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# Derived median line or surface straightness

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## Derived median line or surface straightness

(OP)
Is the callout shown DML or surface straightness?
1.) DML becauase it is associated with the size dimension
2.) Surface straightness because does not have the Ø symbol.
3.) DML because is bigger than the size tolerance.

"Expert" solution was: According to Rule #1 in the Y14.5 standard on Dimensioning and Tolerancing the size tolerance controls the surface form on all rigid parts. Since the size tolerance is a diameter of .0002, it already controls cylindricity (roundness, straightness and taper) to within a diameter of .0002. This is less than the straightness control of .0003 and the roundness control which is .0002 per side or a diameter of .0004. So, both the straightness and roundness geometric controls should be removed.

Do you agree with the advice?
Why shown callout cannot be considered DML ?

### RE: Derived median line or surface straightness

I thought it was figured out that double the diameter tolerance was the equivalent straightness/cylindricity.

Also, there's no control between the two surfaces marked A. And the datum A targets are suspect as well. Not sure that target points are the correct choice.

### RE: Derived median line or surface straightness

(OP)
So, why the straightness requirement cannot be considered derived median line straightness and consequently can be bigger than the size tolerance?

### RE: Derived median line or surface straightness

In this case that the Derived Median Line tolerance is less than the equivalent straightness limit from the size tolerance.

### RE: Derived median line or surface straightness

This straightness tolerance seems to be of the DML type, because the straightness callout is associated with a feature of size.
Thus, it's OK that the straightness tolerance is greater than the equivalent straightness limit from the size tolerance (and yes it should have a diameter symbol).
But... I suspect that wasn't the intent. Other clues make me think that the designer didn't know about the rule that having straightness tagged to a feature of size means DML straightness.

So to get the real answer, you'd have to ask the designer. This seems to be an older drawing, so you probably can't ask anyone about it, right?

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

### RE: Derived median line or surface straightness

(OP)

#### Quote (Belanger)

This straightness tolerance seems to be of the DML type, because the straightness callout is associated with a feature of size.
Thus, it's OK that the straightness tolerance is greater than the equivalent straightness limit from the size tolerance (and yes it should have a diameter symbol).

If shown straightness “IS” considered/decided to be DML straightness (assumed design intent), then circularity could be equal or even bigger than size tolerance? Could it?

### RE: Derived median line or surface straightness

No -- while straightness leads a "double life," circularity is only to be used as a surface form control. So circularity cannot be larger than the size tolerance. (One minor exception is if the size is given as AVG, but that's irrelevant for now.)

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

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