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Datum setup
3

Datum setup

Datum setup

(OP)
It seems there are a lot of problems on the attached print, but I will focus on the datum callout. Here is my understanding on the designer's intent:
The bottom surface is primary datum A
Datum B is a pilot location hole
Datum C is another pilot location hole which is established base on datum B
B-C create the axis orientation
Datum B will be the original point for CMM measuring.

My questions are:
1.    Can we allow two datum symbols (Datum A and Datum D) on a coplanar surface?
2.    Is it a right way on Datum C callout?
3.    Which one is correct on the datum reference frame callout
Primary datum A -> B-C -> Datum B
Primary datum A -> Datum B -> B-C
Primary datum A -> B-C (without tertiary datum)

Thanks for all comments

SeasonLee
 

RE: Datum setup

I reviewed you drawing and:
1 Each surface could be the datum but having a datum A on one surface and a datum D on the other surface does not make sense. One needs a phantom line between both surfaces and then both are datum A. Your drawing in not quite correct here.

2 Datum C should be reflected in a feature control frame referencing datum A (bottom surfaces) and datum B (hole) at MMC. The .000 tolerance is fine. It just states that the size of the MMC and the Virtual condition size are the same.

3  If you change the bottom surfaces to datum A (not A & D), the reference datums should be A| B MMC| C MMC. When one has a hole as the secondary datum, it is the intersecting point of 2 perpendicular planes. All dimensions come from datum B while datum C is only to orient the part (square it up).

Hope this helps.

Dave D.
www.qmsi.ca

RE: Datum setup

Dingy,

I believe that 2 planes intersect in a line, not a point.

RE: Datum setup

"Dual datums" like "B-C" are only appropriate for runout.  Not appropriate for position or profile.  Use |A|B|C| for primary|secondary|tertiary.  The implication is that B positions the part and C clocks the part into position.

|A| and |D| can be individual datums because hey are separately identifiable features.  Inspectors will need to block part to ensure part does not rest on wrong portion.  Better to use datum targets to define datums with more clarity.

Why is there a |D| datum?  It's not used for anything.

RE: Datum setup

(OP)
Thanks for all comments.

When should we need to use dual datum like "B-C"?  where can I find it in the standard Y14.5M ?

I believe the designer intent to specify the profile tolerance .020 is to control the flatness after forming, there is a note "two surfaces" below the profile control, so it must be a coplanar surface,I agree datum D should be deleted.

SeasonLee

RE: Datum setup

Ringster:

I agree with that statement about line rather than point. Actually, the standard states "axis".

TheTick:

One can have 2 planes as a primary datum. One surface could be datum A while the other surface is datum B. When referring to the primary datum in a positional tolerance, one would state "A-B".

It all depends on how the part mounts on the mating part. Above, it the part contacts on 2 surfaces at the same time - thus, datum A & datum B. If the part only contacts on 1 of the surfaces, then we only have a datum A.

 

Dave D.
www.qmsi.ca

RE: Datum setup

SeasonLee,
Multiple Datum Features is defined per 4.5.7 - 4.5.7.1 - 4.5.7.2

Fig 4-19 shows a MDF with Tolerance of Position.
Fig 6-21 illustrates a MDF with Surface Profile.
Fig 6-49 & 6-51 shows a MDF with Runout and Total Runout.

RE: Datum setup

(OP)
I found fig 4-8 in the standard Y14.5M is quite similar as the one I posted earlier, the datum reference frame should be I A I B MMC I C MMC I for sure.

TheTick :

1.    Would you please advise where I can find out the rule of "Dual datums like B-C are only appropriate for runout" in Y14.5M.
2.    Dual datums B-C not appropriate for position or profile. This makes me recall one part I inspected two years ago, all profile tolerance with dual datum B-C in the DRF as shown on the attached

All comments will be appreciated

SeasonLee
 

RE: Datum setup

The example you just posted showing Datums B & C as a feature of size taken from theoretical intersections makes complience with 4.3 quite the stretch of the imagination.

As to the illustration posted 07FEB09, the B(m)-C(m) is legal, but does not provide 'predictable' repeatability.

RE: Datum setup

(OP)

If dual datum B(m)-C(m) is legal in the post dated at 2-7-09, may I ask what is the difference with this DRF A I B (m) I C (m) ?

Thanks again

SeasonLee

RE: Datum setup

Please note the difference between "appropriate" and "legal".

The dual datum |B(m)-C(m)| is legal, but it does not fully convey your intent and, as previously mentioned, is not entirely repeatable.

As for stepped datums like |A-D|, use of a single datum defined by datum targets is more appropriate and much more clear.

RE: Datum setup

What drawing standard are you using?  First picture appears to be 14.5 1994 edition while the second one looks like an older version from the datum designator.

1) You could keep datum D but then your FCF when you intend using a datum derived from both surfaces would be "A-D" as shown in Fig 4-20 & explained 4.5.7.1. of ASME Y14.5M-1994.  All depends on end function.

Datum "B(m)-C(m)" is only a single datum.  While I'm not sure the standard fully supports this my assumption is that it is intended to be the plane running through the theoretical axis of the two holes.

On the other hand, DRF A I B (m) I C (m) gives you all 3 perpendicular datums as shown in figures 4-8 & 4-9.  There is no ambiguity and it fully controls location/orientation of the features.

My 2C


 

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Datum setup

Another interesting thread. After quick review, the only real issue I have with the drawing is datums A and D. However, it really depends on the mating part to answer why separate A and D as shown. But, if A and D are supposed to stop a translational degree of freedom together, it is appropriate to then reference A-D as co-datums is the datum reference frame. Making datum feature A multiple surfaces is ok, I guess, but I prefer using the co-datum technique so that those two datums can be appropriately toleranced in location to each other as well.
The other suggestion I have is to more correctly (imho), specify both datum features B & C using positional tolerance back to a datum of A-B. This will reduce a tolerance stack if the axis or plane intended to be functionally formed by the two holes simultaneously is the true physical reality.

The sequence of a A-D and B-C reference appears in the DRF is entirely up to the physical function. Either way could be correct.  Most likely however, it is my guess that A-D primary and Bmmc-Cmmc secondary is the design intent.

Again, its difficult to judge right, wrong, or best without full information, but none the less, those are my quick thoughts.


Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.gdandt.com

RE: Datum setup

Welcome to the fora, Norm!  Another learned opinion here is always welcome.

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

RE: Datum setup

"Another learned opinion here is always welcome." Sure beats the likes of me making it up as I go alongwinky smile.

So just to test you out, could you explain the ASME Y14.5M-1994 explanation/implication of "Bmmc-Cmmc" I had a quick look in the standard and didn't see explicitly where it would come from.  As I said I would think it came from the plane through the 'axis' of the 2 holes but am not clear on the exact derivation/definition etc.

 

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Datum setup

If I am violating any of the policy, please help me out. I did open and review the reminder and well, that was a lot of rules.
My intent is to comment and within the subject at hand.
So, with that said, an axis established by a Bmmc-Cmmc referenced datum is not explicitly written in the standard. None the less, the principles are there, and in reality, a very common physical occurance. And, I might add, most likely, in this. If the two holes (B & C) are infact clearance holes, then the datum is allowed mobility or shift. It really isn't any different then any other datum feature of size referenced with MMC. It is a pain to inspect with variable or attribute data, but is none the less the physical reality.
So, that is why it is not CLEAR on where the "target" is. It's a potentially moving target and probably is. smile

Norm

RE: Datum setup

B(m)-C(m) have no preference between B or C. In a fixture, they could be two diametrical pins over which the work is placed. The angle of Datum A could vary by the amount of slop between the holes and the pins.

A|B(m)|C(m) establishes the orientation of A, followed by setting B(m) with a diametrical pin, followed by C(m) using a diamond pin.

Fig. 4-8 appears to be far more repeatable/predictable.

As to what is specifically on the drawing - it is the designer's responsibility to be as clear as possible. While the second example may be easier to calculate tolerance stacks from, the first example may be indicating that it is not the primary concern in this case.

KENAT, your statements have the conclusions drawn from the examples cited earlier held now in question. Thanks a lot!

RE: Datum setup

weavedreamer, it seemed the OP didn't fully grasp some of what you'd put so I was trying to clarify.  Also the wording of some other replies was misleading in my opinion.

Not sure I quite follow your last line, no offence meant in any case.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Datum setup

No offence taken. Juat going to have to review the references cited, and try to ensure that personal preferences are not coloring the interpretations.

The fact that you are reading this thread makes it worthy to be posting within.

RE: Datum setup

Kenat - Thanks for clarifying your std signature. And yes, I will review details of rules etc. related to this site. So far, I find it an outstanding forum.


Et al -
I did not realize yesterday that there was a second drawing. So, please keep in mind my previous comments are about the first drawing.

As for the second drawing, I also see it as a stretch. In this case, if functional datum features can't be determined for repeatable inspection, it makes sense to use other means of immobilizing the part. Keeping with how design and inspection may have made decisions on the second drawing, I would probably define datums B & C as a single Datum B with datum target points. Depending on that actual contour of the part, that may in itself stop all 6 degrees of freedom; otherwise known as a saddle datum. But since the flat face opposite datum feature A "appears" to be a mating face to a mating part, I would make that face Datum Feature A to functionally stop the appropriate 3 degrees of freedom and then Datum points for Datum Feature B (the four points of B & C) to stop the other 3.

Norm

RE: Datum setup

In reference to the drawing in the original post...

In Datum C's FCF, why is B at MMC? If datum B is an axis, why does the material condition matter? I see this quite often and could never find an explanation.

RE: Datum setup

When B moves from MMC to LMC, the variance can be added to the positional tolerance.  I'm sure someone else will give you a better explanation.

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

RE: Datum setup

(OP)
Norm

What you said is absolutely true, I don't think datums B-C on 2nd example is repeatable, it makes me spend a lot time to find out the Datum B and C on CMM measuring, its not an easy job to do it precisely.

SeasonLee

RE: Datum setup

With regard to Datum B being referenced at MMC, the REASON should be that the fit of the feature of size is a clearance fit. In other words, using MMC or RFS is not really discretionary. It is based on the physical design. Therefore, the part can and will physically be able to move/wiggle/float. And therefore that mobility, otherwise known as datum shift, allows the part to move within the datum feature simulator for datum feature B as datum feature B departs from it's MMC condition so that datum feature C could then fall within it's stated tolerance. (It will be able to assemble with said movement.)
However, beware! You should not get in the habit of incorrectly adding datum shift to the allowable tolerance of a considered feature. Mathematically it makes sense and works for a single feature, but you wouldn't REPORT in or out of a position tolerance value other then what is actually stated in considered feature's FCF. AND, when you have a pattern of features, which not always obvious, mathematically adding datum shift availability "separately" to a stated tolerance of a single feature can lead to accepting bad parts.

Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com

RE: Datum setup

Thank you, that seems to make sense.

RE: Datum setup

Referencing drawing #1 and guessing at functional mating features... I would--- identify the datum feature "A" beneath the callout [profile|0.20] "2 surfaces" note. Then I would use only one of the "dia .125 +/- .002"... making it "2X dia .125 +/- .002" and including the position callout beneath [pos|0(M)|A]... then beneath that identify the datum feature pattern as "B".

All other callouts would declare |A|B(M)| as the datum reference.

Furthermore... to reinforce the fact that both alignment holes have equal functional prescedent in stopping translation and rotation in relation to their mating studs ... I would place a vertical dimension line at the midpoint of the two hole pattern "B"... and show basic displacements to all the features "in that view" from the center of the pattern "B"... including one to each feature "B".

With the CMM... "A" stops one translational and two rotational "degrees-of-freedom" about the surfaces  and "B" stops one rotational and two translational along the line intersecting the axes of the pattern holes and one at the midpoint of the hole pattern.

An attribute gage... commonly referred to as a "functional gage" would do the same!

In figuring the stack allowances for the pattern @ MMC... I would moderate the liability of tolerance accumulation along the axis intersecting the two hole pattern "B" to +/- .002 {predicting that the pattern will be in interference (compression or tension) with the mating studs most of the time... and I would disregard any rotational stack accumulation for the same reason.

Just a thought... That's how I would do it.
Paul

Great Responses... I would have responded sooner but I was vacationing for a while.             

RE: Datum setup

(OP)
Paul

Thanks for your input, I like your different thought on the datum establishing, may I ask:

1.    You mentioned will place a vertical dimension line at the midpoint of the two hole pattern "B", is it based on the part is designed symmetrically ? Actually the part is not symmetric.
2.    What is the difference between your proposed FCF  IAIB(M)I  and Fig 4-8 in the standard Y14.5M  IAIB(M)IC(M)I ?

Fig 4-8 attached for your quick reference since you are on vacation.

Have a great time on your vacation

SeasonLee
 

RE: Datum setup

Paul is providing yet the cleanest way to re-dimension this part with a simplified way of saying what we have been saying. I tried to stay on the co-datum method for explanation purposes, but if I had to start clean, I would go with IAIBmmcI as explained, with only one minor exception.
On the part detail drawing I would not inlcude the vertical centerline ideally located midway between the two holes.  I find that perfectly find for the inspection process (plan), but is not functional to the part (design intent).  However, I also know too, that industry is putting more process dimensioning on prints and if it ultimately clarifies team understanding, then I can live it too.

 

Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com

RE: Datum setup

Norm,
Functionally... if both of the .125 +/- .002 holes mate with rigid round features from the mating part with identically designed clearances... then neither hole by itself will stop translation along the axis oriented to the line intersecting their centers... they do it as a pattern.

If the spread between the holes is longer than the basic then the pattern will experience minimum clearance inboard and if shorter... outboard. Either way the tolerance for spread is shared by combining each hole's deviation from its AME size.

If the secondary datum feature was declared as a two hole pattern: (as I suspect that it functionally operates in the assembly)

Then the DRF by design would reside oriented to "A" and the pattern "B" at the midpoint between the holes. That is why I would detail the basics from that origin. It has nothing to do with "process dimensioning"

Then the inspection process (plan) would be incorrect if the measurement DRF was coincident with the axis of one of the pattern's holes rather than the center of the pattern.

SeasonLee,
ASME Y14.5 Figure 4.8 is intended to illustrate how rotational orientations can be accomplished using holes.
It doesn't suggest a method to functionally detail products with two holes. The function does that.

It is interesting to me however, that paragraph 4.4.3(b) states "Orientation is established by the width of hole C".
It would seem to me that the width hole C (oriented to the line between B and C) would need to be identified as datum feature C for that to be true... otherwise the axis of the AME or the Virtual Condition Size of C depending on RFS or MMC would establish that rotation. I think that statement is a mistake.

Paul   

RE: Datum setup

Paul,
I understand and agree that the two holes as a pattern restrict 2 degress of translation and 1 rotation when referenced as a secondary. I also agree that if the holes are located in two opposite directions within their stated tolerance zone the actual available datum shift is reduced and could be zero depending on the AME.

However, where I think we will have a continued disagreement is locating the pattern to a centerline being the origin of the pattern. What is the tolerance then? The posiiton tolerance you referenced earlier would be the "feature to feature" location tolerance with an orientation control to datum feature A. I know you know that.  Using IAIBmmc, the origin of the pattern can be anywhere as long as the simultaneous requirement rule is properly applied.
How would we locate the "origin" of an irregular pattern? The point being that if the centerline uses BASIC dimensions, then why BASIC dimensions be applied to an origin anywhere? What would be the diffence?

"Process dimensioning" was probably not my best choice of words as many will refer to that as manufacturing process. All I was saying is that a measurement plan can and often does dictate a consistant origin to use for repeatability etc. and I certainly agree with that.
If there is a reference or figure in Y14.5 that dimensions a pattern to its own origin, I really want to know about. No kidding. I would really want to check that out.

Norm Crawford
GDTP-S
Applied Geometrics, Inc.
www.GDandT.com

RE: Datum setup

Back in about 1973 one
of the advantages to using GD and T was to provide '"Universal Interpretation' of a drawing.  My how times have changed!

RE: Datum setup

ringster, when we start discussing some of the more complex things that aren't as clear in the standard I wonder the same thing.

Even if it is based on extension of principles in 14.5 if most readers of the drawing, even those with a reasonable working knowledge of 14.5, can't readily understand it how useful is it really?

Is it better to use an approximation/simplification whatever the term may be, that is actually in the standard so long as it stands a reasonable probability of achieving what you functionally need?

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?

RE: Datum setup

(OP)
Now, part #1 re-dimensioned :

1. Datum D symbol removed and phantom line added between the two surfaces.

2. The two holes changed as a hole pattern and identify as datum B, all geometric tolerance's DRF changed to |A|B(m)|.

I am not a designer, please let me know if there are any problems on the print.

SeasonLee

RE: Datum setup

I feel the same as KENAT on this issue.  As it is with so many engineering tools today, there is usually more than one way to accomplish something.  We have to keep the KISS principal in mind though, and not loose the forest because of the trees.
  As for using a phantom line in this manner, I disagree.  An extension line should be used.

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

RE: Datum setup

Unfortunately I only have time for a quick post.

I'll comment on the "continued disagreement" between Norm and Paul about locating the DRF at the "origin" or "center" of a pattern.  I seldom disagree with anything Paul says, but in this case I agree with Norm.  The basic dimensions don't need to be laid out with any particular origin or centerline.  It makes no difference.  Any DOF constraints implied from the basic dimensions are just that - implied.  

I'll comment Paul's statement:

"Furthermore... to reinforce the fact that both alignment holes have equal functional prescedent in stopping translation and rotation in relation to their mating studs ... I would place a vertical dimension line at the midpoint of the two hole pattern "B"... and show basic displacements to all the features "in that view" from the center of the pattern "B"... including one to each feature "B"."  

I agree that both holes have equal functional precedent in stopping translation and rotation in relation to their mating studs.  It's a fact.  But showing basic dimensions from a centerline to each hole doesn't reinforce that, and shouldn't be taken as that.  I suppose that if the drawing user is going to (incorrectly) constrain the part's degrees of freedom based on how the basics are laid out, then laying them out from a centerline will let them get a result that is closer to correct.  But if the basics were laid out from one hole, the same drawing user would probably use that hole as a secondary datum feature and the other as a tertiary, which would be wrong.  The two holes would still have equal functional precedent regardless of how the basics are laid out and where the origin is - I believe that iss Norm's point and I agree with it.

This is another example of a larger issue, related to another open thread with the compound-angled cylinder datum feature.

I hope to comment more on these tomorrow.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Datum setup

SeasonLee,
Your new sketch is better than the original in that it declares the missing (implied) primary datum features and it controls the original hole "B" for its orientation to "A" (in addition to pattern spread via the 2X pattern position callout). The original drawing was missing that perpendicularity callout of "B" to "A". Even though that control becomes immeasurable as the depth of "B" approaches zero, these building block dimensioning prerequisites i.e. form of the primary, orientation of the secondary, position of the tertiary... are important for design constraint and variation analysis. Without that control a 3D stack analysis would nearly tip the shallow cylinder of "B" over using its tolerance for size.

The only things that I would change... I have already mentioned... Hang the "A" datum identifier from the leader of the 2 surfaces profile control (personal preference... I think that the phantom line option is less emphatic) and reference the basic dimensions from the center of the 2X pattern "B".

Norm,
I agree that with non-symmetrical patterns placing the origin for declaring basic dimensions is a problem without a desirable solution. Technically the origin for describing the basics can be anywhere on the detail... even at a point totally divorced from the datum feature structure... as long as it equivalently relates the basic displacements among all the features being controlled.  The problem with specifying basic dimensions from origins and rotations other than those defined by the feature control frame is that when inspections (other than attribute) are performed the inspector must re-figure the basics from the DRF specified in the feature control, perform the inspection, and report the deviation. Those reports will show deviations from each of the re-figured basic displacements rather than the ones specified on the drawing. In order to make the two relate the inspector would have to refigure the measured deviations from the alternate origin solely for the benefit of the person that reads the inspection report... it is a pain in the xxx.

For instance... consider a thru hole in a cast structure that is not square with the DRF that it is controlled from... the designer "for expedience sake or lack of understanding of how inspection is performed" will often "snap" X, Y, & Z coordinates square to the DRF at the intersection of the structure's surface and the hole... rather than specifying the coordinate axial displacements and associated angular displacements normal to the hole.
The inspector must figure the latter to perform the inspection of the hole's position to the DRF and then report his measurements and figure the deviation. His measurements will not relate to those specified on the drawing unless he re-figures the X, Y, & Z coordinates of the hole's axis deviations at the point where the specified surface should be!

SeasonLee's design is similar. If the secondary is specified as a 2X pattern the inspector must find the center of the pattern, perform the inspections from that reference and then relate the measured data to the origin specified on the drawing. If the two were the same "the center of the pattern" there would be no manipulation required. If the DRF is coincident with one of the "B" pattern holes then the data must be shifted from the center of the pattern... ½ the basic spread so that it matches what is specified on the drawing.  Both holes "B" in this case will have identical measured deviations reflecting whether the spread is longer or shorter than the specified basic.

I hope you understand that I am not saying that it is illegal or wrong to specify the origins and orientations for basics with respect to the displacements that must be figured to perform the inspection... it is just INMHO a poor design practice.

I said that "I suspect" from the details of the component the 2X  .125 +/- .002 holes probably work as a pattern for location and alignment with the mating part... that my be a false assumption... I don't know the details of the mating part... but if it is then I would reference the basics from the center of the pattern just as an inspector would have to do to measure the 2X  .125 +/- .002 [pos|0 (M)|A] as 7.063/2 displacements to each hole from that reference.

Paul
 

RE: Datum setup

I dont believe that I have seen this application before.  It would seem that if we have 2 seperate datum features, we should apply 2 different datum identifiers.  That being the case we could refer to the distance between them as width B- X, IF NECESSARY.  I thought the idea of centerlines as a datum was PELOSI.  I canat recall seeing an example of it in the Standard, although some trainers, include it.  

Hard to figure this one.

RE: Datum setup

Quote:

(personal preference... I think that the phantom line option is less emphatic)
This has been shown to be definitively covered by the standards here: thread1103-237315: Datum split.  If the standards sre to be followed, follow them.

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

RE: Datum setup

Ringster,

The midpoint of the pattern is not the datum feature... that would be PELOSI as you say... The two hole pattern is the datum feature... a common occurance with dowel holes! Putting a linear dimensional reference line at the midpoint between them is just as legal as putting it at one of the pattern's hole axes or anwhere else on the design... so long as the basics maintain equivalent  displacements and orientations. I just helps down-stream users of the design if the basics originate from the DRF's specified in the geometric controls so that they don't have to manipulate the basic dimensions so much to accomplish their execution or scrutiny of the design.

Paul   

RE: Datum setup

It seems that AXYM wants to place a dimension line on the midpoint between B and B.  What is that about?  It seems to be an attempt to designate the center plane of width BB.  

TEC-EASE had a similar example which I raised a question about.

RE: Datum setup

Ringster,

I want to place a dimension line on the midpoint between B and B?  Where did I say that?  Perhaps it was in Paul's statement that I awkwardly quoted by putting it in quotation marks (I can't find the "quote" function on this forum).  Anyway, I am making no such suggestion and I apologize for the confusing post.

Paul,

I feel much better after reading your two most recent posts.  I like your description that where to place the origin is a "problem without a desirable solution".  I believe we're on the same page regarding DOF constraint and basic dimensions.

One thing I'm still not comfortable with, however, is the idea of the "center of the pattern".  I have found it to be a source of endless confusion, particularly in the CMM world.  It can lead to crazy things like inspectors attempting to find the "center" of a pattern of 4 holes and verifying that the center is within a tolerance zone.  I could rant on this at length - perhaps in another thread.  Suffice to say that working with resolved geometry (like centers of patterns) has many pitfalls and is the often the source of disagreements between CMM results and hard gaging results.
 

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Datum setup

Evan,

Dowels are a special sort of pattern in that most of the variation occurs in a straight line (hence slots). If these locating features were designed as a hole (.125 +/- .002) "B" and a slot width (.125 +/- .002) "C" and liberal slot length... I would not suggest that the dimensional reference be moved to the midpoint between the features in SeasonLee's design. However, since (I suspect) they are both round in a steel plate and they mate (I suspect) with a plastic cell phone housing with identically sized drafted protrusions ... this functional application of a two round dowel hole pattern mating with a two round dowel pin pattern is a highly common design... meant to reduce (or eliminate with interference) variation for location and orientation simultaneously from one component to the other. I know that these sort of dowel systems are used extensively in many precision assembly designs and in every one I would recommend that the origin for the basics related to the secondary dowel pattern be positioned midway between the dowel features.

Not knowing what the mating feature details are... whether they are identically sized as the holes are... or even if they are both round and fixed for spread as the holes are... or whether they are a stud and a floating retainer... I cannot say that the origin for the basic dimensions that must be scrutinized by inspection should originate from the center of the 2X pattern or from the feature that solely constrains more degrees-of-freedom than the other (hence "I suspect").  No one would object to a 10X equally spaced round pattern of features identified as a secondary datum feature having the center of its base circle as the origin for basic dimensions yet they are flummoxed when a two hole pattern with equivalent mating clearances is dimensioned and referenced the same.

For symmetrical patterns with more than two features the form of the pattern complicates the determination of where the center actually is or should be when it is used as a locating and or orienting datum feature but that is not the case with two hole patterns such as dowels... therefore I do not agree with the opposition of referencing basics from the midpoint between 2X fixed spread dowel patterns that are designed with identical clearances among the mating features.  For those patterns with more than two features the actual feature position deviations must be used to predict interference or clearance (and permissible shift) to establish the origin for subsequent measurements from the pattern used as a datum feature (just as it is accomplished with an attribute gage).

Paul
 

RE: Datum setup

AXYM

After rereading the post I believe it was Paul's intention to add the dim line.  My apologies.  Whoever, however, I think it is not proper nor substatiated by the Standard.

RE: Datum setup

(OP)
Here come back again, I can't convince the designer to make a change on the datum setup and selection, anyone who can clearly point out where is the designer's problem, why he insists on |B(m)-C(m)| ? Why datum A and D used as primary datum at same time ?

Thanks for all comments

SeasonLee

 

RE: Datum setup

SeasonLee,

There isn't much difference between the specification you have now and the minor tweaks some of us have suggested. I suppose that you have to consider how adamant you are in getting it done your way and what you will likely sacrifice in support and cooperation from your CAD designers.  If you are ultimately responsible for the design "Approved by SeasonLee" and you understand why you want to make these minor changes then just tell him to do it your way period. Be ready however to keep having to do it your way because the designer is likely to let you pass or fail on your own.

Paul
 

RE: Datum setup

A thought just did occur to me;  What if we had the interface components to see just how the parts mated.  That might answer a lot of questions and guide us towards a proper solution.

Figure 142 of the 1988 Standard was similarly 'thrashed around' for a solution without the convenience of the mating part nor its function.

Comments

RE: Datum setup

(OP)
I really hope I can provide all information I have to facilitate our discussion, but there are some difficulties to get the mating components for an OEM parts supplier.

May be we can have more talks about the meanings and differences in between:
|A|B(m)|C(m)|
|A|B(m)|--- as talked earlier use hole pattern and identify as datum B
|A|B(m)-C(m)|

I am also wondering the FCF on the position callout:
|Position|.000(m)|A|B(m)-C(m)|---left side of top view
|Position|.000(m)|D|B(m)-C(m)|---right side of top view

Can one use different primary datum A and D on the position callout? Can one callout the coplanar surfaces in this way?

SeasonLee
 

RE: Datum setup

SeasonLee,

Quote:

May be we can have more talks about the meanings and differences in between:
|A|B(m)|C(m)|
|A|B(m)|--- as talked earlier use hole pattern and identify as datum B
|A|B(m)-C(m)|

Since both holes B and C have the same tolerances for size and both are modified (M)... a hard attribute gage would have 2 Diameter 0.123 pins spaced 7.063 apart... registering and/or establishing the individual secondary and tertiary (1st DRF) or patterned secondary (2nd and 3rd DRFs) for all three datum references identically. So without knowing the mating part details (interface components as Ringster says) there is no difference for the position callouts since all reference B(M). There is however a difference for the limit dimensions that originate from the center of B since they must originate from B RFS. If the limit dimensions were detailed from the center of the 2X pattern B (2nd DRF) they would originate from the center of the 2X pattern regardless of its spread.   

Quote:

I am also wondering the FCF on the position callout:
|Position|.000(m)|A|B(m)-C(m)|---left side of top view
|Position|.000(m)|D|B(m)-C(m)|---right side of top view

The spread between B and C must be 7.063 at MMC and may vary up to +/- 0.004 units when both holes are a LMC no matter whether it was called out as is... or as... 2X 0.125 +/- 0.002 |POSITION|DIA 0 (M)|A|. The only problem is a house-keeping one, in that diameter B is not controlled for orientation to A... but with a material thickness of 0.07 units "who cares?"
  

Quote:

Can one use different primary datum A and D on the position callout? Can one callout the coplanar surfaces in this way?

Since both surfaces must reside between two parallel planes spaced 0.02 units apart "as specified |SURFACE PROFILE|0.02| 2 SURFACES" for a distance of approximately 8.4 units any measurement differences attributable to the lack of co-planarity of A vs. D ...(their cosine effects on the length and width dimensions considering a material thickness of 0.7) would be negligible or imperceptible. So even if the mating part has one semi-rigid interrupted surface which mates to this part with one semi-rigid interrupted surface the net result of declaring two separate datum feature surfaces is inconsequential to measurement.
Paul
 

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