Help with true position

[[Am...]]

Hi everyone, I have question on what to do with this true position call out I have. It is a composite true position but it seems to me that it is called out wrong.

What is the point of calling this out this way if they are going to use the same datum references for both? These are two cylinders that are at the base of some tangs, the cylinders are about 1 inch apart. My thought was to do the position of them together and then separately even though the way its called out seems like they would have got the same result just asking for the individual true position twice. Am i missing something here?

Should I just do one bore pattern true position and then just true position with each one?

[[Am...]]

I can't post a picture of the drawing but I can try to answer any questions.

[[Am...]]

Here is some more information

Datum A is a curved surface that the cylinders are sitting on, radius of the surface is 23 inches and the surface is only 2 inches of that circumference so I don't know how stable the axis of Datum A is going to be. Should I constrain the radius when probing datum A?

Datum B is one edge of the part and datum C is the adjacent edge perpendicular to datum b. Imagine if you had a rectangle bent it at a radius of 23 inches and had two cylinders coming out from the inside face of the radius. Datum A is that inside face, B is the long side and C is the short side. Datum A is BASIC.

[[SH...]]

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IMG_20200712_000550.jpg

[[Am...]]

That is pretty much my expression looking at Calypso today.

[[Br...]]

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It's really difficult to visualize what you are talking about. But just remember, the DRF in the lower segment of the composite tolerance only constrains rotational degrees of freedom. The upper segment would be treated as you would any other single segment position tolerance. Sometimes it makes sense to reference all 3 Datums in the lower segment if the tertiary datum constrains a rotational degree of freedom.

[[Am...]]

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Apologies for the crappy drawing, that BASIC on the bottom says 1.25, im not using the actual values but this is the gist of it. The radius is about 22 inches and i get all of 2 inches of it, the thickness of the part is less then .1.

Im really gonna need some tips on this one I don't have any confidence in keeping a stable datum A. But how do I go about setting up this true position?

[[Ia...]]

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If the tertiary datum doesn't have a modifier, and all degrees of freedom are constrained, how does that work for the bottom segment?

Just having a hard time visualizing.

[[Br...]]

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Something like this for instance. Datum C controls a degree of rotation.

[[Br...]]

Amel P,
So in your case C does constrain a degree of rotation and it makes sense that all 3 Datums could be referenced in the lower segment.
In DRF ABC:
-Datum A constrains 2 degrees of translation and 2 degrees of rotation
-Datum B constrains 1 degree of translation
-Datum C constrains 1 degree of rotation.

ASME Y14.5-2009 para. 7.5.1(b)(3) states:
"In some instances the repeated
datum feature references may not constrain any degrees
of freedom; however, they are necessary to maintain the
identical datum reference frame..."

[[To...]]

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Brett,

I'm confused. So what actually changes between the 2? In other words, why not just use the 1 line with the tightest tolerance?

[[Br...]]

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The upper segment locates the 2 hole pattern to the Datum Reference Frame. The lower segment controls the Feature-to-Feature relationship of the two holes and additionally the orientation of the pattern relative to the Datum Reference Frame.

Maybe what I posted was confusing. Those constraints were to show what the upper segment is doing so you could see what datums constrain rotational degrees of freedom. I was trying to show why it could be necessary to include all three datum references in the lower segment. The datum reference frame in the lower segment only constrains rotational degrees of freedom. So although Datum B does not constrain a rotational degree of freedom, it is necessary to include it in the lower segment to maintain the identical datum reference frame.

[[An...]]

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Is there somebody who is able to translate this into the "Poor CALYPSO USER'S language?

Calling Mark Foster!

[[Aa...]]

I am not Mark Foster. Nor am I in his league when it comes to explaining GD&T. But I'll take a crack at that question. For composite FCF's, ASME requires that the DRF of each segment of the FRTZF be a truncation (Cutting off datum feature references, but not dropping material boundary modifiers) of the DRF of the segment above it. So, if the PLTZF DRF is A|B|C, the first line of the FRTZF can be A|B|C or A|B, but not A|C, even if B is not constraining any rotational degrees of freedom. The result is that if the DRFs of the PLTZF and FRTZF read idnetically, the coordinate system of the FRTZF segment is necessarily oriented identically to the coordinate system of the PLTZF segment (the difference between them being translation only). That is what is meant by:
Note: I have corrected the explanation above. You can read the original version in Brett's quote of it below.

[[Br...]]

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I think you explained it well.

[[An...]]

Dimensioning as much as necessary.
See attached.

Pic_7-42.pdf

[[Aa...]]

Interesting. Why do you say translation is allowed for the PLTZF, but rotation is not? Both are allowed only to the extent they can be allowed for by the datum mobility from the MMB modifiers on B and C (the same way they would be allowed in a non-composite position FCF). But that has nothing to do with its being a PLTZF.

[[An...]]

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Thank you!
An error has occured.
See attached.

Pic_7-42_a.pdf

[[Am...]]

Wow this blew up, im sorry I have been away for the last two days I need to read through these posts and try to understand before I make any comments.

What do you guys think about that A datum and the best way to go about it, The radius is sitting at 22 inches while I have about two inches of that arc to scan. I don't feel confident with the repeatability of that datum axis trying to scan that surface as a cylinder and projecting 99% of the cylinder. Currently I have tried just measuring a circle on that datum and I get the diameter of that circle at about 33 inches with no repeatability. If I constrain the radius in xy I can get that basic to be almost right on, but I don't know if thats appropriate here and do you think I can get away with using a circle here? seems like I need to measure it as a cylinder because I need the Axis of datum A.

I also have a fixture that i am pushing Datum A into with some spring detents, would it be better if I just measure the fixture radius and use start alignment.

Also the width of that wall is .090" and part of those edges are radiused so I don't really have much room there to create a plane so im only getting 2d lines on B and C, but im not sure if thats ok in this scenario.

I have made a fixture where it pushes this part against Datum A and C, can I just use A and C from the fixture?

These are plastic molded parts and this is going to be a tooling and process development project and I feel so un prepared for this because there is a huge rush and this part has like 89 dimensions on this tiny thing 😕 Im really going to need some help on this one, im purchasing some of the eLearning modules for this because I want to be sure I can align this thing properly and get all the profiles on here and have some repeatability in these measurements. Every dimension is going to need to meet a 1.33 cpk out of a 30 piece sample.

[[Am...]]

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Ok I think im starting to visualize the difference and what each segment is asking for but how do I actually differentiate between those two in Calypso when they are using the same Datums. Whats my characteristics going to look like in Calypso. Do I use the Bore pattern and have my 2 elements be my two diameters, with the DRF being ABC? If so what do I do different to report the bottom segment?

[[Br...]]

So first off, the Datum Feature A here can be tricky. I would probably recall that into a freeform surface and do a best fit. You don't want to treat Datum feature A as a feature of size where you're trying to determine the axis based off of an actual mating envelope. So best fitting the freeform points to their basic shape will better reflect what the actual datum simulator would look like in this case.

Next I would create a secondary alignment that references that best fit of the freeform surface, and let datums B and C constrain their degrees of freedom. So Datum C would take on the planar rotation while B would get one of the X Y or Z origins depending on the orientation of the alignment. Name that "DRF ABC" or something like that.

Then I would create 2 separate position characteristics--One for the PLTZF, and one for the FRTZF. Both will utilize the bore pattern option.

When creating the PLTZF, in the datum selection area of the characteristic, select the Secondary alignment you created. Now open up your bore pattern and select your features. Type in the tolerance and you're done.

When creating the FRTZF, you can literally copy/paste the PLTZF you just created, change the tolerance, and open the bore pattern. you will see a dropdown at the bottom of the bore pattern. Select the "View Tolerances" option, and then check the Translation box. This will allow the pattern to shift in a linear direction, but keep the orientation of the pattern constrained to the datum reference frame.

[[Am...]]

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Thank you, for the Datum A simulator what can I do if I don't have freeform? I unfortunately bought curve back when it didn't come with Freeform and now I can't buy freeform standalone now. If I have to get it for this I will but its gonna be a pain to ask for with the current situation.

[[Br...]]

I would try to measure that Datum feature A surface with 3D curve, then do the same best fit thing. Should work the same.

Edit: Another thing you probably can do if its too tricky to do that as a single 3D curve, is do multiple 3D curves so you can seperate the measurement paths, then do a "Best fit of several curves" alignment. (I think thats what its called.) Then do the same thing I suggested above using that in place of the Freeform best fit.

[[Am...]]

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When I create my PLTZF do I have the Rotation and Translation checked?

[[Br...]]

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No Not for the PLTZF. Think of it the same as any single segment position tolerance.