Tolerance of Positoin - feature positioned to itself ??? 2

[DesignBiz]

Please review attached drawing for comment on the tolerance of position callouts circled.

1. I do not understand how a FCF can reference the feature positioned to itself in a datum reference block. (e.g. datum A is positioned with reference to itself in A-B)

2. I believe the location tolerance and the basic dimension relavent to it should have the same number of decimal places. (Not shown on the drawing) (e.g. if tolerance in FCF is .XX then its basic dimensions should be XXX.XX NOT XXX) Is this in the standard?

DesignBiz

"Quality is in the details"

 

[CheckerRon]

In your example, the drawing is establishing an A-B axis through the center of the tube, using batums A and B as the datum features for establishing the axis. You're not calling the datum to itself, as much as you are making it part of the A-B axis.

Looking at the bends of this part, I would not use this axial dimensioning method. I would use profile-of-a-surface instead, to datums A and B, using the same A and B datum features.

Regarding the number of places, since this is metric and unilateral dimensioning, it is IAW Paragragh 2.3.1(a) of Y14.5-1994.

 

[axym]

1. I've never been a fan of FCF's that reference a datum feature that includes the considered feature. This method is not described in Y14.5. I have seen it in GD&T textbooks, but only for coaxial features and with MMC references on everything.

The presumed intent of the positional tolerances on features A and B is to control their locations and orientations "relative to each other" without making either one the datum feature. Which is fine. This will allow them to function well together as multiple datum feature A-B.

To me, controlling A and B relative to each other doesn't require referencing the A-B datum feature. If the datum reference were left off, then the rule of simultaneous requirements would tie A and B to each other.

This is all I have time for today. More tomorrow.

2. This one is in the standard. The same number of decimal places are not required for millimeter tolerances, only for inch tolerances. See 2.3.1 (d) and 2.3.2 (d).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[dingy2]

An example of this situation is on page 193 of the standard in fig. 6-51 with the exception that circular and total runout are used rather than positional. We have the example but no confirming verbiage.

Unfortunately, this is one of the areas in the standard that just does not make any sense. How could one reference a datum that is not already established??

Using the example that I mentioned, one could extrapolate that positional could also be used as shown in your example. I don't like it personally but it does look somewhat legal.



Dave D.

http://www.qmsi.ca

 

[drawoh]

dingy2,

The example on fig[ ]6-51 makes sense to me, and I can see how I would fixture it. In the absence of a long, round feature, two short, concentric, widely separated features make a good centre axis. I have done this on complex shafts.

The OP's drawing makes no sense to me. I cannot see how I would fixture and inspect it. If only one end had a positional tolerance, it would start to work for me.

I observe that datums[ ]A and[ ]B locate the part completely. I do not see a purpose for datum[ ]C, unless it controls the part's flexibility somehow. I would call up datums[ ]A and[ ]B separately in the FCF, and define the datum[ ]B feature WRT datum[ ]A.

JHG

 

[DesignBiz]

I do appreciate the direction to the section for the decimal place question and to pg 193 by Dingy2.

I would like to make clear this is NOT my idea of dimensioning an exhaust tube using the standard.
It is the companies standard template for dimensioning tubes. I dont like it at all personally.

Like Dingy2 I had the same question. How could one reference a datum that is not already established?
Seems like a paradox to me.

I can appreciate the logic Ron and Evan apply to make sense of the callout, however this is not a rotating
shaft that would make sense to create a single datum using both end diameters or 2 bearing diameters. This is
an exhaust tube and could be done more more simply uses 3 datum references in a more conventional DRF.

Thanks for all of the commentaries !


DesignBiz

"Quality is in the details"

 

[ewh]

Have you checked out thread1103-235809? Very similar situation.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[DesignBiz]

ewh,

The thread you refer to is actually one I started. It is even more bizarre to me when the 2 features that are used to create a single datum are at compound angles. Our tube ends are fairly often at compound angles to each other.

We beat that horse, til about dead.

Thanks for your input.

DesignBiz

"Quality is in the details"

 

[ewh]

Sorry about that, didn't check the details. I just knew that a similar issue had been addressed.
Yes, that one was beat to death.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[MechNorth]

Quite honestly, I don't see a problem with the OP drawing. Per the other thread, once you've got your datums established, you can locate the position of any feature of size. Essentially, you know where in space the feature of size is supposed to be, and you just verify it as you would normally. If the original question was really how do you verify position of Datum-A back to itself, you aren't. You are verifying the position of datum-feature-A to Datum A-B. Two entirely separate things. Review the definitions of Datum and Datum Feature.

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

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[PaulJackson]

Well said! Jim,

I had started a response to the

http://www.eng-tips.com/viewthread.cfm?qid=235809

thread but suspended it because I couldn't say with as few words as well as Evan does what I thought.

It is function!!! that must drive the selection of datum features to predict whether other features located, oriented, or formed well enough to guarantee that they perform as designed. The two ends of the tube (I suspect) are certainly the business ends of the primary datum feature… if the mating parts are more rigid than the tube itself – then theoretically they stop all six degrees-of-freedom since they are nominally not coaxial… practically rotation about a line intersecting the midpoints of each A-B and translation along that line would be a problem for inspection R&R unless some “procedure” or “over constraint via additionally defined features” remedies the variation.

Sidebar: If you think that this stuff is confusing now wait until they allow restrictions on the degrees-of-freedom” that datum features datum features may stop!

Paul

 

[DesignBiz]

I have seen both threads which I have initiated go thru some fairly deep interpretations relative to
the average designer and engineers basic knowledge of ASME Y14.5m 1994.

In the case of the exhaust tube example (both threads) I don’t see the need for adding the complexity
of the A-B multiple feature datum. To me this tube as many tubes is a relative non-critical component.
The two ends connect to other tubes to zig and zag from 'point a' to 'point b' as a conduit for exhaust flow.
These tubes can easily be a quarter of an inch off and they fit. I have been told here at the company, that the A-B callout developed because "both ends" of the tube run are equally important. That really isn’t an effective argument from my point of view. In the case of a gear that may have a spline that connects to a rotating shaft and external teeth that mesh with another gear; which of those features is not equally as important? My point is that on an average component it is common to have many features that are equally important in the overall mounting and function of the part.

From my point of view this dimensioning could be done with a primary datum using an end of the tube diameter
(Preferably the one that the xy bend coordinates originate from) and with a secondary datum as the opposite end of
the tube. A simple vertical line between the A and B vs. the "dash" between them in this FCF could help an awful lot more people involved in the design, manufacture and inspection of the part, understand the drawing AND still do the job. After all, isn’t this the main objective?
I don’t argue using the proper callout regardless of it being a little more difficult to understand, if the part function warrants it. I just don’t see it in this case.

What I have found here (this company) is that when I started asking questions about the tube callouts, is that absolutely NO ONE knew what the callout was supposed to mean nor how it was inspected. It gets deeper. I ask the checker what this callout means to him and he said whatever the GD&T group put on the drawing then it was okay with him. The GD&T group, consists of folks
that are not very good at even getting the basics correct. Basically straight out of college with a 40 hour GDT training class. I asked the supplier how they inspect the part and there has been no response for a couple of months. I ask how the 1st article inspection was achieved and no one knows... back to the supplier.... they should know, and absolutely no response to my
inquiries. It’s actually baffling from an "old school' point of view to get a part designed, manufactured and inspected.

Bottom line is that no one here that knows what the callout means well enough to explain it to anyone else
and therefore pretty much it is "disregarded entirely". Reminds me of the story about the emperor without any cloths.

The company has an on-site GDT consultant who continually had been frustrated trying to explain it to me (the thread(s) I posted) and ended up with a response, "it’s in the computer". This totally is disheartening to me. I have used computers (CAD) to do design work since 1977 and I KNOW the answer is with the designers and engineers.... not in the computer!

I was on the board before computers came around as a tool and I do like GD&T, however there is an old design rule ," keep it simple"; the ole "KIS"rule, and I won’t even use the second "S".

In light of this I dont see a need to go beyond the fundamentals of GD&T at this facility, for this callout. I have seen suppliers automatically add $10k
to quote a part simply because it was dimensioned with GD&T. I dont want to give up on the standard, however as in the case of the tube run, I think common sense and simplicity should prevail so that at least some people can understand the drawing. To me it is better to use a callout that some understand if the callout is appropriate, rather a more sophisticated callout that no one does.

I really do appreciate the comments made especially by the senior certified guys I find it extremely interesting as to the extrapolations or "extensions" that Evan, Jim, Norm, Paul and others have proposed and hope to hear more about it from this website.

DesignBiz

"Quality is in the details"

 

[PeterStock]

DesignBiz's last comment brings up a question that I have been thinking of. What would be the difference in interpetation of using [A]B]C] as he suggests and using [A-B]C] as shown on the drawing?

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services

 

[ringster]

I have a question:

How does one go about establishing an AXIS (LINE) between a set of skewed lines? An axis is usually determined by 2 points.

If I am reading the drawing correctly the datum features A and B are diameters resulting in a pair of datum axes.

 

[ewh]

A star for that, DesignBiz.
The explanations given here for this type of GD&T are good and valid; however, if a simpler scheme can be employed and does not effect the end use of the part, I'm all for it. If you happen to have studied GD&T extensively, this type of scheme will seem the best, but a majority of people who have to interpret these drawings will draw a blank when they see it. If they can't interpret it, how can they achieve it? These are non-critical parts and a more conventional, uniformly interpreted approach seems to me to be a better way to go.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[CheckerRon]

This is also what bothered me about this drawing, ringster.

We generate some compound bent tube parts. On our tube drawings, all our configuration changes are set up to X,Y,Z basic dimensions in a table (X,Y,Z being 0,0,0 at datum A end)for tube bending machines. We put a datum A and B respectively on the two ends, and everything in between is to a loose profile tolerance.

 

[DesignBiz]

Ringster,

This posted drawing was in ref to a feature positioned referencing itself in the FCF, however it is the same drawing callout posted on a prior thread debating the A-B; from cylindrical features of size at compound angles to each other; which would be the 2 axes that I believe you are referring too.
This was my confusion from the beginning of the other thread that a single resulting axis from these two at compound angles to each other is geometrically impossible.

I have accepted the "extensions" of the standard presented by Axym, MechNorth, and others, however it is not readily discernible with average knowledge of the standard, and still gives me rogue thoughts that bring me back to how is this a single axis from 2 at compound angles, or skewed lines as you put it? It can not be and therefore the only sense that can be made of it is by the "extension" that really locks down all DOF's in one datum reference block and is NOT a 'single datum' itself.

Still this does not set well with me.

DesignBiz

"Quality is in the details"

 

[ringster]

Designbiz,

By what authority are these EXENSIONS legitimized. I don't quite understand the how this would be 'discernable' to anyone. Average or especially above average knowlege of the STD.

Perhaps the proper rout to take would be to present for the @015 version of ASME Y14.5 and see if the committee finds it acceptable. Too late for the 2009.

 

[axym]

I haven't finished beating the horse in the other thread! There are still many points to make regarding the A-B issue.

I'm all for keeping things simple where possible, but there are times when the functional reality is more complex and must be acknowledged. This is one of them.

If they changed the callout from A-B to A|B in an attempt to "simplify" things for everyone, it would be a disaster. Why? Because the inspection department would suddenly start rejecting all sorts of functionally acceptable parts (assuming that the parts are being inspected, and inspected correctly). Why? Because A|B doesn't represent function! The part isn't fixtured on one end first and then the other.

To create the A|B datum reference frame correctly, the inspector would need to constrain two rotations and two translations using Datum Feature A, a cylinder which is roughly 5% of the overall length of the part. Small deviations in the part would be magnified into very large apparent deviations. A lot (most?) of the parts would not pass this artificially tight requirement.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[TheTick]

Compound datums give me the heebiest of jeebies. Yes, there are rare instances where they are appropriate. The majority of the time, compound datums signal that the drafter did not know what he really wanted and does not fully understand how datums are defined and used.