Does secondary datum feature-B required?

[Madhu454]

Hi All,
Please see the attached drawing.
1) The Primary datum feature A is the cone surface. (Datum Axis and Datum Point can be derived.) As per the standard, 5 DOF are constrained by selecting the cone surface as datum.
2) I have choosed datum feature-B as the secondary datum, since it is also one of the mating surface.

I am not confident on using the secondary datum feature-B, since primary datum is already controlling 5 DOF (except one rotational constraint). Any way the datum feature-B will not be controlling the remaining 1 DOF.

Anyone please help me

 

[bxbzq]

In '09 std, you can customize your datum reference frame. See fig. 4-45.

 

[greenimi]

Mandhu,
I think here is a good start for your issue: Subject: Conical surface as datum (number four on the list)

http://www.geotolmeadows.com/newsletters/2009/nov2009.htm

I really hope I don't brake any eng-tips.com policy rules. If I did, please delete the post.

 

[axym]

Madhu454,

You're right that datum feature B will not control the last degree of freedom, per your attached drawing. Since it cannot constrain any of the available degrees of freedom, referencing it as secondary is technically illegal.

If you want feature B to control axial translation, then there are a couple of different options:
-Reference datum feature B as primary (this would make B control two rotations and one translation, and make the cone only control two translations)
-Use a customized datum reference frame, as suggested in the other responses

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[Madhu454]

Hi All,
Thanks for the reply. Now it is clear

 

[greenimi]

Re:"Since it cannot constrain any of the available degrees of freedom, referencing it as secondary is technically illegal"

Evan,
Isn't the word "illegal" too strong for this issue?

See Fig. 8.18 from Y.14.5-2009. Having datum B referenced in the profile feature control frame adds something to the profile control? Does the addition of B actually add any value to this control? I am not so sure. But it's "legal" because the standard says so, right
Thank you

 

[axym]

greenimi,

The word "illegal" might be too strong, depending on how you interpret certain statements. Section 4.15 in Y14.5-2009 states that "only the required datum features should be referenced in feature control frames when specifying geometric tolerances". I suppose that this doesn't explicitly say that referencing additional non-constraining datum features is illegal. But I would say that doing so is a poor practice, and will most likely lead to confusion.

Regarding Fig. 8-18, referencing datum feature B does have an effect. Datum feature A is a cylindrical surface, and constrains two rotations and two translations. So B would control the third translation, in the direction parallel to the axis derived from A.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

Evan,
I agree with you, but because is no basic on Ø24 (size defined by Ø.24±0.2) how depicted profile is different versus profile .002 wrt A (no datum B). Wouldn't have the same effect? Just curious.

 

[greenimi]

Also , if you want to clarify which callouts define size, form , location and orientation, to avoid future confusion on this subject. Thank you