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Datum feature and Datum feature reference frame.

Datum feature and Datum feature reference frame.

(OP)
Please see attachment and comment your verified thoughts.

RE: Datum feature and Datum feature reference frame.

1. If I am not wrong, the 8 holes pattern is equally spaced, so you may delete the two basic dimension 4X 1.414 and replace by a pitch circle diameter.
2. Since this part is symmetrical, it does not require a tertiary datum, so you may remove datum C from the Feature Control Frame.
3. The secondary datum feature is too high (2”height), suggest to add an orientation control (perpendicularity control with respect to datum feature A), this will improve the stability between the datum feature and the datum feature simulator.

Season

RE: Datum feature and Datum feature reference frame.

(OP)
Season Lee. Was not the answer I looking for. By chance did you compute any of the other stuff I written down pertaining to the pictorial?

RE: Datum feature and Datum feature reference frame.

[C] is not a necessary reference. A gauge to inspect the part would not be any different with or without the reference. At best is says locate the hole relative to itself. At worst it's a distraction distraction. Like repeating a word in a sentence is a distraction.

RE: Datum feature and Datum feature reference frame.

TOOLS4FOOLS

I did read your attached print and I didn’t answer your question directly, but I do not agree to specify Datum C on the print as I mentioned earlier, sometimes it’s not necessary to fully constrain all six degrees of freedom.

However, for the inspection purpose, you need to setup the datum reference frame, just choose any one hole (I believe this is datum feature C you mentioned) of the pattern to establish the 2nd plane, once it established and the 3rd plane will constructed automatically, this can be done easily on the CMM, I am sure the CMM inspector can to do it well and you don’t need to tell him/her how to do it.

Season

RE: Datum feature and Datum feature reference frame.

Datum C is not needed; that should be pretty clear by now.
But to your main question: It is not necessarily wrong to have a GD&T callout reference a datum that is derived from a datum feature which is included in the callout. This is because the "datum" is distinct from the "datum feature." I think that's the point you were trying to get at.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Datum feature and Datum feature reference frame.

(OP)
Belanger. Yes that was the answer I was looking for. Some what.

To truly lock down the whole part on the inspection stand. Was to elimanate any other movement because Datum feature B is only two points of contact. Someone were to bump into the table this thing has the potential to move. In theory the second and third plane of the Datum feature reference frame is still spinning around 360°. So by selecting datum feature C which uses one point of contact for either/or the 2nd or 3rd plane of the datum feature reference frame would stop the features from revolving 360°.

So all the alphabet letters in the positional tolerance frames only reference Datum features. To restablish the planes that exist in the machining equipment.

So to really keep people from picking at datum feature C. Yes you can remove it and be sufficient with Datum features A and B. But to have a Third Datum feature as a third reference really tunes in the clocking of this part to the datum feature reference frame. That's my explanation. The alphabet letters in the positional tolerance are just references to where the Datum features should be located. I'm trying to illustrate where the datum features are so this part can be checked using any inspection resource that your town city or state has. For my made up company using GD&T. ( I stand corrected on the datum reference frame. I was typing too fast. Glad I didn't call something a datum).

RE: Datum feature and Datum feature reference frame.

(OP)
Lastly datum planes simply do not exist. The term you want to use is called datum features. Datum features are not Datums. The datum planes we refer to only exist in the machining equipment.

RE: Datum feature and Datum feature reference frame.

No -- again -- do not use datum C in this drawing. Why does it matter if the part spins around 360º? The position of the holes are NOT being checked for rotation on the part. They are only checked for rotation relative to each other (which is the point that SeasonLee and 3DDave were making).

Another correction: You wrote that "all the alphabet letters in the positional tolerance frames only reference Datum features." Better to write that "all the alphabet letters in the positional tolerance frames only reference datums" (or I suppose more properly, "the datum reference frame"). This datum reference frame does indeed consist of three planes, but the rotational aspect of the secondary and tertiary planes is not to be constrained on a part like this. Sometimes CMM operators don't like that sentence, but, well... too bad.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Datum feature and Datum feature reference frame.

(OP)
Datum features are not datums. Datum features are not datums. Please show me your resource that allows you to say datums. When using a standard like ASME Y14.5 from 1994 through 2009.
Please!!!!!!! Show me where it says!!!!! Datums... positional tolerance frames reference the datum features on the drawing which correlate to the 1st 2nd and 3rd planes of the datum feature reference frame. Please get your terms correct the only datum planes that exist is in the machining equipment. Thank you, Joe. ( I stand corrected on Datum feature reference. I was typing too fast).

RE: Datum feature and Datum feature reference frame.

(OP)
No -- again -- do not use datum C in this drawing. Why does it matter if the part spins around 360º? The position of the holes are NOT being checked for rotation on the part. They are only checked for rotation relative to each other (which is the point that SeasonLee and 3DDave were making).

Belanger: On the print there is no order to check or any evidence to check for rotation. GD&T is used for inspection purposes only. And this inspection says to locate the datum features for reference before you check the positional tolerance. This will ensure the datum features are properly aligned before you can go and check The positional tolerance zone at each hole. Located by the basic dimensions and the implied 90 degrees location

RE: Datum feature and Datum feature reference frame.

It's true that the letters at the end of the feature control frame (to use the correct terminology) can be called the datum feature reference letters. See ASME Y14.5-2009 paragraphs 3.4, 3.4.1, and others for this simple idea.
However, a position tolerance is not really related to the datum features, and I know that you know that. So according to ASME Y14.5 there is no such thing as a "datum feature reference frame." It's a "datum reference frame." See Figures 4-1 and 4-7, for instance. In 4-7 the holes are not positioned from datum feature C; they are positioned 150 mm from datum C.

Back to your drawing, though. Since we're looking at the Y14.5 standard (2009 edition), check out Figure 4-8. Notice that it doesn't include a tertiary datum. Yes there are 3 datum planes in theory, but the holes are located rotationally only among themselves, not to anything else on the part. So a third datum feature would be meaningless on the print. That's all that I and the other posters here have been trying to convey to you.

One other minor correction: datum planes do not exist in the machining equipment. The fixture/equipment, etc. is merely the physical datum feature simulator. (Check out the entirety of paragraphs 4.5 and 4.6 for this distinction).

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Datum feature and Datum feature reference frame.

(OP)
So to answer my own question Datum feature C can be used as a third reference for clocking. Correct it's not needed for people that don't know what they're talking about. Who use incorrect terminology.

Datum features are not datums. A datum is a letter in a square box. You never dim. Out to a Datum. It just doesn't happen. Datum features are something you can touch.

I don't know what book you're reading that refers to Paragraph so and so. But the drawing requirements manual that I use is the 11th edition which is strictly governed by ASME Y14.5 2005.
So it doesn't matter what book we have you can't deviate from that standard.

Datum features on the inspection table, by using angle plates, gage pins, 123 blocks Etc etc align the Datum reference frame. The datum reference frame simulates the datum planes that are inside the processing equipment. Since 1966 Y14.5

RE: Datum feature and Datum feature reference frame.

There is no 2005 version of the 'Y14.5 standard (perhaps there is a reaffirmation date?) You are looking at a derivative work, which is likely distilled for use as an aid for those familiar with the ASME standard. The paragraph references are to the actual, governing standard. Perhaps you should get a copy.

Your FCF refers to C(M), which means the part is not locked, but allowed to rotate the amount of clearance the as-produced feature has with the virtual condition datum feature simulator.

RE: Datum feature and Datum feature reference frame.

(OP)
Okay ASME Y14.5 2009. I think that is stated on the drawing as well. Happy now?

With Datum feature C being referenced there is no way the 1st 2nd and 3rd planes of the datum reference frame can rotate or shift. So what does that mean? the part on the inspection table cannot move around based on the datum features which was used to reference the datum reference frame. which stimulates the planes in the processing equipment.

RE: Datum feature and Datum feature reference frame.

(OP)
So just curious there Dave does the virtual condition use the perpendicularity established by datum feature A?

RE: Datum feature and Datum feature reference frame.

It isn't the DRF that shifts - it is the part that is able to shift relative to it, a condition known as 'datum shift'

The same shift can happen whether there is a datum feature [C] or not.

Your question about perpendicularity - per the fundamental rules the axes of the holes are considered to be perpendicular to [A].

Draw a picture of the functional gauge that would be used to accept this part.

RE: Datum feature and Datum feature reference frame.

Quote:

Datum feature C can be used as a third reference for clocking. Correct it's not needed for people that don't know what they're talking about. Who use incorrect terminology.
I know what I'm talking about, Joe. I've been a full-time GD&T instructor since 1995 and along the way I sorta picked up these very basic concepts.

Quote:

A datum is a letter in a square box.
No. a datum feature is a letter in a square box. Remember how we all need to use correct terminology?

Quote:

I don't know what book you're reading that refers to Paragraph so and so.
Oh brother. You demanded that I show you exactly where I'm getting my distinctions on these things ("When using a standard like ASME Y14.5 from 1994 through 2009") and so I gave you the exact reference paragraphs in 2009.
And yes, it does matter "what book we have" because I'm using the standard and you're using one supposedly derived from the standard. Guess which book is correct?


I've tried to help. Peace, out

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

RE: Datum feature and Datum feature reference frame.

(OP)
Just because you teach it doesn't mean you can do it.

Getting folks out the door with the wrong terminology is a disservice.

Hey I didn't write the stuff it's under the standard. So you have all these books that are about GD&T. But the specification ASME..2009 dosen't change. Thanks for trying John. Again what book are you Reading that states the ASME....2009 terms and guidelines?

A datum feature is something you can touch. So the next time you hold a part in your hand and you look at the drawing you need to touch the datum feature A B and C or T T Y L.

RE: Datum feature and Datum feature reference frame.

Let's get this out of the way, first...

Quote (ASME Y14.5-2009)

1.3.13 Datum
datum: a theoretically exact point, axis, line, plane, or
combination thereof derived from the theoretical datum
feature simulator.

Quote (ASME Y14.5-2009)

1.3.16 Datum Feature
datum feature: a feature that is identified with either a
datum feature symbol or a datum target symbol.

Quote (ASME Y14.5-2009)

1.3.17 Datum Feature Simulator
datum feature simulator: encompasses two types: theoretical
and physical. See paras. 1.3.17.1 and 1.3.17.2.

Quote (ASME Y14.5-2009)

1.3.19 Datum, Simulated
datum, simulated: a point, axis, line, or plane (or combination
thereof) coincident with or derived from processing
or inspection equipment, such as the following
simulators: a surface plate, a gage surface, a mandrel, or
mathematical simulation. See para. 4.6.

Now.. back to my response:
"The drafting requirements manual 11th edition:"
I don't know what manual you're talking about.

As with Belanger and others, we mainly speak and cite directly regarding ASME Y14.5-xxxx depending on the one at hand. Your drawing references 2009, so that's the one referenced. Directly.

I personally don't understand your question in the attachment. Sentence fragments are too fragmented for me to conclude your point, and you use somewhat-slang terms that aren't completely familiar to me.

You can find many examples in the 2009 standard which have a FCF with a DRF containing self-referential Datum Feature.

The third datum, as others echoed, is unnecessary, as shown in Fig 4-8 suggested by Belanger. I don't think that's even a debatable point. You mention "truly restricting the six degrees of freedom" but, for emphasis, this is not a requirement. In some cases it may not even change anything by calling it out in the DRF. So I suppose I would not agree that it applies as a "3rd reference" unless you make it so. The wisdom of making it so... that is debatable.

As for 'best practices' - I would start not with "dimensioning to the datum plane" but rather your use of a circular hole pattern, but not calling out the diameter of it, and also lacking basic dimensions to the 12, 3, 6, and 9 o'clock holes. You locate only 4 holes, the way it is dimensioned currently.

RE: Datum feature and Datum feature reference frame.

(OP)
Thank you for all the pointers.

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