End Bell ( End Shield ) with shaft extension....GTOL callout

[IM5timer]

to all,

I have an motor assembly with and End Shield machining customer rabbet with a shaft extension thru the end shield.
The A datum in the shaft extension diameter, and datum B is the customer machined diameter bottom surface.

Someone put a GTOL of | position | .006 | A | B |

I want to change it to | concentricity | .006 | A |

Is this an accurate GTOL to change to ?

Thanks
Mark

 

[JNieman]

I don't know why you'd want to change it at all, honestly. It's effectively identical unless I'm mistaken.

I've seen comments on this forum from "people who would know" that concentricity may be going away in future ASME Y14.5 revisions (assuming that's what's in play here) because it's, iirc, considered redundant.

It's quite difficult for me to make out some of the details of your screenshot due to it being shrunk down, so I apologize if I've missed something key.

Edit-to-add - if I would change anything, I'd remove Datum A from the 1/2-20 feature and put it on the precision diameter of the shaft's more-critical centerline.

 

[3DDave]

Concentricity is the axis related equivalent to planar related symmetry, so it is not the same as position.

In the A|B reference frame, B controls no additional frame of reference refinement as A has already taken the same degrees of freedom. The B|A Reference frame would typically make more sense.

 

[geesamand]

I see a significant difference in the datums.

The first method is wrong, because B adds no refinement to A when A is primary.

The second method is weak because you've cut away more than half of -A-. For the machinist this is not a big deal but for an inspection now A has become short in length and a weak datum.

The best method IMHO is to fix the first method. Put B as primary and A as secondary. This makes B drive the vector direction and lets A contribute only a point.

One last point of note: B is probably not the mating surface. The mating surface on this kind of motor is surface surrounding the bolt holes. B surface usually does not touch anything.