how to handle this w/ cmm

[d.evans]

Ive got a large flange (approx 20" by 14") on which, I've got to measure a hole pattern.
Here is my problem. I've seen this sort of datum callout in books, but never had one to deal with myself. How do you handle the B and C datums? The part material is very thin (.050"). I've got it fixtured to a rayco plate and can reach all the external edges.
What I understood to do was this... (PLEASE CORRECT ME IF I AM OUT IN LEFT FIELD)

1) create plane for datum A (constructed plane from vector points)
2) create three points along the left outer edge
3) create three mirrored points along the right edge.
4) create three MID POINTS for each pair of left/right points from step 2 and 3.
5) Do the exact same thing for the top and bottom outside edges creating three more mid-points.
6) create two 2d lines constructed from the mid-points to get a "center-of-the-part" pair of intersecting 2d lines.
7) align: level to plane A, rotate to long "center" line, origin X at long "center" line, origin y at short "center" line, origin z at plane A.

Or should I be using simulated datums? or is there a way to create these datums of size (+/- .020") some other way that I am not thinking of?

Using 3.7MR3, 1mm ruby tip.

[d.evans]

here is the drawing, sorry not in 1st post

here is the drawing

[Wes Cisco]

A sketch would really help. At the very least, please try to let us know what sort of features the datums are and what you measuring to get all those midpoints.

never mind, sketch is up now

[d.evans]

A sketch would really help. At the very least, please try to let us know what sort of features the datums are and what you measuring to get all those midpoints.

i posted it, sorry.

[#2#]

Ive got a large flange (approx 20" by 14") on which, I've got to measure a hole pattern.
Here is my problem. I've seen this sort of datum callout in books, but never had one to deal with myself. How do you handle the B and C datums? The part material is very thin (.050"). I've got it fixtured to a rayco plate and can reach all the external edges.
What I understood to do was this... (PLEASE CORRECT ME IF I AM OUT IN LEFT FIELD)

1) create plane for datum A (constructed plane from vector points)
2) create three points along the left outer edge
3) create three mirrored points along the right edge.
4) create three MID POINTS for each pair of left/right points from step 2 and 3.
5) Do the exact same thing for the top and bottom outside edges creating three more mid-points.
6) create two 2d lines constructed from the mid-points to get a "center-of-the-part" pair of intersecting 2d lines.
7) align: level to plane A, rotate to long "center" line, origin X at long "center" line, origin y at short "center" line, origin z at plane A.

Or should I be using simulated datums? or is there a way to create these datums of size (+/- .020") some other way that I am not thinking of?

Using 3.7MR3, 1mm ruby tip.

What you've got looks good to me.

[Wes Cisco]

Ive got a large flange (approx 20" by 14") on which, I've got to measure a hole pattern.
Here is my problem. I've seen this sort of datum callout in books, but never had one to deal with myself. How do you handle the B and C datums? The part material is very thin (.050"). I've got it fixtured to a rayco plate and can reach all the external edges.
What I understood to do was this... (PLEASE CORRECT ME IF I AM OUT IN LEFT FIELD)

1) create plane for datum A (constructed plane from vector points)
2) create three points along the left outer edge
3) create three mirrored points along the right edge.
4) create three MID POINTS for each pair of left/right points from step 2 and 3.
5) Do the exact same thing for the top and bottom outside edges creating three more mid-points.
6) create two 2d lines constructed from the mid-points to get a "center-of-the-part" pair of intersecting 2d lines.
7) align: level to plane A, rotate to long "center" line, origin X at long "center" line, origin y at short "center" line, origin z at plane A.

Or should I be using simulated datums? or is there a way to create these datums of size (+/- .020") some other way that I am not thinking of?

Using 3.7MR3, 1mm ruby tip.

Your approach looks good to me. I would try it and unless I was being argued with, that is what I would go with. I don't think simulated datums are much of an answer for you and since the are RFS in the FCF I think you have to go with the actual surfaces and not simulated. The only other advice I have is, since this is thin material I trust you have made sure it is not flexing when you take hits and also that you have not distorted it with the Rayco fixturing. Lastly I don't get sheet metal, but I am pretty sure you would want to use edge points with sample hits for establishing your datum edges. HTH

[John Riggins]

Your plan sounds like what I would do.
Happy Trails, D. Evans.
(Give Buttermilk a carrot for me)