Origin of Measurement for Pattern of Holes

[dthom0425]

Hi all,

I'm slightly confused with section 4.12.3 Pattern of Features of Size at MMB in Y14.5-2009. There are two claims; origin of the DRF can be established at the center of the pattern and/or any other location defined with basic dimensions relative to the datum feature simulator.

Say I have a plate with 4 holes. Primary A is the flat face, Secondary B is the pattern of 4 holes. Normally I would throw centerlines into the view on the drawing to simulate measurement from the established center of the symmetric hole pattern.

So I'm confused about the "any other location defined with basic dims..." part of the spec. I recognize not all hole patterns are going to be symmetric and finding the center of an asymmetric pattern of holes is probably not fun.

The gage i'm envisioning for this part is: flat plate with 4 pins set at the VC of the 4 holes. So is the standard suggesting that, rather than find the center, I could touch off on one of the gage pins, zero out and take all of my measurements from there? If this is true, not sure I'm understanding the implications here...at that point, what's the difference between the pattern of holes as a datum vs making one hole in the pattern a datum (B oriented to A) and another hole in the pattern a datum (C located and oriented to A|B).

Thanks for the help.

 

[Burunduk]

The difference is in the constraints of degrees of freedom. If all holes are of identical VC and there is no priority between them when mating with the other part, the entire pattern should be the secondary datum feature. Datum precedence is not about where the origin of measurement is, but about capturing the way the part mates both in functional assembly and when fixtured for inspection. This will influence how part features end up located and oriented relative to their tolerance zones during the inspection.

 

[chez311]

dthom0425,

A common misconception is about the importance of your measurement origin relative to your datum features. The reality is that the selection of this origin is arbitrary, as long as it is kept constant throughout the measurement process* where you choose this to be will not impact the result. Obviously there are certain conventions that we typically follow, but these are a matter of convenience not a requirement. Indeed your origin can even be floating off in space somewhere, as Y14.5-2009 fig 4-28 suggests. Note that your DRF is actually established from your datum feature simulators, and therefore your origin would be based on the simulators.

Additionally, I think you'll find determining the "center" of a bolt pattern of any type (nominally symmetric or asymmetric) on a real as-produced part (ie: with location/orientation variation of the resultant holes) is often a non-trivial endeavor.

What WILL on the other hand have an effect is selection of your datum features, tolerancing of said datum features, specified boundary condition and order of precedence. In short, your datum feature and datum feature simulator relationship (and associated DOF constraint) is what you should concern yourself most with.

*For features held to the same DRF.

 

[CheckerHater]

This paragraph has long and confusing story and you don't have to read thru all of it. It's all about using multiple features as a datum.

It looks like in older standards they had two separate concepts, the compound datum, something that required two datum features to establish one datum; and pattern, which was seen as circular or other form where central axis makes more sense.
In the latest standard the ideas of compound datum and pattern datum are joined together under the common name of “Multiple Datum Features”
Please take a look at the following:

1982:
From 1982 Para. 4.4.5 Compound Datum Features.
Where more than one datum feature is used to establish a single datum, the appropriate datum reference letters, separated by a dash, are entered in one compartment of the feature control frame.
From 1982 Para. 4.4.6 Pattern of Features.
Multiple features of size, such as a circular pattern of holes at MMC, may be used as a group to establish a datum when part function dictates. In this case, individual datum axes are established at the true position of each hole. These are the axes of true cylinders which simulate the virtual condition of the holes. When the part is mounted on the primary datum surface and rotated about the centroid of the secondary pattern of holes, a central datum axis (axis of rotation) is generated for establishing the datum reference frame.
It looks like back in 1982 the ideas of “pattern” and “centroid” were intended primarily for rotating parts with circular patterns like wheels, etc.

1994:
In 1994 they dropped the term “Compound Datum Features”, but it is still occasionally used by some textbooks and Internet sources. Also “rotating parts” were put to rest.
From 1994 Para. 4.5.7 Multiple Datum Features.
Where more than one datum feature is used to establish a single datum, the appropriate datum reference letters and associated modifiers, separated by a dash, are entered in one compartment of the feature control frame.
The following sub-paragraphs further explain the use of MDF:
4.5.7.1 Simulation of a Single Datum Plane
4.5.7.2 Single axis of Two Coaxial Features.
From 1994 Para. 4.5.8 Pattern of Features.
Multiple features of size, such as a pattern of holes at MMC, may be used as a group to establish a datum when part function dictates. In this case, individual datum axes are established at the true position of each hole. These are the axes of true cylinders which simulate the virtual condition of the holes. When the part is mounted on the primary datum surface, the pattern of holes establishes the second and third datum planes of the datum reference frame.
We see both the ideas of circular pattern and centroid are abandoned and it’s now up to us to establish the datum framework.
1994 version still shows the distinction between “multiple features” and “pattern of features” by placing them into separate paragraphs.

2009:
In 2009 version both concepts are included into one single paragraph:
From 2009 Para. 4.12 Multiple Datum Features.
Where more than one datum feature is used to establish a single datum, the appropriate datum reference letters and associated modifiers, separated by a dash, are entered in one compartment of the feature control frame… Where the intent is clear, a datum feature reference letter may be used to define the multiple surfaces as a single datum feature.
Here is very important distinction: two surfaces designated with two letters or many surfaces designated with one letter are all placed together in a single paragraph.
The following sub-paragraphs further explain the use of MDF:
4.12.1 Simulation of a Single Datum Plane
4.12.2 Single axis of Two Coaxial Features.
4.12.3 Pattern of features of Size at MMB
4.12.4 Pattern of features of Size at RMB
First two paragraphs explain what was formerly known as “compound datum”, second two paragraphs explain “pattern of features”, not necessarily circular.
It looks like users of the standard demanded explanation of what happened to centroid, so “center of pattern” was brought back again with further explanation:
From 2009 Para. 4.12.3:
The origin of the datum reference frame may be established at the center of the pattern of the datum feature simulator…, or at any other location defined with basic dimensions relative to the datum feature simulator…

2018:
A set of datum features used to create a single datum (notated A-B) is now called “common datum features” in 2018. This used to be called “multiple datum features” in 2009.

For all it’s worth, I think “historical” view sometimes gives us better understanding of what creators of the standard were trying to achieve.

Sorry for long post


"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[dthom0425]

Thanks for the responses.

chez311: I guess I'm struggling with how selecting an origin out ins pace does not affect the end measurement result. Would you not have accumulated some type of tolerance stack throwing off your measured result?

I attached a picture of a simple part.

The machined plate with the pins in it has the pattern of pins as the datum feature and flat face as another datum feature...all other features on this part are held back to those two datum features.

The bottom plate is me illustrating the gage - so a flat plate with holes set at the VC of the pins. I put a little red sphere out in space on the gage...so, with the spec, it seems like I can take a probe and zero out on this red sphere and start measuring all of the features on the plate..this way, I wouldn't have to locate the center of the two pins in space to measure off of.

If my zero point is out in space, I guess i'm missing how this doesn't affect your measurement. Is it because of gage tolerances that would inherently keep any tolerance accumulation fairly low?

 

[chez311]

dthom0425,

So thats a bit of a different question. My initial response was based on the assumption that you would probe the pattern of datum feature simulators and choose your measurement origin for subsequent measurements to be coincident with one of the simulator axes, the center (possibly centroid) of the pattern, or somewhere else in space - hence my answer is "it doesn't matter as long as its consistent for a given DRF".

Changing your measurement strategy to probe a feature on a gauge other than the datum feature simulators will introduce a stack up of this additional feature to the simulators. FYI specific indication of an origin (as in my example of fig 4-28) does not dictate such a measurement strategy.

 

[dthom0425]

I'm sorry - still not getting this I suppose. So in the picture I provided

1)"choose your measurement origin for subsequent measurements to be conincident with one of the simulator axes": Does this mean I would zero the probe on one of the 2 holes, then place the part into the gauge and begin to touch off on all the features?

2) "the center (possibly centroid of the pattern)": Does this mean I would probe both holes in the gauge to identify the center between the two that is also coincident with the flat top surface of the gauge, place the part on the gauge and then begin to touch off on all the features

3) "somewhere else in space": Im still unclear as to where in space you could go for a part like this. That's why I put that little red sphere there to illustrate "somewhere else in space" which I guess is how I interpreted the spec.

 

[chez311]

Let me rephrase my previous response by adding a word, in bold: "My initial response was based on the assumption that you would probe the entire pattern of datum feature simulators." Ie: probe both holes regardless of where you choose your origin to be. You can probe both holes and still set your zero/origin at the center of one hole, the other hole, between the two holes, or elsewhere.

That's why I put that little red sphere there to illustrate "somewhere else in space" which I guess is how I interpreted the spec.

This is not what is suggested by the statement "or at any other location defined with basic
dimensions relative to the datum feature simulator" in the standard. It can certainly be a valid measurement strategy if you are willing to accept the additional stackup and the possible addition of other strategies for alignment (ie: just probing said sphere would not tell you where the simulators are oriented), but it is not required.

 

[dthom0425]

between the two holes, or elsewhere.

So where is "elsewhere" on the gauge in my picture? Can you only probe on locations confined to the 3 mutually perpendicular planes established by the DRF? i.e. in between the holes (and in line with the holes), out in front or back of the part but centered between the two holes.