Common Datums 1

[AMontembeault]

We're having a bit of a debate here at work.

Suppose I had a piece of plate with 2 holes in it. Each hole is specified as a datum (in this case, datums B and C, where datum A is the large flat face of the plate).

I have another feature with a positional tolerance with respect to A and a common datum defined by B and C (B-C).

The question we're debating is if B-C defines 1 datum wherein the axis of both B and C must lie (call it a horizontal datum), 1 datum between the axes of B and C at an implied center(call it vertical), or actually 2 datums orthagonal to eachother (controls both the horizontal and vertical datums).

Previously, we've been operating under the assumption that it only defines one plane, however reccently I had a conversation with an outside source that claimed that because the datum simulator would basically be 2 pins, it in fact defines 2 datum planes. Unfortunately, I cannot find an example in either the ASME GD+T standard or the ISO standard which can confirm or deny that either way is factual - almost every example I've seen only relates to 2 planar surfaces or 2 nominally coaxial surfaces, not 2 holes at some distance apart. Could anyone shed some light on this, or perhaps show me a published example?

Thank you

 

[greenimi]

http://www.geotolmeadows.com/newsletters/2008/jun2008.htm

See Co-Datums Subject

From the answer I would say one of an important sentance :"The datum axis formed by these two holes would be perpendicular to H and halfway between the two gage pins."

 

[CheckerHater]

If you don't want axis to be "halfway between two gage pins" you can have it anywhere, providing that it's located by the means of basic dimension(s).

 

[fsincox]

Agreed, that is the beauty of basic dimensions. The standard does not show this kind of thing but it is very common in aircraft and automobile manufacturing to have a common universal "origin". The part datums just need to tie back to them.
Frank

 

[CheckerHater]

The standard does not show this kind of thing but it is very common

Actually it does now.

From ASME Y14.5-2009 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…”

It's quite interesting how wording has changed over time, unfortunately I don't have time to search for all the quotes right now.

 

[AMontembeault]

Thanks for the fast replies everyone. One additional question came up based on the example greenimi provided - looking at the datums G and F in that example, the feature control frames define the datums, but how can that be when the feature control frames themselves reference the G-F datum which first has to be defined in part by G and F? It seems like circular reasoning on its face - what subtlety are we missing that makes this permissible?

 

[greenimi]

Quote: "looking at the datums G and F in that example, the feature control frames define the datums, but how can that be when the feature control frames themselves reference the G-F datum which first has to be defined in part by G and F? It seems like circular reasoning on its face - what subtlety are we missing that makes this permissible?"

The standard make it permissible: See Fig. 9-6 page 183
total runout 0.02 to C-D and this feature/cylindrical surface *IS* called datum feature C

Same thing for datum feature D/ same fig. 9-6

 

[greenimi]

Y14.5-2009 for previous post

 

[AMontembeault]

The standard make it permissible: See Fig. 9-6 page 183
total runout 0.02 to C-D and this feature/cylindrical surface *IS* called datum feature C

Same thing for datum feature D/ same fig. 9-6

OK, I see the example (thank you again), and going back a bit I see:
9.5.2 Multiple Cylindrical Datum Features
Figure 9-4 illustrates application of runout tolerances
where two cylindrical datum features collectively
establish a single datum axis to which the features are
related.

But whats the theoretical interpretation behind this reasoning? I see that the standard says its permissible, I'm just trying to understand why its permissible so I can explain it to my collegues.

 

[greenimi]

Quote: "WHY?"

Short answer: Because the guys in the ASME Y14 COMMITTEE said so.

Long answer: probably you can find it in
ASME Y14.5.1M Mathematical Definition of Dimensioning And Tolerancing Principles

 

[3DDave]

To measure runout one needs an axis to rotate about. A practical application is to have two nominally aligned jawed chucks, one clamping on C and the other on D. Clamping on them both orients the part in a way that using either one by itself would not and simulates a commonly used bearing support. Once they are aligned one can check runout to the common axis in a way that mimics hundreds of millions of electric motor and gear shaft installations. Obviously (or not) one has to accept that part of the surfaces interfaced to the chucks will not be available for inspection, but just as not every single atom is checked on a surface anyway, that the parts are sufficiently uniform that the missed areas can be judged adequate.

The parallel holes do not share this characteristic. It isn't clear that there is any value to adding the reference to the compound of the two datums. It certainly isn't clear how to calculate the effect of variations in parts based on the given FCFs.

 

[CheckerHater]

@AMontembeault:

OK, let’s start with good old runout example.
Look at the picture. You see how datum axis A-B is derived from datum features A and B.
You probably will have no problem with the idea of checking relation of axis A wrt axis A-B, because they are definitely not the same.

Now let’s move to parallel cylinders.
Datum A-B is neither features A or B, but the thing of its own, derived from both.
Do you see problem with the idea of checking features A and B wrt datum A-B now?

 

[axym]

AMontembeault,

The debate at your workplace is just the latest in endless debates on the question of "where is the datum for the pattern". If you look to the standards, there were some very misleading explanations in ASME Y14.5M-1994 that (IMHO) caused a lot of confusion in industry. The explanation is better in Y14.5-2009, but there are still some misleading things.

Part of the issue lies in Y14.5's basic datum reference frame concept of a three-plane coordinate system built on datums (theoretically exact plane/line/point geometry extracted from datum feature simulators). This concept is workable and intuitive for very simple datum feature types and combinations, but doesn't hold up well when applied to more complex configurations such as hole patterns. In particular, the step of defining datums becomes difficult and somewhat arbitrary.

Section 4.12.3 in Y14.5-2009 describes a datum feature comprised pattern of features of size at MMB. This is very similar to your original example, except with 4 holes instead of 2. Figure 4-26 illustrates the situation. The misleading part, that I believe is the root cause of a lot of the debates, is that the figure shows "Datum Axis B" as the datum for the 4-hole pattern. This just doesn't make sense. If the datum feature was a single cylindrical hole, the datum would be a single axis and would constrain 2 translational degrees of freedom. But if the datum feature is a pattern of holes that constrains 3 degrees of freedom, the datum can't be a single axis. It should be an axis and centerplane, as shown in Figure 4-3 f) in Y14.5-2009.

At the end of the day, it doesn't really matter what the datums are. The important thing is that the datum reference frame is a coordinate system that is defined in the pattern of simulators. As CH mentioned, the origin of the DRF can be at the center of the pattern of simulators, or at some other location defined by basic dimensions.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

I finally put together some historical data I found interesting.

It looks like they had two original concepts, the compound datum, something that required two datum features to establish one datum and pattern; and pattern 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”

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.
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.
In 2009 version both 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 addition: 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…

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

 

[SeasonLee]

@AMontembeault:

Here is the tip (from Tec-Ease) you needed, its open for member only now, pls see the attached for details.
It is referencing the datum - not itself.

Season

 

[weavedreamer]

A tangental, but related question, using the 1994 standard:
Given the Tec-Ease example in the previous post, would positional, perpendicular and profile callouts referenced to a co-datum call out A(m)-B or A-B(m) be valid?

This example uses runout and A-B as the co-datum, and I've seen A(m)-B(m) referenced in the standard.

 

[greenimi]

If the callouts A(MMC)-B or A-B(MMC) are functional, why not? Just my opinion.

 

[axym]

weavedreamer,

There's nothing in the standard that specifically precludes that, but you're getting into uncharted territory.

One simulator would be fixed at virtual condition size, and the other would grow/shrink to achieve as full contact as possible. Depending on the as-produced geometry of the datum features, you might get full contact on the RMB feature or it might hang up on the MMB feature first. I'm not sure why an assembly would be designed in this way.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

@dreamer:

Are the datum features A and B still nominally coaxial cylinders, or are we talking about something else?

 

[weavedreamer]

Yes, CH, A and B are nominally coaxial (I.D. rather than O.D.) cylinders.

Evan, it is uncharted territory, and if I can't point to an illustration, others don't seem to be interested in venturing. Your explanation is what I am envisioning.