Common Datum with eccentric axis' to control orientation of flats. GD&T

[RevSol]

Howdy all.

Would the following work to control the orientation of these flats to the 2 axial datums that are offset?

If I have datum A establishing 1 axis, datum C (eccentric portion) establishing the other axis.

Then referencing A-C as a common datum to establish clocking of the shaft and then use parallelism and perpendicularity to control the flats?

Or is there a better, cleaner, easier way? I need to know how to best clock this part to the eccentric portion.

Thanks!!!

 

[3DDave]

I suppose adding a diameter symbol for the [A] and datum features will happen later?

Instead of Perpendicularity and Parallelism, use Angularity which is less biased in interpretation.

 

[RevSol]

What would I reference as the datum for the angle callouts for the flats?

and... how would I reference the... zero angle for the vertical flat?

Just sort of stuck on how to create the callout that compares the flats to the eccentric portion.

 

[3DDave]

I also think it would be better to use [A-B|C] so that there is a better orientation control.

You don't need any angle values, 0 degrees and 90 degrees are default, but if you like, the top surface in that view is at [90 degrees] to the plane of the datum references and the front surface is at [90 degrees] to the top surface.

 

[RevSol]

So, just to be clear, there could never be a combination of the two eccentric axis’ like that to form a common datum, in this case A-C? Is that type of combo even possible? Would that make a plane?


Where can I read more about these defaults?

Is that an ISO thing?

 

[3DDave]

You can, but the features are very small and small variations will yield large error chances when magnified by the distance to the features being controlled. [A-C] does make a chitty basis for making the measurement. [A-B|C] makes a far better one. If you want a chitty basis you can use it.

Is there some reason you object to using what seems like the combination of surfaces that will control the orientation of that shaft?

Buy a copy of ASME Y14.5. It's around $250 and has answers about the angles and many more.

 

[3DDave]

The way to tell if a scheme is stable is to see if that would be a good way to hold the part still while machining the feature that is being controlled.

Would a machinist put [A] in a v-block and [C] in a separate v-block and expect those to withstand the machining forces?

You already used [A-B] to control the feature used for [C] because [A] is hardly stable enough on it's own.

I would likely have set features used for [A] and to both be controlled by total runout to [A-B].

 

[Burunduk]

I agree with the angularity recommendation.
Parallelism and perpendicularity are also not wrong in your case but I would recommend using them only in simpler cases where the primary datum is a plane tangent to a planar surface or the axis of a circumscribed/inscribed cylinder. Maybe the axis of a cone too. Your A-C common datum is just a bit more complex than that, so angularity is a good way to control orientation relative to the datum reference frame without making anyone think too hard what exactly the feature needs to be parallel or perpendicular to.