DRF Origin and Clocking for Cylindrical Secondary Datum

[dtmbiz]

The attached jpg has 3 questions regarding a clocking feature.

In another thread Pmarc was trying to convince me that the DRF origin was the center axis of that secondary feature. I argued it was not based on the Rule that a DRF is 3 mutually perpendicular planes, and that the tertiary datum when it is a surface feature that establishes a plane is part of establishing the DRF origin. I now see and concede that my interpretation was in error because I was not considering 4.4.2 which states that the secondary datum axis is the DRF origin. It is confusing that the standard states that a tertiary datum establishes the third datum plane of a DRF; except now I see that in another section there is another rule that overides that concept.

4.4.2
….“This axis serves as the origin of measurement
from which other features of the part are located.
See Figs. 4-5, 4-11, and 4-12”

In the situation that a tertiary datum is used for clocking purposes, it is actually a “fourth plane” involved in establishing the orientation of three planes which define a DRF with only 2 datum features.

My lingering question is seen as Question #1 in the jpg posting. The standard's examples show and the wording indicates that the tertiary datum which serves to establish the orientation for the secondary datum’s 2 datum planes derived from the cylindrical datum axis; is a tertiary plane that “intersects” the secondary datum plane axis. In this jpg and in the other thread the tertiary does not intersect the secondary datum axis. It is parallel and seems possible. Is this acceptable or should it (Datum C) be rejected in this case.

Comments on Questions 2 & 3 shown in the posted jpg would be appreciated also.

 

[PeterStock]

Datum C could be used for clocking by restricting the rotation and as such it is valid. I would not concider it the best choice, I would use the hole pattern as the pattern is probably more inportant than the one flat surface from C.

All the geometric tolerances in the Question 2 view refer to the same DRF A|B and are to be met at the same time (your simultaneous requirement). As such there is no need for a clocking datum. This is how I would probably do this part, possibly changing the position of B to itself to a perp to A.

The 102.98 dia surface in Question 3 has opposing elements and would be a feature of size. It is interrupted by the square feature of size on top of it.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

http://www.infotechpr.net

 

[pmarc]

Hi dtmbiz,

Question #1:
Technically your callout for datum C is correct.
Third datum feature does not have to intersect an axis established by secondary datum feature. I agree that Y14.5 does not give many examples of such situations, but fig. 5-42 from 1994 edition shows that.

Question #2:
As Peter said, since position and profile callouts use the same DRF, simultaneous requirement applies and a clocking datum is not needed if a relationship between part's outline and the pattern of 4 holes is considered.
Theoretically - using the same logic - if datum B reference was removed from every FCF on your left drawing, the goal would be also met. It is not so common and intuitive pratice, but it is worth to know.

Question #3:
Per Y14.5M-1994 the set of 4 surfaces could be rather considered as 2 separate features of size.
However Y14.5-2009 introduces "irregular feature of size" concept which says that irregular FOS "is a directly toleranced feature or collection of features that may contain or be contained by an actual mating envelope that is a sphere, cylinder or pair of parallel planes". I believe your set of surfaces perfectly suits to this definition.

 

[dtmbiz]

Thanks for the input Peter and Pmarc.

The problem I have with the feature of size is that the 1994 standard states it can be "one" cylindrical surface. Even thought the four radii are opposing, they are 4 separate surfaces.

The original dim scheme on the right was done by someone else and I see it with numerous errors. Using concentricity in this application is advisable either IMO. I prefer to tolerance per the left view.

 

[dtmbiz]

...concentricity NOT advisable....

 

[SeasonLee]

dtmbiz

Question #1:
You may use the edge as datum C, since the four straight edges are symmetrical with datum B, so I will make choice of the center plane of the width as datum C, just move the datum feature symbol C in line with the dimension line to establish the center plane. This will stop the last freedom of rotation and establish the origin of the measurement.
Fig 4-5 of 1994 standard is a symmetrical part, it does not require a tertiary datum (A tertiary datum will make no sense at all for this case). But, for your case a tertiary datum is required since it is not a symmetrical part.

Question #2:
Position callout CFC need tertiary datum C to fully define the DRF.

Question #3:
Ø102.98 is called interrupted feature of size in 94 standard, and now is called irregular FOS in 2009 standard.

SeasonLee