Maximum possible contact and a single solution

[greenimi]

Per ASME Y14.5-2018
7.12.4 Pattern of Features of Size RMB
When RMB is applicable in a feature control frame to common datum features of size used to establish a single datum, the true geometric counterpart of each feature shall be fixed in location relative to one another. The true geometric counterparts shall expand or contract simultaneously from their worst-case material boundary to their LMB until the true geometric counterparts make maximum possible contact with the extremities of the datum feature(s). When irregularities on the feature(s) may allow the part to be unstable, a single solution shall be defined to constrain the part.
Questions:
1.) What means maximum possible contact?
2.) What means "single solution" default for unstable/rocking datum features (that replaces the former default of a candidate datum set) ?

 

[mfgenggear]

"the constrained least squares simulator" is only available to users of CMMs, the main reason for the existence of many explanations. It also does not offer any particular connection to the actual installed condition of the features or the entire part because assemblers do not use least squares when installing parts.

"expand or contract simultaneously from their worst-case material boundary" is also a CMM-only function and, strangely, is something that isn't actually done. The mathematics is they start either from 0 or from infinity or, more likely, just do a least squares approximation directly from the captured data with no expansion or contraction whatsoever.

Even in a gauge situation, there would be no need for a collet, for example, to be set with infinite precision, to the worst-case material boundary before inserting the part that will use that as the True Geometric Counterpart/Datum Feature Simulator.

What is done is not what the standard says is done. What would make their statements true is adding "As if" the following were happening. It's as if they expand or contract simultaneously, but they actually do not.

In practice there is some amount of deformation of the datum feature surfaces to provide enough grip to keep the part stationary against external forces, even if they are limited to gravity loads.

Wow practice in theory vs actually.
I believe they are pushing more for automation.
Cnc controlled cmm