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Common GD&T Student Questions: A Pattern of Holes as a Datum Feature
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- čas přidán 27. 03. 2023
- In a recent live instruction webinar, we answered four common GD&T questions that we have received from our students. We are now making this available to everyone in a four part series! In this video, we are answering the second question:
On a pattern of holes being identified as the datum feature, would the datum feature be the single hole, or all holes?
Interested in learning more about Print Reading or GD&T? Check out more of our free GD&T resources here: download.gdand...
Great explanation, this helped put several GD&T puzzle pieces in place for me, thanks!
Very welcome!
THANK YOU SO MUCH!!!!! I am a new Tech Designer and this really helped me.
Glad it helped!
Thanks a million, just what I was searching for.
Fantastic!
Hi, I'm wondering if this works if you set the hole pattern as the A datum and the surface as the B datum? If you do this I'm not sure if you can position the hole pattern (which is datum A) to the surface B because you would be referencing B while establishing A before you have established B? I ask this because I have a part that has blind holes for dowels in a hole pattern so in my mind it would not be possible for the surface to be the primary datum feature if the pins have to align with the dowel hole pattern first and slide in before the surfaces can make contact, hence my reasoning for making the hole pattern the primary datum.
Perhaps I should leave the surface as A and the hole pattern as B and then when I position other features I will make B the primary and A the secondary in the feature control frame?
You certainly can utilize a pattern of features as a primary datum, and then the surface that is perpendicular to the holes as the secondary datum feature. All this implies is that anything that refers to this datum structure is to be held in location and orientation to the pattern of holes first, and then if necessary, to the plane. scena
Hi, could you indicate which standard/standards this points to? I.e, is it just ASME or is it an ISO standard too and if yes to the iso standard which one? Thankyou :)
This is a general practice when it comes to GD&T and is applicable whether you're using ASME or ISO.
how do you specify the orientation of the axis created by the circle pattern? I see you just assumed a vertical/horizontal orientation. But why is it assumed this way?
The pattern of holes has to create a datum axis that parallels the axis of the holes because this pattern as a datum cannot stop translation along this axis. That is why we must include Datum A in our datum reference frame. It can stop that translation. If we were to imagine the axis created by the pattern of holes parallel to datum A this would be incorrect.
Sir, this is a wonderful session,also I have doubt. In DRF only mentioned Datum A&B only.
Q1# Is there not necessary Datum C for centre hub axis.
Q2# can mention Datum C for (dia1.750) instead of Datum B(dia .313)
Q3# Is their any sequence for mentioning Datum according to part feature is producing at first. My concept is these pattern holes are mfg by drilling at final.
Kindly clarify.
Q1: There is no need for a tertiary datum because the position controlling datum feature B (the pattern of holes) is controlling the location of hole to hole for the pattern and there are no other features left over to "clock" the final degree of rotation for this scenario.
Q2: Identifying the 1.750 diameter as datum C does not add any additional control beyond datum B and therefor would be unnecessary
Q3: There is absolutely a consideration for order of selection for datums. For instance, if the 2.490 hole is a press fit diameter, then i would recommend that be datum feature B and the 6x bolt pattern would then be referencing A|B in its datum reference frame. However, more often then not, the bolt pattern is the feature that secures the part in the final assembly and therefor should be used as the datum feature.
@@GdandtbasicsSir thank you so much , to this reply .
Am just referring this note,fine.
Expensive part too =o)
This video is clear, thanks for sharing. One more question, if the pattern of holes of B datum feature are perpendicular to datum feature A, how to decide the original point?
What would be the point to define datum B parellel to datum A? It would not constrain any more degrees of freedom that are not already constrained by plane A. The only use case would be if both A and B would be relevant together as a single reference, so you would then specify the as a common datum [A-B], resulting in a median plane basically.
@@AlexLapugean Yes, you are right. Thanks for your notice.I just realize I asked a wrong question.
What I really want to know is how to decide the original point if the B datum holes are “perpendicular” to datum A? How to setup the datum alignment on CMM?
@@alanchang0118 AH, ok, that is also a common question, and the answer - though maybe not being as satisfactory as desired - is the truth: It does not matter.
Usually it will be defined by how you put the basic dimensions on the drawing, but it really makes no difference either way, as in the end, all features are measured with respect to their true position and the basic dimensions are only used to define that true position.
@@AlexLapugean Thanks for your answer. The datum reference frame is base on the basic dimensions which the drawing set, right? I think I know how to explain to my colleagues now.
@@alanchang0118 Sure, you can think of it like that, that is what the metrologist will usually do. My point however was that it is not something set in stone and it is not really relevant. as long as the true position is the same with respect to the datum references that is used to locate/orient the tolerance zone, it doesn't change anything. You might see sifferent values for the basic dimension that define the true position, but in the end, the deviation from that true position will be identical, and that is what is being checked, not the absolute dimension.