Partial Arc or Cylinder as a Datum?

[randy64]

I searched around a bit and found some conflicting opinions on this.

What I'm referring to is a cylindrical feature that is, let's say, about 30 degrees of an arc.

Can or should this be used as a datum feature?

I've seen the following: "It's an example of a "partial feature of size" which is a classic pain in the CMM world. The partial arc (in this case it's around 1/4 of a circle) has no opposed points anywhere, and is technically not a feature of size. Finding its actual "axis" is highly uncertain if the feature has any form error."

Also seen talk of 4.5.10.1 from -1994 (which is what we use here), Mathemetically Defined Surface showing that it's okay to use a partial cylinder as a datum feature.

What I seem to be seeing is that it is legal, but in reality it is a bad practice that may give bad results. Would you say this is a correct assessment of the situation?

What say you?

Thanks

 

[JNieman]

I would say it is legal but they are correct in stating the inconsistency and difficulty in nailing down, with accuracy, the true center.

For example, tooling balls are technically spherical, but we can only ever use a partial surface (though a vast majority of it) to define the center point of that sphere. It's also a relatively superb surface (hopefully) which helps.

However, 30 degrees of an arc.. of what radius? What process created it? That will affect the ability for a CMM to get consistent results. The less you have, the wider the possible results. The more points to define a feature, the better. At a certain point toward the "fewer" end of the spectrum, the ambiguity of a feature increases. 3 points can define a sphere, but the more points the better defined, more precise, it is.

_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[axym]

randy64,

I would consider the "should" aspect of this first. First of all, does the partial cylinder function as a datum feature? Does this surface constrain some degrees of freedom of the part, or of a mating part? If so, how is the constraint accomplished? If not, why is it being considered as a datum feature?

I wouldn't worry about the CMM aspects right away. CMM's can do some things that are impossible in physical reality (such as finding the "center" of this partial cylinder) and also have certain limitations compared to physical gaging equipment. Unless you're using some sort of metrology-based method for aligning the parts during assembly, I would leave the CMM considerations until later.

I would say that to use the partial cylinder as a datum feature, it would need to be treated as a mathematically defined surface. This was shaky ground in ASME Y14.5M-1994, with no real definition of the simulator or how to establish a datum. The partial cylinder can't be defined as a feature of size, because there are no opposed points and no actual mating envelope.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[drawoh]

randy64,

Your datums are fixturing points. Always ask yourself how you will fixture to your datums.

Two datum targets on one side and one on the other end of the curved face will make everybody fixture the same way. How about doing that?

--
JHG

 

[Belanger]

This is probably a perfect situation for which the new term "irregular feature of size" in the 2009 would apply: a feature that might not be a traditional FOS, but one that "may contain or be contained by an actual mating envelope that is a ... cylinder." With that nuance, it could be used as a datum feature.
Also see Fig. 4-29 in the standard, where datum feature B is a partial arc.

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

 

[CheckerHater]

Randy,

Could be you more specific about your part?

Anything can be a datum, including complicated (mathematically defined) surfaces, even by 1994. Also, as JP mentioned, per 2009 anything you can wrap some sort of envelope around, can be considered irregular feature of size.

So, how your part looks like (sketch will be appreciated)? By datum do you mean primary, secondary, or tertiary? Radius surface can be good enough for clocking if oriented properly.

Right now I could only quote general requirement as "..a datum feature should be accessible on the part and be of sufficient size to permit its use" (1994 Para. 4.3)
So, is your datum feature useful?

 

[randy64]

CH,

The datum feature is useful. It's part of my tool that locates the product piece for a grinding operation.