Datum Question with True Position 3

[Vindicit]

Hi I have a question on datums. In the attachment I have 3 examples and all 3 I have seen done on drawings. I dont show any basic dimensions or datum A, although datum A would be the face of the square shape. If someone could explain these 3 examples and how the datums relate to the true position and where the datums are actually placed it would help me out (and if any are incorrectly represented). I use a Optical CMM table so I am not sure how a Datum -A- is usually represented on those since its typically the face of the part that is lying flat on the table.

Example 1 seems the most basic. Datum B is the top left hole and the true position only references datum A so the analysis should be evenly distributed.

Example 2 Im not sure of. Is datum B again the top left hole or is it the center of the pattern of the 4 holes which is the center of the square also? The topic of datums on patterns I dont find much information on.

Example 3 Since datum B is part of the true position reference how does that top left hole get analyzed? If its a datum wouldnt it just show up as zero true position?.

 

[KENAT]

Quick partial answer,

Cases 1 & 3 you are treating the top left hole as the datum feature. I don't believe case 3 is actually valid as the datum feature is referencing itself.

Case 2, the pattern of holes is the datum. Simplistically the center plane for the entire pattern in both orientations is the datum. ASME Y14.5M-1994 section 4.5.8 7 figure 4-22.

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[drawoh]

KENAT,

Example[ ]3 makes sense to me. The datum applies only to the first hole. The specification is for four holes. For example[ ]2, see figure[ ]4.26 in ASME Y14.5-2009, which shows and explains a hole pattern datum. I am not going to try, and I would not call anything up this way.

Example[ ]1 is ambiguous to me. You need something to locate the other three holes.

--
JHG

 

[CheckerHater]

I would say in Example 1 holes are located to each other as a pattern, the way it works in multiple-single-segment frames.

Don't know if it helps though.

"For every expert there is an equal and opposite expert"
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[Vindicit]

Actually example 3 is very similar to one that Im working on now. Could anyone try to explain that one in a bit more detail? How are each of the holes analyzed? Does the datum hole need to be done separately? Would the feature control frame look different if each hole was called out individually?

 

[pmarc]

Guys,
Let me put it this way (forgive me for repeating some things that have been already said).

Case #1:
- The position tolerance wrt A controls orientation of the pattern relative to primary datum plane A as well as spacing between the holes in the pattern.
- Only the top left hole is the datum feature B.
- In absence of any geometric tolerance referencing B it is unclear at what material boundary the datum B should be simulated.

Case #2:
- The position tolerance wrt A controls orientation of the pattern relative to primary datum plane A as well as spacing between the holes in the pattern.
- The entire pattern is the datum feature B.
- In absence of any geometric tolerance referencing B it is unclear at what material boundary the datum B should be simulated.

Case #3:
- The position tolerance wrt A|B controls orientation of the pattern relative to primary datum plane A as well as location of the pattern relative to datum axis B.
- Because datum feature B is referenced RMB in the positional feature control frame, the datum axis B is the axis of the related actual mating envelope (RAME) of the datum feature B, which is the top left hole only. The RAME is a virtual cylinder perfectly perpendicular to datum plane A that expands within the datum hole B until it gets in maximum possible contact with the physical surface of the feature.

Cases #1 and #2:
- Assuming that all holes in the pattern have been produced perfectly perpendicular to datum plane A, and that the basic distance between the holes in the pattern is 2.000:
* MIN possible distance betweeen the axes of the top right and the top left hole is 1.980 = 2.000 - .020/2 - .020/2. It is when both holes have been produced at LMC.
* MAX possible distance betweeen the axes of the top right and the top left hole is 1.990 = 2.000 - .010/2 - .010/2. It is when both holes have been produced at MMC.

Case #3:
- Assuming that all holes in the pattern have been produced perfectly perpendicular to datum plane A, and that the basic distance between the holes in the pattern is 2.000:
* MIN possible distance betweeen the axes of the top right and the top left hole is 1.990 = 2.000 - .020/2. It is when the right hole has been produced at LMC. Size and location of the left hole are irrelevant.
* MAX possible distance betweeen the axes of the top right and the top left hole is 1.995 = 2.000 - .010/2. It is when the right hole has been produced at MMC. Size and location of the left hole are irrelevant.
- For this abovementioned as-produced condition (all holes in the pattern being perfectly perpendicular to datum plane A) the actual position error of the hole B relative to datum axis B is zero.
- Any as-produced perpendicularity error of the datum hole B will result in the positional tolerance error reading for that hole. In other words, if the hole B is produced with .003 perpendicularity error to A, the position error of this hole relative to datum axis B will also be .003.

 

[mkcski]

pmarc:

My interpration of Case 3 would be the same as KENAT's earlier post which said: "I don't believe case 3 is actually valid as the datum feature is referencing itself." So I ask: How can a feature be located to itself? Logically this make no sense. It reminds me of the "Chicken or Egg, which comes first" dichotomy. I understand the RAME and URAME concepts with RFS datums and position and I follow your discussion above. But I have never seen this explained or supported in any books manuals or other GDT materials. Is this really kosher? Or am I learning something new for my application "tool box"? Can you expand?

 

[pmarc]

mkcski,
While I am not saying I would ever use or recommend using tolerancing scheme from case #3 (as it is hard for me to imagine a functional reason to do it this way), I would not say it is invalid. Take a look at Fig. 9-7 from Y14.5-2009. The concept shown on that figure is pretty similar. Datum feature D is controlled with the total runout tolerance that references datums C and D. Is it self referencing? No, because the datum axis D is derived from RAME of the datum feature D, but the toleranced feature is not the axis of the RAME, but the surface of the datum feature D, which in this case may have certain runout error relative to the datum axis.

In case #3 the datum axis is the axis of RAME, but the controlled axis is the axis of UAME (Unrelated Actual Mating Envelope) of hole B. That is why it is not self-referencing.

If, however, the pattern was controlled with position tolerance that referenced B only, I would agree it would be self-referencing, because the datum axis and the controlled axis would be the very same thing.

I know, it looks weird, but geometrically it holds water. Question is whether functionally it holds water too. As I mentioned, I have some doubts.

 

[mkcski]

PMARC:

As always, your posts are prompt, clear and concise. I believe this "drawing" is good for case study to better comprehend GDT concepts,etc. but not for real-world function and fit-up applications. Again, much appreciated.

 

[pmarc]

You are welcome.

I am just afraid that this conclusion may not make OP any happier, as case #3 is very similar to the one he is working on now.

 

[ashivu123]

Hi pmrc, we have similar situation as case#3, can we call datum C in case#3 even though none of the dimensions are referenced from Datum C. Datum C only for clocking. Let me know how can I explain in better way that datums are not self referencing in the frame per case#3.

 

[ashivu123]

I attached pic let me how these cases will be analyzed. Also I sit always required to show basic dimensions from DRF?

 

[pmarc]

ashivu123,
I can't see your attachment.
Do not use any special characters, like '&' in the filename.

 

[ashivu123]

Please find attachment now.

 

[ashivu123]

In your previous case #3 how datum B hole is located since hole itself datum B.