Datum setup 3

[SeasonLee]

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

 

[ewh]

(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. If the standards sre to be followed, follow them.

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

 

[PaulJackson]

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

 

[ringster]

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.

 

[axym]

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.

http://www.axymetrix.ca

 

[PaulJackson]

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

 

[ringster]

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.

 

[SeasonLee]

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

 

[PaulJackson]

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

 

[ringster]

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

 

[SeasonLee]

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

 

[PaulJackson]

SeasonLee,

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)|

[red]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.[/red]

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

[red]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?”[/red]

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

[red]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.[/red]
Paul