GDT of datums - Putting Datum C on a drawing

#1
Hello all, for a quick background I am a cmm programmer and inspector and I have a question for any and all fellow GD&T intelligent people out there. I am currently in debate with my design team on drawings. On particular drawing datum A is called out to a stamped sheet metal part. Datum B through two tool-holes. So far I am good however they have a center-line through these two holes as their reference point. I asked them to put Datum C through this center-line of these two toolholes so there is no future confusion or assumptions or anything of that nature. He states that putting Datum C on the drawing goes against ASME standards. We are using ASME Y14.5 2009. I asked him how it goes against standard and he couldn't tell me other than example he provided because it wasn't on there. I told him I have to have C on the drawing or else we in Metrology will not and can not do layouts. Reason we refuse is because we do not want to be responsible for assuming and going against design intent even if told by words but we HAVE to have that design intent on clearly stated on drawing. Can anyone provide example or standard that states datum C being marked on center-line through two labeled datum tool holes goes against standard? Sorry for the extremely long question just wanted all the details properly established. I have attached a snippet of the image with the two called out datum Bs and center-line between them.. Datum A is not shown but is on drawing; due to nature I can not show complete drawing. Thanks
 

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Matt's Quality Handle

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#2
A bit rusty on the subject, but I've got some other questions:

What type of feature is datum A? Most of the time, the primary datum is planar, so everything after this will assume -A- is a plane.

Datum features should be physical feature (I can touch it, and know that I'm touching it). I read this drawing as a single hole -B- being the datum as specified on the drawing, and I would use the opposing hole as -C-.

Using the pair of holes as a datum fully constrains the part, and the designer is correct. Adding a third feature into the mix would overconstrain the part.

I'm not 100% on the "legality" of using the centerline, because I've never seen it done (although it very well could be). Bottom line is that functionally, you need to constrain 6 degrees of freedom, which is in effect done by using the two holes. Whether you call both holes -B-, or one hole -B- and the other one -C-, you are constraining it properly. I would not constrain beyond that.

Hope that makes sense, it does in my head, but I'm not sure that it comes through in text.
 

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#3
I have usually seen the centerline between the holes used as the B datum, and the center of one of the holes used as the C datum. However, you could use a hole pattern to accomplish both.
 
#4
Correct Datum A is a plane of 4 datum targets.. datum b is through-line between two holes as on right side marked 2x holes.. and datum b marker..Since axis of hole/circle is a point and line constructed through two points is a line..plane restrains 3 degrees... Line restrains 2 degrees.. Design intent from the group want the vertical center-line to be the reference and he told me that it was against ASME standard to mark/label this vertical line as Datum C; I asked for explanation of why and couldn't get one other than the lone example he supplied. I require ABC on all drawings before I do work so there is no second guessing with engineers approval, programmers after me etc etc. The part is symmetrical is why they want the vertical center-line for datum and all holes refer back to this center-line. Hopefully this all makes sense.
 

Paul F. Jackson

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#5
The 4 surface targets constrain 3 DOF (2 rotation + 1 translation)
The 2X hole pattern constrains the remaining 3 DOF (1 rotation + 2 translation)
Only features can be datums, lines cannot.
Matt is correct.
 
#6
Well the design group has positions all over part referencing this imaginary centerline. So leaving datum C off of the drawing leaves me assuming where and what to use for my last and final origin for my datum reference frame (x). Is there a way to denote this. My preferred method was as Matt suggested as one hole as B and opposing hole on right as C but they wont budge. Thanks for all input.
 

Paul F. Jackson

Quite Involved in Discussions
#7
Any time you use a pattern 2X or more as a datum feature the origin X0Y0Z0 must be fitted to the pattern. No single feature in the pattern defaults as the origin otherwise the pattern is ignored, the chosen feature becomes the secondary datum feature, and the tertiary is undefined.

The fitting origin X0Y0Z0 when using a pattern can be placed anywhere on the axis or plane of the primary datum feature provided there is a basic relationship to the pattern. It can be placed at one hole of a 2X pattern but that hole will always share the deviation in basic spread with the other hole and will not be coincident with X0Y0Z0.
 
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