The meaning of FRTZF without datum reference 2

[SeasonLee]

Hi :

The books I read say the lower segment without datum reference controls the sapcing, the perpendicularity and parallelism are controled by the upper segment. I am confused and I really don't know what is the real meaning of this example.

Thanks in advance for all inputs.

Season

 

[PaulJackson]

The lower part of the composite callout (in this case) is refining the coaxiality of the two in-line holes relative to an axis created by both holes simultaneously. The tolerance is specified RFS so no matter what size either hole is... each hole's individual axis must reside within a 0.01 diameter cylindrical boundary from an axis created by both.

If the tolerance was specified with a MMC modifier then a pin of dia. 0.123 could be used to verify the coaxiality along with separate measurments for size. The collout, however is not modified MMC so the each hole's short axis must be acquired and measured for orientational displacement from an axis created by both.

Paul

 

[ringster]

Paul,

Your explanation appears to be logical enough. However, it seems quite out unorthodox to have a positional tolerance expressed without referencing it to a single datum as a minimun.

Are you not making an 'assumption" when you relate the holes to the axis created by both?

Is there a similar example in the Standard? It appears that Y14.5 is invoked on this drawing although there are a couple minor 'glitches'.

 

[PaulJackson]

ringster,

It is not unusual or unorthodox to see a callout like this without a datum reference in the lower FRTZF. See Y14.5M-1994 paragraph 5.11.1.5 and figure 5-51. I have used these controls often to tolerance a series of coaxial hydraulic control valve bores.

It is unusual however, that the FRTZF's tolerance is not modified "MMC" since the coaxial bores typically mate with a pin-like mating part. I suppose that if these holes mated with an expanding roll-pin and alignment of the holes were critical to achieve surface contact for retention then RFS may be appropriate... but I doubt that... Some one probably just omitted the MMC because it wasn't on the previous design or they didn't consider its relationship to function or gaging.

Paul

 

[dingy2]

ringster:

I agree with Paul on this one.

One really needs the modifier MMC to be be able to confirm the alignment. One would then make a straight pin of the virtual condition size (MMC minus the tolerance) and insert it into both holes. The feature meets requirement if the pins are able to go into both holes with the force of no more than 1 finger.

Without MMC, I really don't know how one would confirm the requirement.

Dave D.

http://www.qmsi.ca

 

[MechNorth]

Paul & Dave are correct, MMC makes most sense here. You could verify the axis of each feature RFS and compare back to the coaxial axis between the two features, but there's not enough wall thickness to do this with great repeatability.

The answer to "why" you don't need a datum reference on the FRTZF is this; the PLTZF (first level of the composite positional control) establishes the general zones based on the datum reference frame where the axes of each feature in the pattern must be located. The second level (FRTZF) is a refinement of the first lefel (PLTZF), wherein the inter-feature positional relationship is being refined. Datum references are permitted on the PLTZF if you are trying to simultaneously restrict the position of the features to each other and the orientation wrt your datum structure.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

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[ringster]

Paul,

If we are to consider the 2 holes as a single axis, why would not perfect form at MMC be applied without the need for the lower segment of the composite tol?

 

[MechNorth]

That would be overly restricting. Envision the top level tolerance being a Dia-1" zone; there are two coaxial tolerance zones which would be 1" dia, and therefore the axis of each feature must be within that zone. Now picture that on the second level, the tolerance zone is 1/4" Dia. That means that the two individual feature axes must be within a 1/4" dia cylindrical zone, but that 1/4" zone floats within the 1" zone.
Per ringster's proposal, I could tighten up to a 1/4" zone on the top level, but why make things tighter than they need to be?

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

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[ringster]

Then we come around to another consideration. What the heck is the assembled relationship? That would be the determining factor, would it not? Otherwise we are guessing at the requirements.

A similar exercise occurred in the 1994 version when resolving the old figure 142 of the 1982 version. IMHO.

 

[PaulJackson]

ringster,

Sorry for the late response I had to reply to Dave... It took a while after church and such.

I disagree that it would be overly restricting as MechNorth stated. It is simply that the two holes are two seperate features and each one's "perfect form at MMC" is considered individually.

GM (in their USCAR addendum) allows features to be delared "interrupted" which makes both one feature. Outside of that... the coaxial position tolerance as was done in this composite example would be required to refine the coaxiality beyond the position tolerance.

Paul

 

[ringster]

I will restate my opinion. We need to know the assembled relationship to adequately evaluate the dimensional controls/requirements for this part.

I do not have at my disposal, currently, the 1994 version but the 1982 for coaxiality does ref a datum feature. Fig 161.

 

[dingy2]

ringster:

Absolutely reassess your assembled relationship. GD&T should apply only to features where there is a function and relationship to another feature on the same part or its mating part. If there is no function and relationship, don't apply GD&T.


Dave D.

http://www.qmsi.ca

 

[SeasonLee]

Thanks for all inputs.

We are just the OEM part supplier and I am not quite sure about the assembly and its function, sorry for I can't provide further information regarding the two holes feature.

But I am interesting to know the meanings, if:
1. Two datums reference A B on the second segment (FRTZF) in this example.
2. And with only one datum reference A on the FRTZF in this example.

Thanks again

Season

 

[ringster]

It is my opinion that there are several issues that need attention on this drawing, and for the forum to attempt a solution to any one brings up another unaddressed issue.

I hope this is not misunderstand, but sometimes there just AINT NO good solution to a problem.

 

[MechNorth]

Paul,
The purpose of the composite positional control is that you don't have to overly restrict the general position of the pattern while allowing the features to be more tightly positioned wrt each other, whether a planar layout of features or coaxial. So, by using only the second level of the composite control, with the tighter tolerance, you would be unduly restricting the position of the pattern as a group. The "uninterrupted" or continuous feature point is valid in this particular application because the holes will likely be fabricated at the same time as a single tool pass, but what if they were different sizes? While they could be fabricated using a stepped drill, the probability of such tooling drops with the decrease in production batch size. As a result, you would need to use a composite control to first locate the two holes wrt the DRF, then second to control the coaxiality of the two holes to each other.

Dave, Dave, Dave my friend... every part needs GD&T.
Without invoking Y14.5, you don't even get Rule #1. With GD&T, you get the size controlling the form at least. With Y14.5, you get documented implied conditions; without it you make assumptions based on "common practice". With GD&T, you have a documented means of controlling size, form, orientation and location of every feature wrt a datum structure. With GD&T, you know how to set up the part for inspection. With GD&T, you minimize tolerance accumulation, and you control the feature under consideration, not the dimension to a point in space.

I've frequently heard people say that they only use GD&T on critical features,; those that they care about. My questions then are:
1) so you don't care how the part is set up for inspection?
2) if you don't care about the feature, why have it?
3) if the feature broaches another feature but meets the +/- spec, is it ok?
4) a square or rectangular position zone is ok, with its biases?
5) you don't care if a rod or pin is bowed, or a rectangular feature is twisted and warped?
6) do you want to use a general tolerance that doesn't conflict with the geometric controls?
7) which is the overriding control ... a conventional +/- tolerance or a geometric control, when both are applied on the drawing?
8) do you think that you don't have to verify any of the conventionally-toleranced features when geometric controls are present?

I agree that designers shouldn't be tossing controls onto the pile just because they're there; they should be conservative in using only the controls necessary to attain the functionality they need. That, however, does not mean not using GD&T, it means judicious application of controls. My perspective is that, as an engineer, I want to fully define all relationships whether they have a tight tolerance value, or a loose tolerance value. By defining all relationships, I can avoid wasted time explaining what the drawing means, and avoid civil action to resolve open interpretation of an engineering print. Now, that being said, there are certain "junk" features that are appropriate applications of conventional tolerances; fillets, rounds & chamfers are the most prevalent.


Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[PaulJackson]

Mechnorth,

I know the purpose of a composite tolerance! Maybe I mis-understood the purpose of your "overly restricting..." comment... sorry if I did.

You stated in the latest response "The "uninterrupted" or continuous feature point is valid in this particular application because the holes will likely be fabricated at the same time as a single tool pass" What are you inferring with this comment? According to the current Y14.5 standard the two "in-line" coaxial holes are two separate features. If refinement of coaxiality is necessary one would need a geometric control to do so. It does not matter how the features were processed... they cannot be considered one interrupted feature and therefore constrained (refined) to some degree for coaxiality by Rule #1 (perfect form at MMC).
BTW...Thanks for responding to Dave's comment... I agree.

Season lee,
You asked:
"But I am interesting to know the meanings, if:
1. Two datums reference A B on the second segment (FRTZF) in this example.
2. And with only one datum reference A on the FRTZF in this example."

The upper part of a composite callot controls as many degrees-of-freedom as the specified datum references are capable of constraining... up to three for translation and three for rotation. The lower portion of the composite control "the refinement control of the pattern" has two purposes:
1) it controls all feature's in the pattern for their relative location and rotation (6 DOF) to each other without regard to specified datum features.

2) If datum features are specified... it controls the "refined" pattern zones for as many orientation degrees-of-freedom (up to 3 DOF) as the specified datum features in the lower segment are capable of constraining.

In your case... if A was specified in the lower composite segment the two dia. 0.010 zones would need to be parallel to surface A.

If both A and B were specified in the lower composite segment, B would do nothing because the only rotation DOF that it is capable of restricting (perpendicular to B) is already constrained by A (parallel to A).

Added note, The control on the print refining the coaxiality could also be done with seperate single segment position control rather than a composite... as long as there were no other identical controls (position without specifed datums) on the drawing because if there were the simultaneous requirement rule would make all features with identical controls one pattern. Unless of course this was a GM print which unilaterally reverses that simultaneous requirement rule by invoking its USCAR addendum.

Paul

 

[MechNorth]

Paul, I figured there was a misunderstanding of my post; no biggie. As for Dave's comments, yup, we just keep bouncing back & forth on that belief.
Your comment on the GM addendum re "uninterrupted" feature is timely in that the next rev'n of the Y14.5 standard looks like it will include what are called "continuous features" or "CFs". Where features are intended to be fabricated and therefore considered as a single feature despite an interruption, then a single control may be applicable to both by invoking "CF" (if I recall correctly at least). That's where I was going with my comment. If the two holes in the example are fabricated in one step and are considered a continuous feature, then a single-segment FCF rather than a composite should do the trick. I've seen this being considered for other corporate addendums also. Also, by extension, "CF" on a size callout would include all interrupted surfaces within a single form limit rather than individually per segment. Without the "CF", you would have to invoke co-cylindiricity by using a surface profile to establish the cylindricity-limiting relationship between the two surfaces.
The final reviews & public comment phase of the Y14.5 revision are winding down, and the next version should be released relatively soon. Some of the "wish list" items for GD&T look like they'll be included ... we'll have to see how the new standard looks & reads, and then everyone gets to decide whether they'll adopt it or not.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[dingy2]

Mechanic North:

Welcome back again. I now have someone other than Paul for intense discussions in this subject.

When I stated that one does not need GD&T on every feature or characteristic, I didn't say that the drawing should not have the statement "complies with ASME Y14.5M-94". It certainly should - absolutely. Rule #1 is now covered.

I didn't say that datums should not be shown since they, again, should be reflected on each and every drawing.

When I learned the subject in the early, mid 80s, I used the book written by Lowell Foster. I always remember the statement about function and relationship and it did make sense.

Should one have positional tolerances on holes to lighten the product. I don't think so. There is no function and relationship whatsoever. Other holes have fixed fasteners and, of course, positional tolerances are applicable.

Should one have profile of a surface on all surfaces of which most have not function or relationship? I don't think so.

Having a drawing with each and every feature containing a feature control frame (or in notes) does not make sense to me. Check the ASME Y14.5M - 94 standard. Many of the drawings in the standard do not have a feature control frame application to all the features. Function and relationship is important.

Dave D.

http://www.qmsi.ca

 

[MechNorth]

Hmm, so a hole used for weight reduction doesn't need a location control? If it breaks out the side of the part, is it ok? If it breaks into another feature, is it ok? How would you check where the hole is wrt the conventional tolerances? Without a position control applied to the feature of size, or a surface profile, how will you verify the location?

GD&T was not used, understood, nor evolved to the level that it is today. You may have used a book for the '66 standard or the '82 standard, and Lowell Foster's consideration of GD&T's uses has surely changed/evolved over those years. The technology that is GD&T has changed, and so must our thinking. To accept that the understanding or perceptions of the past are blindly valid in perpetuity is to say that the world is still flat, the gods show favor or displeasure by good/bad weather, and "Yes, Virginia, there is a Santa Claus". Also, the illustrations in the standard are intentionally left incomplete so that there is not an information overload to the user, as it says at the beginning of the standard.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[ringster]

I don"t believe I have encountered the situation before, but how does a RFS modifier apply to a diameter with a draft angle?