Datum & DOF

[Madhu454]

Hi All,
I saw a drawing where primary datum and secondary datum constraining all the 6 DOF, but they have used 3rd datum also, I felt which is wrong. before I question the Engineer, I wanted to get it clarified with you that, by any change can we have 3rd datum, when we can constrain all DOF with the primary and secondary datums.?

 

[pmarc]

If primary and secondary datums really constrain all 6 DOFs, I do not see a reason of specifying tertiary datum.

 

[fsincox]

I agree, seems it is only going to aadd confusion. Like adding datums to a drawing that are not referenced anywhere.

 

[Belanger]

Are you sure that all three datums are referenced in the same feature control frame? If so, then yes I agree with the other guys.

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

http://www.gdtseminars.com

 

[SeasonLee]

Would you mind to post a sketch? it will be easy for all.

SesasonLee

 

[Madhu454]

Yes Belanger, all the datums are referrenced in the same FCF.
Hi Seasonlee, I dont have the sketch with me right now.

 

[drawoh]

Madhu454,

If I have a concentric part, there is no need to control rotation about the main axis. 5[ ]DsOF will constrain the part adequately. This is true up to the first non-concentric feature, such as a flat face or pitch circle.

When you add the second non-centric feature, you have to control clocking.

--
JHG