Sanity check 3

[forza UTA]

Fig 9-6 – ASME Y14.5 - 2009.

If circular runout is to be used for datum features C and D (instead of total runout), will the meaning of the datum feature definition change?
I am thinking that, because cylindricity within 0.005 is used to refine the form error on both datum features (C and D), the answer to my own question is "NO", but I just need a sanity check (validation of my own thinking).

What would be the value added of using total runout (instead of circular runout) when the form is already enhanced/ smaller form error than allowed by total runout?

 

[CheckerHater]

Runout is composite control covering form, location, orientation, so tighter form simply means larger "wobble" is allowed at the expense of other variations.
But, as runout controls those variation "indirectly", I think it's hard to pinpoint any particular value added.

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

 

[pmarc]

forza_UTA,

There is a geometric difference between having circular or total runout tolerance in addtion to the cylindricity callout. In case of datum features C and D from fig. 9-6 the effect of the difference is a bit harder to visualize compared to a scenario where a single cylinder is controlled with a runout and cylindricity combo relative to a datum axis derived from a different cylindrical feature.

The attached illustration shows the difference when the latter example is considered:

https://files.engineering.com/getfi..._runout_vs_total_runout_plus_cylindricity.pdf

So to answer your question, the value added of using total runout instead of circular runout is in not allowing the scenario from the lower picture of the attachment to happen.

 

[chez311]

pmarc,

Excellent graphic - I was trying to conceptualize this earlier and I knew there might be a slight difference between the two but I couldn't quite get there.

Utilizing your figure to answer OP's question, essentially it looks like changing to circular runout vs total runout when combined with cylindricity will allow additional error equal to the cylindricity tolerance - correct? Ie: in 9-6 if it is changed to circular runout (of the same value .02) the new combination will allow .02 + .005 = .025 total runout error.

 

[chez311]

Apologies - I just realized how obvious the second part of my post was. I didn't realize you had already essentially done exactly that in your example until I posted that and then looked at your example again.

Sorry for restating the obvious!

 

[forza UTA]

Thank you very much all.
Looks like I got it wrong (incorrect assumption on my part).
Well... live and learn....

Thank you pmarc for the graphic.

 

[greenimi]

I have to say:
Another masterpiece from pmarc.
Thanks again.