Straightness Callout of a Datum

[brandnew1]

Hi all,

i was just glancing over another thread and i think it may relate but i just wanted to run this by you to see if this makes sense.

Attached is a blue print based on Y14.5-1994 which calls out datum C to have a straightness with .012, however datum C runs at least 16" in length.

My question relates to the runout callout of .001 to datum C on one side and a .002 perpendicularity callout to datum C on the other. If the part were to take the max tolerance of .012, would the part still have a .002 perpendicularity on one side or a .001 runout on the other?

When looking straightness in the 1994 standard i see figure 6.1, but this points out to an outer diameter rather then in inside diameter such as in the attached pdf.

Thank you for the help and clarification.

 

[pmarc]

brandnew1,
First saying shortly, if the part was to take the max tolerance of .012, it would still have a .002 perpendicularity on one side or a .001 runout on the other.

It is happening like that because for both tolerances, runout and perpendicularity, datum C is called out at RFS (RMB)basis. It somehow means that size and form of datum feature C has no impact on values of geometrical tolerance using datum C as a reference.

For this case however even if there was (M) modifier specified after C in perpendicularity FCF, it would have no impact on perpendicularity allowable tolerance value.

Runout callout can't have (M) in datum portion of FCF so the theoretical situation can't apply at all.

 

[dingy2]

Yes! Once the feature meets the .012 straightness tolerance, it becomes datum C and is assumed perfect. All the other requirements relative to datum C would have to be met.

Dave D.

http://www.qmsi.ca

 

[powerhound]

You need to have a diameter symbol inside the feature control frame that specifies straightness in order to make this callout correct.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[brandnew1]

To add to this, i'm coming from an inspection point of view.

From what i can gather, whatever the straightness value ends up being, so long as it is within the .012 tolerance, this will become the Datum C reference to which all other callouts are to meet.

But when it comes to measuring on the CMM my concern is that it will say it is not meeting those requirements (even though it is), especially if a part ends up looking like a) of the attached pdf.

Would it be a 'fair' assessment to measure the straightness of the entire area of Datum C and as long as this passes, go ahead and take a sub-section of the part that is closest to each side (.002 perpendicularity / .001 runout) to verify that these come within tolerance?

thanks again for the help