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Compound datum feature A(M)-B(M) used as a primary

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greenimi

Mechanical
Nov 30, 2011
2,407
A cylinder has three inside diameters,

ID#1 on one end ½-20UNF thread (called datum feature A)
ID#2 middle section ø.375 and
the other end ID#3 again ½-20UNF thread (called datum feature B)

Datum feature A is ID#1 thread
Datum feature B is ID#3 thread

The middle section width/length is dimensioned with ± and is called datum feature C
Other features on this part (such as holes perpendicular to axis of the cylinder) are positioned to A(M) –B(M) primary and C (RFS) secondary.
The two inside mating parts (one on each end of the cylinder) threaded on ½ -20 thread will create a sealing surface/contact with datum feature C (and that’s why datum feature C is called RFS). In other words, take the mating part and threaded all the way in until touches one side on datum feature C to create a contact surface. The same thing on the other side (just to describe a little bit the functionality)

Is this compound datum (used for threads !!) a good option? I would say it is legal option, but it is a functional one?
 
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Functionally, it's going to be pretty impractical to use the centerline of UNF threads as a datum and expect excellent repetition.

Why not use the centerline of the midsection bore as C, and the opposing faces of that midsection bore as A or B?

_________________________________________
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From the functional description you have provided, it looks like sealing surfaces on sides of feature C have to be controlled INDIVIDUALLY each to its respective datum, rather than to common axis of A and B.
I cannot really suggest replacement for your primary datum as function of the rest of the part is unclear.
 
So, who the relationship between the threads (datum feature A and datum feature B in my original post) is to be controlled then (if the sealing surfaces on sides of feature C is controlled INDIVIDUALLY)
Do I need to use composite position?
By the way, I have a perpendicularity control between the sides of the feature C with its respective datum features (A respectivelly B), but that is for orientation only.
Again, the main question how to control the vertical hole, what is the datum reference frame for its position?

 
greenimi said:
So, how the relationship between the threads (datum feature A and datum feature B in my original post) is to be controlled then

Does it have to be controlled?

I understand you have good reasons not to disclose the form and function of your design in fine detail to the strangers. But you have to understand, the description you provided so far is not very convincing.
I really don't see why threads have to be concentric/ coaxial to each other if each thread is used to plug its own individual hole. BTW you can specify position of threads to each other without making them composite datum.


 
I would say that A(M) –B(M) callout is quite funky because datum feature A and datum feature B do not belong to the same part. My opinion is, if datum feature A and datum feature B are portion of the same rigid part (like a shaft with two journals/bearings surfaces) you can use A-B or A(M)-B(M). If they are not (and in your case surely they are two different parts) you cannot use it as shown. I would not go so far to say it is illegal, but close to it.
 

Again, I am trying to find the correct datum structure (DRF) for the vertical hole.

A cylindrical shaft/stem will be assembled thru the vertical hole and connected to the sphere (located in between the “ two plugs” –red--). Some openings in the sphere (not shown) and in the plugs (shown in red) must be aligned (in some tolerances) for the regulator to function properly.

If vertical hole is not positioned correctly (to its truly functional datum reference frame which I am trying to find out what it is) the cylindrical shaft /stem will not contact the sphere correctly (or will drive the sphere in an unacceptable position) --the main connection between the shaft and the sphere is thru a slot --- and them an improper alignment is established between the above openings/ports (“two plugs” and sphere.

Hope this helps

BTW, why the proposed callout is illegal? Does it HAVE to belong (A(MMC)-B(MMC) compound) to the same part to be “legal” ?
 
Regarding the last question, I don't think that type of datum structure is a problem -- in fact, the MMB modifiers are what make it feasible.

If the M symbols weren't there it would be legal but quite tricky in a practical sense.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
J-P,
What is feasible in deriving single datum axis from two pitch diameters referenced at MMB?

greenimi,
Have you considered making inner diameter primary datum feature, and controlling both threads and the vertical hole relative to datum axis derived from that diameter? Is there a reason for not doing it?
 
Well, I didn't notice that they were threaded features. I was just looking at the picture with the pretty colors.
But the notion was floated that merely having two separate, rigid parts makes the idea of a compound datum unfeasible; that was the point I was addressing.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
 
Why are even we talking about two separate parts? I thought greenimi is asking for proper DRF for the green part. Isn't this a single piece? What am I missing?
 
Yes, I am asking for proper DRF for the green part and IT IS a single piece (the green part)

As far as making the inner diameter (Ø.375) primary (and maybe the width/length secondary) certainly can be an option. The argument has been (to make the threads –compound-primary) the threads are stopping the degrees of freedom (and not the Ø.375 ID). The left plug is threaded into the body and stops 4 degrees on freedom (cylinder) and when contact the sealing surface then stops the remaining translation. The same thing on the other side for the right plug.

But, and here is the dilemma, the plugs must be aligned and based on their alignment (with functional DRF) to position all the other features (including the vertical hole in question) on the same rigid (green) part.
 
greenimi,
Datum features are the features that play the most significant role in orienting and locating a part in an assembly. Are you sure that the threads orient and locate the green part in assembly?
 

Quote pmarc:” Are you sure that the threads orient and locate the green part in assembly?

I would say that IT IS the design intent. And we hope it does (physical reality of the assembly), not just the designer wish that to be true (threads orient and locate the green part in the assembly).

Which feature will really “win the fight” for the superiority in the battle for eliminating degrees of freedom (what are the physics of the assembly—real physics of the real world) that remains to be seen /decided. But that is what the designer is wishing, shooting for… and to be honest we don’t have too many functionality problems with this assembly (because the feature chosen to act as the truly functional datum feature are not chosen properly….. ) We think that the selected datum features are physically capable of holding the parts in the fashion shown.
 
So should I understand that both plugs orient and locate the green part in the assembly?
 

Yes.

How?
Hmmm...... The best they (the threads) can get !
 
greenimi,
Can I have another question?
Are these plugs on both sides rigidly fixed in the assembly or can they move radially relative to each other to some extent? I am asking because if the former, and if the green part is considered rigid, then I am having some troubles to imagine this green part assembled with both plugs in reality (unless the plugs are held very very coaxial to each other in the assembly).
 
pmarc,

They move radially relative to each other to some extent.
The green part as well as the plugs are considered rigid parts
 
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