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more than 3 datums

more than 3 datums

more than 3 datums

My understanding of y14.5 2009 (roughly pg 83 of the std.)
Is that if I have a part being support by things that are compliant in certain directions but not others that I can tell a datum to not control all of it's implied degrees of freedom.
A cone defines 5 degrees of freedom, but in my case it's attached to a ball joint, so it really only controls 3 degrees of freedom. [xyz]
It's also attached to several other things compliant in certain ways that I really need 4 datums to control all 6 degrees of freedom.
I don't see any examples in the standard of something using more than 3 datums, but near as I can tell, that is how the part is really constrained in the real world (its functional interface).

FYI, this is a steering knuckle.

Catia and Solidworks do not allow 4 datums, I can manually make one in NX.

EDIT-I've included an inpcomplete dwg.

RE: more than 3 datums

It is a bit confusing trying to figure out your X, Y, and Z axes.

If you look just couple pages back (page 81 that is), the paragraph 4.22(a) states: "the rectangular coordinate axes SHALL be labeled in at least two views in the drawing..."

So, shall you please?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: more than 3 datums

Ah, sorry, I am aware that i would need to put axes on the dwg. I haven't quite figured out where I might want my 0,0,0. Currently I'm thinking probably at the center of the ball joint about 13mm below the bottom taper. Very open to suggestions.

My main question is: are 4 datums allowed to be referenced by one feature control frame?


RE: more than 3 datums

I don’t think that I’ve seen a limitation to the number of datums/datum features used to establish reference frame.

Generally, you apply your datums until all the necessary degrees of freedom are constrained.
And I am having hard time imagining that 3 datums are not enough.
This is why I was asking about coordinate system you are trying to establish – what are you trying to constrain about what.

Text book definition requires datums to be assigned according to function of the part.
There are also schools of thought saying that you should take machining and/or measuring into consideration.

Your datum scheme seems to be neither of three, so I cannot help myself but ask “why?”

Once again, if we could see what are you trying to establish, maybe we could give you better advice.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: more than 3 datums

CH - H Voeckler's scheme allows control of which freedoms are limited on a per datum scheme; one could have as many as six datum references, each of which could be a compound of 2 individual datums, for a total of 12 references.

While I guess that sometime that could be valuable, it seems to pollute the standard with a difficult and infrequent scheme that will still require a long conversation with the supply chain - the opposite of what standardization is usually intended to do.

RE: more than 3 datums

You say

Quote (checkerhater)

Text book definition requires datums to be assigned according to function of the part.

Quote (checkerhater)

Your datum scheme seems to be neither

Perhaps I'm not understanding ... something.
I'm very much trying for the datums to be assigned according to function of the part. The lower joint only controls translation along xyz, the upper bolts only control the rotation about x, the sides of the upper faces only control rotation about y, and the tie rod taper only control rotation about z.

Agree w/ 3ddave - it will cause confusion. I may write some detailed inspection info and reference the pgs in the std on the dwg to help w/ confusion. Or just slap some datums somewhere that will be easy to inspect and ship it.


RE: more than 3 datums

I think you are taking the whole "functional" thing too seriously.

You only have to produce enough datums to constrain the part. There is no need to emulate workings of a front suspension.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

RE: more than 3 datums

Once the cone is in place and the two bolts are in place, that part cannot rotate unless what the bolts are attached to also rotate; in that case I agree with CH; to expect more is to include a camber and toe-in spec in the notes for this part.

The main thing is that this chain of tolerances doesn't help make a part with obvious limits to variation. Instead it makes the part tolerance stack so extreme I would not expect anyone to generate a useable analysis.

What I would expect is that the dimensioning and tolerancing would allow the stress analyst to confirm the part is sturdy enough for the job and that it weighs as little as is reasonably possible for the cost. The main item that stabilizes the part is the cone at the bottom, which sets the height of the casting and all other machined features at installation - or it is that axis and the top two bolts that set the height.

** I wonder if Herb thought about relaxing constraints in vector directions not aligned with some coordinate system or if he thought there should be multiple coordinate systems and they would be numbered. The mind boggles.

RE: more than 3 datums

I'm not a GD&T guy, but if I read it correctly that part is uninspectable. Tapers are hopeless as a physical reference point, even though they are crucial to the correct function of the part .Height of the OTR relative to the bearing axis is the most critical tolerance. Just from the point of setting it up on the CMM machine a datum plane based on the outer flange of the wheel bearing hole looks more sensible, maybe use the lower ball joint to orient it in the other DOF, or ideally a defined diameter on the OTR's taper. Check with your CMM people how they handle tapers, they may have some clues.


Greg Locock

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RE: more than 3 datums

I just looked at the drawing for an upright/knuckle for a modern vehicle. The final machined part uses three "random" casting datums and eight more datums for other features. The conical surfaces of the upper ball joint, lower ball joint, and outer tie rod are all datums. The "primary" datums are the hub/bearing face, hub/bearing bore, and the upper brake caliper mounting hole. No endorsement that this is preferred or optimum.

RE: more than 3 datums

Thnx Cory,
It sounds like they are using the 14.8 std (just got it!) to help define where the casting datums are to make the machined datums. Interesting. You say 8 datums. Must only be two casting datums A,B? CDE for hub/bearing/caliper and FGH upper/lower/tie rod...makes 8 datums.
I wonder if the the balljoint/tie rod taper datums are being used anywhere in the dwg? ie. Is there a feature control frame referencing those datums? You imply those aren't the primaries though. The point of calling out the taper features as datums being to assist for tolerance stacks?
In a standard knuckle w/ an upper ball joint 3 datums would be enough. Since ours is attached to a mac-strut I still feel that 4 would be required to truly make a functional interface. FYI, we gave up and just slapped some easy to inspect datums w/ tight enough tolerances and shipped it. We put spherical ball simulators into the female tapers to help clarify position of the tapers.

RE: more than 3 datums

Three casting datums (X, Y, Z). A, B and C for the hub/bearing/caliper. D for hub/bearing fastener hole. F for OTR face. E, H and J for conical surfaces. There are feature control frames that use E, G, and H, mostly for perpendicularity of adjacent surfaces to the tapers, although there is one position requirement for UBJ taper to LBJ taper.

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