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Is a hole pattern datum the centeroid, plane, or many axes?

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sendithard

sendithard

Industrial
Aug 26, 2021
186
I encountered a hole pattern datum today and it appears from reading a thread on here a long time ago that if each hole is of equal importance this is proper.

So I begin thinking the datum could be several ideas, but feel free to slap me with a dead fish and get my mind right.

Below is a drawing from the old thread. I tend to think one central datum is created, but then it becomes challenging to manually check dimensions from said center. Aanother part of me thinks two center planes are created b/w the hole basic dimensions. then I wonder what is the .4 tolerance zone actually measured from, seems you need multiple axes.

Below you have a center hole which can be secured, but it is a different feature and needs to be measured to the pattern datum, so using a fixed gage pin doesn't help as it could be off from the pattern. I'm confused on this one, but not out in left field so I think I can follow along quickly. Thanks....


patterndatumdims2_ayom1o.jpg
 
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This is perfectly fine for a fixed gauge. It's a problem for a CMM. Use 4 pins to represent the 4 MMB holes (MMC + position tolerance effects) and on pin to represent the hole, same deal.
 
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3D,

Are you saying Datum D is 4 separate axes or one centroid one?
 
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I'm saying if there is a condition where the 4 pins fit the datum feature and the one pin fits the referencing feature the part is acceptable, providing that they meet all the other limits.
 
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3d,

I understand the fixed gauge your are discussing. You are confining each patterned hole to their own perfect theoretical location. I'm trying to wrap my head around it all, not solely understand from a manual or cmm method of inspection. Is the ultimate datum D a centroid axis or 4 separate axes or are you saying to me in some hidden fashion that it doesn't matter. I'm trying to combine all this standard nonsense with real world inspection both cmm and manual.

My only question is what is Datum D exactly?

 
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Don't think of datum D as a centroid axis because then it would not have any ability to control clocking. The intent on that drawing is that datum D restricts 3 degrees of freedom (after datum A has already constrained 3 degrees of freedom).
Therefore, of the two choices you are thinking of, the better one is 4 separate axes. Then, for positioning the center hole, it doesn't matter which of the four axes you start from.
 
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Belagner,

Thanks for the reply. If you use more than one axis I see you get all DOF locked dowm, but what if a later feature is dimensioned from datum D. Where is datum D? Is is the top left hole axis, the centroid axis, the bottom left axis?

Thanks.
 
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That later feature can be dimensioned from any one of the four axes. Recall that the datum pattern is already spaced apart by basic dimensions. So pick any one to dimension from on the drawing. Then, when measuring that new feature, I can start from a different axis. They're all interrelated so it doesn't matter.
 
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Belager...I understand what we have walked thru, I'd like to ask what you have all seen in the field. Do you see dimensions from patterns? Do you like pattern datums? Can the intent of pattern datums be done without using pattern datums?
 
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Hi, senithard:

I am not Belanger whom I respect a lot. Here are my thoughts on your questions.

I see application of dimensions from patterns and use it many times. Pattern datums are important concept, and have many applications. It will be very difficult to create this datum without using pattern datums.

You need to think there are four expanding pins BASICALLY located 40 and 91.2 apart. You will then expand the pins to locate datum planes (perpendicular) which are derived, or mathematically calculated.

You don't have to use a pattern feature as a datum feature. You can make all the 5 holes as a pattern feature and simultaneous requirement will make them positioned properly as long as you can live with 0.4 positional tolerance.

Best regards,

Alex
 
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