Question regarding True Position Callout - verification 2

[brandnew1]

Quick and hopefully simple question.

i attached a portion of the blue print of a part i'm looking over. The question i have relates to the true position of the 4 diameter holes which is based on a composite feature control frame.

Basically i'm trying to determine if the holes are good in one sense to the part and then to each other.

So my questions / or verifications are:

The .017 true position will utilize the .677BSC dimension as this the primary datum it ties to and then i calculate if the other holes meet the specifications within the .017 & Maximum Material Condition modifier?

But what i'm more concerned about relates to the .005 portion of the true position call out. It has a basic of 1.05 from Datum D / E. But what if the hole location for the .017 true position is on the high end of the tolerance.

For example...say the hole location measured 1.090 from Datum D. The first true position call out of .017 will accept this value. But the true position call out of .005 will not since .004 is already well above this tolerance.
i just want to verify, is the .005 call out based on where the holes are located in reference to the first true position (.017) and then the holes to each other need to be .005 or is the 1.05BSC also dictating where the 4 holes start from each other?

So in other words, first hole called out 1.050BSC can be further than this so long as it is with the .017 tolerance and then from there the holes will need to be within the .005 true position and thus i'm not necessarily using the 1.050BSC dimension to determine the second half of the composite control frame.

 

[SeasonLee]

brandnew1

I think first of all you need to correct your composite TOP callout, according to the composite datum reference rules: The lower segment is a refinement of the upper segment, therefore any datum references must be a subset of the upper segment. Any datum references must be repeats and in the same order of the datums from the upper segment and must be contain the same modifiers.

So, the tolerance Ø.005 should followed with MMC modifier on the lower segment; besides, the reference datum should be |A|D| instead of |D|E|, the secondary datum D on the upper segment can not be changed to a primary datum on the lower segment.

SeasonLee

 

[pmarc]

brandnew1,

1. As SeasonLee stated, your FCF does not fall under category of composite positional callouts.
2. I am having troubles in understanding what are your datum features D and E? It looks like datum feature symbols D, E are attached to some center lines which is against Y14.5 rules (I assume you follow this standard).
3. I would recommend reading proper sections in the standard about composite positional tolerancing. I pretty sure this will help you to find an answer to your questions.

SeasonLee,
There is no "composite positional tolerancing" rule saying that tolerance values in all segments must be expressed at the same material condition. That said, theoretically upper segment may be modified at MMC with lower specified at RFS. Though I cannot picture any practical application of that at the moment, I just wanted to notice that it is possible.

 

[SeasonLee]

pmarc

I am pretty sure that the refinement, same order of datum and same modifier from upper segment is the rule said by Alex on his book “Advanced concept of GD&T”, but it is not with me on my office, will try to show you tomorrow.

SeasonLee

 

[brandnew1]

Just a quick note to this, this is a customer's drawing....thus i'm checking to verify that the part is good

 

[brandnew1]

Also to add, i believe the standard this is following is Y14.5-1982...not sure if that makes a difference as pmarc indicated that the datums attached to the same center line.

i found this: Link

which i believe answers my question.

Based on the images i see on this link, the first true position call outs that tie into Datums A, D & E...give the allowanc that the holes can be further than what is given by the Basic dimension call outs so long as they are within the .017 TP plus bonue given by the MMC.
But the holes to each other will only use the 2.650BSC, 2.775BSC & 2.200BSC dimensional call outs and need to be within the .005 true position tolerance, regardless of feature size.

thanks for the help

 

[pmarc]

SeasonLee,
I have a copy of this book and in figure 19-8, point 5, it says that: "The tolerance value of the lower segment must always be a refinement of the tolerance value of the upper segment".
Example: having 0.8(M) in the upper segment, and 0.2 in the lower segment is not against this rule in my humble opinion.

 

[brandnew1]

Sorry again, i think i'm getting more confused the more i research this, especially based on this link of another example shown:

Link

It says, "Any datum reference in the second segment of a composite feature control frame tightens up the orientation of the pattern".

So does that mean in the case of this (customer) drawing, the second half of the feature control frame is saying these holes need to start off from the 1.05BSC dimension? Thus indicating that the .017 true position call out only is to affect where the holes are in reference to Datum A (.677BSC) (literally saying the holes can be way off either to the left or right of Datum A) but has to be super tight to where Datum E/D is located as the only datums called out to Datum D & E.

i thought what i said made sense but as i look around now i'm not so sure...any guidance would be much appreciated...

thanks

 

[pmarc]

brandnew1,
I am afraid you cannot compare meanining of your feature control frame with the ones shown in the tips, simply because your FCF does not fulfill requirements for being composite due to the reasons mentioned by SeasonLee in his first post.

 

[brandnew1]

thanks pmarc,

2 follow-up questions:
A) If this is not a a composite tolerance frame (even though it is set in a way similar to one = sharing one true position symbol), what is it?
B) Please have a look at the attached pdf, i found this format on another sheet labeled with the same part number except it does not have a slot which makes me think that the reason the two datums (D & E) share a common center point.
But besides that, on this updated drawing the feature control frame is called out this time to Datum D (primary), Datum A (secondary) and Datum C (tertiary) followed by a lower segment to Datum D.
Does this fulfill the requirements of a composite feature control?
C) With the updated drawing, as i brought about before does this mean that the TOP feature controls the location of the holes, thus they need to be within .017 plus potential bonus to the BASIC dimensions.
But now that BOTTOM feature control frame calls out .005 Regardless of feature size, does this only affect the holes in refence to each other. In other words, location feature (TOP segment) allows the holes to be lower than 1.05BSC so long as meeting the true position of .017. Then the holes (BOTTOM segment) controls how the holes are controled to each other?

thanks again for the help and i apologize for the questions...i just want to make sure i'm reading this call out correctly

 

[brandnew1]

opps...i mean't 3 questions...

 

[pmarc]

brandnew1,

A. I would call it poor attempt of specifying composite positional feature control frame. The fact that both segments share common symbol of tolerance of position does not automatically mean the frame is legal composite callout. There are other rules that additionally must be fulfilled. You already know what kind of rules.

B. This callout looks much more like properly specified composite positional FCF.

C. Assuming datum D is the axis derived from half-round opening, the lower segment controls spacing between holes within the pattern plus orientation of their axes (perpendicularity) to datum axis D. Basic 1.050 shall not be taken into account when inspecting the lower segment. In other words, like you said, the holes can be lower than 1.050 as long their axes stay within tolerance zone defined by upper segment.

 

[fsincox]

The rules were not allways so clear in earlier versions thus the need for more definition in later versions.

 

[fsincox]

The 1982 standard does say "repeats the same datum", the 1973 standard was specifies the "the lower entry specifies the applicable datum (generally one for non-circular parts)".
Looks like something created by someone trained (if at all) in between.

 

[brandnew1]

Thanks everyone for your help....its difficult to figure out what is mean't on drawings that apparently don't follow Y14.5 to the rules....

 

[SeasonLee]

pmarc

Attached is a snap shoot from the textbook page 19-11, it said: Any datum references must be repeats and in the same order of the datums from the upper segment and must contain the same modifiers.

For me, the latest post sketch looks reasonable on the composite TOP callout.

SeasonLee

 

[pmarc]

That is clear, SeasonLee. But I was talking about tolerance value modifiers, not modifiers on datum references.

 

[SeasonLee]

You are right . pmarc.

SeasonLee