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# Cylindrical Datums

## Cylindrical Datums

(OP)
Hi everyone,

New to Eng-Tips, hoping you guys can help me out.

I'm a little confused regarding the use of a tertiary datum to constrain rotation on a cylindrical part. ASME Y14.5-2009 Fig. 4-8 makes perfect sense to me as the part is symmetrical. But what happens when the part is not symmetrical?

1. Is it safe to say that any part that isn't symmetrical that uses a cylindrical secondary datum would require a third datum to limit rotation?
2. Does the third datum need to be referenced relative to the second?

Section 4.10.4 "Constraining Rotational Degrees of Freedom" explains how but it doesn't really explain when it's necessary.

See the attached sketch. Is the example on the left unconstrained? Is the example on the right better?

Hope this makes sense. I'm still fairly green when it comes to GD&T

Thanks
Sergio

### RE: Cylindrical Datums

Spari,

Your datums must immobilize the part. The example on the right is fully constrained. It helps to try visualizing your fixture.

--
JHG

### RE: Cylindrical Datums

Spari:

My preference: where the geometry is better defined, control all of the motion of the part. So I pick the example on the right. Although Datum feature C may not be functional, specifying it orients the two perpendicular planes from Datum cylinder B parallel/perpendicular to Datum C. This helps the shop with the machine tool setup and allows QA to select the same features when they inspect the part - no ambiguity. Without Datum C any of the 4 sides (or other features for that matter) could be selected to control rotation.

Certified Sr. GD&T Professional

### RE: Cylindrical Datums

If all other features were to be toleranced with the underlying rule of "simultaneous requirement," then you wouldn't need a third datum. But outside of that special case, I agree with the previous two comments.

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

### RE: Cylindrical Datums

Spari,

Symmetry has nothing to do with whether a datum reference frame is fully constrained. It may have something to do with whether a fully constrained datum reference frame is desired though.

#### Quote (Spari)

1. Is it safe to say that any part that isn't symmetrical that uses a cylindrical secondary datum would require a third datum to limit rotation?
No. Consider ASME Y14.5-2009 Fig. 4-24, except with the hole identified as datum feature C. Optionally, imagine there is some additional feature making the part non-symmetrical. Datum reference frame |A|C| is fully constrained in all six degrees of freedom without a tertiary datum feature reference. Datum reference frame |B|C| is fully constrained in all three rotational degrees of freedom, again without a datum feature reference. Datum reference frame |B|A| leaves one rotational degree of freedom unconstrained, but that is not necessarily a problem.

#### Quote (Spari)

2. Does the third datum need to be referenced relative to the second?
It is generally required to define, at least to some extent, the basic relationship between a datum feature and any higher precedence datum features involved in the same datum reference frame. It is usually desired, and sometimes required, to control that relationship with tolerances.

#### Quote (Spari)

Section 4.10.4 "Constraining Rotational Degrees of Freedom" explains how but it doesn't really explain when it's necessary.
It is necessary if a tolerance does not provide the desired control when using a datum reference frame that has unconstrained rotational degrees of freedom.

pylfrm

### RE: Cylindrical Datums

Spari:

I missed that you questions were generic and not specific to the examples presented.

Considering the limitlessness of designs, you cannot make generic statements about motion constraint. The Standard gives representative examples to clarify GDT concepts/conventions, which may or may not be comparable to the particular part at hand. Each design must be analyzed for datum selection based on "function and fitup" requirements. The type of datum features selected and their sequence will constrain the motion necessary for the part features to function.

Personally, over the past 35 years of being around GDT, I have expanded my knowledge and understanding mostly by buying every book that I came across and studied the examples to see the approach used apply GDT. Recently I have used this fantastic forum confirm my understanding, but I would not rely on it solely to move you down the GDT "road".

Certified Sr. GD&T Professional

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