Best fit alignments

[dwade]

I have never used best fit alignments and am need of help. My part has 4 datum target points on a curved surface (D,E,F and G), then 3 diameters are listed as A, B and C.

The print is showing a DRF | A | B | C | D-E-F-G | to control profile. I've done iterative alignments with no problems but have never had to use 3 diameters for a best fit alignment.

TIA

Duane

[kbotta]

Duane,
Is there anyway you can post a snipet of the drawing - with the other datum features, and FCF?
What is going to be important here, is just what does your other datum targets restrain. To see that (and show you how to use BF properly) would help.
Kev

[dwade]

Duane,
Is there anyway you can post a snipet of the drawing - with the other datum features, and FCF?
Kev

Kev,

There is a PDF attached to my post

[kbotta]

"and FCF?"

I cannot see the FCF in question. (cut off)

From what you typed - it looks as if the ABC DRF already restricts the 6 DOF. That means you CANNOT use the D-E-F-G in the alignment.

Incidently,
Have you ever heard of the "can may must" rule?
If a datum feature CAN restrict a degree of freedom, and MAY (no other Datum Feature is restricting it >or< datum precedence here) than it MUST restrict that DOF.
Your example fails that test.
Looks like a invalid FCF from what you have typed above.

KB

[kbotta]

Maybe it should be |A-B-C |D-E-F-G| ?
Looks like the pattern of holes stops everything except the last translational DOF, which is what the datum target's would stop. Is that correct?
Not with 4 segments in the DRF area os the FCF...right?
Little misunderstanding...sorry.

So, if the precedence described above is correct |a-b-c| [space] |d-e-f-g|
You could BF the hole pattern, and then select the highest point (or lowest depending on how you look at it) to set your last pt of origin (restricts last DOF).
Think of it as a gage would see it. Hole pattern alignment, then it sits on the highest (or lowest) pt of the DT features. It would only be 1 pt though...or course - if datum feature modifiers are used - then that is a whole different ball game. It helps to see the FCF in question....
kb

[dwade]

Incidently,
Have you ever heard of the "can may must" rule?
If a datum feature CAN restrict a degree of freedom, and MAY (no other Datum Feature is restricting it >or< datum precedence here) than it MUST restrict that DOF.
Your example fails that test.
Looks like a invalid FCF from what you have typed above.

KB

Kev,

I've never heard of that and what source did it come from.

Here is more of what I've copied from the drawing. I honestly believe some of the FCF's on the drawing are not legal per the standard.

Duane

[kbotta]

Duane,
moved to Pm's....
Kevin