## Using Maximum Material Boundary (MMB) and Least Material Boundary (LMB)

The ASME Y14.5-2009 standard now specifies how both the size and position or orientation tolerances of a datum feature are to be used in calculating the datum MMB or LMB. Please refer to “ASME Y14.5-2009 Dimensioning and Tolerancing” section 4.11.6, “Determining Size of Datum Feature Simulators at MMB”, and succeeding sections on LMB. Figure 4-16, “Example Calculations of Maximum Material Boundary”, in the ASME standard provides examples of how the MMB boundary is calculated for a datum feature taking into account both the size of the datum feature and position or orientation tolerances on the datum feature.

Following the Y14.5-2009 standard, PC-DMIS now calculates datum material boundaries as follows:

- Maximum Material Boundary (MMB) - When you specify circle-M on a datum feature of size, PC-DMIS calculates the datum maximum material boundary (MMB) by including the effects of size and any preceding position or orientation tolerances on the datum feature, while respecting the datum order of precedence as specified in the ASME Y14.5-2009 standard. Precedence in a FCF moves from left to right.

- Least Material Boundary (LMB) - When you specify circle-L on a datum feature of size, PC-DMIS calculates the datum least material boundary (LMB) by including the effects of size and any preceding position or orientation tolerances on the datum feature, while respecting the datum order of precedence as specified in the ASME Y14.5-2009 standard. Precedence in a FCF moves from left to right.

Stating a Specific Boundary

If you want to define a specific boundary perhaps because the implied boundary is not clear, using the Feature Control Frame Editor you can explicitly state the boundary by enclosing it within brackets immediately following the applicable datum feature reference and any modifier following that datum feature reference. This value represents the size of the material boundary and will be used by PC-DMIS to calculate the bonus on the datum feature, disregarding any preceding position or orientation tolerances on the datum feature:

Feature Control Frame Editor showing the MMB symbol and the square brackets that will hold the boundary value

Note that PC-DMIS does not currently support the use of "BSC" or "BASIC" as discussed in the standard.