GD&T Datums on Ordinate 0-0
GD&T Datums on Ordinate 0-0
(OP)
Hi Everyone -- I have a question about specifying a GD&T datum at the 0-0 of an ordinate dimension scheme. The 0-0 is specified at the center of a hole, so is it correct to call out the horizontal 0 as datum B and the vertical 0 as datum C, assuming a perpendicular surface is already datum A?
We are having many a discussion about this at work, but I can't find examples of this in the Y14.5 spec. According to the spec, I would think you'd want datum A to be a perpendicular surface, datum B to be the hole itself, and datum C to be a vertical or horizontal surface to lock down rotation. That said, we have found examples in released industry specs showing the datum on the 0-0.
So is this a correct place to put a datum? If so, what is the datum? Is it a theoretical vertical (or horizontal) plane through the axis of the hole?
To help visualize the part, you can think of a flat metal plate that has several mounting bosses and features on it. The drawing view would be looking head-on at the mounting bosses and trying to use one of them to define the features control frame (in all directions except into/out of the page).
We are having many a discussion about this at work, but I can't find examples of this in the Y14.5 spec. According to the spec, I would think you'd want datum A to be a perpendicular surface, datum B to be the hole itself, and datum C to be a vertical or horizontal surface to lock down rotation. That said, we have found examples in released industry specs showing the datum on the 0-0.
So is this a correct place to put a datum? If so, what is the datum? Is it a theoretical vertical (or horizontal) plane through the axis of the hole?
To help visualize the part, you can think of a flat metal plate that has several mounting bosses and features on it. The drawing view would be looking head-on at the mounting bosses and trying to use one of them to define the features control frame (in all directions except into/out of the page).





RE: GD&T Datums on Ordinate 0-0
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T Datums on Ordinate 0-0
A datum must be a real feature you can contact and measure from. In your case, the base of your part is the primary datum. The hole is your secondary datum. Your tertiary datum is another hole, or some face that controls rotation. If you are using a feature of size as a datum, such as your hole, it really ought to be an accurate feature of size.
This scheme allows you to draw ordinate dimensions from your hole.
I think I have re-phrased what KENAT said.
--
JHG
RE: GD&T Datums on Ordinate 0-0
Just to clarify the root of the issue KENAT and drawoh are addressing.
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NX8.0, Solidworks 2014, AutoCAD, Enovia V5
RE: GD&T Datums on Ordinate 0-0
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II
RE: GD&T Datums on Ordinate 0-0
See paragraph 3.3.2 (a) through (f) in the 2009 edition of the standard in order to help steer your colleagues in the right direction.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: GD&T Datums on Ordinate 0-0
I'm really curious how the center of the hole is specified as 0,0. That specification is usually handled by the datum features referenced in the FCFs.
To answer part of the original question - yes, there are at least two mutually perpendicular theoretical datums that can pass through a theoretical axis, but they are typically not labeled. Their location and orientation are a composite of the choices made in the datum reference part of the feature control frames.
Look at the figure for an inclined datum feature to see that a theoretical datum doesn't have to be coincident or parallel to the datum feature that is key to its location.
RE: GD&T Datums on Ordinate 0-0
RE: GD&T Datums on Ordinate 0-0
I'm not suggesting you move your datums to coincide with your coordinate origin, but rather the opposite. Your datums should be the wisest choice based upon the function of the part. Your zero does not need to be external surfaces such as if you were to but a part up against two walls (or a fixed vice jaw and a part-stop for the sake of visualization). Your zero can just as easily be the center of a cylindrical feature such as a mounting-boss in your case. Your Z-zero may be the mounting surface level. Your coordinates can origin there. However your Datums may be a derived from two mounting bosses (either a line tangent to both, or center-to-center).
You don't have to sacrifice convenient/logical coordinate origin location to suit Datum schemes. If they overlap, it can have some benefits. Also keep in mind that ordinate dimensions can be "basic" as well.
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NX8.0, Solidworks 2014, AutoCAD, Enovia V5
RE: GD&T Datums on Ordinate 0-0
IMHO...sometimes you do. Convenience should always take a back seat to function. In those cases where convenience has an equal outcome as function then fine. The problem is that it most often does not work out that way. I'm not saying convenience should be disregarded but it should be considered only after functional requirements.
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
SSG, U.S. Army
Taji, Iraq OIF II
RE: GD&T Datums on Ordinate 0-0
It may be entirely appropriate to put the axis of the hole as 0,0 of your ordinate dimensioning scheme.
Since your tertiary datum is only used for 'clocking' then you arguably don't need to worry about your basic dims chaining back to it.
So 0,0 of your ordinate coinciding with hole CL is probably correct.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T Datums on Ordinate 0-0
RE: GD&T Datums on Ordinate 0-0
I do not disagree at all that there are times when coordinate origins cannot overlap with Datums. I didn't meant to imply otherwise. I meant to say that you should design for manufacturing as much as you do for the engineering requirements. Making the manufacturing easier, combined with inspection, will reduce rejected parts. Making the coordinate origin overlap with the datum scheme (when possible) is not about convenience for the designer. It is no more or less difficult to origin your dimensions on any other feature. It's just clicking around to place dimensions, no matter the origin. It's about making the manufacture as seamless and simple as possible in order to get the best rate of quality parts.
Apologies if I was vague or misleading, before.
_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5
RE: GD&T Datums on Ordinate 0-0
The ordinate dimensions would be basic, locating other mounting holes, and we'd have a position tolerance on those holes (and size if they aren't tapped). Other critical features would be dimensioned/toleranced as appropriate relative to that same datum reference frame. Then we want a note to tolerance all undimensioned (CAD) geometry to a standard range relative to that datum reference frame. I'm thinking the feature control frame in the note would be a profile tolerance, as I've seen in some examples.
On a related note: There are multiple coplanar mounting bosses. Would it be more correct to specify the top surface of one as the primary datum and THEN establish coplanarity with the rest? Or would you establish coplanarity and make that coplanar surface the primary datum. The latter makes more sense functionally, since they all mate to the same part, but I'm not sure what effect (if any) it would have on my datum reference frame (or if it's even allowed!). Note: I'm establishing coplanarity by having a profile tolerance that points to a phantom line connecting the bosses and a note "[X] SURFACES" as shown in the spec. Maybe I'm overthinking it, but the idea of coplanar surfaces as a datum was confusing for me.
RE: GD&T Datums on Ordinate 0-0
RE: GD&T Datums on Ordinate 0-0
Yes, see 4.5.7.1 & 6.5.6.1 of the 94 edition Y14.5 std. Not explicitly shown but I believe readily extrapolated.
I believe the newer version of the std may allow other options but don't have a copy to say for sure.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T Datums on Ordinate 0-0
RE: GD&T Datums on Ordinate 0-0
Now how many people really understood/understand the implications is another matter.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: GD&T Datums on Ordinate 0-0
What the 2009 version offers is the suggestion that multiple perfectly located and coordinated expanding features with a frictionless interface that allows the physical datum simulator to glide over the surface of the part and be perfectly constrained in 3 degrees of freedom would allow the use of the RFS modifier to a pattern of holes.
It doesn't go into all that detail because it's just a theoretical inclusion in the standard, but that is what would be necessary to make use of the concept. I still have seen no real mechanism that depends on multiple interference fits and allows understanding where a part will move to when constrained in a similar fashion without taking a significant amount of stress/strain analysis to determine where the deformed interference fit pieces would deflect to and how that would alter the location of the mating parts.
I asked for an example during the public review and received only the suggestion that it was there to allow the concept, not to make use of it.