Assigning datum features to hole patterns
Assigning datum features to hole patterns
(OP)
Guys,
A general question:
I've seen that if a four-hole connector mounting group makes up a datum, say -B-, then a datum axis is created at the centroid of the group e.g. at the center of the hole group and passing through the center of where the connector comes through the panel. But if, say, one or more of those four mounting holes is moved, then the centroid is no longer in the center of the connector hole, at a location that would have to be calculated. The same practice applies to a group of holes falling on a straight line but non-uniformly spaced; the centroid would have to be mathematically determined. For hole patterns without symmetry, can dimensions to features elsewhere on the part be taken off of any hole in the group or does it strictly have to come off the datum axis at the centroid of the group? TIA for your valued input.
A general question:
I've seen that if a four-hole connector mounting group makes up a datum, say -B-, then a datum axis is created at the centroid of the group e.g. at the center of the hole group and passing through the center of where the connector comes through the panel. But if, say, one or more of those four mounting holes is moved, then the centroid is no longer in the center of the connector hole, at a location that would have to be calculated. The same practice applies to a group of holes falling on a straight line but non-uniformly spaced; the centroid would have to be mathematically determined. For hole patterns without symmetry, can dimensions to features elsewhere on the part be taken off of any hole in the group or does it strictly have to come off the datum axis at the centroid of the group? TIA for your valued input.
Tunalover





RE: Assigning datum features to hole patterns
I don't see the term 'centroid' mentioned in the 1994 or 2009 versions of Y14.5; it may have been in the 1982 or earlier version.
RE: Assigning datum features to hole patterns
I'd say yes, you can.
RE: Assigning datum features to hole patterns
Paragraph 4.12.3 of Y14.5 is the source for this. The best sentence for your question: "The origin of the datum reference frame may be established at the center of the pattern of the datum feature simulator where it intersects plane A, as shown in Fig. 4-26, or at any other location defined with basic dimensions relative to the datum feature simulator as in Fig. 4-28" (emphasis added).
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Assigning datum features to hole patterns
RE: Assigning datum features to hole patterns
Does similar verbiage appear in the 1994 standard? Our company is still working to the 1994 standard.
TL
Tunalover
RE: Assigning datum features to hole patterns
I'd go farther and assert that using a pattern of features as a datum is a bad idea, regardless of how their size or interrelationship is accounted for.
My first GD&T instructor, in 1967, asserted that GD&T came out of the gage-making trade, and that it helps to think of datums as being features of a gage or fixture, not features of the part.
Thinking of datums as gage features, consider the complexity necessary for a gage to pick up a group of holes of varying size and location, and calling some reasonable point on that gage a datum. What's reasonable? Centroid only makes sense if the pattern has symmetry. If one hole is intentionally offset for clocking purposes, as sometimes happens, that moves the centroid, but it might be more reasonable to omit the clocking hole from the centroid calculation, and it would be even better to omit any calculation entirely.
Even if you use a CMM instead of fixed gages, the complexity remains, and adds to the normal confusion.
Remember that you are expecting your drawings to be interpreted correctly, every time, by every viewer, by people who may not be GD&T experts, and who may not have much formal education of any kind.
Remember also that the only reason you have a job is because the machinists/ CNC operators/ artisans charged with actually making the parts do not have erasers or 'undo' buttons.
</soapbox>
Mike Halloran
Pembroke Pines, FL, USA
RE: Assigning datum features to hole patterns
Tunalover -- in 1994 check out paragraph 4.5.8. It doesn't say anything about having a choice for where the theoretical center is; it just says that "individual datum axes are established at the true position of each hole [virtual condition]." So the 2009 standard is more clear about it, but I would still say that even in 1994 it doesn't matter which hole the dimension is displayed from, or even from an imaginary center (if one can logically be found).
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Assigning datum features to hole patterns
So there is no need to look for a centroid and it doesn't matter how pattern looks like.
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Assigning datum features to hole patterns
This is the fundamental issue we seem to go back and forth on here, and in industry, that I feel shows how insecure/tentative the support for the whole D&T thing really is. To me the standard has already spoken on the issue, manufacturing people have never liked when we did this and they obviously still don't. Everywhere I have worked that had an internal shop fights it, looses the support and goes back to the “good old way”.
Frank
RE: Assigning datum features to hole patterns
Tunalover
RE: Assigning datum features to hole patterns
I am talking the ideal, the drawing should state the function and let qualified manufacturing-inspection people do their job, figure out how to make it and inspect it.
If I have learned anything in this forum, it is that I may be too idealistic.
Frank
RE: Assigning datum features to hole patterns
In your next-to-the-last post, is the "good old way" with simple, yet confusing and ambiguous, bilateral tolerancing e.g. without geometric controls?
Thanks,
TL
Tunalover
RE: Assigning datum features to hole patterns
Frank
RE: Assigning datum features to hole patterns
I admit to having done this on occasion rather than picking the entire pattern as the datum. While it's still a compromise from true functionally driven datums it's arguably less of a deviation than some other options.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Assigning datum features to hole patterns
If the holes are referenced MMB it really does not "do violence" to the design due to "the datum virtual condition rule" (formerly, rule #5).
Frank
RE: Assigning datum features to hole patterns
Just as an example, picture a situation where related actual mating envelopes (RAMEs) of these two datum features are at different-than-MMB size, and RAMEs of at least two other features within the pattern are at MMB size. With the entire pattern selected as datum feature there would be no datum feature shift available in such case. But with just two features chosen as datum features the inspection will be allowed to take advantage of a datum feature shift which actually will not exist in reality.
As for the discussion about a centroid, I agree that there is no need to look for it and that the origin of DRF can be anywhere as long as basic dimensions define location of the origin relative to datum feature simulator. I would just add that this applies regardless of whether the datum pattern is referenced RMB or MMB - in both cases the DRF origin is derived from datum feature simulator pins (or holes), and in both cases mutual location of those pins (or holes) is the same.