Logical datum features for part function seem to conflict with ASME requirements 1

[Nereth1]

I have a part which is intended to sit in a jig that restraints it on all 6 degrees of freedom, with only the necessary and sufficient points. This is required for part function, as it is manufactured on a jig, tweaked on the same jig, QC'd on an identical jig, then installed with another identical jig that ensures it goes into the final assembly with all the functional parts in the right position.

Which sounds like it would therefore be a GD&T home run, except that the degrees of freedom are locked up using, in order of precedence:

1) One vee notch in the part that slides on a shaft in the jig - locks translation on two axes (some error introduced by the fact that a vee is not really a spherical bushing - but it's close for our purposes).
2) One flat face in the part perpendicular to the shaft axis right next to the vee notch, and a matching flat shoulder on the shaft - locks translation on the final axis.
3) One flat face in the part tangent to the aforementioned shaft, which bears against the same shaft, locking up a rotational axis
4) Two datum targets distant from all of this, which lock up the remaining rotational axes.

So, this all works just fine kinematically - there is only one way that this part sits in this jig. Functionally, gravity makes it sit stably in the jig. We can gauge off it. There is some 'crosstalk' between axes since for example the final two datum targets control the final two rotation axes in a mix depending on the errors. But that isn't a problem for us functionally.

But, this seems to fly in the face of the standard, at least implicitly. Though I can't find explicit statements. Issues like:

1) This frame has at least 4, and maybe even 5 datums in each FCF to build the datum reference frame.
2) We are not anywhere close to a 3-2-1 situation, we're more like 2-1-1-1-1 or 3-1-1-1 or something depending on how disingenuous I get with combining not-really-an-axis datum features into axes (A-B) or similar.

To be clear, the positions and types of these datums is not really negotiable, they've been picked for part function quite carefully. So if I can't make this work with GD&T, it's going to be "Write an essay about it" instead, which I would like to avoid.

Below is a screenshot of the datum features/simulators of the part/jig to try to make the above more clear, and a set of planes that represent what I want to try to create. I've had to delete the part outside of the jig points for IP reasons. I've left the jig solid and the part transparent for clarity. And I've zoomed in the individual parts where the part sits on the jig also for clarity.

Can you guys see a way to make this work per the standard via GD&T? Is doing so a non-issue? Or is it an unsolvable issue?

 

[3DDave]

Label each one as a datum target A1-A6. The 3-2-1 is for planar objects. A cylinder as primary covers 4 degrees of freedom, an irregular surface can control 6, a flexible part that is constrained may have many more than 6.

 

[drawoh]

Nereth1,

Are you designing this part and preparing the drawings, or are you fabricating and inspecting a part as per someone's drawings?

--
JHG

 

[Nereth1]

3DDave,

Ahhhhh I see what you mean. I have seen that about the irregular surfaces but didn't realise I could use datum targets to basically define multiple 'regular' surfaces into an irregular one.

In the interest of my continued professional development - I'd like to re-read the relevant parts of the standard. Would you point me to which parts that is? If not, I'll just use the contents page

Drawoh,

My company designs the majority of it to suit our clients requirements. Then we sit down with the fabricator and work out the best way to fabricate it (the scope of that discussion being primarily, reference points for jigs and checks). The fabricator and I agreed on the best jigging method from a practical perspective, now I'm trying to document that in a way that minimises drama later on in the project. There is some scope to move things still, but not without good cause (I assume that's why you're asking).

 

[drawoh]

Nereth1,

Are you designing the system that contains the part, or are you documenting the part for fabrication?

Ideally, your datum features should be the mount points of your part. Speaking as a designer, I actually don't care how the fabricator fixtures the part for fabrication and inspection. I will use my datum features for inspection, and the part will pass or fail depending on that.

--
JHG

 

[Nereth1]

We are designing the component that goes into someone elses system. We as a business have a long term relationship with both our clients and our suppliers, and it's in our interests to make both parties happy, rather than hands-off "do as you will". Neither of those two parties has good context of the other though.

As part of making the system both manufacturable and installable to the required tolerances, the jigging points (datum features) actually form part of part function - they are placed in proximity to, and control specific degrees of freedom of, parts of the component to which those specific degrees of freedom are relevant. This means if everyone uses the right jig, the tolerances of the component between the jigging points/their related parts of the component, don't actually matter much.

The literal mount points of the part are not used for location. They have sufficient float to provide the component 6 degrees of freedom when mounting.

 

[Nereth1]

One more question,

I've established a datum reference frame using my 6 datum points, shown the axes X,Y,Z in two views to make it explicit, and now I need to decouple some of my tolerances of profile from one of the axes. For example, I have one complex surface that needs to be free to move on the Y axis. Is this a simple matter of explicitly not calling that axis within the FCF?

 

[3DDave]

That is the way it is to be used.