Datum Feature Material Requirements (ISO)
For a
datum reference frame in which a referenced
datum feature is a
feature of size, you must specify a
material requirement (
MMR,
LMR, or
RFS) for that datum feature. Keep in mind that MMR and LMR introduce
datum feature displacement to the datum reference frame.
The use of a datum feature letter in a tolerance frame, along with the material requirement of the size datum feature, provides a clear understanding of how the datum feature is to be simulated.
• MMR, Ⓜ, is placed after the datum letter in the tolerance frame to establish a fixed datum size simulation at the MMVC of the datum feature.
• LMR, Ⓛ, is placed after the datum letter in the tolerance frame to establish a fixed datum size simulation at the LMVC of the datum feature.
• RFS is used to establish a variable datum simulation from the largest inscribed or the smallest circumscribed size of the datum feature.
Note that you may 'Accept' the DRF definition without specifying a value for a datum feature material requirement (i.e., leave the value set to '?') if you want to defer that decision until later. If you do so, there will be a 'DRF specification is incomplete' advisor message associated with the DRF until that value has been specified.