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





RE: Compound datum feature A(M)-B(M) used as a primary
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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Engineer, Precision Manufacturing Job Shop
Tool & Die, Aerospace, Defense, Medical, Agricultural, Firearms
NX8.0, Solidworks 2014, AutoCAD LT, Autocad Plant 3D 2013, Enovia DMUv5
RE: Compound datum feature A(M)-B(M) used as a primary
RE: Compound datum feature A(M)-B(M) used as a primary
I cannot really suggest replacement for your primary datum as function of the rest of the part is unclear.
RE: Compound datum feature A(M)-B(M) used as a primary
Feature C is perfectly square to compound [A-B], but no good for sealing either A or B.
RE: Compound datum feature A(M)-B(M) used as a primary
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?
RE: Compound datum feature A(M)-B(M) used as a primary
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.
RE: Compound datum feature A(M)-B(M) used as a primary
RE: Compound datum feature A(M)-B(M) used as a primary
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” ?
RE: Compound datum feature A(M)-B(M) used as a primary
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
RE: Compound datum feature A(M)-B(M) used as a primary
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?
RE: Compound datum feature A(M)-B(M) used as a primary
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
RE: Compound datum feature A(M)-B(M) used as a primary
RE: Compound datum feature A(M)-B(M) used as a primary
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.
RE: Compound datum feature A(M)-B(M) used as a primary
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?
RE: Compound datum feature A(M)-B(M) used as a primary
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.
RE: Compound datum feature A(M)-B(M) used as a primary
RE: Compound datum feature A(M)-B(M) used as a primary
Yes.
How?
Hmmm...... The best they (the threads) can get !
RE: Compound datum feature A(M)-B(M) used as a primary
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).
RE: Compound datum feature A(M)-B(M) used as a primary
They move radially relative to each other to some extent.
The green part as well as the plugs are considered rigid parts
RE: Compound datum feature A(M)-B(M) used as a primary
Did I answer all of your questions? Does it make any sense how this assembly suppose/ is intended to work?
Thank you for your help
RE: Compound datum feature A(M)-B(M) used as a primary
Allow me to express just one of them in a following way:
Based on the description you provided (that both plugs locate and orient the green part in the assembly, and they are able to move radially relative to each other to some extent), do you think it is functionally justified to have threaded datum feature simulators A and B (used to establish a single datum axis during inspection) fixed in location relative to each other (as this is what default rule says)?
RE: Compound datum feature A(M)-B(M) used as a primary
But, why do you think that the datum feature simulators A and B must be fixed in location relative to each other? Or do you think is better to use translation modifier (as per Y14.5-2009)?
Again, this is what the current print said before someone start questioning DRF.
RE: Compound datum feature A(M)-B(M) used as a primary
Going further, perhaps it would even make sense to untie orientational relationship between datum feature simulators A and B (within defined datumless positional tolerance applied to both threads)? The question is: do both plugs "behave" in the assembly that way? The problem is - how to say that in GD&T language known from Y14.5-2009.