphere, this doesn't mean that it functions like a sphere and we have to somehow extract a center point datum (handle) from the actual part surface. The center point and radius just provide a more convenient way of describing the basic geometry (and the geometry of the theoretical datum feature simulator). If the datum feature surface was a blunt cone, then an an axis and apex could be defined on the drawing but it would have the same non-relevance. If the datum feature surface was a more complex nominal shape, such as a blunt oblong quasi-cone like in 4-3 (g), then an axis, point and center plane handles could be defined on the drawing. But again, they would have the same non-relevance. If the datum feature surface was an blunt arbitrary surface of revolution, similar to 4-28, then the point/axis/plane handles could be defined but they would be completely arbitrary.

So where am I going with this, you may be wondering? I think I'm agreeing that in GD&T it's really all about the surfaces - the fact that the position requirement can be verified with a gage, that only has surfaces and not handles, shows this. So we need to describe datum feature constraints in terms of the surfaces - sometimes the handles get in the way.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca